PPLICATION GUIDELINE O V E RV O LTA G E P R O T E C T I O N
Dimensioning, testing and application of metal oxide surge arresters in low-voltage power distribution systems
Foreword
Up until 1998 no international standards existed for surge arresters in lowvoltage power systems. This situation presented two difficulties: firstly it lead to specifications which were adapted from other standards, for example, IEC99-1 and IEC-99-4, which are applied for high voltage surge arresters, with and without spark gaps; secondly, declared rating, parameters and tests performed by different manufacturers were not clear, and therefore not really comparable. In the past, different committees of IEC worked (and are still working) on standards and guidelines, as in IEC SC 28A: Insulation co-ordination of low-voltage installations; SC 37A: Surge protective devices (SPDs) in low-voltage power distribution systems; TC 64: Electrical installations of buildings; SC 77B: Electromagnetic compatibility – high frequency phenomena; and TC 81: Lightning protection. This did not make a clear and easy situation. Joint Working Group (JWG) 31 of TC 64 has taken the task to co-ordinate the work of the different technical committees and sub-committees under the title: Surge overvoltages and surge protection. In 1998 the standard IEC 61643-1 (First edition 1998-02), was released with the title: Surge protective devices connected to low-voltage power distribution systems- Part 1: Performance requirements and testing methods Mr. Bernhard Richter, Product Manager of the surge arrester division of ABB High Voltage Technologies Ltd, gladly took on the task to describe in a short and clear form the technical bases and application of surge protective devices for lowvoltage power systems, concentrating on Metal-Oxide surge arresters (MO-arresters) without gaps for outdoor and special applications. Mr. Richter is an active member in different working groups of IEC SC 37A and TC 81. His activity field includes mainly the development, testing and application of surge arresters for use in all voltage systems of power supply. We hope, that you as a reader, will find this booklet useful. We welcome amendments, suggestions and qualified hints, which may help us to cover all the demands of our customers.
ABB High Voltage Technologies Ltd Wettingen, April 2001
First published: May 2001 All rights reserved. No parts of this booklet may be reproduced or translated in any manner without the express written consent of ABB High Voltage Technologies Ltd. © ABB High Voltage Technologies Ltd Division Surge Arresters Wettingen / Switzerland 1
Contents
1
Introduction
5
Low-voltage MO-surge arresters from ABB
2
Overvoltages in low-voltage supply networks
5.1
MO-resistors
2.1
Overvoltages due to direct flashes
5.2
MO-surge arresters
2.2
Induced overvoltages
5.3
Technical data of the arresters
2.3
Overvoltages due to coupling
6
Tests
2.4
Transferred overvoltages through transformers
6.1
Type tests
2.5
Probability of overvoltages
6.2
Special tests
3
Low-voltage networks
6.3
Routine tests
3.1
System voltages in low-voltage networks
6.4
Acceptance tests
3.2
Insulation categories
7
Selection of MO-surge arresters
3.3
Low-voltage earthing systems
7.1
Selection of Uc
3.4
Temporary overvoltages (TOV) in low-voltage systems
7.2
Selection of Up
4
Surge protective devices (SPDs)
7.3
Selection of the energy capability
4.1
Principle function of surge arresters
8
Coordination of surge arresters
4.2
Definitions
9
MO-surge arresters for d. c. systems
4.3
Classifications
10
Installation of surge arresters
4.4
Service conditions
Bibliography
2
1 Introduction Overvoltages in electrical supply networks result from effects of lightning strokes and switching actions, and cannot be avoided. They endanger the electrical equipment and due to economical reasons, the insulation cannot be designed for all possible cases. Therefore, a more economical and safer on-line network calls for extensive protection of the electrical equipment against unacceptable overvoltages. This applies to high voltage as well as to medium and low voltage networks.
u
u i
i/2
Overvoltage protection can be basically achieved in two ways:
i/2 Z0
overhead line earth
– Avoiding lightning overvoltages at the point of origin, for instance through shielding earth wires. – Limit overvoltages near the electric equipment, for instance through surge arresters in the vicinity of the electrical equipment.
i : U: Z0 :
In low voltage systems the earth wire protection is generally not very effective. A lightning would hit not only one wire (the earth wire), but all, including the phase wires, and induced and transferred overvoltages could not be avoided.
lightning current generated overvoltage surge impedance of the line
Figure 1 Lightning overvoltage caused by a direct lightning flash to an overhead line.
U = Z 0 x i/2
The most effective protection against overvoltages in low voltage networks is therefore the use of surge arresters in the vicinity of the equipment.
Assuming Z0 = 450 Ω and a typicall current of i = 20 kA (80 % probability, see Table 1), the prospective voltage will reach U = 4500 kV. On low voltage lines, therefore, flashovers will occur between all the line conductors, and usually also a flashover to earth at the closest pole of the line. After flashover the effective impedance is reduced, depending on the earth resistance involved. Even with a low impedance of 10 Ω, and the current being at 10 kA, the voltage will still be U = 100 kV, travelling along the line. Therefore further flashovers can occur along the line.
For general information, and especially with regard to medium voltage networks, we refer to our APPLICATION GUIDELINES: Dimensioning, testing and application of metal oxide surge arresters in medium voltage networks [1]. Overvoltage protection in railway facilities, a. c. and d. c., is described in: Dimensioning, testing and application of metal oxide surge arresters in railway facilities [2]. Lightning overvoltages are the greatest threat to the low voltage networks. Overvoltage protection must be arranged in such a way that the overvoltage is limited to non-damaging values.
