High Voltage Gapless ZnO Surge Arresters

Product Guide High Voltage Gapless ZnO Surge Arresters

ABB HV Components Surge Arresters Ludvika, Sweden 2009 Edition

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TABLE OF CONTENTS

Page

1.

Introduction

3

2.

Definitions

4

3.

Overvoltages

11

4.

Historical Background

17

5.

Features Of ZnO Arrester Design

22

6.

Design Requirements Requirements For ZnO Surge Arresters

36

7.

Arrester Classification as per Standards

49

8.

Standards and Testing

50

9.

Arrester Selection

81

10.

Installation Guidelines

98

11.

Maintenance and Monitoring

101

12.

Special Applications

106

13.

References

115

__________________________________________________________________________________________ _____________________________________________________ _____________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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TABLE OF CONTENTS

Page

1.

Introduction

3

2.

Definitions

4

3.

Overvoltages

11

4.

Historical Background

17

5.

Features Of ZnO Arrester Design

22

6.

Design Requirements Requirements For ZnO Surge Arresters

36

7.

Arrester Classification as per Standards

49

8.

Standards and Testing

50

9.

Arrester Selection

81

10.

Installation Guidelines

98

11.

Maintenance and Monitoring

101

12.

Special Applications

106

13.

References

115


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1.

INTRODUCTION

All electrical systems and equipment are subjected to electrical stresses caused by higher than normal voltages many many times during their lifetime. lifetime. Such overvoltages overvoltages are caused by atmospheric atmospheric disturbances disturbances (lightning), (lightning), switching phenomena phenomena as well as system disturbances, disturbances, and these cannot be avoided. It is vital that the electrical equipment operates fault-free during such abnormal conditions. However, for economic reasons, it is not possible to insulate the electrical equipment with a sufficiently high withstand level to survive all these overvoltages, particularly those resulting from lightning or switching switching surges. Consequently, Consequently, these pose a very real danger danger for causing failure of the the electrical equipment. An economical economical and safe safe on-line network therefore therefore requires requires extensive protection against unacceptable overvoltage loads. Overvoltage protection is not new, and has been used in one form or another for well over 100 years. Today, overvoltage protection protecti on can basically be achieved in two ways (sometimes in combination): • Avoid or limit limit the the overvoltages overvoltages at the the point point of of origin. origin. For example, through the use use of of overhead shield earth wires and lower tower footing resistance as countermeasures against atmospheric overvoltages and pre-insertion resistors and/or controlled switching against switching overvoltages. • Limit overvoltages near the electrical equipment equipment with with surge surge arresters In isolation, shield earth wires wires and pre-insertion resistors resistors offer a degree of protection. However, by their nature, surge arresters provide the primary protection against different types of overvoltages (atmospheric (atmospheric and switching). switching). They are generally connected connected between each phase and ground, in parallel with the equipment to be protected and function to divert the surge current safely to earth; thereby t hereby limiting the overvoltage seen by the protected object. Insulation co-ordination is the art and science of choosing the right insulation strength of electrical equipment taking into account normal and abnormal service conditions as well as the characteristics and location of suitable surge arresters. Despite being a well-established technology, there remains a degree of mysticism about the design, selection selection and application application of surge arresters arresters in electrical networks. networks. This is not made easier through the continual improvement and development of the active elements by leading manufacturers as well as the designs and housing material, ultimately leading to new applications applications for surge arresters. This guide is intended to clear away some of this mystification, and guide the reader to a better understanding of how to select and use modern day surge arresters. It is principally limited to the common application of the protection of transformer insulation between phase and ground in outdoor air-insulated air-insulated substations. substations. Other applications applications are briefly discussed, discussed, but are, for the most part, considered beyond beyond the scope of this Guide. Instead, the reader is referred on to additional additional technical literature which covers covers the topic in in more detail. In addition, addition, International International Standard IEC 60099-5 “Surge arresters - Selection and application recommendations” is recommended reading. Finally, the reader is referred to the ABB surge arresters “Arresters Online” web page (www.abb.com/arrestersonline (www.abb.com/arrestersonline)) for continually updated information on surge arresters.


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2.

DEFINITIONS

To permit the reader to understand the basis for the selection and application of surge arresters, it is important to make a brief review of some of the common terminology used throughout this Guide. The surge arrester standards referred to herein are t he prevailing editions of:

•

IEC 60099-4, Ed. 2.2 (2009-05) Metal-oxide surge arresters without gaps for a.c. systems

•

IEEE C62.11, 2005 together with Amendment 1, 2008 Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV)

Backflashover Occurs when lightning strikes the transmission line tower structure or overhead shield wire. The lightning discharge current, flowing through the tower and tower footing impedance, produces potential differences across the line insulation. If the line insulation strength is exceeded, flashover occurs, i.e. a backflashover. Backflashover is most prevalent when tower footing impedance is high. Continuous current (Ic) The current that flows through the arrester at continuous operating voltage (U c or MCOV). This current is predominantly capacitive (in the range of mA) and is generally expressed as a peak value. Continuous operating voltage It is the maximum permissible r.m.s. power frequency voltage that may be applied continuously between the arrester terminals. This voltage is defined in different ways (verified by different test procedures)in IEC and IEEE. IEC (Uc) IEC gives the manufacturer the freedom to decide U c. The value is verified in the operating duty test. Any uneven voltage distribution in the arrester shall be accounted for. IEEE (MCOV) IEEE lists the maximum continuous operating voltage (MCOV) for arrester ratings used in a table. The value is used in all tests specified by IEEE. Note! MCOV is less stringent as regards uneven voltage distribution in an arrester.

all

Duty-cycle voltage rating (IEEE) The designated maximum permissible voltage between its terminals at which an arrester is designed to perform its duty cycle.

__________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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Earthing Solidly earthed system A system whose neutral points are earthed directly. Impedance earthed system A system whose neutral points are earthed through impedances to limit earth fault currents. Resonant earthed system A system in which one or more neutral points are connected to earth through reactances which approximately compensate the capacitive component of a singlephase-to-earth fault current Isolated neutral system A system where the neutral point is not intentionally connected to earth, except for high impedance connections for protection or measurement purposes. Earth-fault factor (ke) The ratio of the voltages in the healthy phases during and prior to earth-fault conditions. Energy capability The energy that a surge arrester can absorb in one or more impulses, without damage and without loss of thermal stability. The capability is different for different types and duration of impulses. Standards do not explicitly define the energy capability of an arrester. The only measure specified is the Line Discharge Class in IEC. Often, this is not enough information to compare different manufacturers. Therefore ABB presents energy capability also in kJ/kV (U r ). This is done in 3 different ways: Two impulses as per IEC switching surge operating duty test This is the energy that the arrester is subjected to in the switching surge operating duty test while remaining thermally stable thereafter against the specified TOV and Uc. Routine test energy This is the total energy that each individual block is subjected to in production tests. Single-impulse energy This is the maximum permissible energy, which an arrester may be subjected to in one single impulse of 4 ms duration or longer and remain thermally stable against specified TOV and U c. Follow current The current from the connected power source which flows through an arrester with series gaps following the passage of discharge current.


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Hydrophobicity Classification The superior electrical performance of composite insulators and coated insulators stems from the hydrophobicity (water-repellency) of their surfaces. The hydrophobicity will change with time due to exposure to the outdoor environment and partial discharges (corona). Seven wettability (hydrophobicity) classes (WC) have been defined with a value between 1 and 7 (IEC TS 62073). WC 1 corresponds to a completely hydrophobic (water-repellent) surface and WC 7 to a completely hydrophilic (totally wetted filmed) surface. These classes provide a coarse value of the wetting status and are particularly suitable for a fast and easy check of insulators in the field.

Fig. 1

Wettability class (source IEC TS 62073)

Impulse (of current or voltage) A unidirectional wave which rises rapidly to a maximum and falls, a little less rapidly, to zero. Its waveshape is expressed by two numbers (T 1/T2). T1 refers to the virtual front-time and T 2 to the virtual time to half-value of the tail; both expressed in microseconds. Some important current impulses are defined below. Impulse Steep current impulse Lightning current impulse Switching current impulse

Waveshape (T1 /T2) T1 = 1 μs T2 < 20 μs T 1 = 8 μs T2 = 20 μs 30μs < T1 < 100 μs T2 ~ 2T1 (usually designated 30/60 μs) T1 = 4 μs T2 = 10 μs

High current impulse

A special impulse is the rectangular current impulse which is in the shape of a rectangle. Common durations are 2000, 2400, 2800 and 3200 μs. Insulation withstand characteristic A general term for the equipment insulation withstand voltages and comprises: Withstand level Lightning impulse withstand level Switching impulse withstand level Power-frequency withstand

(LIWL) (SIWL) (PFW)

Voltage waveshape 1.2/50 μs 250/2500 μs 50 Hz or 60 Hz sinusoidal

Lightning classifying current (IEEE) The designated lightning current used to perform the classification tests.


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Maintainability prediction Mean-time-between-failure (MTBF) is the average time between failures, typically measured in hours. MTBF is a statistical value and is meant to be the mean over a long period of time and large number of units. In practice, MTBF is only relevant with reference to repairable items, since calculations of MTBF assume that a system is "renewed", i.e. fixed, after each failure and then returned to service immediately after failure. Mean-time-to-failure (MTTF) is the average time expected to the first failure of a piece of equipment and should be used instead of MTBF in cases where a non-repairable item is replaced after a failure. However, MTBF is commonly used for both repairable and non-repairable items. Mean Time to Repair (MTTR) is the total amount of time spent performing all corrective maintenance repairs divided by the total number of those repairs. A common misconception about MTBF is that it specifies the time (on average) when the likelihood of failure equals the likelihood of not having a failure. This is only true for certain symmetric distributions. For typical distributions with some variance and uncertainty, MTBF only represents a statistical value and hence is not suitable for predicting detailed time of failure. Maximum system voltage (Um) The maximum voltage between phases during normal service expressed in kV r.m.s. Mean breaking load, MBL The average breaking load for porcelain-housed arresters. Nominal discharge current (In according to IEC) The peak value of the lightning current impulse which is used to classify the arrester. Normal service conditions The service conditions which the surge arresters should normally be suitable to operate under without any special consideration in design, manufacture or application. Ambient temperature Solar radiation Altitude above sea level Power system frequency Wind velocity Erection

-40 °C to +40 °C < 1.1 kW/m2 < 1000 m (< 1800 m according to IEEE) 48 – 62 Hz < 34 m/s vertical

This should be seen as the minimum requirement for compliance with the Standards, and individual designs may operate in wider extremes, even without the need for special consideration. Overvoltage A voltage level exceeding the maximum allowable continuous operating voltage for an electrical system.


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Protective characteristic The combination of the arrester’s residual voltages for different current impulses. For good protection, the arrester characteristic should lie well below the equipment insulation withstand characteristic at all points. Lightning impulse withstand level (LIWL or BIL) is the equipment’s insulation withstand level against lightning impulses Switching impulse withstand level (SIWL or BSL) is the equipment’s insulation withstand level against switching impulses Lightning impulse protection level (LIPL residual voltage for the nominal discharge current

or

Upl) of

the

arrester

is

the

Switching impulse protection level (SIPL or Ups) of the arrester is the residual voltage for a specified switching impulse current

Note! IEEE standards refer to LIWL as BIL and SIWL as BSL

Parameters of the system

Voltage

Parameters of the surge arrester

BIL / BSL Protective margin

LIWL / SIWL

Protection level LIPL / SIPL Upl / Ups TOV TOV capability COV

Fig. 2

Protective function of a surge arrester

Protective margin The protective ratio minus 1 and expressed as a percentage. As an absolute minimum, the margin should cover the voltage increase due to the connections between the arrester and the protected equipment as well as the increase in the residual voltage due to the discharge amplitude and front-time being different from the nominal discharge current of the arrester.


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Protective ratio The ratio of the equipment insulation withstand level to the corresponding protection level of its arrester. Rated voltage (Ur ) For other apparatus, the voltage that may be applied continuously is usually called its rated voltage. However, this is not the case for surge arresters. An arrester fulfilling the IEC standard must withstand its rated voltage (U r ) for 10 s after being preheated to 60 °C and subjected to two long duration current impulses, corresponding to its line discharge class as defined in the standard. Thus, Ur shall equal at least the 10 second TOV capability of an arrester. Additionally, rated voltage is used as a reference parameter. Reference current (I ref ) The peak value of the power frequency resistive current at which the reference voltage is measured. Reference voltage (Uref ) The peak value divided by √2 of the voltage measured across the arrester at reference current. Residual voltage/ Discharge voltage This is the peak value of the voltage that appears between the terminals of an arrester during the passage of discharge current through it. Residual voltage depends on both the magnitude and the waveform of the discharge current. Shielding Protection of phase conductors from direct lightning strokes; generally by means of additional conductor(s) running on the top of the towers and grounded through the tower structures to earth. Stations can also be shielded by earth wires or lightning masts. Shielding failure Occurs when lightning strikes a phase conductor of a line protected by overhead shield wires. Short circuit (pressure relief) capability The ability of the arrester, in the event of its overloading due to any reason, to conduct the resulting system short-circuit current through it without a violent explosion which may damage nearby equipment or injure personnel. After this operation, the arrester must be replaced. Specified long-term load, SLL A bending moment allowed to be continuously applied during service without causing any mechanical damage to the arrester. Often referred to simply as “continuous load”. Specified short-term load, SSL The maximum bending moment allowed to be applied during service without causing any mechanical damage to the arrester. Often referred to simply as “short-term load”.


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Surge Impedance Studies of transient disturbances can be a complicated process involving many calculations and iterations through travelling wave analysis. A transmission line can be modelled as a distributed parameter network consisting of series inductance and resistance and shunt capacitance and resistance. Partial differential equations are then written and solved for the voltage and current. Computer programs specifically designed for solving these equations are available. A simplified method is to approximate the transient phenomenon by considering that a lightning strike to a conductor or the closing of a breaker will produce a travelling wave of voltage e and current i that are related by a surge impedance Z equal to e/i that travels along the conductor at the speed of light c. The surge impedance Z is purely resistive and therefore e and i have the same shape. Only system components such as a transmission lines and towers, cables or a GIS bus present a surge impedance. The surge impedance and velocity v of propagation can be obtained from their inductance and capacitance, i.e. Z = √ (L / C) and v = 1 / √ (LC). From which the equations can be derived: L = Z / v and C = 1 / (Zv) where L is the inductance and C is the capacitance per unit length. For an overhead line, v = 300 m/ μs and the surge impedance of a single conductor varies in a narrow band between about 400 and 500 ohms. The surge impedance of an oil-paper cable varies from about 30 to 60 ohms and the velocity of propagation is taken to be around 150 m/ μs. In many cases, an equivalent surge impedance or a combined surge impedance of two or more conductors is desired, leading to the need for more complicated calculations. Furthermore, the simplified analysis does not take into consideration line geometry, coupling factor (whereby a travelling wave voltage and current are impressed on only one conductor, leading to a voltage being induced or coupled to the other conductor) points of discontinuity nor corona effects. More detailed guidance is given in Cigré 63 and IEC 60071-2. Temporary overvoltages (TOV) Temporary overvoltages, as differentiated from surge overvoltages, are oscillatory power frequency overvoltages of relatively long duration (from a few cycles to hours or longer). The most common form of TOV occurs on the healthy phases of a system during an earth-fault involving one or more phases. Other sources of TOV are load-rejection, energization of unloaded lines, etc. Temporary overvoltage withstand strength factor (T r or Tc) This is the TOV capability of the arrester expressed in multiples of U r or Uc respectively. Tower footing impedance The impedance seen by a lightning surge flowing from the tower base to true ground (earth). The risk for backflashover increases with increasing footing impedance. Travelling wave Occurs when lightning strikes a transmission line span and a high current surge is injected onto the struck conductor. The impulse voltage and current waves divide and propagate in both directions from the stroke terminal at a velocity of approximately 300 m/ μs with magnitudes determined by the stroke current and line surge impedance.


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3.

OVERVOLTAGES

An overvoltage is defined as a voltage level exceeding the maximum allowable continuous operating voltage for an electrical system. Overvoltages may be of different types, which can be divided into three generic groups: • atmospheric overvoltages (lightning) • switching overvoltages • temporary overvoltages Depending on the origin of the overvoltage, a differentiation is made between external overvoltages (caused by lightning), and internal overvoltages originating from switching operations in the network (switching overvoltages) or faults and other abnormal system disturbances. Overvoltages can cause severe problems for the operation of the system, which is why it is essential to limit these to a low and harmless level. One way of limiting overvoltages is to use surge arresters to protect important apparatus.

3.1 External Overvoltages Atmospheric overvoltages are normally divided into two different groups: those arriving from direct lightning strokes to the lines or equipment and those induced from nearby strokes to ground or between clouds. 3.1.1 Direct lightning strokes A direct lightning stoke to a transmission line will result in two identical travelling waves propagating in either direction along the line. Arrester currents of extreme amplitude and steepness can occur in arresters located on an unshielded transmission line. For lower system voltages, the current in these cases will be approximately a third of the stroke current since flashover to all three phases is likely to occur. When lightning strikes a transmission line, the line itself is usually not damaged but the overvoltage generated may result in flashovers of the line insulators and can also cause insulation breakdowns in apparatus in sub-stations connected to the line. A lightning impulse has a very short front time, microseconds (µs), and the voltage on the transmission line can rapidly increase to several thousands of kilovolts when lightning strikes the line. If the earthing impedances of the towers are not sufficiently low, a lightning stroke to the tower or to the overhead shield wires (if any) might cause a so called “backflashover” across the insulator strings to the phase conductors. Travelling waves are generated at the location where the lightning hits the line and these waves propagate along the line. The insulation is stressed further if the travelling waves reach an open end of the line where they are reflected; causing a doubling of the voltage. It has been acknowledged that many lightning strikes are not a single stroke, but instead consist of a series of consecutive strokes of varying magnitude and intervals. The incidence of multistroke flashes naturally varies considerably by region and season. In some cases, about 70 - 80 percent of lightning strikes may consist of multiple strokes; 3 - 4 pulses on average, with an interval between each pulse of less than 50 milliseconds. Distribution lines are generally unshielded, and hence bear the full force of direct strikes. Consequently, multipulse performance is a decisive measure of survival ability and reliability of distribution arresters near direct strikes.


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3.1.2 Induced overvoltages The induced overvoltage on the line resulting from an indirect lightning stroke is • proportional to the stroke current • inversely proportional to the distance between the line and the location of the stroke • proportional to the height of the line above ground Induced surges are lower in magnitude and the front of the wave is usually substantially longer than for direct strokes. The front steepness is therefore seldom decisive from a protection point of view. However, despite the fact that discharge currents are mostly lower than for direct strokes, induced strokes can nevertheless be decisive for arrester duty requirements. This is especially true for low voltage systems in areas where the lightning intensity is high. Due to a substantially larger collection area compared to direct strokes, the number of arrester operations per year can be substantial. For distribution and low voltage systems it is often the induced overvoltages which cause the most damage to unprotected equipment.

3.2 Internal overvoltages Internal overvoltages, i.e. switching overvoltages and temporary overvoltages, are caused by transient phenomena including, for example, switching of transmission lines or transformers, faults between phases and earth, etc. The duration for these overvoltages can range anywhere from milliseconds to days, depending on the cause of the overvoltage and the system parameters. Due to the common insulation practice with relatively low insulation levels for higher system voltages, switching overvoltages will normally only be of interest for system voltages above 245 kV. Switching overvoltages occur in connection with all kinds of switching operations in a network. The waveshape can be of practically any form, with the fundamental frequency normally in the order of some hundred to some thousand Hertz. Large overvoltages can occur in connection with switching operations, particularly with the following types of loads: • Interruption of short circuits • Disconnection of unloaded transformers and shunt reactors • Switching of long unloaded lines The switching overvoltages are usually defined in terms of per-unit of the system voltage. The overvoltage factor is defined as the ratio of the peak value of the overvoltage to the peak value of the maximum phase-earth voltage. In EHV networks, for example, it is desirable for cost reasons to reduce the insulation level as much as possible, and thus overvoltages higher than 2.5 p.u. are usually not accepted.


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Temporary overvoltages (TOV’s) can be defined as overvoltages which are sustained for a number of cycles. The frequency can either be the network fundamental or a higher frequency determined by system resonances superimposed on the power frequency. Temporary overvoltages typically arise from events such as: • Earth faults • Sudden change of load • Resonance phenomena These overvoltages can normally be kept to acceptable levels with the help of a high shortcircuit power in the supply network, line compensation with shunt reactors, suitable generator control, automatic fault clearing, etc. Hence, this type of overvoltage is normally not of concern for the system equipment itself (although at system voltages of 550 kV and above it may become significant).

1

p.u 6

p.u =

Um x √ 2

Lightning over-voltages

√3

5

Fig. 3 Classification of overvoltages showing duration and amplitudes of stress on insulation in HV networks

Switching over-voltages

4 3

Temporary over-voltages

2

System voltage

1 10-6

10 -4

10-2

10 0

102

104

t

3.3 Protection measures Atmospheric overvoltages are particularly dangerous for low voltage, distribution and even subtransmission systems. Transmission lines for 300 kV and above are usually equipped with overhead shield wires as a protection against direct lightning strokes. These overhead shield wires are installed along the entire transmission line and are earthed at each tower and connected to the common earthing system in the substations at the ends of the line. Lines for lower systems voltages usually lack overhead shield wires along the entire line length. Instead, they are only used in close vicinity (1 – 2 km) out from the substations in order to prevent direct strokes to the phase conductors close to the stations. The amplitudes of incoming lightning surges to the stations will thus be limited. In some cases, earthed crossarms or spark gaps have been used close to substations in an attempt to limit the amplitude of incoming lightning overvoltages. However, such measures tend to increase the likelihood of flashovers near the station with the consequent generation of fastfront surges. Special attention should be given to tower earthing near the station to lower the probability of back flashovers at this location .


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Surge arresters are used as protection in the stations against incoming overvoltages. In close vicinity to the arresters, these overvoltages are reduced to low and harmless levels. However, at some distance away from the arresters, high overvoltages may still occur, which is why it is essential to position the arresters as close as possible to important equipment. More recently, special arresters have been taken into use out on the transmission lines. These so called Line Surge Arresters (LSA) are installed at selected towers along the line in order to prevent lightning and/or switching related faults on the line itself. A commonly used method for limiting line switching overvoltages is to use pre-insertion resistors on the line breakers. Other means, such as synchronized control of breaker closing times and the use of surge arresters (alone or in combination), can also be used to limit these kinds of overvoltages.

Fig. 4

Station protected by surge arresters

Surge arresters are not normally required to protect against temporary overvoltages (although special cases exist), but they must survive them. TOV’s can thus be decisive in selection of the rated voltage for the arresters.

3.3.1 Cable connected to a lighting endangered line If a travelling wave coming from an overhead line enters a cable that is connected to a transformer, reflections will take place at the point where the surge impedance changes. Typically, about 80% of the wave in question will be reflected at the point of changed impedance, which means that the voltage passed into the cable will be about 20% due to the lower surgeimpedance of the cable.

U

Ucable = 20% of U

Ureflect = 80% of U

Fig. 5

Wave reflection with a cable connected to a transformer

For the purpose of analysis of cables connected to a transformer, the surge impedance of the transformer is considered as infinite, so that total reflection occurs at the transformer terminals. To further simplify the matter, the incoming wave is assumed to be rectangular. The result is a step-wise increase in the voltage, up to potentially double the initial incident voltage, U. In contrast, for a cable connected between two overhead lines, the voltage reflected in both ends of the cable will increase to the value of the incoming voltage U. In practice, the wave front is not rectangular, but has a rate of rise. This rate of rise will change the way the cable is charged, but not the value. Generally a cable can be considered as a concentrated capacitance, which will be able to bring down the steepness of the incoming wave, but if the wave energy is sufficiently high, the cable does not limit the voltage rise.


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The flashover to earth of a busbar or line conductor will cause a brief power outage, but subsequent serious damage is rare. In contrast, flashovers (puncture) in cables or at cable terminations can cause severe damage to insulation and require extensive and expensive repairs. The resultant outage times can be very long and costly. Cables should therefore be treated as station equipment and be protected against lightning by well-specified surge arresters.

Fig. 6 Cable termination protected by surge arresters

As a general rule, surge arresters are recommended to be mounted at both ends of the cable; especially where the cable is exposed to lighting from either end. In specific cases where a transformer is connected to an overhead line by means of a short cable, only one set of arresters is sometimes used. However, the effectiveness of this arrangement is less than with double-end protection. The definition of a “short cable” - and thereby the resultant protective zone - depends, among other factors, on whether or not the line is provided with overhead shield wires against direct strokes, the insulation withstand level, arrester protection level and the presence of additional arresters on the incoming line into the substation. The effective protective zone will in turn influence which end of the cable the arresters should be fitted to afford the best protection for both the transformer and the cable itself.

For transformers which are manufactured with internally connected cable boxes, the cable is connected directly to the transformer terminal, without any possibility of connecting arresters at this point. Therefore arresters can only be installed at the far end of the cable. A certain higher degree of associated risk for insulation failure may therefore have to be accepted. In order to achieve the greatest possible protection zone and ensure a sufficient protective margin between the cable/transformer LIWL and the voltage that may occur, special attention should be paid to the following for cables operating at 72.5kV and above: 







The incoming overhead line connected to the cable should have a zone of about 2km out from the substation equipped with shield wires for cables lengths of up to 1000m. For longer cables, the length of line with shield wires should be equivalent to at least double the length of the cable. The shield wires and tower configuration must effectively shield the line against direct strikes to the phase conductors as well as back flashovers. Inside this zone out from the substation, the footing resistance of the towers must be kept sufficiently low, i.e. a few ohms. The incoming lines should be fitted with surge arresters, to further limit the overvoltages actually transferred into the substation. The location of the arresters for cable protection must be as close as possible to the cable terminal in order to achieve the maximum degree of protection.


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3.3.2 Protection of Line-side station equipment High voltage equipment in line bays of a substation are dielectrically designed and tested in accordance with applicable Standards. Sometimes, however, breakdown of insulation on equipment may still occur due to lightning overvoltages with higher amplitude and rate of rise than prescribed by the Standards. Specifically, flashovers are known to occur in cases when the line circuit breaker is open. This risk is most likely to arise during normal operation as a result of subsequent strokes of the lightning flash during the open-close cycle; presuming that prolonged opened breaker condition normally dictates that the breaker disconnecting switches are also opened and grounded. Due to the open breaker the incoming voltage surge will be doubled. As a result, there may be a flashover across the open breaker or an insulation breakdown of other line-side equipment; notably instrument transformers. A backflashover on the transmission line towers adjacent to the substation can also lead to a steep voltage wave propagating on the phase conductor towards the substation. The risk of insulation failures in the substation is proportional to the rate of back flashovers on the lines. Other important parameters are the steepness and the amplitude of the voltage wave entering the substation. When selecting the MTBF for the substation insulation, it is important to consider the consequences of a fault in terms of outage and repair times and costs. Regardless of the cause, some form of protection should be used to prevent surges impinging on the line-side equipment. Arresters should always be located adjacent to the power transformers in the substation. With the line breaker closed, these arresters are often (subject to their effective protective distance) all that is needed to protect apparatus connected to the same line within the substation; presuming a low backflashover rate. However, with the breaker opened, no protection exists for the line side of the breaker, making it and other equipment vulnerable. Occurrence of overvoltages that may lead to flashovers is related to the key design and protection of the incoming overhead line. A Cigré survey made in 1990 showed that almost 60% of all reported violent failures of instrument transformers were due to lightning. The survey concluded that the number of failures due either to an inadequate lightning protection or to an inadequate transient withstand voltage is significant and that this field has to be investigated to reduce the number of failures of Instrument transformers A similar Cigré survey conducted in 1997 regarding circuit breakers concluded: Line insulation and spark gaps cannot be dimensioned to protect the breaker Appropriate shielding and sufficiently low tower resistance gives low risk of flashovers For existing lines, surge arresters placed at the incoming line end gives efficient protection   

There are a number of advantages with additional arresters located on the line side of the station; i.e. in front of line breaker Reduces the overvoltage seen at the station transformer Reduces overvoltages seen at the closed breaker; especially with long distance to the station arresters Protects the open breaker by limiting the effects of multiple lightning strokes Protects all equipment at the line entrance e.g. instrument transformers Removes the risk for backflashover; especially when fitted on transmission line towers with high footing resistance  

  


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4.

HISTORICAL BACKGROUND

Surge arresters constitute the primary protection for all equipment in a network against overvoltages which may occur as the result of lightning or switching operations in the network. The earliest overvoltage protection devices were introduced during the last decade of the 19th Century and consisted of a simple air gap for which the sparkover voltage changed with weather conditions, i.e. temperature, air pressure and humidity. One major disadvantage with this device was that its operation led to a power arc and consequent interruption of power supply on systems having earthed neutral points. The next significant step in the development was the so called conventional arrester, or gapped arrester, developed during the 1930’s. The arrester comprised of voltage dependent silicon carbide (SiC) resistor blocks in series with spark gaps, mounted together in a porcelain housing. The gapped arrester was improved through several generations during the subsequent decades. The voltage across the series connected spark gaps was controlled with grading components comprising non-linear resistors and capacitors and the protection characteristics were improved by introduction of current limiting (active) gaps around 1960. Better protection was achieved through the active gaps permitting the use of SiC resistors with a lower residual voltage. The conventional spark-gap assembly consisted of stacked brass electrodes with steatite spacers and grading resistors (if present) between them. Between each electrode was a device for preionization of the ignition point. This ensured that the ignition was distinct and as free as possible from variations resulting from different surge steepnesses. Active gaps were formed between electrodes riveted to discs of arcresistant material, with several assembled to form a stack. The stack also comprised a blow-out coil with a parallel-connected voltage-dependent resistor. Active gap arresters had better extinguishing capacity, a lower discharge level and a greater discharging capacity for switching surges than conventional gapped arresters.