Negative downward Percentage Current peak value
2 Overvoltages in low-voltage supply networks Lightning surge overvoltages in electrical systems may be classified according their origin as follows [3]:
98 %
95 %
> 4 kA
> 6 kA
80 %
50 %
> 20 kA > 34 kA
20 %
5%
> 55 kA > 90 kA
Table 1 Probability of lightning peak values.
– overvoltages due to direct flashes to overhead lines – induced overvoltages on overhead lines due to flashes at some distance – overvoltages caused by resistive, inductive and capacitive coupling from systems carrying lightning currents.
2.2 Induced overvoltages Due to the electromagnetic field changes caused by a lightning flash, overvoltages are induced in overhead lines of all kinds. As a rough approximation, the prospective overvoltage between the line conductors and earth can be estimated according to Rusck [5]
In [4] is discussed in detail the case of transferred overvoltages through a distribution transformer from the medium voltage to the low voltage side.
Umax = Z0 x Imax x H / D Imax is the peak value of the lightning current Z0 is the effective impedance (assumed to be 30 Ω) H is the height of the line D is the distance of the flash location from the line
2.1 Overvoltages due to direct flashes The overvoltage is determined by the effective impedance of the line and the lightning current. For a flash to an overhead line conductor, the impedance is in the first moments determined by the characteristic impedance (surge impedance) Z0 of the line.
Considering a height of 5 m for low voltage overhead lines, a lightning current of 20 kA, and a distance of 100 m, the induced voltage is calculated
The impedance Z0 is normally in the range of 400 to 500 Ω for one conductor. As shown in Figure 1 the lightning current is diverted in two, each part travelling along the line. The generated voltage is calculated
Umax = 30 kV. 3
With a distance of 1000 m between the line and the flash location, the induced voltage has a value of Umax = 3 kV. The above calculated values of induced voltages in low voltage overhead lines show that this kind of surge is of primary concern for low voltage distribution systems. Lightning induced overvoltages occur mainly between the conductors and earth. The voltage difference between the conductors is initially small, especially when twisted conductors are used. However, due to different loads on phase conductors (depending on low voltage system), interactions of surge protective devices, flashovers, etc., considerable line-to-line stresses can also occur. An example illustrating induced overvoltages line-to-line in low voltage systems is shown in Figure 2. Twisted conductors, including neutral, are assumed. The neutral is earthed on both ends of the line. The voltages at a certain point of the line show a high frequency dumped oscillation (ringing wave). The period of the oscillation corresponds to twice the travel time of a span, a span being the distance between two poles. Furthermore, it is found that the highest voltage occurs in the middle of the span. In the given example the voltage reaches up to 23 kV in the middle of the span, and up to 5 kV directly at the pole, where consumers may be connected.
25 U(kV)
Lightning protection system (LPS) 100%
L1 L2 L3 N
0.5 i
i
∆U
∆U PE
0.5 ⋅ i earthing impedance
LV cable
Figure 3 Example of resistive coupled overvoltages in electrical systems. In the electrical installations in the building, as well as in close installations (and all conductuing parts) in the earth high overvoltages can be generated.
Due to the high electromagnetic fields caused by the lightning current, inductive and capacitive coupling to electrical systems close to a lightning path can also cause overvoltages of concern, causing failures or malfunctions.
32 kA
U2
i
20
150 m
U1
15
2.4 Transferred overvoltages through transformers
0 1 2 250 m
10
Overvoltages generated in the medium voltage (MV) system are transferred to the low voltage (LV) system in two ways,
U0
5 0 1
2
3
4
5
6
7 t(µs) 8
– by capacitive and magnetic coupling through the MV / LV transformer – by earth coupling (see Figure 4).
-5 -10
The magnitude of the transferred overvoltage depends on many parameters and some important differences can exist between different countries, due to differences in the transformer design and the LV earthing systems (T T, T N, IT). Figure 2 Induced overvoltage line-to-line. Calculated values, assuming twisted conductors.
Medium voltage line (MV)
Low voltage line (LV)
A2
2.3 Overvoltages due to coupling A1
A lightning flash to earth can result in an earth potential of high value at the point of the strike, as well as in the vicinity. This phenomenon will cause overvoltages in electrical systems, using this point of earth as reference for their earthing system. Figure 3 shows the principle of this phenomenon. The potential rise of the earthing system is determined by the lightning current and the effective earthing impedance. In the first moment the earth electrode potential is determined by the local impedance, for instance 10 Ω. This means that a high voltage is generated between the earthing system and electrical installations inside the building, or other installations close to the earthing system. With a high probability this overvoltages will cause either flashovers, insulation breakdown or operation of surge protective devices. Following such events, current impulses can flow into the various systems, mainly determined by their impedance to earth. In this way overvoltages are produced in the power supply system as well as in other services (telecommunication, data and signalling systems, etc.). Furthermore, overvoltages are transferred to other buildings, structures and installations.
B2
Transformer
B1
C L1
L1
L2
L2
L3
L3
A1 A2 B1 B2 C
by direct lightning to the MV line by indirect lightning to the MV line (induced voltage) by direct lightning to the LV line by indirect lightning in the LV line (induced voltage) by capacitive coupling through the transformer
Figure 4 Overvoltages in the Low voltage system
4
10
The high frequency components of the overvoltage are transferred capacitively from the MV to the LV side of the transformer [4]. Figure 5a shows a typical wave shape of the overvoltage transferred to the LV line. Being the transferred overvoltage characterized by high frequency oscillations, the natural capacitance of the load can reduce very effectively the peak overvoltages, as shown in Figure 5b. The calculated voltages in the given example reach peak values of 10 kV (without load, Figure 5a), and 3 kV (with load, Figure 5b). In case of direct lightning to the MV line, the surge arrester operation or an insulator flashover diverts the surge current through the earthing system, and can produce a resistive earth coupling between the MV and LV system. An overvoltage is transferred to the LV system as shown in the typical case of Figure 6a. Depending on the earthing impedance, this earth coupling overvoltage can be much higher than the capacitive coupling through the transformer. Separating the earthing electrodes, as in Figure 6b, avoids this problem. Practically it is not possible to have really separated earth systems, due to the short distance and the conductivity of the earth.
a)
U (kV)
5
0 0
5
15
10
t (µs)
20
-5 10
b)
U (kV)
5
0 0
5
15
10
t (µs)
20
a) without representation of the users installation (no load assumed) b) user installations represented by lumped capacitances
2.5 Probability of overvoltages The frequency of lightning flashes to an overhead line, or in the vicinity of the line, depends on the local flash density, line type (especially the height) and possible shielding effects of the surroundings [3], [4], [5]. For lines in an open area the number of flashes can be calculated as follows
Figure 5a / 5b Typical wave shape of overvoltage transferred to the LV line (calculated).
N = A x Ng x 10-6 A=6xHxL A H L Ng
U = U0 + R⋅i + L di/dt
MV Arrester i
Equipment Soil
(R, L) Transformer Earthing
For a line of 5 m height and assuming Ng = 1, N is found to be 0,03 per km of line and year, that means three direct flashes per 100 km of line length and year. This gives a rough estimate of number of direct flashes to low voltage overhead lines.
Installation Earthing
a) MV and LV side of the transformer have same earthing point. This generates, in case of arrester operation, an overvoltage Ug on the LV system (∆Ug = R ⋅ i + L ⋅ di/dt) (no load assumed)
The number of induced and transferred overvoltages is certainly much higher than the overvoltages due to direct flashes in the line. Especially the local lightning density and the different possibilities of generating overvoltages, including switching, has great influence on the occuring number of dangerous overvoltages. In Figure 7 a typical low voltage system with overhead line is given. Calculated figures are presented for induced overvoltages which may be expected in this network, [3]. The ground flash density was assumed to be 2,2 flashes per km2 per year, all loads were modelled by frequency independent resistors. Table 2 shows the calculated results. The last column (> 20 kV) shows high levels of overvoltages, but these occur only in case of direct lightning to the low voltage line. The probability of occurrence of such surges in this example is once in 22 years. But the overvoltages in the range of 1,5 kV to 6 kV can occur several times a year in a low voltage network, depending on the type of installation.
U = U0 MV Arrester i
Equipment Soil
Arrester Earthing
Transformer Earthing
= effective area for direct lightning to the line (in m2) = height of the line ( in m) = length of line (in m) = local flash density per km2 and year
Installation Earthing
b) Separate earthing for MV and LV side of the transformer.
Figure 6a / 6b Overvoltage on the low voltage side due to earth coupling.
5
Voltage line-to-neutral derived from nominal voltages a.c or d.c. up to and including
230/400 V line twisted cable (3 phases + neutral)
MV/LV station
Line connection 20 kV line
V 25 m 240 m
Neutral earthing 30 ohms
Conductive parts earthing 30 ohms
with earthed neutral E
unearthed
V
V
Installation earthing 50 ohms
Figure 7 Typical low voltage network with overhead line. Arrangement used for calculating the values in Table 2.
> 2,5 kV
> 4 kV
> 6 kV
> 20 kV
Unloaded TT system
6
3
18
1
0,045
Loaded TT system
4
17
1
0,5
0,045
Loaded TN system
1
0,6
0,35
0,25
0,045
V
Single-phase three-wire systems a.c. or d.c.
V 24 30 48
30–60
100
66/115
66
60
150
120/208* 127/220
125, 120 127
110, 120
110–220 120–240
300
220/380, 230/400 240/415, 260/440 447/830
220, 230, 240 260, 277, 347 380, 400, 415 440, 480
220
220–440
600
347/600, 380/660 400/690, 417/720 480/830
500, 577, 600
480
480–960
600 690, 720 830, 1000
1000
1000
> 1,5 kV
Single-phase two-wire systems a.c. or d.c.
12,5 25 42
Consumer’s earthing
30 m
Neutral earthing 30 ohms
Three-phase three-wire systems
50
MV arresters 240 m
Three-phase four-wire systems
* Practice in the United States of America and in Canada.
Table 3 Nominal voltages presently used world wide.
Table 2 Line-to-earth prospective overvoltage levels in the LV installation, occurrences per year. Note 1: The numbers shown in the table were obtained for an overhead twisted cable distribution system. For a distribution system with overhead open conductors in air, the voltage levels can be expected to be twice as high for the same probabilities. Note 2: In this example, when performing a variation of the model to represent a TN system, it was found that the value of the earthing impedance had no significant influence because the LV neutral is directly connected to earth.
3.2 Insulation categories The concept of overvoltage categories is used for equipment energized directly from the low voltage mains. For the different categories the insulation levels are specified. According to [6] the definitions of the categories are as follows: – Equipment of overvoltage category IV is for use at the origin of the installation (e. g. overhead lines, cables, bus bars, meters, primary overcurrent protection equipment).
3 Low-voltage networks Around the world very different low voltage networks exist. They differ in the system voltage, the number of wires, the handling of the neutral and the protective measures. The nominal voltages of the supply systems are basically given in publication IEC 60038 (1983-01) and amendments: IEC standard voltages.