Fig. 7 Conventional spark-gap assembly

The most advanced gapped SiC arresters in the middle of the 1970’s gave good protection against overvoltages, but the technique had reached its limits. It was difficult, for example, to design arresters with several parallel columns to cope with the very high energy requirements needed for HVDC transmissions. The statistical scatter of the sparkover voltage was also a limiting factor with respect to the accuracy of the protection levels.


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The metal-oxide (also called MO, zinc-oxide or ZnO) surge arrester was introduced in the mid to late 1970’s and proved to be a solution to the problems which could not be solved with the old technology. The protection level of a surge arrester was no longer a statistical parameter, but could be accurately given. The protective function was no longer dependant on the installation or vicinity to other apparatus - as compared to SiC arresters, whose sparkover voltage could be affected by surrounding electrical fields. The ZnO arrester could be designed to meet virtually any energy requirements by connecting ZnO varistors in parallel (even though the t echnique to ensure a sufficiently good current sharing, and thus energy sharing, between the columns is sophisticated). The possibility to design protective equipment which could handle extremely high energy stresses also opened up new application areas; protection of series capacitors, for example. Some of the first arresters with ZnO blocks utilised spark gaps in series with the ZnO blocks or in parallel with sections of the block column (shunt gaps). These designs reflected, to some extent, a concern for the long-term stability of the ZnO material. Using spark gaps in series or parallel consequently decreased the voltage stress on the blocks. These designs are not found on the market any longer for HV applications. With experience, the elimination of gaps permitted the building of very compact, reliable, low profile arresters compared to what was possible with the old technology. Ures Ur

The ZnO technology was developed further during the 1980’s and 1990’s through to present day, towards improved protection levels, higher permissible voltage stresses on the material, greater specific energy absorption capabilities and better current withstand strengths.

E (kJ/kV)

3.0

15

2.0

10

1.0

5

ZnO

Active spark-gaps

Passive spark-gaps

1930

40

50

60

70

80

90

2000

Vintage Polymer housings

Fig. 8 Surge arrester development

New polymeric materials, superseding the traditional porcelain housings, started to be used in the mid 1980’s for distribution arresters. By the end of the 1980’s, polymer-housed arresters were available up to 145 kV system voltages, and today polymer-housed arresters have been accepted even for 800 kV system voltages.

Many of the early polymeric designs utilized EPDM rubber as an insulator material, but during the 1990’s more and more manufacturers changed to silicone, which is less affected by environmental conditions, including UV radiation and pollution.

4.1 Operation of gapped and gapless surge arresters A non-linear resistor type gapped arrester, commonly known as a silicon carbide (SiC) arrester, comprises SiC valve resistor blocks in series with either passive or active (current limiting) spark gaps. The purpose of the gaps is to protect the valve elements, give an exact sparkover voltage, carry the arc during the discharge without being damaged and to deionize the arc sufficiently at the short time at zero passage to avoid a reignition of the gap. The active gap has the additional function to create an arc voltage drop resulting in a counter voltage, and thus a current limitation, during the follow current and extinction interval. In series with the active gaps, a coil is connected electrically in parallel with a non-linear resistor valve block. See Figure 9.


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The operating principle for SiC arresters with passive (non current limiting) gaps and active (current limiting) gaps differs. For the passive gaps, the overvoltage wave creates an increasing voltage across the gaps until sparkover occurs and, during a short period of time, an impulse current rushes through the arrester. Thereafter, the normal power frequency voltage will force a follow current through the arrester of several hundreds of amperes. Due to the non-linearity of the resistor blocks, the current is reduced much faster than the voltage, and when the voltage approaches zero, the current is choked and the arc extinguishes. Practically the entire voltage is across the blocks, with only some small percentage being across the gaps as an arc voltage drop. When the current has been reduced to below about 1 ampere, occurring some electrical degrees before zero, the arc voltage drop is suddenly increased since the low current cannot support any plasma. The arc is transformed to a corona discharge and is extinguished some hundred microseconds before the zero crossing. The function of an arrester with active gaps is somewhat different. A lightning overvoltage, which has a high steepness, causes a sparkover of the gaps and the impulse current passes through the non-linear resistor blocks in parallel with the coils, since the impedance of the coil for the steep wave is much higher than that for the non-linear resistor. The follow current is, however, much lower, both in steepness and magnitude, and the current is forced into the coil and a magnetic field is built up.

A. Stack of spark-gaps B. Coil C. Shunt resistor D. Valve resistor E. Grading resistor

Fig. 9

One section of an active-gap arrester

The magnetic field results in an electromagnetic force acting on the arc, which is forced from the initial ignition point out into a narrow chamber where the arc is lengthened 50 - 100 times. The arc is cooled against the walls and starts to take up voltage. The resulting voltage reduces the follow current and, as soon as the momentary value of the power frequency voltage falls below the arc voltage, the follow current ceases. This is in contrast to a passive gap, which must wait until the voltage is almost zero before it can interrupt the current.


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Voltage distribution for steeper waves is determined by the capacitance of the arrester. The function of the grading resistors in gapped arresters is to distribute the voltage evenly across t he gaps in the event of relatively slow voltage variations. The sparkover voltage at power frequency and for switching surges is then determined by these grading resistors. There are two kinds of grading resistors, those with linear resistance and those with non-linear resistance. Generally, the sparkover voltages for this frequency range needs to be fairly high to prevent false operations for normal service voltage variations. Service under polluted conditions has always been a problem for gapped arresters. The formation of so called “dry bands” on the porcelain surface under such conditions leads to a disturbed voltage gradient, which affects the internal gaps by means of coupling capacitance between gaps and porcelains. As a result, some arresters may then even sparkover at service voltage during periods of heavy pollution. Repeated sparkover may result in overheating when the gaps fail to reseal, leading to complete failure of the arrester. Improved reliability under conditions of high contamination requires a strong grading, which can be achieved with highly non-linear grading resistors. Should a SiC resistor be placed on high service voltage without series gaps, it would draw a continuous current of some hundreds of amperes and thus quickly destroy itself. A “gapless SiC arrester” is therefore not a possibility. Zinc-oxide (ZnO) varistors, in contrast, represent a high impedance at normal service voltage and draw only a small leakage current (predominantly capacitive), with the resistive component of the current in the order of only 50 to 250 μ Apeak (depending on the varistor diameter). Such a low “leakage” is neither dangerous to the varistor nor uneconomic for the system. Therefore ZnO varistors can be placed directly on voltage, and it is possible to remove the series gaps entirely from the arrester. ZnO varistors have an extremely non-linear, but well defined, volt-amp operating characteristic. The working principle of a gapless ZnO arrester is therefore very simple: When an impulse occurs, the arrester’s impedance reduces via its operating characteristic and subsequently changes over from conducting a small, predominantly capacitive current to a large resistive current. Due to the passage of the impulse current, a voltage is consequently built up across the arrester (residual voltage), the magnitude of which is determined by the volt-amp operating characteristic of the arrester for the applied impulse current and waveshape. Once the impulse has been dissipated, the arrester thereafter immediately returns back along its operating characteristic to its non-conducting state. Even though a lightning overvoltage causes an impulse current through a gapless ZnO arrester as for the gapped arrester, the normal power frequency voltage after the discharge is not high enough to force a follow current through the arrester. Hence, a ZnO arrester is only subjected to the energy from the lightning, in contrast to the SiC arrester, where a large energy contribution is obtained from the follow current. See Figure 10. Protection levels for gapless ZnO arresters depend only on the residual voltages determined by the operating characteristic for the respective waveshapes and currents and thus are better defined and more stable compared with gapped types. In general, the protection levels are lower (i.e. better) than for gapped SiC arresters of equal rated voltage. This improvement is particularly marked when steep-fronted impulses and switching surges are considered.


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Function of a silicon carbide (SiC) arrester with passive gaps Fig. 10

Function of a metal oxide (ZnO) arrester without series gaps

Comparison in operation of a silicon carbide (SiC) and a gapless metal-oxide (ZnO) arrester without series gaps

arrester

with

passive

gaps

The following Table 1 gives a summary of the major differences between gapless ZnO and gapped SiC arresters. Metal-oxide type (gapless)

Gapped type

No sparkover, current flows as per U-I

Sparkover, afterwards power frequency follow

characteristic

current

Small scatter band for residual voltages,

Usual scatter band for spark-gaps (up to 15%

typically ± 3%

scatter; even higher for poorly graded arresters)

Excellent steep-front wave characteristics

Strong rise (>25%) in sparkover voltage due to

(only approx. 10%)

steep-front overvoltages

Temporary power frequency load above

Continuous voltage at power frequency, always

Uc possible

lower than rated voltage

Energy absorption capability can be

Restricted energy absorption capacity, parallel

increased (arresters in parallel)

connection has no effect

Simple active part with few components

Complex structure for active part

Practically no ageing effect

Ageing of spark-gaps due to arc erosion

Table 1 Summary of the major differences between gapless ZnO and gapped SiC arresters


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5.

FEATURES OF ZnO ARRESTER DESIGN

A zinc-oxide (ZnO) surge arrester for high voltage applications comprises the following main components: • ZnO varistors (blocks) • Internal parts • Housing of porcelain or polymeric material with end fittings of metal (e.g. flanges) • A grading ring arrangement where necessary The internal parts can differ considerably between a porcelain housed arrester and a polymerhoused arrester. The only certain commonality between these two designs is that both include a stack of series connected zinc oxide varistors, together with components to keep the stack together.

Surge arrester with porcelain housing (left)

Fig. 11

Surge arresters with silicone-housing in an open-cage (centre) and tubular design (right)

Cut-away view of three principal designs for ZnO surge arresters


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5.1 ZnO varistor The most important component in the arrester is the zinc-oxide (ZnO) varistor itself, which gives the arrester its protective characteristics. All other components are simply used to protect or keep the ZnO varistors in place. The ZnO varistor is a densely sintered block, pressed to a cylindrical body. The block consists of approximately 90% zinc oxide and 10% of other rare earth oxides (additives). During the manufacturing process a powder is prepared, which is then pressed to a cylindrical body under high pressure. The pressed bodies are sintered in a kiln for several hours at a temperature in the order of 1200 °C. During the sintering, the oxide powder transforms to a dense ceramic body with varistor properties, whereby the additives form an intergranular layer surrounding the zinc oxide grains.

ZnO Grains 10–15 μm

Fig. 12

ZnO varistor blocks and their microstructure

These layers, or barriers, give the varistor its non-linear characteristics. Metal is applied on the end surfaces of the finished varistor to improve the current carrying capability and to secure a good contact between series-connected varistors. An insulating layer is also applied to the cylindrical surface to give protection against external flashover and chemical influence. Before the blocks are assembled in an arrester, they must be subjected to a variety of tests to verify their protection performance, energy and current capability as well as long term electrical stability.

5.1.1 How does a ZnO varistor work? With reference to the following Figure 13, the voltage-current characteristic for the varistors can be divided into three different regions with respect to the slope. In the low current region, called the ”prebreakdown region” (Region 1), the resistivity of the material is temperature dependant. The normal continuous operating voltage is found in this region. Here the surge arrester acts as a capacitor, with only small resistive currents through it. This is mainly due to the metal-oxide barrier (intergranular layers) between the zinc-oxide grains acting as insulating barriers. However, the varistor’s temperature influences the insulation capability slightly, i.e. an increased temperature leads to a higher resistive current.


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In the “breakdown region” (Region 2), when the voltage stress has increased due, for example, to temporary overvoltages or switching overvoltages, the intergranular layers switch from insulating barriers to conducting layers and the current carrying capability of the varistor increases many-fold. For example, if the voltage stress increases from 200V/mm to 300V/mm, the current increases 10 000 times. This acts as a voltage limitation, and gives the arrester its protective characteristics. At even larger current densities, the arrester is working in the “high current region” (Region 3) and the curve turns upwards, which determines the impulse behaviour of the surge arrester. The barriers between the ZnO grains are electrically broken down and the current increase is solely limited by the resistivity of the ZnO grains. When the voltage across the arrester is reduced to a normal level, the working point returns again to Region 1, without delay.

Voltage (p.u.) Min protection levels in kV (peak) according IEC60099-4

Region 1

Region 2

Region 3

Protection against lightning overvoltages 2.3 2.0

Protection against switching overvoltages Rated voltage (U r )

1.0 x √2

Ires, resistive current

Continuous operating voltage (U c)

0.8 x √2 Ires

Effect of increased block temperature on Ires

Icap

Icap, capacitive current (no influence from temperature)

10-5

10-3

102

103

104

Log scale

Current (Ampere) Fig. 13

Current-voltage characteristic of a ZnO-varistor .


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5.2 Housing of a surge arrester The main purpose of the insulator housing is to : • Keep the internal parts together • Protect against external flashovers • Secure that the function of the arrester is independent of external influences An arrester must also be equipped with fastening devices to ease the erection. This is achieved by assembling flanges (or similar) at one or both ends of the insulator. If the arrester consists of several series connected units, the flanges are also used to mechanically and electrically secure arrester units to each other. Insulators can be manufactured with different mechanical fracture values. The required fracture value for a specific insulator is determined by the design and intended use of the arrester.

5.2.1 Mechanical design A surge arrester consists internally of series-connected ZnO varistors (blocks), plus additional hardware as necessary for individual designs: metal spacers, assembly plates, sealing rings, pressure relief device, etc. To ensure a controlled environment for the blocks, the internal parts must be shielded against the ambient environment, and this is achieved by housing the blocks in a well designed and securely sealed insulator. The insulator housings for surge arresters have traditionally been made of porcelain. However, today there is a strong trend, and even a preference, towards the use of silicone insulators for arresters at all system voltages. There are a number of reasons why silicone is seen as an attractive alternative to porcelain, including: • Better behaviour in polluted areas • Better short-circuit capability with increased safety for other equipment and personnel • Low weight • Better earthquake withstand capability It is incorrect, however, to believe that all polymer-housed arresters automatically have these features just because the porcelain has been replaced by a polymeric insulator. The design must be scrutinised carefully for each specific type, which can be grouped generally into the following categories: • Open or cage design • Closed design • Tubular design These are discussed in more detail at the end of this chapter.


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5.2.2 Pollution performance The creepage distance is the total length of the outer contour of the insulator. Simplified, the longer this length is, the more severe environmental conditions the arrester will be able to operate under without increasing the risk for an external flashover. Surge arresters can be supplied with different creepage distances, and one type of insulator frequently used has a long-short shed-form, thus enabling a short assembly height for a given creepage distance. Silicone insulators generally perform better in polluted environments compared to a porcelain insulator. This is mainly due to the hydrophobic behaviour of the silicone material, i.e. the ability to bead water and prevent wetting of the insulator surface. Hydrophobicity results in reduced creepage currents during heavy pollution episodes, minimising electrical discharges on the surface; thereby reducing the effects of material ageing. However, it should be noted that whilst most polymer materials are hydrophobic when new, not all polymeric insulators necessarily retain their hydrophobic properties over their service lifetime.

Fig. 14 Insulators made from silicone retain their hydrophobic properties over their in-service lifetime

Two commonly used polymeric materials for the arrester housing are silicone and EPDM rubber (Ethylene-Propylene Diene Monomer), and both exhibit hydrophobic behaviour when new. Polymer materials may lose their hydrophobicity during an extended period of severe pollution, such as salt in combination with moisture. Silicone, however, will ultimately recover its hydrophobicity, through diffusion of low molecular silicone oils to the surface restoring the original material behaviour. EPDM rubber, in contrast, lacks this ability. Hence the material is very likely to lose its hydrophobicity completely with time, and is consequently often regarded as a hydrophilic insulator material, similar to porcelain.

Polymeric materials can potentially be more affected by ageing due to partial discharges and leakage currents on the surface, UV radiation, chemicals, etc, compared to porcelain, which is a non-organic material. For this reason, the raw material is often blended with a variety of additives and fillers to achieve the desired material features: UV stability, anti-tracking, flame-retardancy, etc. Silicone, as a material, has a natural resistance against these effects, and thus such additives simply aid in further improving the material’s inherent properties.

5.2.3 Short-circuit capability A correctly selected arrester can divert surges to ground almost endlessly, provided the energy to be dissipated is within the capability of the arrester. In the event that an arrester is required to dissipate more energy than it is capable of, it will sacrifice itself by failing short circuit. Most commonly, arresters are connected between phaseground and the resultant earthfault will immediately collapse the voltage on that phase, thereby protecting other equipment on the same phase. The upstream protection will initiate a breaker trip to clear the fault, and the failed arrester can then be replaced.


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If steps are not taken, there is a likelihood that arresters which contain an enclosed gas volume might explode due to the internal pressure increase caused by the heat generated from the short circuit arc. This leads to the need for these arresters to be fitted with some type of pressure relief system which will open quickly to release the enclosed gas volume to the outside. Such arresters are normally supplied with devices at the top and bottom of each unit, which operate as soon as the internal pressure reaches a certain value. The ionized gas will subsequently be evacuated to the outside of the arrester, and when the two gas streams meet the internal arc will commute to the outside, thus preventing a continual internal pressure increase. Fig. 15 Operating principle of the pressure relief device of an ABB type EXLIM porcelain housed arrester. (1) Arrester in its healthy state (2) Arrester has failed short-circuit, pressure relief plates open and gas begins to be expelled through the venting ducts (3) The two gas streams meet and the internal arc is commuted safely to the outside

(1)

(2)

(3)

The sealing cover in ABB’s high voltage EXLIM porcelain-housed arresters also acts as an overpressure relief device. Other manufacturers may have other solutions; a blast plate for example. During normal service, the sealing cover tightens against the porcelain. At an internal short-circuit of the arrester, an open arc occurs across the block column. Due to the heat from the arc, the internal pressure increases and would soon reach a value that could cause an explosion of the insulator if no pressure relief device was present. The sealing cover is designed such that it will open, both at the top and bottom, as soon as the internal pressure reaches a certain value (significantly below the bursting pressure of the porcelain) and the enclosed gas volume can be evacuated to the outside of the arrester. The internal pressure is thus relieved, and a violent shattering of the porcelain is avoided.

Pressure relief and Sealing plate

Flange cover Venting duct

O-ring Indicating cover Flange Cementing

Fig. 16 The position of the overpressure relief device on an ABB type EXLIM T porcelain housed arrester. The figure shows the key parts of an arrester with the pressure relief and sealing plate, block column, spring device and the cemented metallic flange.


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Safer short-circuit performance is not, however, automatically achieved simply by replacing the porcelain housing with one made of polymer. In the past, there has been the incorrect belief that all polymer-housed arresters, irrespective of design, were capable of carrying enormous short-circuit currents. Standardised short circuit test procedures within IEC (for both porcelain and polymer-housed arresters) now take into consideration what might happen at failure of the ZnO blocks for individual designs. Fig. 17 Operating principle of “ pressure relief” for an ABB type PEXLIM moulded open-cage design. (1) Arrester has failed short-circuit and gas begins to be expelled through the soft silicone housing (2) The gas streams trigger an external flashover and the internal arc is commutated safely to the outside

The short circuit capability for surge arresters (porcelain and polymer) is verified by tests to minimize the risk for damage to surrounding equipment and personnel. However, the risks related to an open arc in service can also be influenced by the physical positioning of the equipment as well as by the circuit connections.

5.2.4 Internal corona A low corona (partial discharge, PD) level during normal service conditions is essential for all apparatus designs intended for high voltage applications. Arresters with an annular gas-gap between the active parts and the external insulator may have large voltage differences between the outside and inside of the arrester during external pollution and wetting of the housing surface. To fully avoid corona under such conditions is not technically or economically feasible. Instead the internal parts, including the ZnO blocks, must be able to withstand these conditions. In order to prevent internal corona during normal service conditions for these type of arresters, the distance between the block column and insulator must be sufficiently large to ensure that the radial voltage difference between the blocks and insulator will not create any partial discharges. For polymer-housed arresters lacking such annular space in their design, the radial voltage difference is entirely across the rubber insulator. In order to avoid puncturing of the insulator, the rubber must be sufficiently thick. It is also very important that the insulator is free from voids to prevent internal corona in the material which might lead to problems in the long term. The maximum voltage stress occurring across the polymer material is proportional to the length of the insulator. A longer insulator therefore requires that the thickness of the material is proportionally increased with respect to the increase in length. Another solution is to reduce the height of the individual units in a multi-unit arrester, since the maximum voltage across each unit is limited by the non-linear current-voltage characteristic of the ZnO blocks.


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5.2.5 Voltage grading The performance of a ZnO surge arrester is defined by its protective levels, its temporary overvoltage and energy discharge capabilities and the long term-stability of the zinc oxide material. The temporary overvoltage and energy handling capabilities are closely related to the temperature of the ZnO blocks during normal operation. This temperature depends on the power losses, which increase rapidly with voltage stress, due to the block material ’s non-linear voltage-current characteristics. Therefore, the most essential parameters to minimize the temperature during normal operation are inherently low power losses in the zinc oxide material, together with a linear voltage distribution along the block column. Under normal operating conditions and voltage, the ZnO blocks act like a capacitor. The voltage distribution along the block column then depends on the capacitance of the ZnO blocks and the influence of stray capacitances. The stray capacitances are strongly dependent on t he height of the block column. Short arresters - up to about one meter in height - usually have a sufficiently linear voltage distribution along the block column, as the self-capacitance of the ZnO blocks is relatively high. For taller arresters, the influence of stray capacitances makes the voltage distribution less linear. If no measures are taken to prevent an uneven voltage distribution on a tall arrester, the local voltage stress at the top may reach (or even exceed) the knee-point of the voltage-current characteristic of the zinc oxide material. This leads to a localized increase in the power losses, with high temperatures in the block column as a consequence. Above the knee-point of the current-voltage characteristics, the blocks start to conduct large currents, which would ultimately lead to the failure of the arrester. The amount of this current is determined by the applied voltage and the total stray-capacitance of the arrester to earth and can be considerable; particularly for high-voltage arresters. Further, the localized heating of the ZnO blocks (hot-spots) leads to a reduced energy absorption capability of the arrester. Tall arresters therefore must be equipped with some type of voltage grading. This can be achieved by additional grading capacitors and/or grading rings. Provision of suspended grading rings is the most common way of improving the voltage distribution.

Corona ring

Grading rings

It should be noted that it is only grading rings hanging down from its electrical connection point that helps to improve the voltage grading of the arrester. Large metallic electrodes, including metallic flanges or rings to reduce corona without any suspension from its electrical contact point to the arrester, actually increases the stray-capacitances to earth, thereby amplifying the uneven voltage distribution. An important point, which often remains unconsidered, is that an actual surge arrester installation constitutes a three-dimensional problem with three phase-voltages involved together with certain stipulated minimum distances between phases and to grounded (earthed) objects. All this must be considered when making electrical field calculations. To not consider the influence of adjacent phases, for example, will lead to an underestimation of the maximum uneven voltage distribution of up to 10%.

Fig. 18 Example of grading ring and corona ring arrangement on an ABB type EXLIM surge arrester for 550KV system voltage


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Both IEC and IEEE standards require the maximum voltage stress to be taken into consideration in accelerated ageing tests on ZnO blocks. However, it is not possible to determine the correct voltage stress to be used in these tests without proper calculations of the maximum voltage stress occurring in practical three-phase installations. If no such calculations have been performed, the tests should therefore be carried out with a voltage stress corresponding to the knee-point of the voltage-current characteristics, i.e. at the reference voltage. Type tests in accordance with Standards to verify the long-term stability of the ZnO blocks are hence not valid if the actual voltage stress on the arrester during service is allowed to exceed the applied voltage stress proven in the type tests. When grading arrangements for surge arresters are based on complete electrical field calculations for each arrester design at the maximum continuous operating voltage and with the maximum possible three-phase influence taken into account, this guarantees that the voltage stress remains below the critical level at all points along the block column. This maximum voltage stress level is then used in accelerated ageing tests on the ZnO blocks. In this way, the long-term stability of the ZnO blocks is verified at the highest possible voltage stress found in any installation under normal service conditions. A guide for the determination of the voltage distribution along surge arresters using simplified representations of arrester geometries and boundary conditions (applied voltage, proximity and voltage applied to other objects in the vicinity) is given in IEC 60099-4. 5.3 Polymer arrester designs The potential weight reduction for polymer arresters can be considerable compared to porcelain housed arresters. As an example, one of the standard ABB type EXLIM arresters with porcelain insulator for a 362 kV system voltage has a mass of approximately 430 kg. A PEXLIM siliconehoused arrester for conventional up-right erection, with the same rated voltage, has a mass of only approximately 125 kg. This leads to the obvious benefit of lighter structures with subsequent reduced costs, and even the possible complete elimination of the need for a structure at all if alternative mounting arrangements are acceptable; e.g. suspended mounting.

Fig. 19 Two examples of possible mounting arrangements for ABB type PEXLIM silicone housed surge arresters


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Since the soft outer polymeric insulator does not have the necessary mechanical strength to keep the ZnO column together, other insulator materials must be used in the design. The most common material used for this purpose is glass-fibre reinforced plastic. There are then several types of mechanical designs in common use: loops or rods, cross-winding and tubes. These designs can be grouped generally into three basic categories: • Open or cage design • Closed or wrap design • Tubular design 5.3.1 Open or cage design This design may consist of loops of glass-fibre, glass-fibre rods or a cage of glass-fibre weave around the block column. It is worth noting that there are solutions which can be considered “open” or “cage” or a combination “open-cage”. What defines this type of design is that the active components are not fully enclosed by hard materials. Instead, a body of soft polymer material directly surrounds the internal components. An outer insulator with sheds is required over the inner body, with two common methods for achieving this being: • A pre-moulded polymer insulator is made in a separate process, and then slipped over the internal component assembly (which itself may be enclosed in soft polymer). The boundary between the internal assembly and the outer polymer insulator is usually filled with grease or gel, generally of silicone.

•

The outer housing is moulded directly onto the internal components to form a void-free, sealed housing along the entire length of the insulator.

Such designs lack enclosed gas volume. Should the arrester be stressed in excess of its design capability, an internal arc will be established. Due to the design principle, the arc will easily tear or burn its way through the polymer material, permitting the arc, along with any resultant gases, to escape quickly and directly. Hence, special pressure relief vents or diaphragms are not required for this type of design. However, it is important that the design is not “too open”, otherwise the internal active elements may be violently cast out. It is of great importance that these designs are totally “void-free” and no air pockets are present, otherwise partial discharges might occur, which would lead to the destruction of the insulator over time. Penetration of water and moisture must also be prevented, which places strict requirements on the sealing of the insulator at the metallic flanges (in the case of a pre-moulded housing) and adherence or bonding of the rubber to all internal parts (in the case where the polymer is directly moulded onto the inner body). ABB employs a unique, patented design for the PEXLIM arrester to enclose the ZnO blocks of each module under pre-compression in a cage formed of glass-fibre reinforced loops fixed between two yokes which form the electrodes. A special mineral-fibre is wound as belts around the loops resulting in an open-cage design for the module. This achieves high mechanical strength and excellent short circuit performance, through the belt-windings preventing explosive expulsion of the internal components.


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Each module is then passed through a computer-controlled cleaning and priming process. Thereafter, the module is loaded in a highly automated vulcanising press, where silicone is injected at high pressure and temperature to completely bond to the active parts, leaving no internal voids or air spaces.

5.3.2 Closed or wrap design Surge arresters in this category incorporate a “void-free” (partial or total) polymer housing around the internal assembly, while surrounding the active components themselves with hard material. In contrast to the open design, they have been mechanically designed to not include a direct path for externalising the arc during internal short circuit. Typical designs include a glass-fibre weave wound directly on the block column or a separate tube in which the ZnO blocks are mounted. A soft polymer insulator is then fitted (either pre-moulded or directly moulded) over this internal component assembly; often together with grease or gel to fill the interfaces. In order to obtain a good mechanical strength, the weave/tube must be made sufficiently strong, which, in turn, might lead to a too strong/closed design with respect to short-circuit strength. The internal overpressure could rise in the tube design to a high value before cracking the tube, which may lead to an explosive failure with parts being thrown over a wide area. To prevent a violent shattering of the housing, a variety of work-around solutions have been utilised, e.g. slots in the tube. When glass-fibre weave is used, an alternative has been to arrange the windings in a special manner to obtain weaknesses that may crack. These weaknesses are intended to ensure a pressure relief and commutation of the internal arc to the outside; thus preventing an explosion. Note that such alterations do not inherently then make these an “open/cage design”, as the arc path is not considered to be direct and the internal components are still, in practical terms, completely surrounded by hard material. Sealing and partial discharge issues also require consideration in a similar manner as for the open or cage design.

5.3.3 Tubular design The tubular design incorporates a distinct annular gas-gap between the active parts and the external insulator. It is designed in more or less the same way as a standard porcelain arrester, but with the porcelain housing having been substituted by an insulator of a glass-fibre reinforced plastic tube, moulded with an outer insulator of silicone or EPDM rubber. The internal parts are, in general, almost identical to those used in an arrester with porcelain housing. In particular, the arrester must be equipped with some type of sealing and pressure relief devices, similar to what is used on porcelain-housed arresters. This design has the prime advantage that high mechanical strength is possible (potentially even higher than for porcelain). Among the disadvantages compared to other polymeric designs is less efficient cooling of the ZnO blocks and, if appropriate precautions are not taken in the design, an increased risk of exposure of the polymeric material to corona that may occur between the inner wall of the insulator and the block column during external pollution.


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5.4 Silicone for use as an insulator There is a worldwide trend towards the use of polymer housings for surge arresters because of their light weight, flexibility and non-shattering performance. Silicone is recognised and proven world wide as the technically superior polymer for all high voltage outdoor applications. It's exclusive use is prevented only by it's comparatively high raw material cost. Nevertheless, an increasing number of utilities are specifying the more expensive silicone composite insulators; especially for more demanding or polluted environments; recognising silicone's long-term cost benefits over porcelain and other polymer materials. The unique properties of silicone makes it ideal for use as an electrical insulator due to its superior properties in the following key areas: Fig. 20

•

Natural resistance to Ultra Violet (UV) Silicone demonstrates a natural resistance to UV radiation without the need for additives. Of all the polymers, it best resists UV energy induced polymer chain scission. Because the main polymer chain has silicon-oxygen bonds instead of carbon-carbon bonds (which are subject to oxidation) silicone polymer molecules do not split, and therefore surface properties do not degrade to any significant extent. This results in inherently superior UV performance compared to other polymers.

•

Stable pollution/anti-tracking performance

Silicone polymeric chain

Methyl

CH 3 Si

O

Oxygen

CH 3

Silicone

O Si

CH 3

CH 3

n

n > 1000

EP-rubber

Carbon

Hydrogen

H

H H

H

H

H

C

C

C

C

C

H

H CH 3 H CH 3 H

C

m > 1000 m

Silicone has a wet-polluted withstand strength much greater than a similarly dimensioned porcelain or alternative polymeric insulator. Experiments show that even with the deposit of extreme pollution, silicone materials provide superior performance with respect to leakage current, electrical withstand and anti-tracking when compared with other polymers and certainly porcelain.

•

Unique hydrophobic recovery mechanism Silicone gains the advantage in pollution performance due to its superior hydrophobic properties, i.e. the ability to bead water. Importantly, this property is maintained with age. Whilst other polymeric materials may demonstrate the ability to bead water when new, they can eventually degrade under contaminated conditions over a period of time. Not so with silicone rubber - even with a large amount of contamination on the surface, the hydrophobic recovery properties of silicone rubber reverses degradation of its surface properties. This exceptional pollution performance is achieved due to silicone’s unique dynamic surface properties resulting in low molecular weight silicone oils diffusing towards the surface to encapsulate the pollution layer, so that no electrolyte is present that can conduct current. As a result, silicone-housed arresters exhibit lower leakage and consistently higher flashover values over the total lifetime of the arrester. Fig. 21 Principle of silicone hydrophobicity recovery


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•

Fire self extinguishing and freedom from any toxic combustion products. Due to an inherently high temperature index (temperature required to sustain burning in air), silicone is the last to catch fire with increasing temperature and the first to self-extinguish. This natural performance of silicone can be further enhanced by the addition of functional fillers – typically Aluminium Trihydrate (ATH) - which releases bound water at high temperature to cool the arc resulting from the short-circuit after an arrester overload.

•

Negligible thermal ageing Silicone has low surface energy, resulting in it remaining stable over a wide temperature range. Due to its high continuous rating, no significant embrittlement occurs.

•

Well tested and proven Silicone materials have been used in various applications for insulators and cable terminations since 1960. ABB has been using silicone housings on surge arresters since 1985. PEXLIM silicone-housed arresters tested according to IEC 60099-4 for 1000 hours salt–fog and 5000 hours multi-stress show no deterioration in material performance or change in electrical characteristics from before to after the test. Similar superior performance apply with other customer-specific tests; including long-term site tests.

5.4.1 Comparison between types of silicone Silicone covers a wide range of materials, with those used in high voltage applications including dielectric fluids, greases, coatings and polymers. These in turn can be grouped based on their form and application. All nevertheless have the same silicone-oxygen backbone structure that provides the inherent stability and low reactivity common to all silicones. RTV is the generic group of “Room Temperature Vulcanised” silicone materials, which includes many of the commercially available household sealants, grease coatings, etc. 

RTV 1 component

Condensation curing at room temperature through catalyzation by the moisture in the air diffusing into the rubber. Typical applications are gluing and sealing.



RTV 2 component

Addition curing with platinum catalyst; permits faster curing at higher temperatures. Typical applications are casting and encapsulating.

LSR is the generic group of “Liquid Silicone Rubbers”. These are two component addition cured by platinum catalyst at high temperature. Typical application is injection moulding. HCR is the generic group of “High Consistency Rubbers”. These are a one component solid rubber compound which crosslinks at high temperature (130 – 190 oC) with the aid of either a peroxide or platinum catalyst. Typical applications are injection moulding and extrusion, and includes the HTV (“High Temperature Vulcanised”) silicone used for ABB type PEXLIM surge arresters. HCR currently holds around 85% of the insulator market at high voltage, but LSR market use is growing (particularly in niche markets). HCR is a solid rubber material while LSR has a pasty to liquid consistency, which lends them to use in different applications and different processing methods.


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5.4.1.1

LSR

Processing • Allows the use of low pressure injection systems, leading to lower investment costs • Can be fully automated • Fast cycle times at relatively low temperature Physical properties compared to HCR • Higher mechanical strength • Lower specific gravity (weight) • Lower hardness, more flexible • Permits more angled shed profile Electrical properties compared to HCR • Lower dielectric constant • Higher dielectric strength • Similar tracking resistance • Erosion resistance similar to common HCR, but not as good as the best HCR

5.4.1.2

HCR

Processing • Needs high pressure & clamp force injection machine, leading to high investment costs • Platinum cured HCR faster cure rate than peroxide cured HCR, but slower than LSR • Mould may not need as close tolerances as for LSR, leading to lower cost Physical properties compared to LCR • Elongation and tear strength relatively low • Higher hardness • Specific gravity is typically between 1.5 – 1.65 Electrical properties compared to LCR • Higher tracking resistance • Best material shows no erosion at 3.5, 4.5, 6.0kV tracking and erosion test

5.4.1.3 Application All groups belong to the silicone family and therefore each permit the fundamental and generic advantages of silicone to be utilized. However, since there is an expectation of a long in-service life for HV surge arresters, a high degree of security is required. The manufacturer must carefully consider the complete design as a whole – including the choice of housing material - to ensure the final product has adequate performance in all key areas of concern for a given application and not just excellent in one specific area at the expense of others. For example, ABB type PEXLIM surge arresters utilize a housing moulded from a specially formulated HTV-silicone in order to ensure excellent performance in all key areas; including: ● ● ● ●

Physical properties Wet electrical performance Tracking and erosion resistance Salt-fog performance

● ● ● ●

Electrical properties Hydrophobicity recovery Environmental ageing resistance Flame retardancy


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6.

DESIGN REQUIREMENTS FOR ZnO SURGE ARRESTERS

There are a variety of parameters influencing the dimensioning of an arrester, but the demands as required by a user can be divided into two basic categories: • Protection against overvoltages • High reliability and a long service life Additionally, there is the requirement that the risk of personal injury and damage to adjacent equipment shall be low in the event of an arrester overloading. Users are also beginning to put requirements on environmental aspects, for example that arresters should be separable, recyclable and only contain non-hazardous materials. The above two main requirements are somewhat in contradiction to each other. Aiming to minimise the residual voltage normally leads to the reduction in the capability of the arrester to withstand power-frequency overvoltages. An improved protection level may therefore be achieved by slightly increasing the risk of overloading the arresters. The acceptance for increase of this risk is, of course, dependent on how well the amplitude and duration of the temporary overvoltages (TOV’s) can be predicted. The selection of an arrester is therefore always a compromise between protection levels and reliability. A more detailed classification could be based on what stresses a surge arrester is normally subjected to and what continuous stresses it shall withstand. For example: • Continuous operating voltage • Ambient temperature • Rain, pollution, sun radiation • Wind and possible ice loadings as well as forces in line terminal connections and additionally, non-frequent, abnormal stresses, for example: • Temporary overvoltages, TOV’s • Overvoltages due to transients, which affect • thermal stability and ageing • energy and current withstand capability • external insulation withstand • Large mechanical forces (e.g. from earthquakes) • Severe external pollution and finally, what the arrester can be subjected to only once: • Internal short-circuit For transient overvoltages, the primary task for an arrester is to protect. But it must also normally be dimensioned to handle the current through it, as well as the heat generated by the overvoltage. The risk of an external flashover must also be very low. Detailed test requirements are given in International and National Standards, where the surge arresters are classified with respect to various parameters such as energy capability, current withstand, short-circuit capability and residual voltage.


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6.1

Designing for continuous stresses

6.1.1 Continuous operating voltage Maximum continuous operating voltage, denoted as U c in the IEC standard, is the maximum r.m.s. voltage level the arrester is designed to operate under during its entire lifetime. The arrester shall act as an insulator against this voltage. The entire voltage is across the ZnO varistors and these must be able to maintain their insulating properties during their entire lifetime. The continuous operating voltage for AC surge arresters is mainly at power frequency, i.e. 50 Hz or 60 Hz with some percent of superimposed harmonics. For other applications, e.g. HVDC, the waveform of the voltage might be very complex or even a pure DC voltage. It must therefore be verified for all applications that the ZnO varistors are able to withstand the actual voltage under their technical and commercial lifetime; normally stated to be in the order of 30 years. The basis for the dimensioning is the result from ageing procedures where possible ageing effects are accelerated by performing tests at elevated temperature. 6.1.2 Ambient temperature All arresters, according to the IEC standard, must be designed to withstand an ambient air temperature of -40 °C to +40 °C without impairing the surge arrester’s function. Due to the varistor current-voltage characteristic, higher temperatures may be decisive for the arrester’s design as resistive leakage current increases with higher temperatures at U c. In order for a manufacturer to verify that the arresters are capable of withstanding the highest possible temperatures, certain type tests must be performed. It is, however, worth noting that the ambient air temperature surrounding the arrester is not necessarily the temperature of the ZnO blocks themselves. Arresters installed outdoors, for example, will always have a proportion of their housing in the shade. Thus, even if the ambient temperature is considered higher than +40 °C, it is the average ambient temperature of the blocks themselves which should be the determining factor for the evaluation of verifications made for thermal stability during the type tests. Further, this temperature is normally considered to be the average over a 24 hour period. 6.1.3 Rain, pollution, sun radiation A contaminated insulator surface in combination with moisture causes a creepage (external leakage) current on the insulator surface that can reach high values. This leakage current may negatively influence the arrester with respect to internal corona, heating of ZnO blocks and external flashovers. Thus the arrester must be designed in such a way that the internal parts will endure, during a limited time, a high internal corona level (for arresters with an annular gap between insulator and block column), and that the blocks will withstand a higher grading current, and subsequent higher power losses, than normal without failing. The risk of an external flashover must also be minimized. Heating of the arresters due to direct sun radiation and self-heating is normally a minor problem. The influence from the sun radiation is sometimes thought to be significant, as one might assume that sun radiation can result in considerably high surface temperatures. However, it is the average surface temperature of the complete arrester that counts, and sun radiation falls on less than half of the insulator surface at any point in time. In fact, the closer to the equator an arrester is situated, the smaller the fraction of the insulator surface that is subjected to direct radiation due to the sheds. The effects of direct sun radiation are included in the Operating Duty test of arrester sections. Heating from sources other than sun radiation must be checked separately for each case.


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For a given pollution level, the performance of gapless arresters can be generally improved by employing any or all of the following measures: • Increasing the creepage distances of the housings to reduce external leakage current • Using ZnO blocks of larger volume to improve the energy absorption capability • Improving the TOV capability, i.e. by increasing the rated voltage (U r ) for the same arrester type • Improving the heat transfer mechanism • Using blocks with lower losses at U c A well-designed arrester should already employ blocks with very low losses and the heat transfer mechanism should be optimized for these blocks. Thus, in practical terms, pollution performance is usually improved by using one or more of the first three methods. In this regard, it is important to note that an increased Uc without corresponding improvement in TOV capability is not effective. 6.1.4 Wind, ice, external forces A surge arrester is not a post insulator and should not be used as such, since normally its mechanical strength is limited. In all but extreme cases, wind and ice loads are usually not a problem for surge arresters. It is only if hurricanes (gales) could be expected at the arrester location, that a detailed check is necessary of whether or not a standard arrester will mechanically withstand the wind forces. By suitable selection of the housing strength or its physical mounting, a surge arrester may be designed to withstand very high winds. Similarly, severe ice storms are normally required to build up sufficient ice to load the arrester significantly. The most suitable way to connect an arrester to the overhead line is to arrange the tee-off vertically and slack to the line terminal of the arrester to minimize the bending moment on the arrester. Since surge arresters have a certain maximum bending moment for each design type, expressed in Nm, the maximum force at the line terminal is lower for a tall arrester than for a shorter one of the same type. 6.1.5 Considerations for polymer arrester designs The design for continuous stresses on polymer arresters must also take into consideration their effect on the behaviour and characteristics of the polymer material. For example, polymeric materials can potentially be more affected by ageing due to partial discharges and leakage currents on the surface, UV radiation, chemicals, etc, compared to porcelain. Further, polymers, as a rule, become softer at higher temperatures with a higher degree of creeping (cold flowing), while at cold temperatures the material becomes brittle. Many of these characteristics are strongly dependent on temperature and load time. It therefore is of great importance that the arrester design is tested with different temperature and load combinations to verify that all possible sealings operate adequately over the entire temperature interval. Composite materials, such as glass-fibre joined in a matrix with epoxy or other polymeric materials, can exhibit behaviour changes at high loading. The rate of this material degradation is determined by temperature, applied force, velocity of the applied force, humidity and the time during which the load is applied. It is therefore not sufficient to simply dimension the arrester with respect to its breaking force, but rather consideration must also be taken to how the arrester withstands cyclical stresses.


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6.2

Designing for non-continuous stresses

6.2.1 Temporary overvoltages (TOV) TOV’s in networks are primarily caused by earth faults, load rejection, energising of unloaded lines and resonance. By definition, a TOV is above Uc and normally will last from some few cycles up to some seconds. However, in certain isolated systems, the duration of an earth-fault may last several days. Further, the TOV's may be preceded by a switching surge. A ZnO arrester is considered to have withstood a TOV if: • the ZnO-blocks are not destroyed due to energy under the TOV i.e. cracking, puncturing or flashover of the blocks does not occur, and • the surge arrester is thermally stable against Uc after cessation of the TOV Since the resistive leakage current through the arrester is temperature-dependent, achieving thermal stability is also dependent on the final block temperature. If, for example due to a prior switching surge, the arrester already has a high starting temperature before being subjected to a TOV, it will naturally have a lower overvoltage capability. This is exemplified in the TOV characteristic given below (Fig. 22), which shows the ability of a specific ZnO arrester to withstand overvoltages with and without a preceding energy absorption. The lower curve is valid for an arrester which has been subjected to maximum allowable energy, for example from a switching surge prior to the TOV. The upper curve is valid for an arrester without prior energy duty. For ZnO arresters, the TOV amplitudes are normally at, or immediately above, the knee-point of the current-voltage characteristic. If the arrester is designed to fulfil the IEC standard, it shall be able to withstand a TOV equal to the rated voltage of the arrester for at least 10 seconds after being subjected to an energy injection corresponding to two line discharges as per relevant line discharge class of the arrester. This voltage level is also designated as the ”rated voltage” of the arrester in compliance with IEC. Fig. 22 Example of TOV-capacity for a specific ZnO surge arrester. The upper curve is valid if the arrester has not been subjected to any energy prior to the TOV and the lower curve is valid if the arresters has absorbed maximum allowable energy prior to the TOV. The TOV capability is normally based on the lower curve, being the “worst case”.


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The TOV is generally regarded as a stiff voltage source, i.e. the surge arrester cannot influence the voltage amplitude. For the dimensioning to fulfil a certain TOV level, the varistor characteristic must be chosen such that the current through the arrester, and consequently the energy dissipation, will not result in a temperature above the thermal instability point. The TOV capability given for a certain surge arrester should always be assumed with a stiff voltage source. However, if this is not the case, the TOV capability of the arrester is, generally, significantly higher. An important parameter concerning the dimensioning for TOV's is to accurately control the kneepoint voltage, since the non-linearity of the characteristic is at its most extreme in the TOV range. This is best achieved by defining a reference voltage close to the knee-point on the voltagecurrent characteristics, and then checking through routine tests that every arrester has a reference voltage above a guaranteed minimum voltage. A manufacturer is relatively free to assign any data for the arresters. A given arrester with ZnO blocks capable of absorbing a certain amount of high energy could therefore be assigned a high line discharge class with low TOV capability or, conversely, a low line discharge class with high TOV capability. The ideal should naturally be to assign the highest line discharge class with the highest possible TOV capability.

6.2.2 Transient overvoltages - Protective function The arrester shall, for an expected maximum current, limit an overvoltage to a level well below the insulation withstand level of the protected equipment. The protective characteristic for a ZnO varistor is slightly dependent on the steepness of the expected current. The below Figure 23 shows the characteristics for a specific arrester for three different current shapes given in the Standards.

Fig. 23 Example of protective characteristics for a specific ZnO surge arrester. The protection level is given in % of the residual voltage at a current impulse with wave-shape 8/20 µs and amplitude 10 kA.


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As can be noted from the diagram, the protection level for currents having a front time of 1µs are approximately 10% higher compared to currents with a wave form 8/20µs or longer. However, even more important than this marginal increase for steep current waves, is the effect of positioning the arrester in relation to the protected equipment and the length of the connections. There is also an effect as a result of the arrester’s own height (length). These effects add inductance (L) into the circuit, typically 1µH/m for outdoor arresters, which results in a further increase in the overall residual voltage against steep current impulses according to the formula U = L.di/dt. In order to obtain an efficient protection against fast transients, for example caused by backflashover close to a substation, large margins are therefore required between the protection level of the surge arrester and the protected equipment’s insulation level. A ZnO block with larger diameter normally has a better protection level with maintained overvoltage capability. A better protection level, in this case, also automatically results in a better energy capability. Computer programs are used to make accurate calculations of the resulting overvoltages in a substation originating from lightning and detailed models of the transmission line and substation are made. In these type of calculations, a ZnO arrester may be modelled as shown in Fig. 24.

HIGH FREQUENCY MODEL OF A SURGE ARRESTER

L1=(LENGTH OF CONNECTING CABLES +

L1

ARRESTER HEIGHT)*1 µH/m

L2=0.029 µH/kV RATED VOLTAGE

L2

R1 R1=0.06 /kV RATED VOLTAGE Ω

ZnO

ZnO=U-I CHARACTERISTIC FOR 8/20 µs CURRENT IMPULSES

Fig. 24

Equivalent scheme for ZnO arresters used in computer calculations Apart from the standard current-voltage characteristic for an arrester (”ZnO” above) a circuit is included for modelling the increase of the residual voltage for shorter times than 8/20 µs (”R1” and ”L2” above). The effects from connection leads and arrester height is modelled with the inductance ”L1”.


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6.2.3 Transient overvoltages - Energy capability and current withstand strengths In service, a surge arrester may be subjected to different energy impulses originating from such sources as lightning, faults in the network, switching of lines or capacitor banks, etc. The arresters must be designed in such a way that the ZnO blocks will withstand the energy or current without failing. Additionally, the arrester must be able to withstand the thermal energy, i.e. it must be able to cool against U c after an energy absorption. High voltage arresters are normally designated according to IEC with a specific line discharge class. The below Figure 25a shows relative energies in kJ/kV rated voltage for the different line discharge classes. The intention with this classification is naturally that a higher class should represent a higher energy capability for a given arrester. Hence, the energy absorbed during a single line discharge is approximately: • Class 1 1 kJ/kV (Ur ) • Class 2 2 kJ/kV (Ur ) • Class 3 3 kJ/kV (Ur ) • Class 4 4 kJ/kV (Ur ) • Class 5 5 kJ/kV (Ur ) However, this is only valid if the ratio between the switching impulse residual voltage, U ps, to the rated voltage of the arrester, Ur , is approximately a factor of 2.0. If the ratio differs greatly from this, the line discharge class becomes a useless measure, i.e. the higher the residual voltage for a given rated voltage, the less energy the arrester is required to absorb during the line discharge, and vice-versa. Specific energy kJ/kV (Ur)

Specific energy kJ/kV (Ur)

7

Class Class Class Class Class

6

7

1 2 3 4 5

6

5

5

4

4

3

3

2

2

1

1

0 1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

Class 72.5 - 150kV Class 151 - 325kV Class 326 - 400kV Class 401 - 600kV Class 601 - 900kV

3

0 1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Relative protective level Ua/Ur


Fig. 25a Relative energy stresses for different line discharge classes according to IEC 60099-4. U ps is designated as U a.

Fig. 25b Relative energy stresses for different line discharge classes according to IEEE C62.11. U ps is designated as U a.

Potentially even more confusing for the ordinary user is the classification as per the IEEE standard, as depicted in the above Figure 25b. The diagram is drawn for lowest used rated voltage on highest existing system voltage in each class. The highest relative energy occurs for the Class 326 to 400 kV. In general, the energy is lower in IEEE than for the IEC classes. On the other hand, the Line Discharge test as per IEEE shall be performed with 18 impulses in only 3 groups of 6 impulses, compared with IEC which prescribes 6 groups with 3 impulses in each group. The interval between the impulses in each group shall be 50 - 60 seconds and full cooling is allowed between groups. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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The energy absorption capability of an arrester is only defined in IEC as per the previously mentioned Line Discharge classification. Different manufacturers assign the energy capability in different ways. For example, the energy capability may be given as:

•

kJ/kV Ur , kilojoule per kilovolt rated voltage which is possibly complemented with the shortest time during which the energy can be absorbed

•

kJ/kV Uc, kilojoule per kilovolt continuous operating voltage which is complemented with the shortest time during which the energy can be absorbed

•

the sum of the energy resulting from two line discharges separated 50 – 60 seconds in compliance with IEC’s line discharge classification

possibly

Therefore a surge arrester may be described with at least three different energy values, which is why it is essential to state how the energy for a specific arrester has been given. As an example, the following energy capabilities can be given for the same ABB arrester type EXLIM P (Class 4):

•

7.0 kJ/kV Ur , rectangular current impulse with a duration of at least 4 ms

•

8.8 kJ/kV Uc, rectangular current impulse with a duration of at least 4 ms

•

10.8 kJ/kV Ur , two line discharge impulses in compliance with IEC 60099-4

The ZnO blocks are normally able to withstand considerably higher energies with longer durations (seconds), compared to shorter durations (milliseconds). Expressions like ”kJ/kV U r ” or ”kJ/kV” are therefore meaningless unless the shortest time for which the arrester can be subjected to the given energy is also stated. As mentioned previously, a high voltage arrester is normally designed in compliance with a chosen line discharge class as per IEC with respect to energy. For non-standard stresses, such as capacitor discharges or high energies due to lightning, the design may need to be made with a lower energy stress per varistor. Aside from withstanding the energy from current impulses, the ZnO blocks must also have a sufficiently high dielectric withstand so as to ensure that the voltage across the block will not result in a puncture or a flashover across the block. To ensure a sufficient insulation withstand margin for normal stresses, the ZnO blocks (together with all internal parts in a high voltage arrester) are dimensioned to withstand current impulses with an amplitude of at least 100 kA, having a wave form of 4/10 µs. Requirements for high energy absorption capability can be solved by increasing the block volume - either by using blocks with larger diameter or by paralleling block columns and/or arresters. To ensure that the latter designs will operate correctly during service, a very careful procedure is required to ensure a good current sharing between the block columns and/or arresters connected in parallel. Furthermore, possible changes of the block characteristic due to the normal applied service voltage as well as energy and voltage stresses must be extremely small.


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6.2.4 Transient overvoltages - External insulation withstand The primary function of an arrester is to limit, and thus render harmless, overvoltages to which the protected equipment is exposed. It is obvious therefore, that its own insulation (both external and internal) is the best-protected of all. In contrast to other HV apparatus, the insulation level for surge arresters therefore does not need to fulfil a standardised insulation class since the arrester will effectively protect its own insulation against overvoltages. Distance effects need not to be considered. Instead, the Standards stipulate a specific safety margin between the residual voltage of the arrester to the voltage withstand level of its external insulation. The voltage across an arrester can never be higher than that determined by the arrester's protective characteristics. Only the need for an additional (statistical) safety factor (margin) including correction for installation altitude can technically justify a higher external insulation strength. Generally, the risk of an external flashover less than or equal to 10 -3 is considered as acceptable; which leads to a factor of approximately 1.10 to 1.15 (excluding altitude correction) between the arrester protective levels and the LIWL and SIWL of the housing. Both the IEC and IEEE standards clearly stipulate that such a margin is sufficient. IEEE stipulates that the external LIWL of the housing shall be 20 % above the discharge voltage at 20 kA, 8/20 μs impulse plus an altitude factor of 9% per every 3000 feet (roughly equal to 10 % per every 1000 m). IEC stipulates a LIWL margin of 15 % above the discharge voltage at nominal current plus an altitude factor of 13 % for up to 1000 m. A longer arrester may, in fact, lead to less effective protection for steeper surges for which the inductance of the arrester itself becomes more significant. Thus, the stipulation of high external insulation withstand values (e.g. equal to that for the protected equipment) may thus be disadvantageous for the protected equipment. The complete arrester, including possible grading rings, must be designed to give a reasonable safety margin against external flashovers. With the specified margins in the IEC Standard, an acceptable low risk for external flashovers is obtained up to an altitude of 1000 m. For higher altitudes, special consideration needs to be given on a case-by-case basis. 6.2.5 Large mechanical forces It is relatively simple to calculate the maximum bending moment at the base of a self-supported arrester from loads caused by wind and terminal pull. For the earthquake forces, however, the situation can be a lot more complicated. The US-based IEEE’s Recommended Practice for Seismic Design of Substations (IEEE 693) is often taken to be the “worst case” earthquake criteria since it provides a comprehensive prescription for the seismic qualification of equipment. It allows for equipment to be certified as meeting one of three levels of seismic resistance; with the appropriate level selected by considering the peak ground acceleration for the 2500-year return period hazard spectrum at the site, i.e. 2 % probability of occurrence in 50 years. Nevertheless, actual criteria for a specific location should always be used in any analysis and caution is required when applying earthquake design spectra from different design codes. True requirements demand consideration with respect to the applicable zero period acceleration, return period, damping, subsoil type, structural performance factors, assumed ductility, location, near-fault factors, design methodologies and expected performance given the nature of the equipment.


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When the earthquake is defined as a maximum horizontal acceleration, the bending moment can be easily calculated when considering the arrester as a rigid body. When more accurate calculations are necessary, the elasticity and damping of the arrester must first be determined in a snap-back test. Knowledge about elasticity, resonance frequencies and related damping is also required when the earthquake is specified by a frequency spectrum, In such cases, a specially developed computer program will need to be used. However, a reliable calculation needs to also have adequate information about the structure on which the arrester is erected. For example, mounting the arrester on a support structure which has a sufficiently high natural frequency (e.g. a large power transformer) may reduce or remove the seismic loading on the arrester. Since polymer-housed arresters are more or less elastic, temporary loads - including short-circuit forces and earthquake forces - can be looked upon differently compared to rigid bodies like porcelain insulators. The reason for this is that the forces do not have time to act fully due to the elasticity of the material and mass inertia, i.e. the forces are spread out in time leading to the arrester not encountering any high instantaneous values. These advantages, combined with a design with small mass participation, have been fully utilised by ABB for the 550 kV arrester shown opposite in Figure 26. This arrester withstands a ground horizontal acceleration in excess of the highest seismic demands as per IEEE standards. Seismic qualification testing has also been successfully made on other standard ABB type PEXLIM surge arresters (without additional bracing), even at the arduous 1.0g ZPA level. Alternatively, suspending polymer surge arresters directly from the overhead line is a viable mounting alternative to eliminate seismic and other large cantilever loads altogether. Experience has shown that loads from short-circuit forces, wind and ice have not had a significant influence during past earthquakes and therefore arresters need not be designed for such loads to act concurrently with an earthquake.

Fig. 26 ABB type PEXLIM for 550 kV system voltage. The arrester is designed to meet extreme earthquake requirements in the Los Angeles area (USA).

6.2.6 Severe external pollution For AC outdoor insulators under dry conditions, the voltage and electric field distribution are determined by capacitive elements (both internal and external capacitances to ground). Under completely wet conditions the voltage distribution is determined by the surface resistance. Under partially wet conditions the electric stress distribution fluctuates between those determined by capacitive and resistive elements. The geometry of the insulator, both shape and surface property, cause outdoor insulators to be partially wet more often than completely. High radial voltage stresses may occur between the block column and the outside of the insulator during severe external pollution. Generally, external pollution may influence a surge arrester in the following ways: • Possibility of internal corona • External flashover • Heating of the blocks • Tracking and/or erosion of the insulator __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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The problems for arresters with porcelain housings installed in extremely polluted areas have historically been solved by greasing the insulator, thus improving the pollution performance. The aim of the greasing is to reduce the leakage currents on the insulator surface. Hydrophobic materials, like silicone, give a similar effect. This is one of the strongest motivations for why silicone has been seen as an attractive insulator material. A common belief is that all polymer-housed arresters have better pollution performance compared to arresters with porcelain housings. However, a more correct statement would be that hydrophobic materials (like silicone) have better performance in polluted areas due to reduced external leakage currents. In contrast, EPDM rubber, which can lose its hydrophobic properties quickly, should be designed in the same manner as porcelain from a pollution performance point of view. It is very difficult to avoid internal corona during severe external pollution on arresters containing an annular gap between the ZnO blocks and the insulator, irrespective of whether the insulator is made of porcelain or a polymeric material. The design of such arresters must therefore be able to withstand corona during such pollution episodes. Some rules-of-thumb for designs such as these are: • ”No” corona during dry conditions • Minimise the use of organic materials in the arrester. When organic materials are used, they must have been thoroughly tested and subjected to realistic corona tests • Prevent the possibility of electrical discharges directly onto the ZnO blocks For polymer-housed arresters which do not have any annular gap, large radial stresses may occur between the blocks and the outside of the insulator during severe external pollution episodes. It is therefore very important that the rubber insulator is sufficiently thick to avoid a puncture of the material. Furthermore, steps need to be taken to avoid large air pockets or cavities, otherwise corona may occur that would eventually lead to an arrester failure. To avoid external flashover, the creepage distance of the arrester, i.e. the shed form and the length of the insulator, is typically designed in compliance with the same criteria valid for other insulation at the actual site. Possible thermal stresses are determined by the leakage currents that might be present on the outer surface of the insulator. For porcelain arresters, it has been shown that the integral of the leakage current, i.e. the charge, can be regarded as independent of the creepage distance, and instead is approximately linearly dependent on the diameter of the housing. An insulator with a larger diameter thus may give rise to higher thermal stress during conditions with external pollution, provided the service conditions are otherwise the same. For applications requiring arresters with parallel housings and several units connected in series, the general rule is that the units should not be connected in parallel except at the top and bottom. This is because, during pollution episodes, the ZnO blocks in one unit could conduct the external leakage current from all of the parallel connected arresters which consequently may give an increased thermal stress on that unit. Since the ZnO blocks have a negative temperature coefficient in the leakage-current region, i.e. the leakage current increases with increased temperature, the heating of one unit will lead to a reduction of the voltage characteristic with subsequent increase of the current. An increased current through the unit leads to higher power losses with increased temperature, and so the cycle continues. Not even a careful current-sharing test (matching) of the arrester units will be of help below the knee-point of current-voltage characteristic. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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For a given pollution level, the performance of gapless arresters can generally be improved by employing any or all of the following measures: • Increasing the creepage distances of the housings to reduce external leakage current • Using ZnO blocks of larger volume to improve the energy absorption capability • Improving the TOV capability, i.e. by increasing the rated voltage (U r ) for the same arrester type • Improving the heat transfer mechanism • Using blocks with lower losses at U c Lower leakage currents on the insulator surface is achieved with a hydrophobic surface, i.e. the use of silicone insulators. The below Figure 27 shows leakage currents as measured on a porcelain insulator and a polymer-housed arrester having a silicone insulator. The values are taken from testing at NGC’s test station at Dungeness on the English Channel. As can be noted, the amplitudes of the leakage currents on the silicone insulator are roughly half to a third of the corresponding leakage currents on the surface of the porcelain insulator during this specific measuring interval. Fig. 27 Leakage currents for surge arrester PEXLIM Q108-VV145M and porcelain insulator at Dungeness test station. The leakage current for the arrester includes an internal leakage current of around 1 mA. The creepage distance for the polymeric arrester is 5148 mm and 4580 mm for the porcelain insulator.