– Equipment of overvoltage category III is equipment in fixed installations and for cases where he reliability of the equipment is subject to special requirements (e. g. mainly fixed indoor installation). – Equipment of overvoltage category II is energy-consuming equipment to be supplied from the fixed installation (e. g. appliances, portable tools and other household and similar loads).
In IEC 60664 [6] is given a good overview of the nominal voltages presently used in the world, depending on the type of network, see Table 3.
– Equipment of overvoltage category I is equipment for connection to circuits in which measures are taken to limit transient overvoltages to an appropriate low level (e. g. protected electronic circuits).
3.1 System voltages in low-voltage networks As seen in Table 3, there is world wide a variety of existing voltages. The standard voltages in Europe, for instance, are given in [7]. The system voltages, according to the harmonization document, are 230 / 400 V, where 230 V is the line to neutral voltage, and 400 V is the line to line voltage. Other existing common voltages in Europe are 240 / 415 V and 220 / 380 V. Considering an allowed tolerance of 10 %, the highest voltages to be expected for the 400 V system are
U0max = 253 V (line to neutral voltage) and UNmax = 440 V (line to line voltage).
6
Table 4 gives the four insulation categories. The rated impulse voltage gives the insulation withstand capability for the different categories, depending on the line to neutral voltage of the systems derived from the nominal voltages a. c. or d. c., based on IEC 60038.
Electrical Equipment
LV line Electrical Power Source
Voltage line-to-neutral derived from nominal voltages a.c or d.c. up to and including
Rated impulse voltage for equipment
Combined PEN Conductor
L Customer Connection Point
TN-C System
N&E
V Insulation category
V
I
II
III
IV
50
330
500
800
1 500
100
500
800
1 500
2 500
150
800
1 500
2 500
4 000
300
1 500
2 500
4 000
6 000
600
2 500
4 000
6 000
8 000
1 000
4 000
6 000
8 000
12 000
Figure 8b
TN - C-S system (Figure 8c) The supply neutral is earthed at the source and points in the network. Supply lines have a combined neutral and earth wire. Supply within the customer premises would have separate neutral and earth wire, connected only at the service position. A protective neutral bonding (PNB) arrangement may be used to provide an earth terminal connected to the supply neutral. With this arrangement, the neutral will be connected to earth at the source point only, at or near to the customers supply point. The arrangement is generally restricted to a single customer with it`s own transformer. See Figure 8d.
Table 4 Insulation categories for low voltage systems.
3.3 Low voltage earthing systems There are a number of methods used to provide an earth connection or system. The different arrangements and standard definitions are given below. Each is defined by a coding which contains the following letters: T : terre, direct connection to earth N : neutral C : combined S : separate The different principle earthing arrangements are shown in Figure 8. For simplification single line diagrams are used.
Electrical Power Source
Combined PEN Conductor
Electrical Power Source
TN-C-S System
N
Figure 8c
Electrical Equipment
LV line
Electrical Equipment L Customer Connection Point
L Customer Connection Point
E
TN - S system (Figure 8a) The incoming supply has a point of connection between the supply neutral and earth only at the supply transformer. The lines have separate neutral and earth protective conductors.
LV line
Electrical Equipment
LV line
Electrical Power Source TN-S System
Combined PEN Conductor
N
L Customer Connection Point N
TN-C-S System (PNB)
E E
Figure 8d Figure 8a
TT system (Figure 8e) The transformer is connected directly to earth, the customers installation is earthed via a separate electrode. This will be independent of any supply point electrode.
TN - C system (Figure 8b) The neutral and earth wire are combined within the premises, and are earthed at the supply transformer or close to it.
7
SPDs connected to:
Electrical Equipment
LV line Electrical Power Source
L Customer Connection Point N E
TN-systems Connected L- (PE)N or L-N Connected N-PE Connected L-L TT-systems Connected L-PE Connected L-N Connected N-PE Connected L-L IT-systems Connected L-PE Connected L-N Connected N-PE Connected L-L TN, TT and IT-systems Connected L-PE Connected L-(PE) N Connected N-PE Connected L-L
TT System
Alternative Location for Earth Terminal
Figure 8e
IT system (Figure 8f) This arrangement has no direct system connection between live parts and earth, but the exposed conductive parts of the customers installation and its equipment is earthed.
Electrical Power Source
L Customer Connection Point N E
TOV values for 0,2s:
1,45 ⋅ U0 – –
– – –
√3 ⋅ U0 1,45 ⋅ U0 – –
1200V + U0 – 1200V –
– 1,45 ⋅ U0 – –
1200V + U0 – 1200V –
√3 ⋅ U0 1,45 ⋅ U0 – –
1200V + U0 – 1200V –
Table 5 TOV values in low voltage systems.
nonlinear component that is intended to limit surge voltages and divert surge currents. The discussed SPDs are typically for use in low-voltage power systems, providing protection from the low-voltage bushing of the MV / LV transformer up to the plugs in buildings. In the course of this guidelines we will talk mainly about metal oxide surge arresters (MO-arresters) without gaps for outdoor and indoor application.
Electrical Equipment
LV line
Minimum UT for 5s:
IT System
Alternative Location for Earth Terminal
4.1 Principle function of surge arresters There are two different designs for surge arresters: a voltage limiting type, and a voltage switching type. The voltage limiting type is a nonlinear resistor, generally a metal oxide resistor, without any spark gap in series. This types are sometimes called MOV, which is an abbreviation of metal oxide varistor. The voltage switching type is a spark gap, or a spark gap with a nonlinear resistor (MO or SiC) in series or parallel.