Daily maximum currents inover a 16adays period at Daily maximum currents 16 day period Dungeness test station at Dungeness test station 30 Arrester with silicone insulator Porcelain insulator 25

)20 A m ( t15 n e r r u 10 C 5

0

Days

6.2.7 Thermal stability Thermal stability is one of the most important application criteria for ZnO arresters, and hence a thermally stable arrester is a pre-condition for the safe protection of equipment. The majority of the previously mentioned stresses are potential sources of heat input to the arrester, which must withstand them without loss of thermal stability. i.e. • Continuous operating voltage • Temporary overvoltages • Transient overvoltages • Ambient temperature • Pollution effects • Non-linear voltage distribution • Uneven current sharing between parallel columns


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The concept of thermal stability can be depicted with the help of a heat loss – input balance diagram, as depicted in the following Figure 28. This shows principally how the ability of an arrester encapsulation to dissipate heat and the temperature dependent power losses of the blocks result in a working temperature at a certain ambient temperature and chosen voltage stress (”A” in the Figure). An upper maximum temperature also exists (”B” in the Figure), above which the design is no longer thermally stable for a given voltage stress. It can also be seen from Figure 28 that the instability threshold is very much dependant on the applied power frequency voltage. As the power losses curve is non-linear, a lower applied service voltage than verified in test, for example, would shift the upper intersection point further to the right, thereby increasing significantly the temperature limit at which thermal runaway becomes a risk.

Fig. 28 Thermal capacity for an arrester housing and power losses for ZnO blocks at different relative voltage stresses (ambient temperature +40 °C)

Relative power losses 5 B 4

Porcelain curve 3 Losses at 0.9*Uref 2

Losses at 0.8*Uref Losses at 0.7*Uref Losses at 0.6*Uref

A

1

0 40

A = Service temperature at 0.8*Uref B = Thermal limit at 0.8*Uref 60

80

100 120 140 160 180 200

Varistor temperature (°C)

To explain the concept further: The power losses of a typical ZnO varistor (curved line) due to a constant applied power frequency voltage is extremely temperature dependent. At the same time, the ability of the arrester assembly to dissipate heat is generally linear (straight line) and proportional to its thermal design and temperature rise above the ambient temperature. Consequently, there are two intersections of the two curves: one at low temperature – a so called stable operating point, and the other at high temperature – a so called instability threshold. To obtain thermal stability, the temperature rise due to power losses in the ZnO varistors must be balanced against heat dissipation to the environment. If power losses exceeds heat dissipation, then excess energy is stored in the varistors and their temperature slowly increases. Conversely, if heat dissipation exceeds power losses, the temperature of the varistors decreases. The varistor temperature may well increase significantly due to the application of transient or temporary overvoltages, but will always ultimately settle back at the stable operating point, as long as the varistor temperature does not exceed the instability threshold. As the two characteristics diverge beyond the instability threshold point, a thermal runaway will invariably occur from varistor temperatures above this point, whereby the temperature will continue to increase until the arrester ultimately fails. Some “rules-of-thumb” for ensuring a design with good heat dissipation, and thereby low risk for thermal runaway: • Low-loss blocks • Reduced voltage stress/mm • Increased block size • Homogenous block material • Non-ageing blocks • Good mechanical design with regards to thermal heat transfer


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7.

ARRESTER CLASSIFICATION AS PER STANDARDS

According to the IEC 60099-4 standard, surge arresters are classified by their nominal discharge current. The test requirements and performance characteristics related to the different classes must be upheld. These currents do not, however, reflect the limits of the characteristics. For example, an arrester with nominal discharge current of 10kA can withstand current impulses of significantly higher amplitude without damage or deterioration. Standard nominal discharge current 20 000 A

10 000 A

5 000 A

2 500 A

1 500 A

Rated voltage, Ur (kV)

360 < Ur < 756

3 < Ur < 360

Ur < 132

Ur < 36

Under consideration

Line discharge class

4 or 5

1, 2 or 3

-

-

-

Table 2

Classification as per IEC standard

The classification as per the IEEE C62.11 standard is shown below (Table 3). Arrester classification Station Station Station Intermediate Distribution, Heavy Duty Distribution, Normal Duty Distribution, Light Duty Table 3

Max. system voltage (kV)

Impulse value crest (kA)

800 550 < 550 All All All All

20 15 10 5 10 5 5

Lightning impulse classifying current as per IEEE standard

The switching surge protection level is defined at a current impulse with virtual front time of 30 to 100 μs (IEC) or 45 to 60 μs for time to actual crest (IEEE). The current amplitudes are given in Table 4 below. Arrester classification IEC, 20kA, LDC 4 and 5 IEC, 10kA, LDC 3 IEC, 10kA, LDC 1 and 2 IEEE, Station IEEE, Station IEEE, Station IEEE, Intermediate Table 4

Max. system voltage (kV)

Peak current (A)

----

500 and 2000 250 and 1000 125 and 500

326 – 900 151 – 325 3 – 150 3 – 150

2000 1000 500 500

Current peaks for switching surge protection level.


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8.

STANDARDS AND TESTING

In order to fulfil the requirements of users, Standards specify uniform tests and test methods aimed at verifying an arrester’s: • ability to protect against overvoltages • reliability and long lifetime The protective function is verified with different measurements of the voltage level for different current amplitudes and current waveforms (residual voltage tests) and the reliability is checked through a number of electrical and mechanical tests. An important part of the electrical tests is the operating duty tests in which an arrester, or a pre-scaled model of the arrester, is subjected to a combination of stresses representing anticipated service stresses that an arrester might be subjected to during its lifetime. The lifetime is further verified by subjecting the ZnO blocks to an accelerated ageing test procedure. According to Standards for testing of arresters, the tests can be divided into three main categories: • Type tests (Design tests according to IEEE) • Routine tests • Acceptance tests (Conformance tests according to IEEE) These test categories can be defined as follows: Type tests are performed after completion of the development of a new arrester design to establish representative performance and to demonstrate compliance with the relevant standard. Once made, these tests need not to be repeated unless the design is changed in a way which may negatively influence the performance. Only the relevant tests need to be repeated in such a case. Routine tests are made on each arrester or arrester unit, as well as components, as a quality control integrated in the production. Their aim is to ensure that the products meet the design specification. Acceptance tests are made on a number of randomly chosen arresters from a delivery lot when it has been specially agreed between the manufacturer and the purchaser at the time of ordering. Acceptance tests should not be confused with routine tests. Specifically how surge arresters shall be tested is defined in detail in the Standards, with the two most widely accepted being IEC 60099-4 (International Standard) and IEEE C62.11 (American National Standard). Since the IEC standards are international and thereby have a wider scope for use, the coming sections focus primarily on the requirements specified by IEC 60099-4. For comparison purposes, a summary of the major differences between the IEC and IEEE standards is given in Table 5.


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Table 5

Comparison of IEC and IEEE Standards for Surge Arresters

IEC 60099-4 Product Arrester classification and (ANSI/IEEE) impulse classifying currents

Rated voltage, Ur Continuous operating voltage

Relation between Ur and Uc/MCOV

Impulse classifying current Switching surge classifying current

Low current, long duration current withstand

Insulation withstand

Only for metal oxide surge arresters without spark gaps In terms of nominal discharge current 20, 10, 5, 2.5 or 1.5 kA

Defined by the operating duty test Uc Maximum permissible continuous operating voltage across the arrester with consideration of voltage distribution Not a fixed relation. Ur and Uc can be selected independently depending on temporary overvoltages and actual continuous operating voltage across the arrester in a particular system. See Arrester classification Arrester classification

Peak currents (A)

20 kA, 2000 LDC 4 and 5 10 kA, LDC 3 1000 10 kA, 500 LDC 1 and 2 For 20 and 10kA arresters a line discharge test with 5 classes. 20 kA arresters either class 4 or class 5. 10 kA arresters class 1, 2 or 3. For 5kA arresters a rectangular current impulse test with 75 A and 1ms duration. For 2.5 kA arrester a rectangular current impulse with 50A and duration 0.5ms. See Table 9

IEEE C62.11 For metal oxide surge arresters with and without spark gaps Station 20 kA (System voltage ≤ 800kV) Station 15 kA (System voltage ≤ 550kV) Station 10 kA (System voltage < 550kV) Intermediate 5 kA Distribution, heavy duty

10 kA

Distribution, normal duty

5 kA

Distribution, light duty

5 kA

Duty-cycle voltage rating defined by the duty-cycle test MCOV Maximum permissible continuous operating voltage across the arrester with consideration of voltage distribution Tabulated; for each duty-cycle rating an MCOV value is given. For Ur ≥21kV, MCOV = 0.79*Ur to 0.81*Ur

See Arrester classification System max. voltage (kV) 326-900

Station class (A) 2000

Intermediate class (A) -

151-325 3-150

1000 500

500

For station and intermediate arresters a transmission line discharge test with parameters defined for classes based on system voltage (6 classes for station and 2 for intermediate, 3-72 and 72.5-150 kV system). For distribution arresters a 2ms rectangular current impulse with 250 A for heavy duty and 75 A for normal and light duty arresters See Table 9


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IEC 60099-4 High-current impulse

IEEE C62.11

Arrester classification

Peak current (kA)

Arrester classification

Peak current (kA)

20 kA 10 kA 5 kA 2.5 kA

100 100 65 25

Station Intermediate Distribution, heavy duty Distribution, normal duty

65 65 100 65

1.5 kA

10

Distribution, light duty

40

A quite complex test procedure intended to determine possible temperature rise with respect to the site pollution classification. A simplified method is allowed to determine if a pollution test is necessary or not. No test specified yet for polymer housed arresters Test procedure well specified for different designs and housing types. Includes high, intermediate and low test currents. Clear distinction made regarding specific requirements particular to polymer-housed arresters.

Pollution test (IEC) Contamination test (ANSI/IEEE)

Pressure relief tests (short-circuit current tests)

Test requirements specified for all arrester types

Polymer housed arresters

Two test cycles with application of a pollutant to half the length of the arrester and thereafter energizing at MCOV. Test applicable to porcelainand polymer-housed station, intermediate and distribution class arresters.

Test procedure well specified for different designs and housing types. Includes high, intermediate and low test currents. Clear distinction made regarding specific requirements particular to polymer-housed arresters. Intention was to harmonize with the IEC standard, but some slight differences remain. Distribution arresters considered separately, but otherwise test procedures only for salt fog, cantilever load and moisture ingress. Short circuit current test requirements also specifically considered in common clauses. Extended weather ageing test in an Annex as informative.

Line discharge classes as per IEEE for station class arresters (lowest class for 3-72 kV systems not shown)

Line discharge classes as per IEC Specific energy kJ/kV (Ur)

Specific energy kJ/kV (Ur)

7

Class Class Class Class Class

6

7

1 2 3 4 5

Class 72.5 - 150kV Class 151 - 325kV Class 326 - 400kV Class 401 - 600kV Class 601 - 900kV

6

5

5

4

4

3

3

2

2

1

1 0 1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3


0 1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8



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3

The Standard IEC 60099-4 supersedes the old Standard for gapped silicon-carbide surge arresters, IEC 60099-1, which is not applicable to ZnO arresters. Many countries also have their own National Standards which more or less comply with IEC or IEEE. Changes to the IEC Standard (from Amendment 2 in 2001) deal with specific issues of importance, including: polymer housed arresters, short-circuit tests, accelerated ageing, voltage distribution, environmental and weather ageing tests and mechanical testing. The tests in the IEC 60099-1 standard are not generally applicable to ZnO arresters, and IEC 60099-4 reflects a completely different approach on how to select test sections and verify the protection characteristics. Some of the major differences between these two IEC Standards are listed in Table 6.

IEC 60099-1

IEC 60099-4

Type Tests Sparkover voltage test

No gaps in a ZnO arrester and thus no sparkover voltage tests Conditioning test

Residual voltage tests on prorated tests sections verifies the absolute value for a complete arrester

Residual voltage test at type testing gives a relation to routine tests values

No accelerated ageing tests procedures prescribed

Accelerated ageing test for 1000 hours. Consideration is taken to possible ageing in the Operating duty tests

Changes of the residual voltage level ≤ 10%

Changes of the residual voltage level ≤ 5%

Tests on open sections

Tests on open sections and thermal models

No pre-heating

Pre-heating to +60 °C Strictly specified how the test sections shall be selected and how the rated voltage of the section shall be determined

Routine Tests 50 Hz sparkover voltage test

Table 6

Reference voltage measurement Residual voltage measurement Corona test (PD measurement) Tightness check Current sharing test on arresters with parallel block columns

Comparison between test requirements according to IEC 60099-1 and IEC 60099-4


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Standard nominal discharge current 20 000 A

10 000 A

5 000 A

2 500 A

1 500 A

360 < Ur < 756

3 < Ur < 360

Ur < 132

Ur < 36

Under consideration

Lightning and switching impulse voltage test

If Ur > 200 kV Lightning and switching impulse voltage test. If Ur < 200 kV Lightning impulse and power frequency voltage test

Lightning impulse and power frequency voltage test



X

X

X

X

X

b) Lightning impulse residual voltage test

X

X

X

X

X

c) Switching impulse residual voltage test Long duration current impulse test Operating duty test a) High current impulse operating duty test

X

X

Not required

Not required

Not required

LDC 4 or 5

LDC 1, 2 or 3

75 A, 1 ms

50 A, 0.5 ms

Not required

Not required

LDC 1 with 100 kA

With 65 kA

With 25 kA

With 10 kA

X

LDC 2 and 3

Not required

Not required

Not required

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

---

X X

X X

X X

X X

X X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

---

---

---

Rated voltage, Ur (kV) Insulation withstand tests on the arrester housing

Residual voltage test a) Steep current impulse residual voltage test

b) Switching surge operating duty test Short circuit Arrester disconnector (when fitted) Polluted housing test for porcelain-housed multi-unit arresters Internal partial discharge test Bending moment Environmental tests Seal leak rate test for arresters with enclosed gas volume and separate sealing system Radio interference voltage (RIV) Moisture ingress test for polymer-housed arresters Weather ageing test for polymer-housed arresters for outdoor use Lightning impulse discharge capability for arresters to be installed in overhead lines

Table 7

Type test requirements for gapless ZnO arresters as per IEC 60099-4


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8.1 Type Tests In general. set requirements on arresters and their dimensioning are considered to be satisfactorily verified by subjecting the arresters to the following generic tests:

• • • • • • • • • • •

Residual voltage measurement at different current amplitudes and wave-shapes Current impulse withstand Operating duty Accelerated ageing External insulation withstand Short circuit (pressure relief) Artificial pollution Partial discharge Radio interference voltage (RIV) Sealing Mechanical

The above tests are considered to be type tests (design tests) but some of these may instead be performed during the manufacturing process and/or assembly as part of a manufacturer’s quality assurance. This is acceptable, and even preferable, provided that the type test criteria are fulfilled during the routine testing. ABB has chosen to do this for testing of internal partial discharge, seal leak rate and current distribution (multi-column arresters), as applicable for specific designs. Regarding polymer-housed arresters, the test procedures in IEC 60099-4 differ somewhat from previous tests on porcelain designs. The above tests by topic are also generally applicable to polymer designs, with the main exception being that there is no artificial pollution test yet specified for polymer arresters. Instead, a Weather Ageing test for the polymer material has been devised. Further, the sealing test requirements are more well defined in the form of a Moisture Ingress test, as are the criteria for mechanical loading and short circuit safety.

8.1.1 Test sections (prorated test sections) In order to verify guaranteed arrester data, tests are made on both complete arresters as well as on units of arresters and on components. It is both customary and accepted that some of the tests are made on scaled-down models of the arresters, thus making it possible to also scale-down the requirements on the test equipment. These scaled-down arresters units are called ”section of an arrester” or ”prorated section”. According to the definition in the IEC Standard, the arrester section intended for a particular test must correctly represent the performance of the complete arrester during a specific test. An arrester section may therefore look different depending on the intended tests. In some tests it is sufficient, for example, to perform the test on series connected ZnO blocks while other tests require that the ZnO blocks are encapsulated in a thermally correct model of the complete arrester. The requirements set on the tests sections, according to IEC 60099-4, are listed in Table 8.


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Type of prorated test section

Valid for tests

Non-encapsulated ZnO blocks *)

Residual voltage tests Tests with long duration current impulses

Encapsulated ZnO blocks

**)

ZnO blocks in a thermal model of an arrester

Accelerated ageing test procedure Operating duty tests where thermal stability must be verified

*)

Residual voltage and long duration current impulse tests are performed on three new samples, which may be either resistor elements, arrester sections or complete arresters.

**)

For porcelain housed arresters the blocks must be in the same atmosphere as found in the actual arrester. For polymer-housed arresters the blocks must be surrounded by the same material as used in the actual arrester.

Table 8

Requirements on prorated test sections at different tests according to IEC 60099-4

For all tests with energy injections, it is important that the test section fulfils the following requirements:

• The block volume shall not be greater than the minimum block volume specified for the complete arrester, scaled down with respect to the rated voltage of the prorated test section

• The energy injected into the test section must correspond to what a test section comprising ZnO blocks with a minimum voltage-current characteristic would have been subjected to It is equally important during tests with temporary overvoltages that the test voltage is scaled down with respect to the reference voltage of the test section and the minimum reference voltage assigned to the complete arrester. Test sections comprising non-encapsulated ZnO blocks are well defined, but verification tests are necessary to design a thermally correct test section. A thermal section shall, in principal, be a cross section of the complete arrester. However, the heat transfer in the middle of a long arrester unit takes place mainly in the radial direction, and hence a conservative model of the arrester must be thermally insulated at both ends to avoid heat transfer axially. The principal design of a thermal section for polymeric arresters is shown in the following Figure 29.


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Electrical connection connection Electrical

Sheets of compressed Sheets of wool

Glass bolt fibre Glasfiber

bolt

Insulator Insulator

compressed wool Sheets of compressed wool

ZnO block ZnO blocks

Electrical connection

Electrical

connection

Insulation

Fig. 29

Principal design of a thermally pro-rated section for polymer-housed arresters.

It is, however, not sufficient to specify only the design of a section; it must also be verified through tests. The verification of the thermal section is made by heating a complete arrester unit and a thermal section to around +120 °C by the application of voltage. Thereafter, the ZnO block temperatures in the unit and the section are measured during the cooling time. A correctly designed thermal section shall not cool faster than the arrester unit. The below Figure 30 shows cooling curves from a test on a thermal section and a complete arrester. The Figure additionally shows the cooling for a section designed according to requirements given in IEEE C62.11, which also requires that verification tests be carried out. Relative temperature above ambient temperature 1

Complete Completesurge surgearrester arrester Thermal Thermalsection sectionaccording accordingto to IEC IEC Thermal Thermalsection sectionaccording accordingto to IANSI/IEEE IEEE

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

20

40

60

80

100

120

Time (minutes)

Fig. 30

Verification of thermal section and comparison of specifications between IEEE and IEC


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8.1.2 Residual voltage tests (Discharge-Voltage Tests according to IEEE) The purpose of these tests is to verify the protection level of the surge arrester. All residual voltage tests are made by subjecting the arrester or a section of the arrester (usually some ZnO blocks) to current impulses with different amplitudes and waveforms, and measuring the residual voltage across the test object. The measured voltage represents the protection level of the arrester for the actual current and waveshape. The Standards make a distinction between different current impulses, based on different events in the network:

•

Currents caused by lightning (lightning impulse current) The testing is made with a current impulse having a front time of 8 μs and a halfvalue time of 20 μs. The impulse is normally designated as an 8/20 μs impulse.

•

Currents caused by switching overvoltages (switching impulse current) The testing is made with a current impulse having a front time of 30 – 100 μs and a half-value time on the tail of roughly twice the virtual front time. The impulse is normally designated as an 30/60 μs impulse.

•

Currents having a steep front (steep current impulse) The testing is made with an impulse with a front time of 1 μs while the half-value time may be any value. However, normally a test circuit generating a half-value time of approximately 2 μs to 20 μs is used, i.e. a 1/(2-20) μs impulse.

It is of course possible that switching events or a fault can result in steeper current pulses than 30 μs, or that the current at lightning overvoltages may show both shorter or longer front times than 8 μs. For switching surges with longer front times, the deviation in the residual voltage from the 30/60 μs wave for the same current amplitude is very small; within a few percent. For lightning surges having a shorter front time, the residual voltage shows an increase of less than 10% with a reduction in front time from 8 to 1 μs. Residual impulse Residualvoltage voltage for for 1/2 1/2 μcurrent s current impulse Residualvoltage voltage for for 8/20 8/20 μcurrent s current impulse Residual impulse

Voltage (kV) 12

8

Fig. 31 Comparison between residual voltage levels for current pulses 8/20μ s and 1/2μ s.

4

0

Currentimpulse impulse Current 8/20μs 8/20

The upper curves show the voltages and the lower the currents.

-4

Current impulse Current impulse μ 1/2 s 1/2

-8

-12

Current (kA)

0

10

0

1

20

30

40

50

2

3

4

5

Time (microseconds)

Note that the curves have been misaligned for clarity. The lower time scale is valid for the 1/2μ s impulse, while the upper scale is valid for the 8/20μ s impulse


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By testing with different current amplitudes for each of the current-shapes, a complete protection characteristic is obtained for each waveform. For current impulses with the same amplitude, the residual voltage level increases slightly for shorter front-times. This frequency dependence is illustrated below (Fig. 31), showing results from a test with 10 kA for waveforms 8/20 μs and 1/2μs. The steeper front, 1 μs, may be the result of a lightning stroke very close to a substation protected by surge arresters. Further, inductance effects can become significant with steep current impulses, and IEC specifies that the steep current impulse residual voltage tests may need to be corrected to account for the possible inductive voltage drop between the arrester terminals. In order to generate the specified current pulses an impulse generator is needed with the capability to create currents up to 40 kA. To be able to create such currents through a complete arrester at high voltages would require very large impulse generators, since the test equipment must principally be able to simulate full-scale lightning. Tests on complete arresters are however not necessary, nor desirable for reasons of accuracy. IEC therefore recommends that the residual voltage tests are made on scaled-down models of the arrester and specifies also how the measured values shall be re-calculated to be valid for a complete arrester. According to IEC, the objective of the residual voltage type tests is to verify the claimed protection levels by checking the relationship of protection levels at different current waveforms and amplitudes to a level which is checked in routine tests on all arresters. Normally the residual voltage at 10 kA with waveform 8/20 μs is used as a reference. This means that the 10 kA level with this waveform must be verified in a routine test and given for all manufactured arresters. The requirement for a routine test can be fulfilled by measuring the residual voltage for each individual block within the arrester and summing up the result. This procedure will be correct, since all blocks in a single column arrester will be subjected to the same current.

8.1.3 Long Duration Current Impulse Withstand Test A surge arrester limits incoming overvoltages by diverting the surge current. The energy the arrester absorbs is given by the equation:

t

W = ∫ (u * i ) dt 0 where

u = voltage across the arrester i = current through the arrester

The arresters must withstand this energy without thermal instability or damage to the blocks in any way. It is equally important that the characteristics of the arrester are not changed due to repeated energy stresses. This could not only jeopardize the protection function of the arrester, but also the current sharing between parallel block columns in an arrester, or between several parallel arresters, that have been matched with respect to current sharing to cope with large energy requirements. Requirements for very high energy capabilities are solved by utilizing many parallel block columns. For such designs, it is required that changes in the protective characteristic of the blocks is low. From a protection point of view, it is acceptable that the residual voltage decreases with repeated current impulses, but if blocks are connected in parallel, the acceptable changes are much lower than what is allowed by the Standard. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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Aside from discharge of capacitor banks, the highest arrester energies for high voltage AC arresters are obtained from switching of long transmission lines. Simplified, the arrester will be subjected to a current impulse of rectangular shape, with the duration of the impulse being determined by the length of the line. The current amplitude through the arrester is given by the prospective overvoltage (without a surge arrester), the surge impedance of the line and the characteristic of the arrester. By tradition, the energy capability of an arrester has been defined with respect to the withstand capability for rectangular current impulses. The test with long duration current impulses is made on arrester sections. For arresters having a nominal current class 10 kA and 20 kA, the tests are defined as line discharge tests where the test circuit’s wave impedance, charging voltage and duration of the current impulse are defined in the Standards. The resulting energy is dependent on the protection level of the arrester, which is why the energy must be defined and be given in the test report. According to IEC 60099-4, the test energy must be higher than or equal to a value determined by a formula based on the specified test parameters and the protection level of the test sections. The required protection level is according to the lowest value of the switching impulse residual voltage measured for the lower current value specified in IEC 60099-4 for the respective Line Discharge Class. The actual current applied in the long duration test is thereafter determined by the choice of circuit parameters in order to obtain the required energy. As a result, the value of this applied current is of lesser consequence - what is the determining factor is the amount of energy applied and absorbed by the test sample in order to fulfil a given Line Discharge Class. In fact, the value for "withstand current" is not an IEC 60099-4 term. It is however common to be listed by manufacturers, since it serves as an indirect measure of the arrester's single-impulse energy absorption capability - also undefined in IEC - when applied in multiples (18 discharge operations, divided into 6 groups of 3 operations). Hence, to keep in line with IEC, a direct analysis can best be made by considering the amount of energy absorbed, rather than the unspecified current applied. A desired energy capability for the arrester can then be given indirectly by defining line parameters, or directly in kJ/kV rated voltage. However, it must be emphasized that any value given as kJ/kV rated voltage without specifying test procedures is undefined and thus of little value. Arresters having a nominal current class of 2.5 kA and 5 kA are not tested with line discharges, and instead tests with rectangular current impulses are specified with given amplitudes and durations.

8.1.4 Operating duty test The purpose of this test is to verify that the arrester withstands all the kinds of electrical stresses which are likely to occur during its lifetime. This is schematically shown in the following Figure 32. The standardized operating duty test therefore includes different stresses and sequences of current pulses and voltage amplitudes representing possible events in a power system. Originally, the operating duty test was used to verify an arresters’ ability to handle lightning currents while being simultaneously subjected to maximum allowable operating voltage. For gapped arresters this meant that, apart from the lightning current stress, they were subjected to a power frequency follow current before the gaps were able to extinguish the arc at voltage zero. If the arc was not extinguished at the first voltage zero, the arrester normally failed.


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ZnO arresters do not contain any gaps, but an operating duty test is nevertheless still useful to check that the arrester is thermally stable after having absorbed large amounts of energy under severe ambient conditions with respect to temperature and voltage. These energy inputs could come from energy discharges as well as from Temporary Overvoltages (TOV) on the system. How the operating duty test shall be carried out for different Line Discharge Classes is illustrated in the following Figures 33 and 34.

ZnO surge arrester

~

Uf = power frequency voltage across arrester

Type of fault

Stress

Duration

Normal service

Uf = Um / √3

Continuous

Lightning

impulse

μs-impulse

Earth fault

Uf < Um / √3

≈ 0.1 - 10 s *)

Breaker operation

Uf ⇒ 0

≈ 0.3 - 1 s

Re-closing

switching

ms-impulse

Voltage profile

Possible faults: Earth fault on other phase

Uf = ke*Um /√ 3

≈ 0.1 - 10 s

Breaker operation

Uf ≤ Um /√ 3

≈ 0.3 - 1 s

Re-closing

switching

ms-impulse

Normal service

Uf = Um / √3

Continuous

*)

In some Countries even longer earth-fault times are allowed.

Fig. 32

Examples of stress sequences on surge arresters during different fault conditions. The system is directly earthed having an earth-fault factor ”k e” ≤ 1.4.


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1

2

Residual voltage measurement at I n , 8/20 μs Time interval not specified Conditioning test. Four groups of five impulses at In , 8/20 μs, superimposed on the continuous operating voltage + 20% Time interval not specified, +20 °C ± 15 °C High current impulse, 4/10 μs Pre-heating to +60 °C ± 3 °C Time as short as possible, not exceeding 100 ms

3

Elevated rated voltage, U r *, 10 seconds Elevated continuous operating voltage, U c*, 30 minutes Cooling to ambient temperature, +20 °C ± 15 °C Residual voltage measurement at I n , 8/20 μs

4

Visual check of the test objects

Elevated rated voltage U r * and continuous operating voltage U c* only if the accelerated ageing test procedure gives increased power losses. Otherwise, U r and Uc are applied. In = Nominal discharge current Explanation of the numbers: 1 2

Fig. 33

Preparatory measurements Conditioning

3 4

Operating duty test with high current impulses Measurements and checking

Operating duty test on 10 kA surge arresters with line discharge class 1 and arresters of class 1.5 kA, 2.5 kA or 5kA.


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1

2

Residual voltage measurement at I n , 8/20 μs Time interval not specified Conditioning test. Four groups of five impulses at In , 8/20 μs, superimposed on the continuous operating voltage + 20% Time interval not specified, +20 °C ± 15 °C Conditioning with high current impulse on a pro-rated test section, 4/10 μs Cooling to ambient temperature Conditioning with high current impulse on a pro-rated test section, 4/10 μs Kept for future testing

3

4

Pre-heating to +60 °C ± 3 °C Line discharge impulse Time interval 50 - 60 seconds Line discharge impulse Time as short as possible, not exceeding 100ms Elevated rated voltage, U r *, 10 seconds Elevated continuous operating voltage, U c*, 30 minutes Cooling to ambient temperature +20 °C ± 15 °C Measurement of residual voltage at I n, 8/20 μs Visual check of the test objects

Elevated rated voltage U r * and continuous operating voltage U c* only if the accelerated ageing test procedure gives increased power losses. Otherwise, U r and Uc are applied. In = Nominal discharge current Explanation of the numbers: 1

Preparatory measurements

3

Operating duty test with long duration current impulses

2

Conditioning

4

Measurements and checking

Fig. 34

Operating duty test on 10 kA surge arresters with line discharge class 2 or class 3 and 20 kA arresters with line discharge class 4 or 5


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The test sequence starts with a conditioning phase, where the test objects are subjected to a large number of current pulses to take into consideration possible changes of the block characteristic during actual service conditions due to repeated stresses. After this sequence, the part of the test commences which shall verify the arrester’s thermal stability and designated rated voltage. This part of the test shall therefore be made fulfilling the following requirements:

•

The prorated test section must be thermally equivalent to a complete surge arrester

•

The test objects must be heated before the test to a temperature being representative of the worst possible service conditions

•

The test must be made on previously non-tested blocks. Consideration must be taken of possible ageing of the blocks by applying correction factors (giving U r * and Uc*) according to the guidelines in IEC 60099-4.

Finally, it is required that the arrester withstands the operating duty test without change to its electrical properties beyond acceptable limits. The residual voltage level at nominal discharge current is therefore checked before and after the test sequence. The operating duty test is normally performed on arrester sections. To fulfil the requirement for thermal equivalency, the section is principally a cross-section of the complete arrester. IEC 60099-4 requires preheating of the thermal pro-rated section to +60 °C before the energy injections. This temperature is thought to represent an ambient temperature of +40 °C together with solar radiation, self-heating of the blocks due to power losses and some influence from pollution. For ZnO blocks with low power losses at normal service voltage, +60 °C is a conservative value, and the operating duty test consequently gives a safety margin with respect to thermal stability limits.

8.1.5 Accelerated ageing test procedure One of the key basis for the dimensioning of an arrester is the result from the accelerated ageing test procedure, where an acceleration of possible ageing effects is obtained by performing the test at an elevated temperature. Surge arresters limit overvoltages by conducting current, but during most of the arresters’ lifetime it shall act as an insulator. The entire continuous operating voltage is across the ZnO blocks and these must keep their insulating properties during their lifetime. IEC 60099-4 specifies an accelerated ageing test during 1000 hours at an elevated temperature of 115 °C as a type test. For arresters filled with air, the ZnO blocks need not be encapsulated during the test. If the surrounding atmosphere is something else (e.g. nitrogen or other gas) the test must be performed with the blocks in that particular atmosphere. For polymer-housed arresters, where the blocks are in direct contact with other materials, the ageing test must be made including all materials which are in direct contact with the blocks to show that the blocks are not negatively affected (i.e. aged) due to influence from the other materials. The accelerating ageing test is based on the Arrhenius law, which provides good confidence on life expectancy of ZnO blocks. When tested according to the IEC requirements, the equivalent minimum demonstrated lifetime is predicted to be 110 years at the conservative average ambient temperature of 40 °C.


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An important parameter during the ageing test is the selection of the voltage stress on the blocks. The test voltage must reflect the highest possible local voltage stress in the arrester when it is energized at the highest possible continuous operating voltage, U c, assigned to the arrester. A thorough electrical field calculation therefore must be made for each arrester type and rated voltage which, in turn, is the basis for determining the relevant voltage stress during the accelerated ageing test procedure. Influence from all phases in a three-phase configuration must also be taken into account when performing the calculations. A c c e l e r a te d a g e i n g t e s t o n Z n O v a r i s t o r s ty p e P E X L I M Q a t 1 1 5 ° C w i t h a v o l t a g e s t r e s s o f 0 . 9 7 * r e f e r e n c e v o l t a g e , U re f P o l y m e r i c i n s u l a t o r m o l d e d d i r e c t ly o n t h e Z n O b l o c k T e s t t im e : 1 1 2 2 h o u r s

Relative powerlosses P/Po (P o=losses after 1.5 hour)

1 .2 1 0 .8 0 .6 0 .4 0 .2 0 0

100

200

300

400

500

600

700

800

900

1000

1100

1200

T i m e ( h o u r s)

Fig. 35

Example of power losses during an accelerated ageing test procedure for ABB’s ZnO blocks.

ZnO blocks are normally manufactured in batches of some thousands of blocks, and variations (even minor ones) may have a negative influence on the block characteristics. From a quality point of view, it is thus necessary to perform ageing tests as sample tests on blocks from each manufactured batch. Separate from the type test, ABB further verifies the stability of every production batch of ZnO blocks by routinely performing an accelerated ageing test on some blocks picked out randomly from the whole batch. Power losses after 1000 hours, extrapolated from a test with shorter duration, at an elevated temperature of 115 °C at 1.05 x Uc shall not exceed the losses at the start of the test and not more than 10% above the lowest losses occurring during the test period. Batches in which unapproved blocks appear are rejected. It is however not sufficient to check only the characteristics of the blocks, but rather the entire arrester must be seen as a unity. The ability of the arrester housing to dissipate heat must also be adjusted to the power losses of the blocks during different service conditions with respect to voltage, temperature and even frequency. This is necessary to ensure that the average block temperature will not considerably exceed the ambient temperature, and thereby remain thermally stable. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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8.1.6 External insulation withstand tests The external insulation of arresters need not fulfil a certain standardized insulation class since the arrester effectively protects its own insulation against overvoltages, both external as well as internal. This is also reflected in the Standards, where the insulation requirements for arresters are based on the arrester’s protection levels with a reasonable safety margin added. The ZnO blocks can naturally not be assembled in the arrester during such a test on the housing, since no laboratory equipment exists which is capable of generating the very high currents that would be needed. The tests are therefore performed on empty unit housings. For multi-unit arresters, grading capacitors can be used in place of the ZnO blocks. The following minimum values for the external insulation must be kept according to IEC 60099-4:

• Arresters with rated voltage < 200 kV For short impulse, 1.2/50 s The arrester housing shall withstand 1.3 times the residual voltage value at nominal discharge current (10 kA or 20 kA) with waveform 8/20 μs, i.e. the lightning impulse protection level For power frequency voltage, 50 Hz or 60 Hz The housings for 10 kA and 20 kA arresters shall withstand a voltage (peak value) of 1.06 times the residual voltage level at classifying current for switching overvoltage (0.125 kA up to 2 kA) with wave form 30/60 μs. Arresters with nominal current of 1.5 kA, 2.5 kA and 5 kA shall withstand a voltage (peak value) of 0.88 times the lightning impulse protection level.

• Arresters with rated voltage 200 kV For short impulse, 1.2/50 s The arrester housing shall withstand 1.3 times the residual voltage value at nominal discharge current (10 kA or 20 kA) with waveform 8/20 μs, i.e. the lightning impulse protection level. For long impulse, 250/2500 s The arrester housing shall withstand 1.25 times the residual voltage value at classifying current for switching surges (0.125 kA up to 2 kA) with wave form 30/60 μs. Note: IEEE does not use the same correction factors as IEC, and therefore IEEE requires other withstand levels; due partly to the difference in maximum required design altitude (1800 m for IEEE compared with 1000 m for IEC). Refer Table 9. IEC 60071-1, for insulation co-ordination principles and rules, states that when it has been demonstrated that one condition (dry or wet) or one polarity or a combination of these produces the lowest withstand voltage, then it is sufficient to verify the withstand voltage for this particular condition. Hence, insulation withstand tests shall be wet tests for outdoor arresters where wet conditions are expected to lower the withstand voltages. Experience shows that this is the case for power frequency tests and switching impulse tests, but not for lightning impulse tests. If the arresters are intended for indoor use, dry tests are considered sufficient in all cases.


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IEC (installation < 1000 m)

Power frequency IEC: 50 Hz, 1 minute IEEE: 60 Hz, 10 second

SIWL LIWL

Table 9

Ur < 200 kV 1.06 /√2 x Ups for arresters with In = 10 or 20 kA or 0.88 /√2 x Upl for arresters with In = 1.5, 2.5 and 5 kA

Ur > 200 kV Not applicable

Not applicable 1.25 x SIPL 1.3 x Upl for all arresters

IEEE (installation < 1800 m)

0.82 x Ups

Not defined 1.42 x Upl at 20 kA for all arresters

Comparison of IEC and IEEE requirements for insulation withstand voltages

All distances between the arrester’s own parts, e.g. grading ring to flanges, must be checked with respect to voltage stress and withstand, either by calculation or test. If actual tests are required on complete multi-unit arresters, the blocks must be replaced with something giving the same internal voltage grading as the blocks would give. Normally capacitors are used to replace the ZnO blocks during such tests to model actual service conditions as closely as possible.

8.1.7 Short circuit (pressure relief) tests As the primary requirement for an arrester is to protect under all circumstances, this leads to the higher possibility for a failure (overload) compared to other high voltage equipment. This is also generally accepted, and should not be considered as a “failure” in the design. As a result, special requirements are set on arresters to ensure that a possible arrester failure will not cause consequential damage to other equipment or injury to personnel. The Standards therefore require tests where a deliberate internal short-circuit has been made to check the shortcircuit / pressure relief capability. Previously, tests were made as specified in the ”old” IEC 60099-1 Standard for gapped SiC arresters. In these test requirements, it was taken for granted that an arrester fulfilling a certain current class with respect to pressure relief capability automatically also fulfilled all lower current classes. It was subsequently realized that this was not always the case (particularly for porcelainhousings), and a design may include ”grey zones” if it is only tested against the highest possible current amplitude. In order to avoid this uncertainty, IEC 60099-4 requires that arresters must not only be tested with the highest short-circuit current (100%), but also at approximately 25 % and 50 % of the highest current. In addition, similar to the “old” standard, a low current test shall be performed at 600 + 200 A. For the high current tests, the test samples should be the longest mechanical section with the highest rated voltage of each different design of arrester. It is accepted that approved high current tests made on the longest housing also covers all shorter insulators of the same design. For the low current test, however, the test sample may be a mechanical section of any length with the highest rated voltage used for each different design and chosen length of test sample.


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High current Arrester class = nominal discharge current A 20 000 or 10 000 20 000 or 10 000 20 000 or 10 000 20 000 or 10 000 20 000 or 10 000 20 000 , 10 000 or 5 000 10 000 or 5 000 10 000, 5 000, 2 500 or 1 500 10 000, 5 000, 2 500 or 1 500 Table 10

Rated shortcircuit Current Is A 80 000 63 000 50 000 40 000 31 500 20 000 16 000 10 000 5 000

Low current

Reduced short-circuit Currents (+ 10 %) A

Short-circuit current, with a duration of 1 s A

50 000 25 000 25 000 25 000 12 000 12 000 6 000 6 000

25 000 12 000 12 000 12 000 6 000 6 000 3 000 3 000

600 + 200 600 + 200 600 + 200 600 + 200 600 + 200 600 + 200 600 + 200 600 + 200

3 000

1 500

600 + 200

Short circuit (pressure relief) test currents (Source: IEC 60099-4)

8.1.7.1 Classification of arrester designs Two basic designs, designated “Design A” and “Design B”, have been defined in IEC 60099-4. They differ in the relative volume of an enclosed gas channel that runs along the length of the arrester. Arresters with "Design A" have a gas channel running along the entire length of the arrester unit and fills >50% of the internal volume not occupied by the internal active elements. For this design, the probability of a failure initiated in the gas volume is much higher than in the solid material. This type of design makes use of the internal overpressure which is built up due to the internal arc resulting from the short-circuit of the active elements. The overpressure is created by heating the enclosed volume of gas, which expands, leading to bursting or flipping of a pressure relief device (including pre-fabricated weak spots in the housing). In this case the tests are sometimes called “pressure relief tests”. The arrester housing is dimensioned to not violently break before the overpressure is relieved. Typically, these arresters are porcelain-housed or polymer-housed with a composite hollow insulator incorporating an annular gap, i.e. tubular design. Arresters with "Design B" are of a solid design with no enclosed volume of gas or having an internal gas volume filling <50% of the internal volume not occupied by the internal active elements. For this design, there is a higher probability of failure initiated in the solid material. The short-circuit performance of this design depends on the arc directly burning through or tearing the housing without explosively expelling the internal components. Typically, these are void-free polymer-housed arresters without any separate pressure relief device, i.e. open, cage, closed or wrap design. In “Design A”, a breakdown or flashover in the gas channel will develop an arc very quickly over the entire length of the arrester. This may generate an intensive shock wave in the gas channel, stressing the housing over its entire length and imposing high requirements on pressure relief devices to open quickly. For “Design B”, in case of a breakdown in the solid material, the arc will develop more slowly.


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8.1.7.2 Test procedure – Failure mode There has been a lot of discussion over the years whether the short-circuit current should be initiated by a fuse wire along the ZnO block surface, a fuse wire through a drilled hole in the centre of the ZnO blocks or by pre-failing (electrical overloading). A short-circuit test has to consider worst-case scenarios, but at the same time the test should represents the most relevant failure scenario without placing too harsh/simple requirements on the design. For "Design A" arresters, it is generally agreed that the fuse wire in the gas volume along the surface of the ZnO block column represents the most relevant failure scenario, since this design has mainly to prove its ability to handle the shock wave caused by the internal arc. For "Design B" polymer-housed arresters, a fuse wire along the ZnO block surface can generally not be accepted since this does not represent the worse case scenario for this design (too simple) and may result in unsafe arresters being considered reliable from a short-circuit point of view. On the other hand, a fuse wire through holes drilled in the blocks is conversely a too harsh scenario for this kind of arrester, as it extremely unlikely that all ZnO blocks of a failing polymer arrester with this design will be punctured.

Fig. 36

Comparison of modes for short-circuit initiation

It is therefore justified to specify the pre-failing method for “Design B” polymer-housed arresters, which among the alternatives gives a reasonable compromise with regard to test severeness and realism, and it automatically covers possible influences of material homogeneity. Conversely, the pre-failing method may be less severe for a porcelain-housed “Design B” arrester in the case where the arc develops elsewhere than in the solid material. To cover for the worst-case scenario, “Design B” porcelain-housed should have a fuse wire drawn along the surface of the ZnO block column as far away as possible from the gas channel. Design Design A

Housing Porcelain Polymer

Design B

Porcelain Polymer

Table 11

Initiation of short circuit current Fuse wire along surface of ZnO blocks; within, or as close as possible to, the gas channel Fuse wire along surface of ZnO blocks; within, or as close as possible to, the gas channel Fuse wire along surface of ZnO blocks; located as far away as possible from the gas channel Pre-failing by constant voltage or constant current source. See Note 1)

Summary of short circuit test pre-failure modes

Note 1) The overvoltage shall be a voltage exceeding 1.15 times U c leading to an electrical failure of the arrester within 2 – 8 minutes; after which the arrester is subjected to the short-circuit current test within 15 minutes.


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8.1.7.3 Test procedure – Circuit arrangement The required arrangement for connection of the test circuit is also specified in IEC 60099-4. Either the so-called C-connection or Z-connection should be used in such a manner that would represent the worst-case scenario for a particular design. Refer Figure 37.

Porcelain-housed arresters (C-connection) Fig. 37

Polymer-housed arresters (Z-connection)

Circuit layout for short circuit testing (source IEC 60099-4)

For porcelain-housed arresters, the C-connection provides the most unfavourable conditions during the initial phase of the test before venting occurs. Once the arc is externalized, it may then be kept in close proximity to the arrester housing, resulting in a thermal shock effect causing excessive chipping and shattering of porcelain sheds. During the remaining arcing time, this routing forces the arc to move away from the arrester, and thus reduces the risk of the arrester catching fire. For polymer-housed arresters, the conductors should be routed as per the Z-connection. In this way, the arc will stay close to the arrester during the entire duration of the short-circuit current, thus creating the most unfavourable conditions with regards to fire hazard. A specific exception is “Design A” arresters with polymeric sheds which are not made of porcelain or other hollow insulator, but which are as brittle as ceramics. These shall be considered and tested as porcelain-housed arresters. It had been observed in many cases that after non-violent thermal breaking of the porcelain housing the active part, completely intact, fell down such that its top end collided with the enclosure (which previously had a radius equal to the arrester height) and some parts of its top end fell just outside the enclosure. Though in these cases the short-circuit performance was good, the arrester did not pass the test. In order to avoid this conflict the radius of the enclosure has now been chosen to be 20% larger than the arrester height; dimensioned according to the below formula or, in any case, at least D = 1.8m.

D = 1.2 * (2*H + Darr ) where

D H Darr

diameter of the enclosure (or side in the case of a square) height of the test arrester diameter of the test arrester


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8.1.7.4 Test duration and “Asymmetry factor” The test duration for the high current and reduced current tests shall be 0.2 seconds, reflecting the time it takes for a circuit breaker to disconnect a fault. To avoid an explosion of the housing, especially for a “Design A” arrester, it is generally considered necessary that the internal arc is transferred (commuted) to the outside of the housing within the first half-cycle of the short-circuit current. As it usually takes more time than up to the first current peak until the arc has fully commutated to the outside of the housing on “Design A” arresters, a certain amplitude for the first peak of the current is defined in the test procedure for the rated short-circuit current - which in the case of IEC 60099-4 has to be at least 2.5 times the r.m.s value of the symmetrical component of the prospective short-circuit current. Many difficulties have arisen in trying to achieving a value of 2.5 for the “asymmetry factor” on certain polymer arrester designs. These difficulties lead to a limitation of laboratories that could perform the tests or a modification in the testing procedures (too severe/simple); neither of which is truly desirable. Specifically for “Design B” arresters, it has been proposed that a symmetrical current transfers more energy during the first 2 – 3 ms, which is the typical time of the housing to open for this design. Regardless, skipping the asymmetry factor on these designs permits the arresters to be tested in full length, and this is considered to be more significant to verifying short circuit behaviour than any marginal energy differences because of a first peak factor. Thus, for “Design B” arresters, the prospective peak value of the first half cycle of the actual test current shall be at least √2 times the r.m.s value, but otherwise there is no specific requirement for asymmetry factor with this design. Similarly, for the reduced short-circuit currents and regardless of design, the prospective peak value of the first half cycle of the actual test current need only be at least √2 times the r.m.s value, In the case of the low current test, the actual first half cycle peak of the test current must be at least √2 times the r.m.s value. The low current shall flow through the arrester for 1 second or, for “Design A” porcelain-housed arresters, until venting occurs. 8.1.7.5 Test evaluation The basic pass criteria are that no violent shattering occurs and open flames shall be selfextinguished within 2 minutes. However, for practical reasons, it is considered unrealistically hard to have such requirements as “remain completely intact” or “no piece shall be ejected” as the pass criteria for surge arresters undergoing short-circuit tests. These would judge the arrester as having failed the short-circuit test, though the arrester basically performed well. Very often fragments of hard material such as porcelain or ZnO blocks just "jump" over the enclosure of the test circle, without any dangerous kinetic energy. Similarly, soft parts of polymeric material can do little or no damage should they be cast off. IEC 62271-200 for metal-enclosed substations and IEC 61330 for medium-voltage/low voltage prefabricated substations explicitly deal with internal arc testing with respect to the safety of personnel and public in case of failures within such stations. Two levels of accessibility have been adopted, where the accessibility B refers to stations that are directly accessible to the public. Although these standards deal with the safety of persons and are therefore very restrictive concerning possibly arising danger, they permit projection of parts out of the station up to a weight of 60 g. Since it is advantageous if all standards dealing with similar parts of a power system specify similar requirements, it was decided that the same criteria be also adopted for the short-circuit testing of surge arresters. Hence, fragments of ceramic material (ZnO block or porcelain) of up to 60 g are allowed to be found outside the enclosure, as are pressure relief vent covers and diaphragms and soft parts of polymeric materials.


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8.1.8 Artificial pollution test Artificial pollution tests are intended to provide information on the behaviour of external insulation under conditions representative of pollution in service, although they do not necessarily simulate any particular service conditions. A number of different methods have been trialled for artificial pollution tests on surge arresters, with those specifically intended for porcelain-housed arresters having the intention of determining • risk for external flashover • effect of partial discharges inside the surge arrester due to radial fields between the external surface and the internal active elements • adverse temperature rise of the internal active elements due to a non-linear and transient voltage grading caused by the pollution layer on the surface of the housing Different methods are intended to test for one or more phenomena. Further, artificial pollution tests aimed at determining localised temperature rise are only considered applicable to multiunit arresters, since single-unit arresters do not have a direct electrical connection between inside to outside along their length. However, the risk of puncture exists for very long units. The conclusion is, of course, that it is necessary to have an arrester design (both internal and external) which minimizes such stresses and/or their effect under all anticipated conditions. A problem with many of the pollution test methods is that their relevance to real conditions during arrester life is questionable. Such methods test the arresters behaviour in more-or-less irrelevant respects, and thus help neither users nor manufacturers to judge between arrester designs with respect to pollution performance. A meaningful test program for surge arresters must therefore start with an investigation of the pollution conditions which arresters can see in real life and what effect these conditions will actually have on arrester designs. Consequently, field-tests of arresters in areas with severe natural pollution have been performed to sort out the relevant mechanisms for arrester performance under polluted conditions. Since 1982, ABB has ZnO surge arresters installed at different sites with known severe pollution conditions (marine, desert, tropical, industrial, etc). The testing has been carried out in collaboration with recognized leading power utilities around the world. The field tests included conductivity measurement of natural-polluted layer, recording of external and internal currents with counting of current pulses and temperature recordings. The results and experience gained from these field-tests has contributed greatly in the designing of ABB type EXLIM porcelain arresters to ensure their optimum pollution performance, even under the most severe conditions. For example, the results show that the temperature rise during real pollution episodes has not been seen to be sufficient to increase the block temperature to such an extent as to create a risk for thermal runaway in the EXLIM design. The value of this experience is recognized also in the IEC Standard, with the possibility for agreement between user and manufacturer to omit performing an artificial pollution test, based (for example) on service experience in specified environments. IEC 60099-4 further acknowledges that artificial pollution tests, as prescribed for porcelainhoused arresters, are not strictly applicable to polymer-housed arresters. Instead, for the time being, only a weather ageing test for the polymer material is specified.


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8.1.9 Weather ageing test In contrast to porcelain which, as a material, can reasonably be expected to remain unchanged over its lifetime, there exists concerns (real or perceived) about the longevity of polymer insulators in extreme weather conditions. The weather ageing test in IEC 60099-4 is thus applicable only to polymer housed arresters. The test is intended as a continuous test with a duration of 1000 hours under salt fog conditions at constant power frequency voltage equal to U c. It shall be performed on the longest electrical unit with the minimum specific creepage distance and the highest rated voltage recommended for the arrester type. This test is primarily intended to age the polymer material so as to cause tracking, erosion or puncture; although other failure mechanisms may occur. Interruptions due to flashover are therefore permitted. If they do occur, the arrester shall be washed with clean tap water and the test restarted with a lower salt content for the fog. As an alternative, in case of severe environmental conditions - intense solar radiation, frequent temperature inversion with condensation, heavy or very heavy pollution (as defined in IEC 60815) - a 5000 hour continuous multi-stress test may be performed after agreement between the manufacturer and the user. This test consists of constantly energizing the arrester with constant power frequency voltage equal to Uc and then applying various stresses in a cyclic manner: • solar radiation simulation • artificial rain • dry heat • damp heat (near saturation) • high dampness at room temperature (saturation has to be obtained) • salt fog at low concentration Furthermore, temperature variations may cause some degree of mechanical stress, possibly leading to sealing failure, and also give rise to condensation phenomena. As this test is intended to accelerate ageing from weather conditions seen in service, flashovers should not occur. If the 5000 hour test is performed on the longest electrical unit with the minimum specific creepage distance, then the 1000 hour test may not be considered necessary to perform. Both tests are considered passed if no tracking occurs, erosion does not occur through the entire thickness of the external housing to the next material layer, the sheds and housing are not punctured, and the electrical performance of the arrester is substantially unchanged from before to after the test.


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8.1.10 Partial discharge and RIV tests A low corona level – both internal and external – is essential for all surge arresters to achieve during normal operating conditions. As a consequence, manufacturers may put more significance on checking for corona as a routine test on all produced units rather than as a single type test. The IEC partial discharge test is intended to detect internal corona which could lead to problems in the long term. The arrester unit must be first pre-stressed at significantly higher than its normal operating voltage in order to create the potential for initiation of discharges. Thereafter the voltage is reduced to a value somewhat higher than the arrester unit’s continuous operating voltage, at which the internal partial discharge level is recorded (measured as apparent charge in pico-coulomb, pC). Radio interference voltage (RIV) testing, as the name suggests, is aimed primarily at detecting external corona which can cause interference with communication equipment. In contrast to internal partial discharge tests, which are performed on individual arrester units, an RIV test needs to be performed on a complete arrester, fully assembled with all fittings (since the aim is to detect discharges from sharp edges, bolts, pins etc). After voltage pre-stress, the value of RIV is measured at different applied voltage levels. RIV instruments measure the voltage drop (recorded in microvolts) caused by a partial discharge just within a narrow frequency band, transforming it by a weighting circuit according to the sensitivity of the human ear. IEC 60099-4 permits that RIV testing may be omitted if the same arrester has passed a partial discharge test; provided both internal and external discharges are recorded. 8.1.11 Environmental tests The environmental tests are intended to demonstrate by accelerated test procedures that the sealing mechanism on porcelain-housed arresters and the exposed metal components, e.g. flanges and terminals, are not impaired by environmental conditions. For polymer-housed arresters, it is considered that the weather ageing test imposes sufficient environmental stress, and hence no additional tests are needed on these types of arresters. The test requirements consist briefly of the following, with the criteria described in more detail in the relevant IEC 60068-2 documents:

•

Temperature cycling test (IEC 60068-2-14) The specimen is exposed to changes of temperature in air by exposure in a chamber to prescribed temperatures varied at a controlled rate.

•

Salt mist test (IEC 60068-2-11) This test is applied to compare the resistance to deterioration from salt mist of specimens of similar construction. It can be useful for evaluating the quality and the uniformity of protective coatings.

The arresters shall be considered satisfactory provided no degradation in the sealing has occurred.