Figure 8f
3.4 Temporary overvoltages (TOV) in low-voltage systems
Figure 9 shows the principle difference in the function of the two types. In case of a failure on the medium voltage side of the MV / LV transformer, due to an internal fault of the transformer or a sparkover of a gap or insulator, a current flows through the earthing impedance of the transformer. Depending on the connection between this earth impedance and the low voltage network a temporary overvoltage with power frequency can stress the low voltage network for a given period of time, equal to the clearing time of the fault in the medium voltage network. This can be between some 10 µs up to some hours. For a detailed discussion of temporary overvoltage conditions see IEC 60364 [8].
Gapped arrester
v
MO-arrester
v
t Depending on the earthing system of the low voltage network different TOV can occur. Table 5 gives an overview about the considered systems and the possible TOV between the different lines. Two values are given, the minimum TOV value for 5 sec, and the TOV values for 0,2 sec. Corresponding test procedures are described in the amendment of IEC 61643-1 [9].
v
v
time scale 10 µs/div
The test procedure depends on the intended application of an SPD in a low-voltage power installation system according to the installation instructions given by the manufacturer.
t
time scale 25 µs/div
Figure 9 Difference in function of gapped arresters (left), and MO-surge arresters without gaps (right). Both types were tested with switching voltage impulses of the wave shape 250/ 2500 µs . The voltage scale is the same in both cases. It is to be seen that in case of the MO-arrester the residual voltage is only half of the one given by the gapped arrester (same Uc for both types of arresters).
4 Surge protective devices (SPDs) SPDs are devices for surge protection against direct and indirect effects of lightning or other transient overvoltages. They contain at least one 8
Surge arresters which contain only spark gaps, or spark gaps with nonlinear resistors in series, have the disadvantage that the voltage collapses suddenly when the sparkover-voltage of the device is reached. This very high du/dt may cause EMC problems in data-lines which are close to the power lines, or lead to failures in inductive loads. Furthermore, the spark-over voltage depends on the steepness of the overvoltage. Because the spark gap fires only at very high voltage levels, it can happen that overvoltages bypass the surge arrester, and downstream connected instruments or installations are over-stressed. Surge arresters containing only MO-resistors have no sparkover-voltage. The turn on time is in the range of 15 ns, and the voltage is limited according to the extremely nonlinear voltage-current characteristic of the MOmaterial. A bypassing of these arresters is not possible.
Nominal a. c. voltage of the system U0 U0 is the nominal line to neutral voltage of the a. c. system (rms value). Continuous operating current Ic The current flowing through the arrester when energized at the maximum continuous operating voltage Uc. Follow current If Current supplied by the electrical power system and flowing through the arrester after a discharge current impulse. Note: the follow current is significantly different depending on the design of the arrester. For MO-surge arresters without gaps the follow current is generally in the range of some 10 mA in maximum.
The advantages of MO-surge arresters are mainly the constant low protection level independent on the steepness and polarity of the incoming surge, the very good ageing behaviour, and the high energy capability. Possibilities of coordination of parallel MO-surge arresters are described in chapter 8.
Reference current of an arrester Iref The reference current is the peak value of the resistive component of a power frequency current used to determine the reference voltage of the arrester. The reference current should be high enough to have a clear dominating resistive component, so that capacitive influences can be neglected. The reference current is specified by the manufacturer, and generally in the range of 1 mA to 10 mA, depending on the cross section of the MO-resistor used in the arrester.
4.2 Definitions In the new standard family of IEC 61643 the special requirements for surge arresters for application in low-voltage power systems are considered. In the following, the most important definitions are given with reference to [9], concentrating on MO-surge arresters without gaps. For the purpose of this guidelines some definitions with reference to [11] are added.
Reference voltage of an arrester Uref The reference voltage of an arrester is the peak value of the power frequency voltage divided by √2 which has to be applied to the arrester to obtain the reference current Iref. The reference voltage at a given reference current is used to determine a point on the u-i-characteristic of an arrester in the low current range.
The surge arresters addressed in this guidelines are to be connected to 50 / 60 Hz a. c. and d. c. power circuits, and equipment rated up to 1000 V a. c. (rms) or 1500 V d. c.
Voltage protection level Up A parameter that characterizes the performance of the arrester in limiting the voltage across its terminals, which is selected from a list of preferred values. This is generally the guaranteed value given by the manufacturer.
Surge Protective Device (SPD) A device that is intended to limit transient overvoltages and divert surge currents. It contains at least one nonlinear component. Note: as mentioned above, in the course of this guidelines this is the same as a surge arrester, or short arrester.
Residual voltage Ures The peak value of voltage that appears between the terminals of the arrester due to the passage of discharge current.
Nominal discharge current In The crest value of the current through the arrester having a current wave shape of 8 / 20 µs. This is used for the classification of the arrester for classII test and also for preconditioning of the arrester for class I and II tests.
Protection ratio Up / Uc The protection ratio gives the relation between the voltage protection level Up at In and the maximum continuous operating voltage Uc. Up is given as a peak value and Uc is given as a rms value. The lower the ratio Up / Uc , the better the protection given by the arrester.
Impulse current Iimp It is defined by a current peak value Ipeak and the charge Q, tested according to the test sequence of the operating duty test. This is used for the classification of the arrester for class I test. A typical waveshape that can achieve the parameters is that of a unipolar impulse current with a waveshape of 10 / 350 µs. An other waveshape or impulse combination is acceptable, as long as they obtain the peak value Ipeak within 50 µs and the charge Q within 10 ms.