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8.1.12 Sealing and Moisture ingress test Sealing breakdown has historically been a major cause of arrester failure, particularly for distribution arresters. Sealing tests on arresters with enclosed gas volume and a separate sealing system should be made using a sensitive method that can detect very low leakage rates (for example, max 1μW = 1 x 10-6 Pa. m3/s = 1 x 10-5 mbar.litre/s according to IEC 60099-4). Example of test methods which are commonly used include: • Helium-mass spectrometer • Vacuum over water • Pressure or vacuum decay • Halogen detection Provided the method and criteria used during routine testing of seal tightness on individual units also fulfils the type test criteria, many manufacturers prefer to waive performing a separate type test on porcelain-housed arresters, as it will not give any additional information. A moisture ingress test is included in IEC 60099-4 which applies to polymer arresters only, and demonstrates the ability of the arrester to resist ingress of moisture after being subjected to specified mechanical stresses. The test includes subjecting the arrester to both thermal as well as mechanical cycling, as depicted in Figure 38. After the cycling, the arrester is placed in boiling salt water for 42 hours, and thereafter moisture is given time to possibility penetrate the arrester (Fig. 39). Electrical measurements are made both before and after the test sequences to verify that the specimen has not absorbed any moisture. If the electrical characteristic of the arrester has changed during the tests, the most likely conclusion is that moisture has penetrated inside the housing, which would imply that the arrester no longer fulfils the original requirements. Temperature

+60 ºC

+45 ºC

24 h

48h

72 h

96 h Time

-25 ºC

-40 ºC Load direction: 0º

Fig. 38

Load direction: 180º



Thermo-mechanical preconditioning (Source IEC60099-4)

Fig. 39 Water immersion test (Source IEC60099-4)


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8.1.13 Mechanical tests Surge arresters are normally self-supported, and consequently will be subjected to a bending moment when mechanical forces are applied. These forces can originate from various sources, including: • line connectors • wind, ice and snow • seismic accelerations (earthquake) • arrester’s own weight These forces will cause a bending moment, which typically has its maximum at the base of the arrester. The arrester must withstand this moment. In the case of multi-unit arresters, individual units must also withstand the moment at their length resulting from the applied forces. Bending moment tests are performed by fixing the housing to the floor and subjecting it to a horizontal force at the top. The force is then slowly increased until the housing breaks, or in the case of verification, that the declared value is reached. The test may be performed on complete arresters or arrester units. According to IEC 60099-4, several sample tests should be performed on porcelainunit housings to determine the mean value of breaking load (MBL). It is then possible to assign the housing a maximum specified short-term load (SSL), i.e. the 100% value in Figure 40, which can be considered its maximum withstand moment against dynamic loads such as short circuit forces, gust Fig. 40 Definition of mechanical loads according to IEC60099-4 winds, earthquake, etc. This should not be confused with the breaking limit proven during testing, which is an average of 20% above this value. The specified long-term load (SLL), which is the maximum static (continuous) moment, should be limited to 40% of the SSL. 8.1.13.1 Considerations for polymer arrester designs Polymer arresters have historically lacked common rules for the definition of dynamic and static service loads, which strongly depend on the arrester design. For example, IEC 61462 “Composite Insulators” gives alternative definitions and criteria for specified mechanical load (SML) and maximum mechanical load (MML). While this standard may be applicable to the housing of polymer arresters with enclosed gas volume (i.e. tubular design), it is not appropriate to use for all polymer arrester designs, and especially those for which the internal components have an influence in determining the mechanical performance of the complete arrester. If potential effects of “mechanical ageing” during continuous loading are not considered, load levels may be chosen at very high levels compared to what the arrester design can actually handle. In other words, the short-term load level could be set just a little below the breaking load. However, it is in doubt whether the arrester would handle these high mechanical loads at the given load levels (continuous and short-term) under actual conditions during its service lifetime. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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There are a number of potential problems that can arise by exposing an arrester to a continuous load that is too near to the breaking load. These include: • Damage to the housing of the arrester, which could cause the arrester structure to completely collapse and break • Damage to internal parts of the arrester, for example damage to the ZnO blocks causing electrical malfunction • Cracks in the polymer housing or sealing, which could lead to moisture ingress and electrical failure Of specific interest is the performance of polymer arresters under continuous loading of a cyclic nature. Due to their construction, polymer arresters of all designs may flex under mechanical load and, when this is repeated cyclically (as would occur over their service lifetime), may be the primary factor which determines the true limit of permissible mechanical loading. A specified short-term load verified on new arresters not previously subjected to any tests may thus give a too optimistic value. Subjecting the arrester, in a cyclic way, to continuous load may result in significant deflection which in turn may affect the likelihood of moisture ingress and/or jeopardise the mechanical integrity of the metal oxide blocks. Additionally, insulation withstand clearances to other equipment may be compromised if the deflection is extreme. Furthermore, the maximum short-term load that can be applied without breaking may be significantly reduced after the arrester has been subjected to a continuous load in a cyclic manner. Hence, a test is required to verify that an arrester, even after many years in service and having potentially been mechanically fatigued, can both remain sealed and still be capable of withstanding a serious mechanical incident that occurs; for example a short circuit or earthquake. IEC 60099-4 specifies a mechanical test to be performed on three complete arrester units with the highest rated voltage of the unit, whereby each is subjected to a cyclical bending moment at the specified long-term load (SLL) for 1000 cycles. Thereafter, two of the arrester units are subjected to a bending moment test at the short term load (SSL), while the third undergoes thermomechanical preconditioning as discussed in section 8.1.12. All three units are then subjected to a water immersion test (see also 8.1.12).

Fig. 41 Consideration of deflection during cyclical loading

If the arrester passes 1000 cycles at the SLL and subsequent water immersion and evaluation tests, it is considered likely that the arrester can continuously be subjected to the SLL Furthermore, the test validates that the SSL is a load which the arrester could be subjected to even after many years in service. For short polymer arresters, i.e. arresters for system voltages not exceeding 52 kV, a cyclic load test has not been considered necessary; neither for porcelain-housed arresters, considering the long experience of this type of arrester.


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8.1.14 TOV versus time characteristic There is no requirement within IEC 60099-4 to perform a specific type test to determine the arrester’s capability to withstand power frequency overvoltages (TOV’s) of different durations. That said, the application of rated voltage for 10 seconds after absorption of two line discharges during the Operating Duty test could be seen as one point on a TOV characteristic. A characteristic curve shall nevertheless be established – by test or calculation – to show the allowable duration of TOV which may be applied to the arrester after it has first been heated to 60 oC and thereafter had rated energy applied, without leading to damage or thermal runaway. The time range should cover the period 0.1 to 100000 seconds to account for the majority of inservice situations. Since there are various definitions of “rated energy”, it is important that the impulse energy consumption prior to application of the power-frequency overvoltage is clearly stated. Often, for completeness and comparison purposes, a separate curve is also provided which is “without prior energy” duty.

8.1.15 Other tests Several other type tests are specified by IEC 60099-4 which have not yet been covered. However, since these do not directly relate to HV surge arresters used for transformer protection they are considered outside of the scope of this Guide. For completeness, they are briefly listed below.

•

Arrester disconnector / fault indicator tests For arresters fitted with such devices – typically for use on distribution systems and transmission lines – these tests demonstrate the correct operation of the device. The device should withstand, without operating, the respective long duration current impulse test, operating duty test and lightning impulse discharge capability test. An operating characteristic should also be established to show the time delay to achieve effective and permanent disconnection with different power-frequency short-circuit currents.

•

Lightning impulse discharge capability Line surge arresters (LSA) are potentially subjected to high energy and current stresses from lightning strikes to the transmission line. Further, the current waveforms – with duration of several 10’s of microseconds for shielded lines to 100’s of microseconds for unshielded lines – are considerably different to the waveforms used in other tests with line discharges. Arresters intended for use as a LSA on system voltages exceeding 52kV need to verify their ability to withstand a test sequence incorporating impulses with duration of 200 μs (considered as a compromise to cover both typical applications as well as the effect of multiple strokes). Thereafter the arrester shall be given a rated lightning impulse discharge capability; corresponding to the lowest current, energy and charge applied during the test.


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8.2 Routine Tests The ambition with the routine tests is to ensure that the produced arresters meet the design specification. All test results have to be within preset limits to qualify the arresters for delivery. The routine tests are consequently an integrated part of the quality control during manufacture. As a minimum requirement for routine tests, IEC 60099-4 specifies the following to be performed on each arrester or arrester unit:

•

Reference voltage measurement o

•

Lightning impulse residual voltage test o

•

This test serves to prove that the arrester will be free from internal corona at normal service voltage. IEC permits a maximum level of 10 pC at a test voltage of 1.05 x Uc, but manufacturers may set their own tougher criteria.

Leakage check of the sealed housing (for arresters with a sealed housing) o

•

This proves that the guaranteed protection level of the arrester is not exceeded. If not measured directly on the complete arrester, residual voltage can be measured on the individual ZnO blocks or arrester units at a suitable lightning impulse current and then summed together to give the value for the complete arrester.

Internal partial discharge check o

•

The measured value of the reference voltage U ref must lie within the range allowed by the manufacturer. The lower limit of the U ref guarantees the thermal stability of the arrester. The higher the value of U ref , the smaller the power losses at U c and therefore better stability during network operation. The practical upper limit is determined by the operating characteristic in order to pass the residual voltage test.

This test proves that the housing hermetically seals the active parts of the arrester. Common test arrangements are helium-mass spectrometer, vacuum over water, pressure or vacuum decay and halogen detection. This test is not applicable to certain polymer arrester designs, when the active parts are directly sealed in the housing material.

Current distribution test on multi-column arresters o

Where an arrester consists of multiple-columns, adequate current and energy sharing between each column is to be verified by application of a suitable impulse current across groups of parallel ZnO blocks. The highest current through any one column is not to exceed the upper limit set by the manufacturer.

IEEE similarly specifies minimum requirements for routine tests: • Power-frequency test • Discharge-voltage test • Ionization voltage test • Seal test • Current sharing test Manufacturers may choose to perform additional quality checks. For example, all ABB type EXLIM and PEXLIM arresters are subjected to the above tests (as applicable), plus a measurement of power losses and grading current at U c (MCOV). In addition, ABB also routinely undertakes energy withstand tests on all produced ZnO blocks (with those having insufficient capability being automatically rejected), together with accelerated life and impulse current tests as well as a check of low current characteristics, protection characteristics and capacitance on samples from every produced batch of ZnO blocks. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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8.3 Acceptance Tests An acceptance test, as per IEC vocabulary, means a complete surge arrester should be tested, i.e. preferably with all individual units connected in series as a fully operational arrester. The number of arresters to be tested is the nearest lower whole number to the cube root of the number of arresters in the delivery lot. Acceptance tests according to IEC 60099-4 incorporate: • Measurement of power frequency voltage of the complete arrester at reference current • Lightning impulse residual voltage test on the complete arrester or arrester units • Internal partial discharge measurement on the complete arrester Additionally, a Special thermal stability test is given as an option, which has to be specially agreed upon. This is, in principle, a shortened version of the Operating Duty test, performed on blocks from the same batch (or similar) as those used in the arresters from the delivery lot. IEEE calls these conformance tests, and specifies the following: • Discharge voltage test, on the complete arrester or individual units • Internal ionization voltage (IIV) and Radio-influence voltage (RIV) on the complete arrester 8.3.1 Value of acceptance tests There exists a degree of confusion as to the meaning, and thereby the value, of acceptance tests on surge arresters. Unlike some other high voltage apparatus, acceptance tests on surge arresters are not the same as repeated routine tests, particularly in the case of multi-unit arresters. That said, the routine tests made after assembly of a single-unit arrester could perhaps be regarded as an acceptance test, since the routine tests are then performed on a “complete arrester”. However, to fulfil the requirements of the Standard, an additional lightning residual impulse voltage test on the unit may be required if this is not performed as routine. It is permitted, for example, as a routine test to measure residual voltages on individual ZnO blocks for a specific applied current and then sum up the values to give the total residual voltage for the unit. Because of the lower voltage required at the block level, this permits testing with high lightning impulse currents (e.g. 10kA) and good measuring accuracy. Conversely, testing a complete unit (or complete multi-unit arrester) at higher voltages can present problems regarding circuit capacity to achieve high lightning impulse currents, as well as potential loss of accuracy in the measured values compared with performing the test on individual blocks. For a multi-unit arrester, consisting of several individual units, the units may be regarded as impedances connected in series, where each individual unit has a specific voltage drop (or residual voltage) for a specific applied current. Thus, measured values on units when summed up can be regarded as valid for the complete arrester. Specifically in the case of reference voltage measurement at reference current, provided that the current is high enough to not be affected by stray capacitances during the measurement, then the summed values on individual units can also be regarded as valid for the complete arrester. Similarly, for the internal partial discharge test, provided the pro-rata voltage used during the routine test on individual units is equal to or higher than the required test voltage during the acceptance test, then assembling the units together will not influence the result with respect to internal PD measurement. Consequently, acceptance tests need not be considered necessary provided already performed routine, batch and sample tests are sufficient to ensure that the acceptance test criteria are fulfilled. If this is the case, acceptance tests will then not give any additional information about the surge arrester characteristics than obtained during the other tests, nor add value or security to the arresters from a delivery lot.


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9.

ARRESTER SELECTION

This section contains only a brief guidance for selection of the most important parameters of HV surge arresters used for standard applications such as transformer protection For a more comprehensive guide, reference is made to IEC 60099-5 and ABB Application Guides. For specialized applications, a more detailed system analysis or insulation co-ordination study may be necessary to permit selection. It is vital that the correct arrester is selected which will provide the desired protection, as well as withstand normal and specified abnormal service conditions. The basic selection is carried out in two major steps:

•

Match the electrical characteristics of the arrester to the system’s electrical demands

•

Match the mechanical characteristics of the arrester to the system’s mechanical and environmental requirements

9.1 Matching the electrical characteristics In selecting arresters in a 3-phase network, it is first of importance to know if they are to be connected between phase-ground, neutral-ground or phase-phase. The most common practice is to connect arresters phase-ground. The simplified process for selection of the electrical characteristics is depicted in the following flowchart (Fig. 42). Uc > Um / 3

Electrical selection complete

Fig. 42 Flowchart for simplified electrical selection of surge arresters


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9.1.1

Selection of continuous operating voltage and rated voltage

a) Obtain System Parameters The maximum highest system voltage (U m) should be known. But if not, it may be estimated as 5 to 10% higher than the nominal system voltage. The most commonly occurring TOV is that at a single line earth fault. The amplitude is given by multiplying Um/√3 by the earth-fault factor ke, which in turn is determined from the earthing conditions. The below Figure 43 gives the value for k e based on the system sequence reactances and resistances for the most unfavourable fault resistance. Should these system parameters be unknown, k e is often assumed to be 1.4 for directly earthed systems and 1.73 for resonant earthed or isolated neutral systems. Fig. 43 Curves showing relationship between R0 /X 1 and X 0 /X 1 for constant earth fault factor k e and zero fault resistance (Source: IEC 60071-2)

R0 = zero sequence resistance X0 = zero sequence reactance X1 = positive sequence reactance

The duration of the applied TOV during earthfault depends on the fault-clearance time. If this is not known, it may usually be estimated to be in the range of 1 to 3 seconds for directly earthed HV systems and 3 to 10 seconds for directly earthed distribution systems. For isolated neutral or resonant earthed systems, the duration is important to determine more specifically, as it may vary from a few seconds to some hours or even days; depending on whether fault-clearing is used or not. For an anticipated fault duration over 2 hours, the TOV should generally be considered (in most cases) as continuous, and the arrester rating chosen accordingly. For the most common 3-phase systems, specific TOV and durations are proposed in Table 12. The assumptions made for directly earthed neutral systems include some combined effects of earth faults and load rejection; considering if an earth fault occurs during a load rejection, the TOV on the healthy phases tends to rise further than it would if the events occurred separately. TOV in p.u. of U m / 3

Fault duration

Um < 123 kV

1.55

1s

Um > 123 kV

1.5

1s

1.73

10 s or 2 h

Directly earthed neutral systems

Resonant earthed & isolated neutral systems

Table 12

Common choice for earth fault factor and fault duration


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Generally, only the TOV during earth-faults and at load rejection are of interest. Certain network configurations can however give resonance overvoltages. These may also arise during nonsimultaneous operation of breaker poles. Nevertheless, resonance overvoltages should be avoided by proper system design (especially for normal AC transmission and distribution systems) and usually should not need to be the basis for selection of the arrester TOV capability. In some cases, efforts are made to reduce the earth-fault current by selectively earthing the neutrals of only a few transformers, yet maintaining an effectively-earthed system overall. In such cases, there is a possibility that some parts of the system may become non-effectively earthed (i.e. increase in value of k e) for certain periods of time when one or more of the earthedneutral transformers are out of service. An earth fault during this time may lead to higher TOV and subsequent arrester failure if this contingency is not taken into account. Since such occurrences are rare, it may be justified to accept a risk of arrester failure instead of selecting an arrester with higher TOV capability and thus a higher protective level. b) Select the Continuous Operating Voltage In a 3-phase system with arresters connected phase-ground, the actual continuous operating voltage, Uca, is not higher than Um/√3. If the system does not have any abnormal service conditions, Uc should therefore be equal to or higher than U m/√3. Special consideration applies to an arrester on the delta tertiary winding of a transformer where one corner of the delta is permanently connected to ground. In such applications, the continuous operating voltage applied to the arrester will be the full phase-phase voltage even though the arresters are connected phase-ground. Should a considerable amount of harmonics (> 10%) be present in the system, a safety factor of 1.05 (i.e. 5%) is recommended (IEC 60099-5) to account for the increase in peak value of U ca. However, in systems with short automatic fault-clearance times, a safety factor of 1.0 is often sufficient nevertheless since the limited duration is normally covered by the TOV characteristic of the arrester. It should be noted that any arresters with U c > U ca are generally equally suitable, with regards solely to continuous operating voltage. The manufacturer should be consulted if abnormal service conditions exist which are outside of those specified by the Standards: such as ambient temperature below –40 °C or above +40 °C, frequencies under 48Hz or above 62Hz, presence of heat sources (e.g. furnaces) near the arrester, etc. Such abnormal service conditions may lead to the need for selection of higher Uc and/or rated voltage (Ur ), unless the arrester has been designed and verified to withstand the specified service conditions. All ABB arresters, for example, can withstand wider ranges of temperature (–50 °C to +45 °C) and frequency (15 Hz to 62 Hz), without the need for special consideration. c) Select a sufficiently high Rated Voltage to meet TOV demands In general, surge arresters are not used to protect equipment against TOV as this would require an enormous number of parallel columns of blocks. Such applications may be considered only in cases of limitation or elimination of resonance TOV, and carefully detailed studies are then required to select arresters with suitable energy capability.


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Factors affecting the TOV capability of an arrester are pre-energy absorbed (i.e. the initial temperature of the blocks) prior to the application of TOV and the applied voltage following the TOV. For a given arrester type, the rated voltage (U r ), defined as per IEC, is a measure of its overvoltage capability. capability. Hence, the additional additional TOV capability of the arrester can be specified as a multiple of Ur . A different philosophy philosophy adopted adopted by some manufacturers manufacturers is to give give the TOV capability in multiples of U c. The following procedure is suggested to select an arrester with sufficient TOV capability:

•

Select a preliminary preliminary rated voltage (Ur0) based on U c, with Ur0 = Uc/0.8 where 0.8 is the “design factor” for ZnO arresters

•

Determine the TOV TOV amplitude amplitude and duration at earth fault as TOVe = ke * Um/√3 ke < 1.4 normally for for directly earthed systems (effectively earthed) earthed) ke = 1.73 normally for resonant earthed and isolated systems (non-effectively (non-effectively earthed) For specific cases, determine the actual k e factor.

•

Determine other temporary overvoltages overvoltages TOV 1, TOV2, … TOVn with amplitude and duration as calculated or estimated, e.g. using the common choice guide in Table 12.

•

Consider the possible possible energy energy absorption absorption W (in kJ) prior to the TOV and calculate calculate W/U W/U r0. For each TOV, determine the minimum required rated voltages U re, Ur1, Ur2, … Urn by dividing the determined TOV amplitude by the temporary overvoltage strength factor T r for a selected type of arrester for the actual duration of the TOV and the calculated energy absorption W/Ur0. If the calculated calculated specific specific energy energy absorption absorption W/Ur0 is higher than the value given for the first choice of arrester type, then increase U r0 or select an arrester type with a higher energy capability. Thus Ure = TOVe/Tre, Ur1 = TOV1/Tr1, Ur2 = TOV2/Tr2, etc

•

Select a final rated voltage, Ur , which is the highest of the values values Uro, Ure, Ur1, Ur2, etc. If this is a non-standard rating, choose the next higher rating.

9.1.2 Selection of nominal discharge current It is often difficult to calculate the arrester current, especially those caused by lightning. Therefore, rough estimations are mostly used. The relatively small variations in discharge voltage with current waveshape and amplitude makes this estimation less critical with ZnO arresters. Important parameters affecting the selection of the nominal discharge current are: • the importance of the protected object • number of lines connected to the station • the line insulation • ground flash density in the area • line performance performance with respect respect to backflashes backflashes and shielding shielding failures failures some spans spans out from the station


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As a general guidance, nominal discharge currents with an 8/20 μs waveshape as given in the following Table 13 are proposed. Maximum system voltage (kV)

Nominal current (kA)

800 550 245 < Um < 420 36 < Um < 245 < 36

20 20 (or 15 as per IEEE) 10 or 20 10 5 or 10

Table 13

Common choice selection of nominal currents

9.1.3 Selection of Energy Capability The case generally considered to be decisive for energy capability is fast reclosing against a trapped charge on a transmission line with the arrester installed at the open far end. Z1

~

Transmission line with: Surge impedance Z Travel time T Initial voltage -1 p.u p.u

1 p.u

Fig. 44

Surge arrester

Simple single-phase model of energy decisive case

If the surge travel time of the line is short compared with one cycle of power frequency and Z 1 presents a low impedance, the current through the arrester will have a rectangular shape with a duration equal to twice the travel time T of the wave on the line. The current through the arrester and its residual voltage at this current are given by the intersection of the relevant switching surge characteristics and the load line, and can be determined by plotting a load diagram, as depicted in Figure 45.

Fig. 45 Load Diagram UL

UL Z Ia Ups

Ups

Prospective overvoltage Line surge impedance Surge arrester current Surge arrester switching surge protection level (residual voltage)


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In reality, the arrester current current does not have have a purely purely rectangular rectangular waveform. waveform. The source impedance, Z1, will affect the voltage imposed on the line at breaker closing, the voltage wave will be distorted during its travel on the line, return waves will cause reflections at the sending end and, for multi-phase systems, the phases will interact. However, this simple single-phase single-phase model model is useful in many cases. cases. To avoid expensive expensive computer studies, the simplified method can be applied as a first attempt to estimate the arrester stresses caused by switching. switching. If these calculations calculations reveal higher higher energies and the need for more qualified studies than had been considered initially, a more accurate study would be justified. Different types and makes of arresters could also be easily compared when high absolute accuracy in calculated stresses is not required. In order to use the simplified method, the parameters in the above figure must be determined in some way. Typical values for different different system voltages are given in the following following Table 14.

System voltage (kV)

Surge Impedance, Z (ohm)

Prospective overvoltage without arresters, UL (p.u)

Under 145 145 to 345 362 to 550 765 (800)

450 400 350 300

3.0 3.0 2.6 2.2

Proposed system parameters Table 14 The base for the per-unit values is the peak value of the highest system voltage phase-to-earth

The prospective overvoltage (U L) depends on a number of parameters such as location of arresters, type of switching operation, presence or absence of pre-insertion resistors, the feeding network and the parallel compensation. The wave propagation time (T) depends on the line length and the velocity of wave propagation. For aerial lines and GIS bus ducts the propagation velocity (v) is approximately equal to the velocity of light (0.3 km/ μs). For cables, the velocity velocity is much lower lower (around 0.15 km/ μs). The energy (W), given in J, absorbed by the arrester is given by the equation:

W = [(UL – Ups)/Z] * Ups * 2T * n where UL Ups Z T

n

prospective overvoltage or line-charging line-charging voltage (kV) switching surge protection level (residual voltage) of the arrester (kV) surge impedance (ohm) wave propagation time (μs) = l/v, where l = length of line (km) v = velocity of propagation (km/ μs) number of consecutive discharges

It can be seen that the energy energy absorbed also depends depends on the protection protection level. Thus, a higher protection level reduces the demands in kJ/kV. __________________________________________________________________________________________ _____________________________________________________ _____________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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The energy absorption capability of ZnO arresters has to be proven in the so called line discharge tests. The energy absorbed by an arrester in a line discharge test is a function of both the line discharge class and the switching impulse protection level of the arrester . For a given arrester, an estimate of the energy absorbed in the relevant line discharge test could be obtained by using the arrester’s switching surge protective level from the catalogues and checking for absorbed energy from the Line Discharge Class characteristic curves (e.g. as per IEC; Fig. 46 below). This value is then compared with the required discharge energy (W) calculated from the above equation. The IEC and IEEE line discharge tests comprise repeated discharges and the thermal stability of the arrester has to be verified for two consecutive discharges with 50 to 60 seconds between them. For single operations, many arrester types could be stressed with a higher energy, equal to the single impulse energy capability. Usually, the design case has a very low probability of occurrence, and it may therefore be sufficient to design for one single operation and not for two consecutive discharges.

7

) r U ( V k / J k , Y G R E N E C I F I C E P S

CLASS 5 6 5 4

CLASS 4 CLASS 3

3

CLASS 2 2

CLASS 1 1 0

1 .0

1 .4

1 .8

2 .2

2 .6

3 .0

R E L A T I V E P R O T E C T I V E R A T I O , Ups/Ur U a /U r

Fig. 46 IEC Line Discharge Class characteristic curves

If the chosen energy capability is not sufficient, the most economical solution is to increase the arrester rated voltage. If this leads to an unacceptable protection level, then select another type with a higher energy capability. For very high demands, parallel ZnO columns and/or arresters may be needed to meet the energy requirements. In these cases, proper and careful matching must be undertaken to ensure sufficiently equal current sharing, as full current sharing is not necessarily assured with standard arresters. At lower system voltages (below 245 kV), the energy due to switching will generally be low. At the same time, less attention is often paid to effective grounding and shielding of such systems. Hence the design capability will be determined by lightning stresses. A conservative estimate for the arrester energy capability for lightning surges is obtained in the high current test using a 4/10μs impulse with standardized peak amplitudes of 100kA or 65kA (IEC/IEEE). This wave subjects the arrester to high energy during a very short time and hence to a thermal shock as well. It is worth noting that discharges of the amplitudes stipulated in the tests rarely occur in reality, and the real impulse durations seen in service may be longer than the stipulated test impulse duration. An arrester with blocks of larger diameter will withstand the lightning stresses better for two reasons: • the current density will be lower • the residual voltage will be lower and consequently also the energy discharged Hence, it is advantageous to choose an arrester with larger diameter blocks (and consequently higher discharge capability) for • areas with high lightning activity • important installations and apparatus • lines and stations where grounding or shielding conditions are inadequate __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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9.1.4 Check of protection levels (Upl and Ups) For insulation co-ordination purposes, consider the lightning impulse protection level (U pl) at the selected nominal discharge current (5, 10, 15 or 20kA according to Table 13). Similarly, the switching impulse protection level (U ps) for co-ordination purposes is taken with a 30/60 μs waveshape having a current amplitude ranging from 0.5kA to 2kA, depending on the system voltage. Refer Table 15. Maximum system voltage (kV)

Maximum current (kA)

420 – 800 145 – 362 < 145

2 1 0.5

Table 15

Common choice switching surge co-ordinating currents

On occasion (notably according to IEEE), special consideration may need to be given to protection levels for a time to voltage crest of 0.5 μs; referred to as “equivalent front-of-wave discharge voltage”. For this type of very steep voltage wave, the effect of connection leads as well as the distance between the arrester and the protected object must be considered in order to accurately determine the voltage stress on the equipment. Withstand curves can then be plotted to check that a sufficient safety margin exist. 9.1.5 Protection margins Protection margins (in %) calculated at co-coordinating impulse currents, are defined as follows:

• Margin for lightning impulses = ((LIWL/Upl) -1) *100 • Margin for switching impulses = ((SIWL/Ups) -1) *100 Margins are normally excellent for ABB arresters due to the low U pl and Ups, and also the fact that most equipment at present has a high LIWL and SIWL. However, depending on the electrical distance between the arrester and the protected equipment, the margin for lightning impulses can become reduced, and thus arresters fail to protect equipment that is not in close vicinity (i.e. within their protection zone). The BIL % flexible erection alternatives for polymer arresters may be of benefit in reducing the distance effect. Additional line-entrance arresters may also help. It is recommended that the protection margins (after taking into account the ”distance effect”) should be in the order of 15 - 20% or more to account for uncertainties and possible reduction in the withstand values of the protected equipment with age. Should the selected arrester type not give the desired protection margins, the selection should be changed to an arrester of a higher line discharge class, which automatically leads to a lower protection level. Note! It is not recommended to use a lower than selected rated voltage (U r ) to improve the margins, as this may lead to an unacceptably low TOV capability.

Years

Fig. 47 Insulation withstand with time for paper and oil insulated power transformers. Ageing reduces insulation withstand of equipment and thus the rotection mar in.


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9.1.6 Consideration of distance effect One may well wonder why it should be necessary to have a protection margin at all, when it would seem sufficient that the protection level of the arrester was equal to the insulation withstand of the equipment (after consideration of possible ageing effects on the insulation). The reason is that the calculated protection levels and margins are only valid across the arrester itself, i.e. if the arrester is mounted directly on the protected object. When there are connection leads and a distance between arrester and object, then the protected object will be subjected to a higher overvoltage. This is illustrated in Figure 48. Fast-fronted overvoltages spread out along a line in the form of travelling waves. When a travelling wave reaches a point where the surge impedance changes, reflections and refractions take place. If the surge impedance is considered infinite – for example a transformer winding or an open circuit breaker – then a total reflection will occur. The positive instantaneous sum of the resultant oscillations cause the voltage at the remote end to increase step-wise to as much as double the value of the initial incoming voltage. When surge arresters are connected in front of the protected object, complex interactions and oscillations will take place between the two with their different surge impedances. Via the travelling wave process, the value of the voltage seen by the protected object can be considerably higher than at the arrester itself. How much higher depends to a large extent on the electrical distance between the arrester and the protected object and the front-steepness of the incoming wave.

S v

L a

Upl

U

Surge arrester

h b

Fig. 48

Voltage increase due to distance effect (simplified method)

The generally used formula to estimate the voltage increase due t o distance effect is:

U = Upl + (2 * S * L) / v where U Upl S L v

voltage at the protected object (kV) lightning impulse protection level of the arrester (kV) steepness of the incoming voltage wave (kV/ μs) electrical distance between arrester and protected object including connection leads (a + b) and arrester height (h) velocity of wave propagation (m/μs); approximately equal to the velocity of light 300 m/ μs, except for cables for which 150 m/ μs may be used

Note! The distance effect reduction does not apply to the U ps margin since the front-time of a switching surge impulse is longer. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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The protection margin will therefore dramatically reduce with increased separation distances, as well as with increased steepness of the incoming wave. The latter is a function of how close to the substation the lightning strikes the transmission line and risk for backflash or shielding penetration. Steepnesses of 1200 kV/μs and 2000 kV/ μs have been well established in Standards and practical insulation co-ordination studies for HV sub-stations, and are often used as reference surge steepnesses. Nevertheless, the determined strike rate leads to the choice of actual steepness for a given application. This simplified method must be used with caution as it is only an approximation. It does not take into account any capacitance of the protected object, nor inductance effects nor the initial voltage at the instant of surge. This simple method may not be sufficient in the case of small margins between the arrester protection level and the object’s LIWL; whereby more complex computer modeling may then be necessary. In all cases, the importance of short distances and connection lead lengths cannot be overemphasized. 9.1.7 Neutral-ground arresters In those cases where efforts are made to reduce the local earth-fault currents by not earthing the neutral of the transformer, each such neutral brought out through a bushing should be protected against lightning and switching overvoltages by an arrester. For neutral-ground arresters protecting fully insulated transformer neutrals, the recommended rated voltage is approximately the maximum system voltage (U m) divided by √3, assuming a relatively long fault duration. Short or very long fault durations may warrant selection of a different rated voltage, after taking into account the specific TOV requirements. In addition, special considerations must be taken for resonant-earthed systems with long radial lines, as a higher rated voltage may be necessary. Alternative selection criteria may also apply in cases with unusually low BIL or for neutrals of transformers with non-uniform insulation. The neutral-ground arresters should preferably be of the same Line Discharge Class as the phase-ground arresters on the same transformer. The electrical characteristics are then usually identical to standard catalogue arresters with the corresponding rated voltage. However, for arresters connected neutral-ground, U c is usually zero, as they are not subjected to any continuous voltage stress during normal service conditions. Consequently, demands for creepage distance and voltage grading do not normally apply to these arresters. For neutral-ground arresters specifically, it is a further advantage that the insulation withstand of the arrester is approximately equal (or even below) that of the transformer neutral so that, in the unlikely event of a flashover, it should occur in preference at the arrester. Consequently, these arresters would typically be assembled in the shortest housing possible. 9.1.8 Special cases In special applications – e.g. transformers in arc furnace installations – switching overvoltages can occur which are not sufficiently limited by arresters between phase-ground. For these cases, three arresters connected phase-phase are used in addition to three arresters phaseground. For the arresters connected phase-phase, U c > Um. Protection of special electrical equipment such as motors, generators, capacitor banks, etc, typically require more detailed evaluation than afforded by the simplified approach, and are therefore beyond the scope of this Guide.