Temporary overvoltage The maximum a. c. (rms) or d. c. overvoltage that exceeds the maximum continuous operating voltage of the network for a specified time duration. Note: It has to be made a clear distinction between the temporary overvoltage UTOV occuring in the network at a given location, and the temporary overvoltage UT an arrester can withstand. The power frequency voltage versus time characteristics of an arrester (TOV-characteristic), provided on request by the manufacturer, is in low-voltage systems normally used only in case of special applications of the arrester.
Maximum discharge current Imax for class II test Crest value of a current through the arrester having a 8/20 µs wave shape and magnitude according to the test sequence of the class II operating duty test. Imax is greater than In and declared by the manufacturer. It is used in the operating duty test to prove the correct function and thermal stability of the arrester.
Combination wave The combination wave is delivered by a generator that applies a 1,2 / 50 µs voltage impulse across an open circuit and an 8/20 µs current impulse into a short circuit. The voltage, current amplitude and waveforms that are delivered to the arrester depend on the impedance of the arrester to which the surge is applied.
Maximum continuous operating voltage Uc The maximum a. c. (rms) or d. c. voltage which may be continuously applied to the arresters terminals. This is equal to the rated voltage. 9
Thermal runaway An operational condition when the sustained power dissipation of an arrester exceeds the thermal dissipation capability of the design, leading to an increase in the temperature of the internal elements culminating in failure. Thermal stability An arrester is thermally stable if after an energy input causing a temperature rise the temperature of the arrester decreases with time under applied continuous operating voltage. Degradation The change of original performance parameters as a result of exposure of the arrester to surges, service or unfavourable environment.
Class I
Class II
Class III
lightning current arresters
surge arrester
surge arrester
lightning protection in connection with lightning protection structures
overvoltage protection energy supply
overvoltage protection “down stream”
I imp (10 / 350 µs) 1 kA … 20 kA
I max (8 / 2 0 µs) > 0,05 kA … 50 kA
Uoc (2 Ω)
Table 6. Classification of low-voltage surge arresters. The given values are typical ratings.
Arresters tested according to class II test methods are generally subjected to impulses of shorter duration than class I arresters. The typical application is the overvoltage protection of low-voltage overhead lines and cables, as well as the protection of indoor installations. The expected stresses are originated by direct or indirect lightning to overhead lines or cable junctions. Required information is the nominal discharge current In and the maximum discharge current Imax.
Disconnector A device for disconnecting an arrester from the system in the event of arrester failure. It is to prevent a persistent fault on the system and to give visible indication of the arrester failure. Type tests Tests which are made upon the completion of the development of a new arrester design. They are used 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 so as to modify its performance. In such a case, only the relevant tests need to be repeated.
Arresters tested according, to class III test methods are subjected to impulses of lesser energy content than class I and class II arresters. They are recommended for locations with less exposure, mainly indoor. The information required is the open-circuit-voltage Uoc of the combination wave generator.
Routine tests Tests made on each arrester or parts of it to ensure that the product meets the design specifications.
4.4 Service conditions Acceptance tests Tests which are made when it has been agreed between the manufacturer and the purchaser that the arrester or representative samples of an order are to be tested.
The normal service conditions are – the applied continuous voltage between the terminals of the arrester should not exceed the maximum continuous operating voltage Uc – frequency between 48 Hz and 62 Hz a. c., or d. c. voltage – altitude up to 2000 m – operating and storing temperatures normal range: - 5 °C to + 40 °C extended range: - 40 °C to + 70 °C – relative humidity up to 90 % for indoor temperature conditions
4.3 Classification In [9] the surge protective devices (or short arresters) are classified according – the number of ports (one or two) A one port device has two terminals, a two port device has four terminals. The two port device may contain internal decoupling elements. – the design topology (switching type, limiting type, or combination type) – the test method (class I, class II, or class III test method) – the location (outdoor or indoor) – the accessibility (accessible or out-of-reach) – the mounting method (fixed or portable) – the disconnector (with or without) – the backup overcurrent protection (specified or not specified) – the temperature range (normal or extended) As long as the arresters are installed at different locations in a system or installation the stresses to be expected are very different. Therefore, the arresters are classified with respect to the expected stresses, and consequently the test methods, in three classes. See Table 6.
Exposure of the arrester to abnormal service conditions may require special considerations in the design or application of the arrester, and should be called to the attention of the manufacturer. Abnormal conditions may be extreme ambient temperatures (minus or plus), mechanical stresses, shock and vibration, etc. For outdoor arresters exposed to solar radiation, air pollution, bad weather conditions, additional requirements may be necessary.
5 Low-voltage MO-surge arresters from ABB A MO-surge arrester is made of two parts: the active part, which consists of a MO-resistor, and an insulating housing including the terminals.
5.1 MO-resistors
The class I test is intended to simulate partial conducted lightning current impulses. Arresters subjected to class I test methods are generally recommended for locations at points of high exposure, e.g. line entrances to buildings protected by lightning protection systems (LPS). These devices are called lightning current arresters. In addition to nominal discharge current In, information is required for the impulse current Iimp.
The voltage-current (u-i) characteristic of a metal oxide resistor is extremely nonlinear. That is the reason why arrester designs without spark gaps are possible [1], [10]. Figure 10 shows a typically u-i-characteristic of a MO-surge arrester with In = 10 kA. The voltage is normalized to the residual voltage at In. 10
The diameter of the MO-resis-tors decides the carrying capacity of the current, the height of the voltage, and the volume of the energy capacity. Table 7 shows the main data of the MO-resistors. For low-voltage application the same high-quality MO-material is used as for distribution and high voltage application. MO-resistors are compressed and sintered in the form of round blocks of different metal oxides in powder form. The diameters of the MO-resistors from ABB for low-voltage application are between 30mm and 75mm, covering even the highest energy requirements. The height of the blocks is between 1 mm and 10 mm, covering a voltage range from 120 V a. c. to 1500 V d. c. For special applications MO-resistors with a rectangular shape can be produced.