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9.2 Matching the mechanical and environmental characteristics he simplified process for selection of the mechanical characteristics is depicted in the following flowchart (Fig. 49).

Mechanical selection complete

Fig. 49 Flowchart for simplified mechanical selection of surge arresters

9.2.1 Selection of external creepage distance IEC 60815 defines four levels of pollution (from light to very heavy) and stipulates the required minimum creepage for porcelain housings as indicated in the following Table 16. Pollution level

Specific creepage ( mm/kV Um )

Light (L) Medium (M) Heavy (H) Very Heavy (V)

16 20 25 31

Table 16

Pollution levels according to IEC 60815 (1986)

For porcelain-housed arresters, select the housing to give the desired creepage - generally the same as for the other equipment in the same location. If the creepage demand exceeds 31 mm/kV, a special design may be required. Silicone-housed arresters, being highly hydrophobic, are better suited for extremely polluted areas than porcelain- or EPDM-housed arresters. Based on the experience from long-term testing of silicone apparatus insulators, it is considered possible to reduce the creepage requirements for silicone housings by at least one step lower specific creepage, i.e. 20 - 30% less than for porcelain/EPDM.


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The background to this conclusion is presented in the Cigré document A3-104 “Optimized use of HV composite apparatus insulators: field experience from coastal and inland test stations” [17]. IEC 60815-3 furthermore accepts that reduction of creepage distance is possible and permissible from a pollution withstand or flashover point of view on specific polymeric insulators based on field trials, test stations or historic data with the same design, materials and electric stress. Hence, for example, if a specific creepage distance of 31mm/kV (Um) is deemed necessary at a site based on porcelain-criteria, then 25mm/kV is considered adequate with the silicone used in conjunction with the ABB’s PEXLIM design. With such reasoning, 31mm/kV would only be necessary on PEXLIM arresters in the case that the site conditions dictate higher than 31mm/kV for porcelain. Note that IEC 60815-3 introduces the new term “unified specific creepage distance” (USCD) defined as the creepage distance of an insulator divided by the r.m.s. value of the highest operating voltage across the insulator. This definition differs from that of specific creepage distance where the line-to-line value of the highest voltage for the equipment is used (for a.c. systems usually Um/√3). For line-to-earth insulation, this definition will result in a value that is √3 times that given by the definition of specific creepage distance in IEC/TR 60815 (1986). Regardless, the same reasoning for acceptance of reduced creepage distance by one pollution level nonetheless applies.

9.2.2 Selection of mechanical strength The cantilever strength (bending moment) of the arrester must be sufficient to withstand specified mechanical loads. These loads will cause a bending moment, which typically has its maximum at the base of the arrester - except perhaps in the case of a multi-unit arrester utilizing different strength housings for individual units; in which case the bending moment at the bottom of each unit should be considered separately. Mechanical loads on surge arresters can be divided into either static or dynamic loads. Static loads are those which are applied continuously (e.g. weight of line conductors, normal wind, etc), whereas dynamic loads are often higher in magnitude, but need only be withstood for short periods (e.g. short-circuit current forces, gust winds, earthquake, etc). Consideration should also be given to the fact that some loads may act alone or in combination. Since a surge arrester is an active protective device, permanent mechanical loads should always be minimized. Static loads are therefore kept relatively low. Dynamic loads by definition are only short term, and should therefore not be treated as permanent loads for the sake of dimensioning the mechanical strength of the arrester. Further, a higher degree of risk may be accepted if the chances are low that all loads could occur at the same time and orientation. Recognition of the difference in load types should always be accounted for in the selection of required mechanical strength for surge arresters. The maximum permissible horizontal load for individual forces is calculated as the maximum moment which the arrester can withstand, divided by the distance between the base of the arrester and point of the applied force. Loads at the line terminal connections can be considered to act at the centre of the terminal, whilst wind loads are assumed to act generally about the arrester’s centre of gravity. For areas with high seismic risk, different specifications and verification methods exist, and the manufacturer should be consulted to verify the arrester’s withstand capability. Notably, mechanical strength requirements are different for seismic loads and it may be permissible to exceed SSL.


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In the case of multiple loads acting in combination, the horizontal loads from individual forces should be used to calculate the vector sum of the bending moments acting about the base, to determine if the arrester housing can withstand them when applied simultaneously. Importantly, this calculation should consider realistic combinations of loads (as would be applied in service) against appropriate safety margins to the arrester’s static and dynamic mechanical strength, and not simply as a sum of the maximum cantilever load applied at the top terminal. This is illustrated in Figure 50, which shows that the arrester in question can withstand the specified loads (safety factor > 1.0) when like loads are correctly considered to act about different moment arms and not simply summed together and located solely at the line terminal. Loads resulting from tensile and compression forces are not usually of concern, as these are normally limited for standard applications and arrester housings are also typically strong in these directions. Torsional loading on the arrester is also considered an abnormal service condition, but may need closer consideration should it exist. For connecting arresters to the line, a common solution is to use the same conductor as for current-carrying equipment connected to the same line. However, this is often unnecessarily large and over-dimensioned for the purpose - the continuous total current through an arrester is of the order of only a few milli-Amps. The result is undue mechanical loading on the arrester. Connecting the arresters to the line instead by light, vertical and slack tee-offs, can considerably reduce the demand for mechanical strength, without requiring significant deviation from common practice. See Figure 52. Due to their flexible construction, there may be a visible deflection at the line-end of polymer arresters under mechanical load. This may ultimately determine the limit of loading which is able to be applied. However, since polymer arresters are light compared to equivalent porcelainhoused arresters, they permit innovative erection alternatives which could reduce the loading; for example suspended or under-hung erection or special bracing.

Fig. 50 Example of Estimated Loading Table


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9.2.3 Selection of short circuit capability The arrester’s short-circuit (pressure relief) capability is chosen on the basis of the prospective symmetrical short circuit in the system at the arrester location or calculated from the formula:

I = Sk / (√3 * Um ) where I Sk Um

prospective symmetrical short–circuit current (kA) 3-phase short-circuit power in MVA at the point where the arrester is to be installed maximum system voltage (kV)

If Sk is not known, the breaking capacity of the associated circuit breaker can be used as a guide for the short-circuit current.

9.2.4 Specification of high ambient temperature Customer specifications occasionally state tough requirements for maximum ambient temperature under which surge arresters are to operate. Whilst at the same time it is generally understood that the average temperature over a 24 hour period will be less. The influence from the sun radiation is sometimes thought to be significant, as one might assume that sun radiation can result in considerably high surface temperatures. However, it is the average surface temperature of the complete arrester that counts, and sun radiation falls on less than half of the insulator surface at any point in time. In fact, the closer to the equator an arrester is situated, the smaller the fraction of the insulator surface that is subjected to direct radiation due to the sheds. Temperature extremes are tracked and measured daily from multiple locations on earth. Highest average annual mean temperature ever recorded was 34.4 °C in Dallol, Ethiopia during the period October 1960 – December 1966. The following list shows the highest temperature ever reported on a continent. Keep in mind that the data in older dates may be skewed because standard temperature measurement techniques and equipment may have changed. Also note that "Oceania" indicates any small location such as an island in the vast oceans of the world. Continent Africa North America Asia Australia Europe South America Oceania

Location El Azizia, Libya Death Valley, California Tirat Tsvi, Israel Cloncurry, Queensland Seville, Spain Rivadavia, Argentina Tuguegarao, Philippines

Temperature extreme o 57.8 C o 56.7 C o 53.9 C o 53.3 C o 50.0 C o 48.9 C o 42.2 C

Date September 13, 1922 July 10th, 1913 June 22nd, 1942 January 16th, 1889 August 4th, 1881 December 11th, 1905 April 29th, 1912

Noteworthy is that none of the temperature extremes have occurred since the introduction of modern gapless arresters in the 1970’s and hence specified requirements for other locations are normally overstated. ABB surge arresters have low power losses and hence self-heating is negligible. The effects of direct sun radiation are accounted for in the IEC Operating Duty test of arrester sections, whereby energy and thermal capacity are verified at a starting temperature of 60 °C. This is actual ZnO block temperature, which adds additional safety margin by ignoring that the average block temperature will actually be well below the localized surface temperature possible from direct sun radiation. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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9.2.5 Altitude correction It is well known that the dielectric withstand of insulation in air decreases with altitude. The design altitude according to IEC 60099-4 is 1000m above sea level. Arresters which are also designed according to IEEE criteria are then likely suitable for use up to an altitude of 1800m above sea level. However, since the arrester’s own insulation (internal and external) is well protected by its inherent protective characteristic, many standard designs of ZnO arresters are suitable for use at much higher altitudes without requiring an additionally extended flashover distance. Naturally, every case must be considered separately to ensure satisfactory performance. Noteworthy is that some equipment standards define altitude correction in terms of increased creepage distance. For surge arresters – and specifically silicone-housed designs – this is not as critical and instead the required physical clearance in air across the arrester with consideration to its protective performance is deemed the defining criteria (see also 12.1).

9.3 Guide to Selection of ABB Surge Arresters The ABB factory in Ludvika, Sweden has a long history as a pioneer in overvoltage protection, dating back to 1938. ZnO technology was first introduced in 1979. ABB’s current family of porcelain-housed gapless ZnO HV surge arresters is called EXLIM (EXcellent voltage LIMiters) while the family of silicone-housed arresters is correspondingly called PEXLIM. Within each family, different housing designs and mounting arrangements are available to meet various mechanical, physical and environmental demands. Each family utilizes ABB’s own ZnO blocks that fulfil or exceed the demands of the respective IEC Line Discharge Classes (LDC) from 2 through to 5. These are in turn given a letter designation based on block type which forms part of the arrester type designation, e.g. PEXLIM R = LDC 2. LDC

Block type

2 3 4

R Q P

5

T

ABB type Porcelain housing Silicone housing EXLIM R PEXLIM R EXLIM Q PEXLIM Q EXLIM P PEXLIM P HS PEXLIM P EXLIM T HS PEXLIM T

Table 17

Overview of ABB surge arrester types

For ABB surge arresters, the type designation furthermore gives detailed information of the rating and its application, as indicated below. Suffix letters: E Non-standard electrical data M Non-standard mechanical data P Parallel columns H Underhung mounting L Line arrester

As a guide, the following diagrams give an overview of the typical applications by system voltage for which the respective arresters would normally be applied. EXLIM porcelain-housed (51a) and PEXLIM polymer-housed (51b). Specific applications may dictate the need for a certain type outside of the common choice. The respective arresters are nevertheless normally able to be tailored to cater for virtually any rating or application.


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Fig. 51a)

Fig. 51b)

EXLIM common choice guide

PEXLIM common choice guide


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With reference to the considerations discussed in Chapter 9, the following rated voltages (Ur ) are recommended for the most common 3-phase systems with maximum system voltage (Um) 52 – 550kV. System Fault clearance time Mechanical load Housing LDC ABB type

Um (kVrms) 52 72 100 123 145 170 245 300 362 420 550

Medium Porcelain 2 or 3 EXLIM R EXLIM Q

Um (kVrms) 52 72 100 123 145 170 245 300

Silicone 4 or 5 PEXLIM P-Y HS PEXLIM P HS PEXLIM T

192 228 276 330 420

Resonant earthed or isolated neutral Max 10 seconds Medium Porcelain 2 or 3 EXLIM R EXLIM Q

High Silicone Porcelain 2, 3 or 4 3, 4 or 5 PEXLIM R EXLIM Q PEXLIM Q EXLIM P PEXLIM P EXLIM T Rated voltage, Ur (kV rms) 48 48 66 66 96 96 120 120 132 132 162 162 228 228 276 276

48 66 96 120 132 162 228

System Fault clearance time Mechanical load Housing LDC ABB type

High Silicone Porcelain 2, 3 or 4 3, 4 or 5 PEXLIM R EXLIM Q PEXLIM Q EXLIM P PEXLIM P EXLIM T Rated voltage, Ur (kV rms) 42 42 60 60 84 84 96 96 120 120 132 132 192 192 228 228 276 276 330 330 420 420

42 60 84 96 120 132 192

System Fault clearance time Mechanical load Housing LDC ABB type

Um (kVrms) 52 72 100 123 145 170

Directly earthed Max 1 second


228 276

Resonant earthed or isolated neutral Max 2 hours Medium Porcelain 2 or 3 EXLIM R EXLIM Q

60 84 120 138 162 192

Table 18 a), b), c)

High Silicone Porcelain 2, 3 or 4 3, 4 or 5 PEXLIM R EXLIM Q PEXLIM Q EXLIM P PEXLIM P EXLIM T Rated voltage, Ur (kV rms) 60 60 84 84 120 120 138 138 162 162 192 192


Common choice guide to surge arrester rated voltage


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10.

INSTALLATION GUIDELINES

Upon arrival to site, the contents of all packages should be checked against the respective packing lists and any shortages identified. During unpacking, a visual inspection should be made for any obvious signs of transport damage. Reference shall be made to the assembly and special instructions provided for details of correct installation and these shall always be followed and take precedence. Since ABB undertakes such extensive routine, batch and sample tests on the ZnO blocks, surge arresters and counters/monitors (in excess of the requirements of the applicable standards), additional testing or commissioning checks are not considered warranted or necessary at installation or before taking EXLIM or PEXLIM arresters into service. 10.1 Conductor dimensioning Under normal operating voltages the arrester represents a high impedance and hence only milliamps of current are typically flowing constantly through the connecting conductors. Even under surge conditions, although the current can be significant (10's of thousands of Amps) it is only present for a very short time (microseconds). Such currents will have a negligible heating effect on the conductor. Consequently, the question of conductor size and cross-sectional area is perhaps not as important for surge arresters as it is for other high voltage apparatus. The true criteria comes when the arrester has overloaded and the system short-circuit current is thereafter flowing through the arrester and its conductors. If the cross-section is thermally insufficient for this condition, the connection may be destroyed, i.e. melt before the protection has operated to clear the fault. However, this may be able to be accepted, since the arrester has to be replaced anyway. If this is not acceptable, the crosssectional area for the conductors must be based on the system short-circuit current and duration. For the line conductors, the simple practical solution is often to use the same conductor as for highcurrent carrying equipment connected to the same line, e.g. dropper to current transformer. However, as noted above, this is typically unnecessarily large and may result in undue mechanical loading on the arrester. Lighter droppers, connected as slack vertical tee-offs, may be preferable, and even recommended, for this reason. Surge arresters shall be connected to the same common earth grid as for the other HV apparatus in the substation. The earth conductor cross section shall be overridingly chosen in accordance with local regulations and earth fault current requirements.

Fig. 52 Example of mounting and connection arrangement


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Connections to surge arresters in distribution systems are sometimes made via 600/1000 V PVC insulated copper cables. The temperature rating for this type of cable is generally V75, i.e. 75 oC. Two important factors should be considered before using this type of cable; especially in HV applications:

•

Above 200 oC, copper anneals and loses about 70 percent of its strength, thus compromising its ability to withstand short circuit forces. This increases the risk of breakage from whipping.

•

Above 90 oC, standard PVC softens. With an ambient temperature of 40 oC, this permits only 50 oC temperature rise. PVC begins to melt at 80 oC temperature rise, at which point the risk of catching fire is very high.

a) Connection between arrester earth terminal and surge counter When a surge counter is mounted on an earthed pedestal structure, it is necessary to insulate the cable/busbar connecting the arrester to the counter, both to avoid parallel current paths and the risk for flashover during surges. Otherwise the counter will not register as it should. The required insulation level for this connector is based on foreseen lightning levels. The voltage drop due to the internal resistance and inductance in the cable itself will be negligible in the case of lighting impulses and what dominates is the circuit inductance. In the general case, the lightning surge current generates a magnetic flux in the circuit comprising the insulated base, the support pedestal and the insulated conductor. The voltage induced is proportional to the magnetic flux in the closed loop and is little affected by the size of the conductor. For this reason the same insulation level is usually required for all earth connectors, regardless of their cross sectional area. The following general guidelines are recommended:

•

The earth conductor between the arrester and counter should be insulated for at least 5 x L kV (LIWL), where L is the conductor length in metres between the arrester earth terminal and the surge counter terminal. Note that the maximum permissible length L of the earth conductor between arrester and surge counter is determined by the LIWL of the insulated base which the arrester is mounted on as well as the counter itself

•

The LIWL of the insulating base and the counter must also withstand this induced voltage; otherwise it will flashover and the impulse will be earthed through the structure without passing through the counter.

•

Even if the LIWL of the insulated cable is sufficient, this lead must in any case be kept as short as practicable since its inductance-drop adds to the protection level of the arrester.

b) Connection between surge counter and earth The conductor between the counter and earth should be the same as for other earthing conductors in the station and connected to the common HV earth grid. The selection of crosssectional area is generally based on the system short-circuit current and duration or as per local regulations. This conductor should also be kept as short as possible, however whether or not it is insulated has no relevance with regards to the registration of surges by the counter.


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10.2 Mounting angle The vast majority of standard installations for surge arresters require vertical mounting on a ground-mounted pedestal support (see Fig 52). However, on occasion, angled mounting may be advantageous or even necessary. In these cases, mounting angles anywhere between “vertical” (0o) to “upside-down” (180 o) are to be considered. Single-unit porcelain arresters have been mounted horizontally for some years (commonly as cable terminations) without any obvious distress having been reported. Issues may arise during actual installation - mainly relating to the logistics of placing a heavy porcelain on a pole - but as long as care is taken, it seems to have been able to be done successfully. Nevertheless, it is not generally recommended to mount arresters utilizing hollow insulators (porcelain or composite) horizontally, since the internal stack of ZnO blocks is typically held under springcompression and there is then the risk that they may become displaced if not kept vertical. Also, inclining the arrester makes its own weight work against the inherent mechanical strength, since the cantilever loading capability is reduced by an amount equivalent to the arrester mass times gravity applied about the centre of gravity. This can significantly reduce the permissible load when heavy and long porcelain housings are involved. Further, full horizontal mounting may affect the performance under short circuit conditions as the external arc may not meet as quickly as when the arrester is vertical; making for a longer fault clearing time. As a consequence of the arrester's placement, if the porcelain is weakened during such an event, there is a greater risk for it to fall down. There is also the issue of the uneven washing of the sheds, as the protected creepage distance becomes 50% of the total when hung horizontally. In addition, there is then zero protected creepage on the top side, which runs the risk for flashover under heavy pollution, rain, fog, etc. The less the angle, the less the effect of all of the above concerns. But nevertheless there is still a degree of risk. Angling at 45 degrees is considered a compromise and limits the risk of the adverse effects (cantilever loading, displacement of blocks, uneven washing of the sheds, etc). Multi-unit hollow-insulator arresters will just compound the issues and so angled-mounting is not to be recommended for these longer arresters. In contrast, direct-moulded silicone-housed arresters (eg PEXLIM) by design are not affected in the same way Fig. 53 Example of angle-mounted PEXLIM as for hollow-insulator arresters. Therefore, these designs of arresters may be mounted at any angle, regardless of length and number of units. Normal consideration of mechanical loading nevertheless continues to apply. In particular, since the "self-weight" of the arrester will apply a permanent moment this needs to be deducted from the permissible loading (both SLL and SSL). Furthermore, with significant ”self-weight", a polymer arrester may also have considerable deflection. Even if loads are kept within acceptable limits, it can look bananashaped which will likely cause concern for the user; a factor which may restrict its practical use. Hanging lightweight polymer arresters from the line is another option, which then removes concerns regarding cantilever loading. When the arrester is to be installed completely upside down (inverted mounting) on an overhead structure, this is possible provided the design is arranged during production to ensure, amongst other matters, that the sheds are "right way up" in order to assist with water run off. It is therefore not usually possible to mount a standard arrester upside down. As a rule, standard arresters may be used for mounting angles from 0 – 90 o. For angles between >90 –180 o, arresters specially designed for inverted mounting are to be used. __________________________________________________________________________________________ ABB HV Surge Arrester Product Guide Edition 2009en

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11.

MAINTENANCE AND MONITORING

A surge arrester does not contain any moving parts or items that can break. Consequently there is nothing to maintain, adjust, correct or repair, which is why there is normally no need to perform any form of periodical checking or monitoring. In general, a correctly chosen and installed arrester is regarded as maintenance free during its entire lifetime. A correctly chosen arrester in this context means that its electrical and mechanical characteristics are matched to actual service conditions. 11.1 Mean Time Between Failure (MTBF) and Mean Time To Repair (MTTR) The question of MTBF and MTTR is often asked for substation equipment. However, such an analysis has no meaning or direct relevance for surge arresters. The design-life of a modern gapless ZnO arrester can reasonably be expected to be at least as long as the equipment it is protecting (nominally accepted to be 30+ years). However, this does not mean that it will necessarily last as long as the primary plant. It must be remembered that a surge arrester is, in principle, a sacrificial protective device, designed to operate to protect other electrical equipment. During the normal course of events, it may need to sacrifice itself at any time to protect the primary plant. It is therefore often very difficult to distinguish between a failure and a correct operation as, depending on the nature of the electrical surge, an arrester could "fail" internally during correct operation. But this should not be considered as a "failure" if it occurs for genuine reasons; rather it is the function of a surge arrester. Finally, an arrester which has “failed” must be taken out of service and disposed of according to local regulations. It cannot be repaired or reworked. 11.2 Cleaning Periodical cleaning of porcelain-housed arresters is usually only necessary after periods of heavy marine or industrial pollution. Surge arresters may be washed under voltage (livewashing), following the same safety regulations as for any other high voltage equipment, plus with the following additional precautions:

• surge arresters normally employ shorter flashover distance compared to other insulators, leading to an increased risk for external flashover during the washing

• surge arresters with series connected units must have all units washed simultaneously to avoid overheating of any unit Arresters with silicone housing should, in general, not need to be washed at all. Nevertheless, it is acknowledged that silicone insulators exposed to heavy pollution for long periods may become discoloured and appear dirty over time. This is as a result of low molecular-weight silicone oils diffusing to the surface, ultimately encapsulating the pollution layer and making the housing appear dirty and difficult to clean. This function permits the housing to ultimately recover its hydrophobicity, even after a temporary loss; a unique feature amongst insulators. Of importance is that, unlike other types of insulators, this discolouration does not necessarily mean that a silicone insulator's in-service pollution performance is affected. In fact, cleaning of a silicone insulator can actually have the disadvantage of washing away the silicone oils deposited on the surface, thereby reducing its hydrophobicity for a period.


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Should washing be undertaken on a silicone-insulator in any case – to remove large amounts of solid layer deposits, for example - then only plain water at low to moderate pressure should be used to prevent damage to the soft housing. If a cleaner housing than can be achieved by livewashing is desired, then hand-washing with plain water and a soft cloth may be necessary. No form of detergents, cleaning agents, abrasive cloth or hard brush should be used, unless approved by the arrester manufacturer. WARNING! The arrester must be de-energized and out of service before any work requiring handling is undertaken. Regardless of how dirty the insulator appears, what is of interest is whether or not the surface of the housing is hydrophobic or not. A class scale exists for measuring the degree of hydrophobicity, and tests can be undertaken for determining the extent to which the surface of the arrester has become hydrophilic. Seven wettability (hydrophobicity) classes (WC) have been defined with a value between 1 and 7. WC 1 corresponds to a completely hydrophobic (water-repellent) surface and WC 7 is a completely hydrophilic (totally wetted filmed) surface. By definition, a composite insulator can be considered hydrophobic in the range WC 1 - 4. Silicone-housings typically exhibit WC 1 – 2 when new. In contrast, a porcelain insulator exhibits WC > 5 when clean and new and WC 7 after a time in service, i.e. completely hydrophilic, without the ability to recover. If desired, this class scale provides a coarse value of the wetting status and is particularly suitable for a fast and easy check of insulators in the field. Refer IEC TS 62073 for further details. In areas with extreme pollution, a silicone insulator’s hydrophobicity may become temporarily reduced from its original level. However, even under such extreme pollution conditions, the hydrophobicity transfer mechanism of the silicone results in the silicone housing performing better than porcelainhousings with equivalent creepage distance and shed profile. Unlike a porcelain insulator, a siliconehoused insulator is not necessarily at risk for flashover just because the surface is covered with pollution. Of importance is the extent to which the hydrophobicity recovers via transfer of low molecular weight silicone oils through the pollution layer to the surface. This is denoted as Hydrophobicity Transfer (HT) and is the relationship between the ESDD (equivalent salt deposit density, being the total amount of salts on the surface) and ASDD (apparent salt deposit density; being the portion of the pollution not covered by the low molecular weight silicone oils). The difference between the values of ESDD and ASDD represents the part of the pollution layer that does not conduct any current. Fig. 54

Note! This is not a simple test to perform in the field. However, it can be undertaken on an individual insulator removed from service as a means to evaluate the pollution performance of silicone insulators under specific site conditions.

Wettability class (source IEC TS 62073)


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11.3 Condition monitoring Despite being “maintenance free”, external factors can place stress on surge arresters (as with all HV apparatus), leading to a risk for their deterioration over time and potential failure. As businesses strive to remain competitive, unplanned outages are increasingly unacceptable, and it can therefore be of advantage to regularly check and/or monitor the condition of HV surge arresters connected to the network, so that they can be taken out of service before the situation becomes acute. Periodical external visual inspection can be undertaken to detect obvious evidence of deterioration which could affect the arrester’s in-service performance, e.g. physical damage, connections, flashover, tracking, erosion, puncture, etc. However, since arresters are delivered as sealed units from the factory, they cannot be disassembled for any internal inspection or tests, as doing so would be the same as destroying the arrester. For system voltages above approximately 100 kV, surge counters are often installed in series with the arresters. The main reason for the use of surge counters is to check if a particular transmission line or phase suffers from an exceptionally high number of overvoltages leading to arrester operation - lightning faults on a line, for example. If this is the case, some preventative counter-measures may be necessary to limit the number of surges. A sudden increase in the counting rate may also indicate an internal arrester fault. Conversely, a steady high counting rate from the beginning may indicate an unsuitable choice of arrester rating. In either case, the arrester should be investigated further. If a surge counter is used, the surge arrester must be equipped with an insulating base; thus ensuring that the discharge current is passing exclusively through the surge counter and not discharged directly to earth. However, surge counters tell only part of the story, as they simply register the number of surges according to their operating characteristic. The user therefore has no way of telling the magnitude of the surge and if it was significant, nor when it occurred and if it was coincident with a system event. A complete check of an arrester can only be made by measurements under laboratory conditions. There is no simple way to check an arrester during service, and normally there is no such need either. If, however, it is decided to perform a check on an arrester, it is desirable that the measurements can be made without disturbing the normal service, i.e. without disconnecting the arrester from the phase conductor. Many measuring methods have been employed over the years for gapless ZnO arresters, with the simplest method utilised being the connection of a standard mA-meter in series with the arrester to measure leakage current. The AC leakage current through the arrester can be divided into a capacitive and a resistive part. At continuous operating voltage (U c), a ZnO surge arrester acts as a capacitor, leading to a predominantly capacitive component of current and a significantly smaller resistive part.

It

Ic

U

Ir

Fig. 55 Principle diagram for a gapless ZnO arrester, where U is the voltage across the arrester, I t is total leakage current and I c and I r are the capacitive and resistive components, respectively, of the leakage current


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The specific capacitance of a ZnO varistor results in typical values of the capacitive current ranging from 0.5 to 3mA peak , depending on the varistor diameter. For a complete surge arrester, the capacitive current is further dependent on stray capacitances, pollution currents on the insulator surface, number of varistor columns in parallel and the actual operating voltage. Meanwhile, the resistive component of the leakage current of a varistor is at the same time in the range 50 to 250 μ Apeak, and is temperature and voltage dependant. Since the capacitive component of the current dominates so greatly, the total leakage current measured on a simple mA-meter will be very sensitive to the installation; making interpretation of the readings difficult. Further, there is no evidence that the capacitive current would change significantly due to deterioration of the voltage-current characteristic of the surge arrester. Consequently, measurement of capacitive current cannot reliably indicate the condition of ZnO arresters. Although increasing values may be of some use in indicating that cleaning of the insulators is necessary. Instead, it is generally recognised (IEC 60099-5) that the only reliable indicator for the condition of a gapless arrester which can be assessed during normal service is to measure the resistive component of the leakage current (or estimate it from the 3 rd harmonic), and compare it with the maximum allowable resistive current, as given by the manufacturer, under prevailing service conditions i.e. temperature and applied voltage. Ageing of the ZnO varistors will generally cause a gradual increase of the resistive leakage current with time. Because of the order of magnitude difference ( μ A vs. mA), a significant change in the resistive current would be required before it could be noticed on a milliamp meter. Therefore, special measuring apparatus are necessary to separate out the two components, and give a reliable detection method for the analysis of the leakage current through gapless ZnO surge arresters. Two such devices are ABB Surge Arrester Monitor EXCOUNT-II and TransiNor Leakage Current Monitor LCM-II.