All used materials are UV-resistant and perform well under extreme weather conditions. Safety and ecological aspects are specially taken into consideration with all arresters. Figures 11 to 14 show a selection of different types of MO-surge arresters from ABB.
U
4/10µs
[p.u.]
1/5µs 8/20µs
1.0 30/60µs 2000µs
Figure 11 MO-surge arrester type LOVOS. This type was developed for outdoor application and can be used under all weather conditions. It is available with In = 5 kA or 10 kA, with or without disconnector. Uc = 280 V, 440 V and 660 V.
0.5
0 10
-4
-3
10
-2
10
-1
10
0
10
10
1
2
10
3
10
4
10
I [A]
Figure 10 Normalized voltage-current-characteristic of a MO-surge arrester with In = 10 kA.
Diameter of blocks in mm
30
41
47
75
Nominal current In 8 / 20 µs in kA
5
10
10
10 / 20
Imax 8 / 20 µs acc. class II test in kA
25
40
32
50
Ipeak (10 / 3 5 0 µs) acc. class I test in kA
–
–
–
10
2,5
4,0
4,5
12,0
Energy capability in kJ / k VUc
Figure 12 MO-surge arrester POLIM-R. Very high energy capability. Can be used for a. c. and d. c. networks. This type is, besides other applications, used in d. c. railway networks. Uc range from 140 V d.c. to 1000 V d. c., and 110 V a. c. to 780 V a. c.. Tested according test class I and test class II.
Table 7 Main data of ABB MO-resistors used in ABB MO-surge arresters for low-voltage application. The values are given as tested in the operating duty test to prove the thermal stability of the respective surge arrester. Other values are possible in other arrester designs.
5.2 MO-surge arresters As long as very different applications and ratings for low-voltage surge arresters exist, different designs are needed. ABB offers a great variety of different arrester types for all kind of applications. The main design principle is always the same: A MO-resistor, as the active part, and the terminals are moulded completely in an insulating housing. Depending on the application and rating of the arresters the physical shape and housing material may be different. The general, surge arresters for outdoor application (e. g. overhead lines, MV / LV transformers) have a housing of polyamide; arresters for outdoor and indoor applications (e. g. railway applications) have a housing of silicon, and arresters of older design have housings of PUR. All arresters are moulded to be completely sealed and waterproof.
Figure 13 MO-surge arrester MVR. Used in low-voltage systems and railway equipment. For a. c. and d. c. application. Available for In = 5 kA and 10 kA, with Uc = 440 V, 660 V and 800 V.
11
The standard IEC 61643-1 does not mention a high current impulse with a waveshape of 4/10 µs and a rectangular current with a time duration of some ms. The high current impulse 4/10 µs, as known from IEC 60099-4 [11], was intended to represent a severe direct lightning to the line very close to the arrester location. Direct lightning, and the relevant parameters, are covered more realistically by the impulse current Iimp, which is used for testing lightning current arresters (class I test). Rectangular currents are generated by discharges of a loaded transmission line of typically some hundred km of length. Such a current waveshape, coming from a line discharge, is not relevant for low-voltage networks.
6.1 Type tests Type tests are performed after completion of the design to prove the performance and specified characteristics of the product. The type tests are described in detail in the relevant standards. In the frame of this guideline, the main electrical tests for MO-surge arresters without gaps for outdoor application are described briefly. In general each test series is performed on three new test samples. The tests are performed in free air at room temperature (20 °C ± 15 °C)
Figure 14 MO-surge arrester MVR...ZS. For low-voltage systems. Only indoor application. Suitable for fixing on DIN racks. In = 5 kA, Uc = 140 V, 250 V and 440 V.
5.3 Technical data of the arresters
Test procedure to measure the residual voltage with 8/20 µs current impulses The voltage-current characteristic of the MO-surge arrester is measured with 8 / 20 µs current impulses in the range 0,1 to 2 times In. The result is given in form of a table or curve to show the protection performance depending on the current magnitude.
Table 8 presents main electrical data of the arresters. The ratings are given according to [9], see also the definitions in chapter 4.2. All described MO-surge arresters are of the voltage limiting type. The energy capability, as given in the table, is the value as tested in the operating duty tests to prove the thermal stability of the arrester with the maximum continuous operating voltage applied. It is not the limiting value that would destroy the arrester.