Fig. 56 EXCOUNT-II surge arrester monitor


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11.4 Replacement of gapped surge arresters Since the statistical chance of a malfunction is greater for very old arresters, these should be identified and removed from service as soon as possible as the first step in any replacement program. In general, aged insulation has a lower withstand level from its original capability. This means that the margin of protection is reduced and the possibility of equipment failure increases with age. Hence, a replacement program should also identify older equipment, and replace the arresters protecting the most valuable equipment first. Further, when systems expand, there may be a need to upgrade the arresters connected to them; a fact that is often overlooked. The result is heavier than designed operating duty and increased failure risk. Arresters manufactured even as late as 1960 to 1970 may not be provided with any suitable pressure-relief mechanism for safe operation during internal short circuit. Even where such mechanism exists, it may not function satisfactorily if the short-circuit capacity of the line has been increased after the original installation and is now higher than the arrester capability. Such arresters almost certainly would not fulfil today’s tough requirements for short circuit safety and would fail violently in the event of their malfunction; causing damage to equipment nearby as well as posing a serious risk of injury to any personnel in the vicinity. As there are still many gapped silicon-carbide (SiC) surge arresters in service worldwide, it is worthwhile mentioning what can be done to assess their condition, since aged gapped arresters can malfunction due to a number of reasons, including: • sealing failures • arc erosion • grading component failures Monitoring may be undertaken on-line as a first step by scanning the arrester with an infrared camera to reveal any unusual hot spots. After the arrester is disconnected from the supply source, additional information can be gained off-line by the following tests: • Physically examine the arrester units externally to see if the gaskets have deteriorated or there is any sign of moisture ingress. • Megger each unit separately to detect any shorted units. However, when grading components are present, the readings should not tend to infinity, otherwise a discontinuity may be suspected in the unit. • If the grading current of an arrester is known at the time of its manufacture or installation, this figure can be used to compare with the value after it has been in service for some time. Considerable deviations from the original recording should motivate further investigation or replacement of the arrester. • Perform a spark-over test at power frequency (50 or 60 Hz) and compare the results with the values obtained during routine tests. If the results are more than + 10% from that given in the data sheet, the unit should be replaced. The sparkover voltage for an arrester must be measured in a high voltage laboratory to obtain the necessary sensitivity and control, and thus this t est cannot be performed on site. With consideration to the age and residual life of most gapped arresters, versus the time and cost to remove them from site, install replacements, perform tests in a HV lab, interpret the results and then possibly reinstate them in service, many users decide it is better to simply undertake a replacement program of all installed gapped arresters of a certain age without further analysis.


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12.

SPECIAL APPLICATIONS

With increased focus on system reliability, together with ongoing developments occurring in the field of overvoltage protection, new and innovative applications are continuing to be found for the use of surge arresters. Many of these are, however, quite specialised and require more in depth discussion than is considered possible within the scope of this Guide. Nevertheless, this section briefly discusses a number of these topics, and refers the reader to other ABB technical information for further reading should they be of interest. See References. 12.1 Reduced clearance distances In order to reduce the risk of insulation failure to an economically and operationally acceptable level, the insulation withstand of substation equipment is selected with regard to expected overvoltages, taking into account the protective characteristics of the surge arresters. The insulation withstand of the surge arrester itself has to be co-ordinated with its own protective characteristics. The arrester has to be positioned with respect to grounded objects and surge arresters in adjacent phases, without increasing the total risk for insulation failure. The insulation withstand properties of surge arresters in a substation can be divided into: • insulation withstand of the surge arrester itself, including the insulation between flanges and grading rings, etc. • insulation withstand between the surge arrester and grounded objects • insulation withstand between the surge arrester and other equipment connected to the same phase, e.g. bushings • insulation withstand between surge arresters in adjacent phases The insulation withstand should be the only constraint when selecting suitable clearances for properly dimensioned surge arresters. Any effects which various phase-to-ground and phase-tophase clearances may have on the voltage distribution along the ZnO block column should have already been accounted for in a well-made design. The insulation withstand of the surge arrester itself should also have been thoroughly considered at the design stage. Spacing between metal flanges, as well as spacing between flanges and grading rings, should be designed to be sufficiently large to withstand overvoltages appearing during current discharges; at least up to the design altitude (and perhaps more). a) Phase-to-ground clearance The phase-to-ground clearance in substations is usually based on the selected standard rated lightning and switching impulse withstand voltages. International Standard IEC 60071-2, for example, recommends minimum clearances. In general, the clearance between a grounded object and a surge arrester should be the same as the phase-to-ground clearance selected for other high voltage equipment in a substation. If it is not possible to use the normal phase-to-ground clearance in special applications, a smaller clearance may be chosen, considering the protective characteristics of the arrester, and after correction for altitude. However, this is generally only possible if there is a fairly big margin between the standard rated withstand voltage for a substation and the protective level of the arresters.


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b) Other equipment in the same phase The clearance between a surge arrester and other high-voltage equipment connected to the same phase, e.g. bushings or post insulators, is usually not of importance during normal operating conditions. In polluted conditions, however, the transient voltage distribution on the insulator surfaces may become extremely uneven. This creates high voltage stresses between the surge arrester housing and any high-voltage insulator positioned nearby. It is recommended therefore to choose half the phase-to-ground clearance as the minimum metal-to-metal clearance between the upper (energized) end of the surge arrester and the top (energized) end of other high-voltage equipment. Furthermore it is recommended to use the phase-to-ground clearance also for the spacing between the lower (grounded) end of the surge arrester and the bottom (grounded) end of other high-voltage equipment. c) Phase-to-phase clearance The phase-to-phase clearance for high-voltage equipment in a substation is normally based on the selected standard rated lightning and switching impulse phase-to-phase withstand voltages. International Standard IEC 60071-2, for example, recommends minimum phase-to-phase clearances. Note that the normal selection of surge arrester protective levels does not directly protect the phase-to-phase insulation. In general, the clearance between surge arresters in adjacent phases should be the same as the phase-to-phase clearance selected for other high-voltage equipment in the substation. If it is not possible to use the normal phase-to-phase clearance in a special application of surge arresters, the minimum clearance with regard to lightning overvoltages can be derived, and should include altitude correction. Similarly, the minimum phase-to-phase clearance for arresters with respect to switching overvoltages should also, if possible, always be based on the selected standard rated switching impulse phase-to-phase withstand voltage for the substation. If a special application requires a minimized phase spacing, a favourable electrode configuration established by the grading rings may permit a reduction of the phase-to-phase clearance in certain cases. More information on the selection of surge arresters for this application is available in the ABB Technical Information document “Insulation withstand and clearances with EXLIM and PEXLIM surge arresters”.


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12.2 Station protection When lightning surges enter a station, reflections occur and oscillations are set up due to the capacitance of the station apparatus and inductance of busbars and connection leads. For steep incoming surges, the difference in voltage shape and amplitude at different locations in the station will be significant. A station should be designed for a low probability of failure, and thus the protection against lightning surges is not only a question of which arrester to choose, but even more important, is to determine the number and location of arresters needed in order to obtain an adequate protection. Two examples of this application include: a) Line entrance arrester If, due to any reason, it is impossible to install an arrester in a sub-station as close to important equipment as ideally necessary, the protective distance of the station arrester may be improved by installing an additional arrester at the entrance into the station of the incoming line. This arrester also fulfils a second function as protection for an open line breaker. b) Protection of open breaker In over half the ground flash cases, the first lightning stroke will statistically be followed by one or more successive strokes. The first stroke may lead to a single-phase or multi-phase ground fault on a line, causing the relay protection to operate and to open the line breakers. If a rapidly following successive stroke hits the line, the lightning surge may reach the breaker in open position before the breaker has fully recovered its dielectric strength across the contacts. A restrike and possible breaker damage may occur. The normal arresters in the station cannot protect the breaker against this event, and instead a separate set of arresters on the line side of the breaker are required. Such additional breaker arresters give the additional benefit of improving the overall overvoltage protection of the station .

Surge Arresters marked with blue

For a full treatment of the problem of station protection, many parameters must be considered concerning probability-distribution of lightning currents, station layout, power frequency voltage, grounding, shield wires, protection levels and connection leads of surge arrester, insulation levels, etc. In view of the complexity of the problem, practical engineering has long been based mainly on “rules of thumb” or on simplified formulas, which often disregard many of the important parameters. The final choice is always left to the system engineer to decide upon an acceptable level of risk; taking into account additional parameters such as the importance of the station and the cost of a failure compared with the cost of improved protection.

Fig. 57 Arrester placement

More information on the selection of surge arresters for this application is available in the ABB Technical Information document “Application guidelines for station protection”.


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12.3 Lightning protection of transmission lines Transmission lines in the lower system voltage range, 72 kV - 245 kV, are often sensitive to lightning overvoltages for the simple reason that one or more of the following situations exist: • the insulation withstand is relatively low • the transmission line often lacks shielding wires • the footing impedance of the towers is high • the transmission line lacks a continuous counterpoise (shield earth wire) Despite this, meshed networks with rapid re-connection of faulted lines for the most part give satisfactory operational continuity. Short-time disturbances (around 0.5 seconds) must be tolerated in radial networks, as well as the voltage drop during the fault time (around 0.1 second) occurring in the meshed networks. There are, however, some types of loads where even brief disturbances can have a severe impact for the on-going process - e.g. steel mills, paper mills, refineries, etc. The cost for such an interruption, both in terms of value of lost production and the costs to re-start the production, are unacceptable. In today’s deregulated energy market, such costs will be more visible to the network operator than before, since the buyer can set high demands on delivery security. The traditional methods to reduce the number of faults caused by lightning have been: • installation of shield wires • improvement of the earthing impedance of the towers • increasing the insulation level Unfortunately, implementing these methods gives only marginal improvements of the delivery security, especially if the earthing conditions are difficult due to a high earth resistivity. A better alternative to reduce the number of line faults caused by lightning is to install ZnO arresters with polymeric insulators in parallel with the line insulators. These line surge arresters (LSA) normally consist of standard polymer-housed arresters together with a disconnecting device and fastening equipment for installation on the line itself or on the tower.

Fig. 58 LSA on HV transmission line

LSA’s give complete protection against lightning flashovers for the actual line insulator. Insulators in adjacent phases and in other towers, however, are not protected; which is why LSA’s are mainly installed on all phases on the towers that are intended to be protected. In reality, LSA’s are seldom installed throughout an entire line length, but instead only in areas where lightning gives most problems due to exposed position, bad earthing conditions etc. Modern localisation systems for lightning-storms in combination with traditional fault statistics are excellent tools to identify towers where LSA’s should be installed to be of most effective use. The dimensioning of a LSA generally follows the same criteria as for an arrester in a substation. However, it is of particular importance that the LSA is designed correctly with respect to energy and TOV capability, since the stresses on the arrester at lightning are highly dependent on the earthing conditions, presence of shield wires, etc. Similarly, the complete solution must be physically and mechanically robust, since it is fully exposed to the elements and typically inaccessible for close inspection or maintenance.


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Line Surge Arresters (LSA) of a gapless design offer a robust, efficient and cost-effective alternative to those with series gap; External Gap Lightning Arrester (EGLA). EGLA have the following inherent disadvantages in their functional operation: • Needs tailor-made design for each situation; making installation difficult and prone to error • Insulation co-ordination is difficult; since the gap characteristic is prone to change • Unpredictable energy sharing; increasing the risk for arrester failure • No switching impulsive overvoltage control; meaning outage rate remains high • Pollution effects unpredictable; increasing the risk for arrester failure • Signalling device needed to indicate failed arrester; but is rarely provided • Failed SA is ”weak point” for repeated ”short-circuits” The final point is perhaps one of the most commonly overlooked. It is promoted that the series gap will hold against power frequency voltage in the event of an arrester overload; which may be true. However, the gap cannot be dimensioned to withstand lightning impulses. Hence, a subsequent lightning strike to the line nearby the failed arrester will cause the gap to fire and a permanent earthfault will occur, with consequential line outage. Without some kind of signalling device (rarely provided) this failed arrester will be practically impossible to locate and repeated flashovers and line trips will occur. The problem will be multiplied in the case that several arresters along the line have failed. In comparison. LSA have the following unique advantages: • Easy insulation co-ordination for both lightning and switching overvoltages • Flexibility in application; may be mounted in a way that best suits the tower configuration • User-friendly installation; simply connected without individual adjustment • Predictable energy sharing; predictable estimate of risk for overload • Self-disconnection means no permanent fault / weak-point • No signalling device needed In contrast to EGLA, an LSA can be suspended from the line or mounted on the tower in a way that best suits the overall configuration. By so doing, the arrangement can be made to ensure that, in the event of an arrester overload, adequate clearance is assured to withstand not only PFWL but also LIWL and SIWL. This is achieved through the clever use and placement of the self-disconnecting device. Manufacturers of EGLA’s sometimes promote the disconnecting device on LSA’s as a “weak point”; prone to damage and premature disconnection. On some designs this may be the case. However, ABB’s LSA design is different since it uses common line hardware as well as specially designed fittings to ensure the robustness of the solution as a whole. Furthermore, in contrast to some solutions, ABB uses station class PEXLIM arresters for its LSA, meaning that the same high quality and secure performance expected in substations is also achieved out on the transmission lines. This is exemplified in the high values achieved during the lightning Impulse discharge capability test according to Annex N of IEC 60099-4 (since Ed 2.1, 2006-07). More information on the selection of surge arresters for this application is available in the ABB brochure “PEXLINK: Transmission-line protection for disturbance-free system operation”.


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12.4 Switching surge control in EHV systems In any complex electromagnetic system, a sudden change in state gives rise to transient oscillations which, in turn, can cause high overvoltages unless suitably damped. For EHV systems it has been common practice for many years to equip circuit breakers with closing resistors, as a means of controlling such system transient interactions during closing or re-closing operations. The closing resistors are inserted in series with the load circuit being switched for a short period of time before closing the main contacts of the breaker – thereby damping the transient overvoltages. Without any form of control, switching overvoltages during reclosing of a fault-cleared line could, under certain circumstances, rise as high as 3 – 4 p.u of the phase-ground peak voltage. Pre-insertion resistors typically function to limit this overvoltage to in the order of 1.5 – 2.0 p.u. Optimum overvoltage control requires correct choice of the resistor value in relation to the source impedance level, the line length and the line parameters. Although a well-proven technology, pre-insertion resistors can lead to a number of problems in mechanical design and operation; with adverse impact on overall system reliability. As robust and efficient alternatives, used either alone or in combination, the microprocessor-based ABB relay type Switchsync and PEXLINK Line Arresters could be substituted instead. The "intelligent" Switchsync relay makes it possible to connect the load to the network at a predetermined instant, which gives optimum transient suppression.

Fig. 59 Switchsync and CAT relays in conjunction with PEXLINK

PEXLIM silicone-housed surge arresters (forming part of the PEXLINK concept), located at line ends and along the line at selected points, function to limit switching surge overvoltages and thus line insulation requirements. To locate arresters along the line has previously not been a practical solution due to the fact that only porcelainhoused arresters with high discharge energy capability have been available. Now with lightweight polymer-housed arresters available for use even on EHV systems, a very efficient overvoltage control along long transmission lines is possible. Different line and switching configurations lend themselves to one or more stand alone solution, or a combination. Switchsync and/or PEXLINK will, in most cases, provide a cost effective, more reliable and efficient method of controlling line-switching transients than pre-insertion resistors. More information on the selection of surge arresters for this application is available in the ABB Technical Information document “Application guidelines for transmission line switching overvoltage control” and ABB brochure “PEXLINK: Transmission-line protection for disturbance-free system operation”.


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12.5 Shunt capacitor banks Shunt capacitor banks are used to an increasing extent at all voltage levels. Three-phase capacitor bank sizes vary from a few tenths of MVAr to several hundreds of MVAr, with both ungrounded wye and grounded wye banks in use. It is common practice today to use ”restrike-free” breakers. However, since many banks are switched on a daily basis, the probability of obtaining high transients associated with capacitor switching increases. Furthermore, the standardized procedure to verify that the breaker is restrike-free includes only a limited number of tests. The use of arresters in this application not only gives protection if a restrike does occur, but also decreases the probability of multiple restrikes since the trapped charge on the capacitors is reduced. Generally speaking, capacitor protection by surge arresters has been a difficult task before ZnO arresters became available. The high discharge currents and possible energies associated with an arrester operation at a capacitor bank heavily stressed the spark gaps in a SiC gapped arrester. The possible high energies could also result in overstressed SiC blocks. Once a sparkover occurred, the arrester which sparked-over had to discharge the whole energy stored in the capacitor bank and also carry a power-frequency follow current before a resealing at the next voltage zero was possible. With the introduction of ZnO surge arresters, it is possible to meet any energy demand by simply paralleling the necessary number of blocks, even if the procedure to ensure current sharing is quite sophisticated. Many capacitor banks are operated without surge arresters. However, there are a variety of beneficial reasons to install arresters:

• • • • • • •

To prevent capacitor failures at a breaker restrike or failure To limit the risk of repeated breaker restrikes To prolong the service life of the capacitors by limiting high overvoltages To serve as an ”insurance” against unforeseen resonance conditions which otherwise would lead to capacitor failures For overall limitation of transients related to capacitor bank switching which can be transferred further in the system and cause disturbances in sensitive equipment For upgrading of capacitors by preventing high overvoltages and/or for increasing the service voltage To serve as protection against lightning for capacitor banks and filters connected to lines

More information on the selection of surge arresters for this application is available in the ABB Technical Information document “Guidelines for selection of surge arresters for shunt capacitor banks”.


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12.6 Series Compensation (SC) and Static Var Compensation (SVC) Series capacitors have been applied for more than 50 years on EHV transmission lines in order to increase the possible power transfer and improve the transient and steady state stability of the power transmission system. The ever-growing need for electrical power, high costs and difficulties to obtain ”right-of-way” for new lines, together with the availability of ZnO varistors as highly effective overvoltage protection, have resulted in a boom for series compensation in the last decade. In addition to the old, but still valid, arguments for series compensation, the possibilities to use adjustable capacitors for load-flow control and balanced loading between parallel lines make series compensation even more interesting for the future. An extremely vital component for the series compensation scheme is its overvoltage protection. Historically, it comprised a single spark gap (for moderate demands on capacitor reinsertion speed) or a dual spark gap protective scheme (for faster reinsertion or other tougher requirements). With the availability of ZnO varistors, the protective schemes have been further improved by using the varistors in parallel with the spark gaps, and ultimately even without the spark gaps. This has led to simple and robust protection with ultra-fast re-insertion speeds, low re-insertion transients and low protection levels. Modern all-film capacitors have low losses, but their overvoltage withstand capability is less than that for the old type of paper-film capacitors. This leads to requirements of low protection levels to obtain an economical capacitor design. Low protection levels, however, may be difficult to achieve with spark gaps alone, since reinsertion transients can give unwanted gap operations. With ZnO varistors, this problem is easily solved and, in addition, the reinsertion of the capacitor will be instantaneous as soon as the voltage across the capacitor decreases below the conduction ”knee-point” of the ZnO varistor. The spark gap is used as overload protection for the varistor and is also usually used to quickly by-pass the capacitor/varistor for internal faults in order to limit the necessary design energy capability for the varistor. For higher protection levels, it may be necessary to use two gaps in series. With further improvements in varistor energy capability and faster by-pass breakers, it is possible in most cases to dispense completely with the spark gaps. More information on the selection of surge arresters for this application is available in the ABB Technical Information document “Overvoltage protection of series capacitors”. Static Var Compensation (SVC) schemes often require arresters with energy capability exceeding that provided by a single ZnO column. However, they are much smaller than those for SC schemes. Standard surge arresters (EXLIM and PEXLIM) are normally easily adapted to cope with these requirements after due consideration to ensure adequate current and energy sharing.


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12.7 HVDC arresters Surge arresters are applied in many different locations within a HVDC scheme, where the normal service voltage waveforms differ widely - from pure power frequency and DC voltages to mixed wave-shapes with commutation overshoots. The introduction of ZnO-technology had a great impact on the insulation coordination for for HVDC-converter stations. stations. With gapless ZnO arresters, arresters, it has been possible to reduce drastically the protective levels, especially as the coordinating cases originate from internal faults and/or switching events. These result in rather low discharge currents (some kA) compared to the usually considered lightning lightning currents (tens of kA) for general AC applications. ABB pioneered the world’s first gapless ZnO DC arrester, with a DC-line arrester installed in the Skagerrak HVDC transmission between Norway and Denmark, and has subsequently built on this success to gain extensive unique experience in this extremely specialized field. More information on the selection of surge arresters for this application is available in the ABB Technical Information document “ Overvoltage protection of HVDC-Converter stations”. stations”.

Fig. 60 UHVDC arrester

12.8 Current sharing considerations To meet very high energy requirements, parallel columns of ZnO blocks have to be used in surge arresters, and/or several arresters in parallel, so as to share the current and thus the energy. Typical high energy applications are protection of series capacitors and arresters used in HVDC schemes, with as many as 400 parallel columns of high energy varistors having been commissioned. commissioned. However, even more traditional traditional applications applications sometimes warrant warrant the use of parallel columns, where the energy demands are beyond the capability of a single column arrester. With ZnO arresters, the energy capability can be increased to meet any possible energy requirements by simply adding sufficiently many parallel columns; provided that no series or parallel spark gaps are used. To make full use of this benefit of ZnO arresters, however, it is necessary to ensure a good current, and thereby energy, sharing between the parallel columns. The columns can be mounted all in the same housing or in separate housings, depending on the necessary number of block columns. For special cases it can also be necessary to ensure that several different arresters share the energy in order to avoid overloading of the arresters. Such matched arresters have to be specially requested, since standard arresters may not necessarily achieve full current sharing. Whenever multi-columns are supplied, additional routine testing is required to verify adequate current and energy sharing between each column.

Fig. 61 Multi-column, high energy application

More information on the selection of surge arresters for this application is available in the ABB Technical Information document “Current “ Current sharing considerations”. considerations”.


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13. [1]

REFERENCES International Standards and Guides IEC 60060-1 IEC 60068-2-11 IEC 60068-2-14 IEC 60068-2-42 IEC 60071-1 IEC 60071-2 IEC 60099-1 IEC 60099-4 IEC 60099-5 IEC 60507 IEC 60815 -1,2,3 IEC 61166 IEC 61462 IEC TS 62073

[2]

American National Standards IEEE C62.11 IEEE C62.22 IEEE 693

[3]

High-voltage test techniques. Part 1: General definitions and test requirements Environmental testing - Part 2: Tests. Test Ka: Salt mist Environmental testing - Part 2: Tests. Test N: Change of temperature Environmental testing - Part 2: Tests. Test Kc: Kc: Sulphur dioxide test for contacts contacts and connections Insulation co-ordination - Part 1: Definitions, principles and rules Insulation co-ordination - Part 2: Application guide Surge arresters - Part 1: Non-linear resistor type gapped surge arresters for a.c. systems Surge arresters - Part 4: Metal-oxide surge arresters without gaps for a.c. systems Surge arresters - Part 5: Selection and application recommendations Artificial pollution tests on high-voltage insulators to be used on a.c. systems Selection & dimensioning of high-voltage insulators insulators intended for use in polluted conditions High-voltage alternating current circuit-breakers - Guide for seismic qualification of high-voltage alternating current circuit-breakers Composite insulators - Hollow insulators for use in outdoor and indoor electrical equipment - Definitions, test methods, acceptance criteria and design recommendations Guidance on the measurement of wettability of insulator surfaces

IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV) IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems Systems Recommended Practices for Seismic Design of Substations

Cigré Technical Brochures and Guides Cigré Technical Brochure 57, “The paper oil insulated measurement transformer”, 1990 Cigré Technical Brochure No. 60, ”Metal Oxide Surge Arresters in AC Systems”, Systems”, 1991 Cigré Technical Brochure Brochure No. 63, “Guide to Procedures Procedures for Estimating Estimating the Lightning Performance Performance of Transmission Lines”, 1991 Cigré WG33.11 Task Force 6, Application Procedures for Station and Overhead Line Insulation Coordination, “Flashovers of open circuit breakers caused by lightning strokes”, 1997

[4]

ABB Technical Information and brochures 2200en 2300E 2310E 2311E 2312E 2350en 2353en 2354E 2364E 2380E 2381E 9100en 1HSA 954312-10en

High Voltage Surge Arresters Buyer’s Guide Selection Guide for ABB HV Surge Arresters Application guidelines for station protection Application Guidelines for Transmission Line Switching Overvoltage Control Guidelines for selection of surge arresters for shunt capacitor banks Physical properties of zinc oxide varistors Voltage grading of EXLIM and PEXLIM surge arresters Insulation withstand and clearances with EXLIM and PEXLIM surge arresters Current sharing considerations Overvoltage protection of HVDC-Converter stations Overvoltage protection of series capacitors Silicone rubber in outdoor insulators PEXLINK: Transmission-line Transmission-line protection for disturbance-free system system operation


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[5]

STRI AB, Guide 1, 92/1, Hydrophobicity Classification Guide

[6]

A.R. Hileman, Insulation Coordination for Power Systems, Marcel Dekker, Inc. 1999

[7]

M. Mobedjina, L. Stenström, “Improved Transmission Line Performance using Polymer-housed Surge Arresters”, presented presented at CEPSI Seminar, Manila, Manila, October 23-27, 2000

[8]

M. Mobedjina, L. Stenström, “Limitation of Switching Overvoltages by use of Transmission Line Surge Arresters”, Cigré SC-33 International Conference, Conference, Zagreb, 1998, Technical Paper P.30 P.30

[9]

L. Stenström, J. Lundquist, ”Selection, Dimensioning and Testing of Line Surge Arresters”, presented at the the Cigré International Workshop on Line Surge Arresters and Lig htning, Rio de Janeiro, Brazil, April 24 -26, 1996

[10]

L. Stenström, Stenström, J. J. Lundquist, ”Energy Stress on Transmission Transmission Line Arresters Arresters Considering the Total Lightning Charge Distribution”, presented at the IEEE/PES Transmission and Distribution Conference and Exposition, Los Angeles, September 15-20, 1996

[11]

M. Mobedjina, Mobedjina, B. Johnnerfelt, L. Stenström, Stenström, “Design and Testing of Polymer-housed Polymer-housed Surge Arresters”, Arresters”, presented at GCC Cigré 9th Symposium Abu Dhabi, October 28-29, 1998

[12]

L. Stenström, Stenström, J. Lundquist. Lundquist. ”New Polymer-housed ZnO Arrester for High Energy Applications”. Cigré 1994 Session August 28 to September 3, Technical Paper 33-202

[13]

S. Vitet, L. Stenström, Stenström, J. Lundquist. ”Thermal ”Thermal Stress on ZnO Surge Arresters Arresters in Polluted Conditions – Part I: Laboratory test methods”, presented IEEE, PES 1991 T&D Conference and Exposition, Dallas, Texas September 22-27, 1991

[14]

S. Vitet, Vitet, A. Schei, L. Stenström, Stenström, J. Lundquist. ”Thermal Behaviour of ZnO Surge Arresters in Polluted Polluted Conditions – Part II: Field test results”. presented IEEE, PES 1991 T&D Conference and Exposition, Dallas, Texas September 22-27, 1991

[15]

S. Vitet, M. Louis, Louis, A. Schei, L. Stenström, Stenström, J. Lundquist. ”Thermal Behaviour of ZnO Surge Arresters Arresters in Polluted Conditions”. Cigré 1994 Session August 30 to September 5, Technical Paper 33-208

[16]

J. Lundquist, L. Stenström, Stenström, A. Schei, B. Hansen, ”New ”New method for measurement measurement of the resistive resistive leakage currents of metal-oxide surge arresters in service”, presented at IEEE SM, Long Beach, California, July 9-14, 1989

[17]

L. Gutman, Gutman, L. Stenström, D. Gustavsson, Gustavsson, D. Windmar, Windmar, W.L. Vosloo. ”Optimized use of HV composite composite apparatus insulators: field experience from coastal and inland test stations”. Cigré 2004 Session, Technical Paper A3-104

[18]

R.P.P. Smeets, Smeets, H. Barts, W.A.Van Der Linden, L.Stenström. L.Stenström. ”Modern ”Modern ZnO surge arresters arresters under short-circuit current stresses: Test Test experiences and critical review of the IEC Standard”. Cigré 2004 Session, Technical Paper A3-105

[19]

C. Neumann, Neumann, V. Aschendorff, G. Balzer, Balzer, H. Gartmair, E. E. Kynast, Kynast, V. Rees, “Performance of the switched gap of SF6-HV circuit-breakers stressed by lightning overvoltages”, Cigré 1996 Session, Technical Paper 13-102.

[20]

S. Narita, A. Sawada, Sawada, H. Watanabe, B. Johnnerfelt, Johnnerfelt, L. Strenström, “Design and testing testing of polymer-housed surge arresters with special emphasis on seismic stresses and selection of specific creepage in coastal areas”, Cigré Tokyo, 2005, Technical Paper A3-108.

[21]

J. Taylor, Taylor, ABB Power Products, Products, Ludvika, Ludvika, Sweden, “Short circuit circuit behaviour – Surge arresters and counters”, counters”, presented at INMR world congress and exhibition, Brazil, May 13 -16, 2007.

[22]

L. Stenström, Stenström, J. Taylor, F. Persson, N.T. Osiptsov, Osiptsov, “Installation of LSA on a 400kV double-circuit line in Russia”, presented at Cigré colloquium “Application of Line Surge Arresters in Power Distribution and Transmission Systems”, Cavtat 2008

[23]

ABB AB, AB, High Voltage Products, Products, Ludvika, Ludvika, Sweden, “Mechanical testing and demands on HV surge surge arresters”, arresters”, June 2008


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High Voltage Gapless ZnO Surge Arresters

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