Arrester Type
In
Test class II for a. c. systems
8/20 µs kA
LOVOS - 5 LOVOS - 10 POLIM-R...1N POLIM-R...2N
Test class II for a. c. systems and special applications MVR...-5 MVR...-10 MVR...ZS Test class I for a. c. systems and special applications POLIM-R...1N POLIM-R...2N
Up /Uc
I max 8/20 µs kA
Operating duty test The operating duty test has two parts: the preconditioning and the evidence of the thermal stability of the MO-surge arrester. It is a test in which service conditions are simulated by the application of a stipulated number of specified impulses to the MO-surge arrester while it is energized at the maximum continuous operating voltage Uc. For the preconditioning test, 15 times In in three groups of five impulses each, are applied to the test samples which are energized at Uc. Each impulse shall be synchronized to the power frequency. Starting from 0° the synchronisation angle shall be increased in steps of 30° intervals. The interval between the impulses is 1 min; the interval between the groups is 25 to 30 min. For practical reasons it is not required that the test sample is energized between the groups. In the operating duty test itself, e.g. to prove the thermal stability, the test sample is energized at Uc, and current impulses up to Ipeak (test class I) or Imax (test class II) are superimposed. The power frequency voltage is applied for 30 min after each impulse to prove the thermal stability. The superimposed current impulses should be of positive polarity and initiated in the corresponding positive peak value of the power frequency voltage. The value of the current impulse is increased from 0,1 to 1,0 Ipeak or Imax. The intermediate values are 0,25; 0,5 and 0,75 Ipeak or Imax. The arresters have past the test if thermal stability was achieved and the residual voltage at In measured before and after the test sequence has not changed by more than ± 10 %.
Energy Capability kJ/kVUc
5 10 10 20
4,1 4,1 3,1 3,1
25 40 50 100
2,5 4,0 12,0 24,0
In 8/20 µs kA
Up /Uc
I max 8/20 µs kA
Energy capability kJ/kVUc
5 10 5
3,5 3,64 3,5
15 32 15
3,0 4,5 3,0
In 8/20 µs kA
Up /Uc
10 20
3,1 3,1
I imp (10/350 µs) I peak Charge Q kA As 10 20
5 10
Table 8 Electrical main data of the ABB surge arresters for low-voltage systems. The arresters of type POLIM-R have been tested according both class I and class II tests. The arresters of type MVR and POLIM-R can be used in d. c. systems as well, see chapter 9.
Disconnector tests Arresters with an integrated or external disconnector are tested together in the operating duty test. During the complete sequence of preconditioning procedure and operating duty test the disconnector remains nonfunctioning.
6 Tests All tests for ABB low voltage arresters follow internationally agreed upon recommendations. For low voltage arresters in power systems the international standard IEC 61643-1 [9] is valid. For some special cases, for instance surge arresters for railway systems with d. c. voltage, other standards are applicable [2].
Thermal stability test (of disconnector) This test shows the disconnecting characteristic and the safety performance of overstressed surge arresters with disconnectors. The arrester with the disconnector is heated electrically with constant current untill 12
thermal equilibrium is reached or the disconnector operates. If the disconnector functioned, there should be clear evidence of effective and permanent disconnection by the device. The surface temperature of the device during the entire test should be below 120 °C, and there should be no evidence of burning or ejected parts. The pass criteria depend on the classification of the arrester, e. g. whether it is indoor, outdoor, accessable or not accessable.
To ensure the long term stability of the MO-resistors, from each produced batch two MO-resistors are taken and tested in a time-reduced accelerated ageing test.
6.4 Acceptance tests Acceptance tests are made upon agreement between manufacturer and customer. If acceptance tests are agreed upon they are then to be performed on the nearest lower number to the cube root of the number of arresters to be supplied.
6.2 Special tests Additionally to the type tests given by the applicable standard, it may be necessary to conduct tests covering special requirements, (i. e. long term behaviour of the MO-material or the behaviour of the housing material under severe weather conditions).
If not otherwise specified, the following acceptance tests are performed: – verification of identification by inspection – verification of marking by inspection – verification of electrical parameters, for instance repetition of routine tests.
Accelerated ageing test This test has to show that the power losses of the arrester in the network under applied continuous operating voltage does not increase with time. An increase of the power losses would lead with time to a thermal runaway, and consequently to a failure of the arrester. In the accelerated ageing test the complete arrester is to be tested under increased stress, e. g. under increased ambient temperature of + 115 °C. During the whole test period of 1000 h the power losses are measured. It is vital that the power losses do not increase with time, but remaining constant at the lowest reached level. Because the material around the MO-resistor may influence its long term performance, it is important that the complete surge arrester is tested and not only the MOresistor. The test has to be performed with power frequency voltage for surge arresters with a. c. systems, and with d. c. voltage for surge arresters for application in d. c. systems. Ageing tests carried out with a. c. voltage are not transferable to the application in d. c. networks. The accelerated ageing test is performed with reference to the test procedure given in [11].
7 Selection of MO-surge arresters For selecting a MO-surge arrester three main electrical parameters have to be evaluated: – continuous operating voltage Uc – voltage protection level Up – energy capability Additionally we need to be informed which modes should be protected. Table 9 shows the possible modes of protection, depending on the earthing practise in the low-voltage network. Depending on the application and the environment it has to be decided whether a disconnector is needed, which mechanical requirements need to be fulfilled (vibration and shock resistant, other mechanical stresses), and which ambient conditions have to be considered (increased temperature, solar radiation, rain, saltfog, etc.).
All ABB MO-resistors or MO-surge arresters, which are to be installed in d. c. networks, fulfill the most strict demands towards the long-term stability under d. c. voltage stress. UV radiation test In regions with strong solar radiation it is important to determine the behaviour of polymeric materials under UV radiation stress. The energy of the radiation can crack the surface of the insulator made of a synthetic material, and as a result the insulator may erode and finally fail. ABB surge arrester housing materials (silicon, polyamide and PUR) have successfully withstood UV radiation tests with time duration of 1000 h.
Power system type
SPD connected between:
TT
Line and neutral
X
Line and PE
X
Line and PEN
TN-C
TN-S
IT
X
X*
X
X
X
X*
X
X
X
Neutral and PE
X
Line to line
X
X
* When the neutral is distributed
Water immersion test This test is performed to show the tightness of design against water permeation. It is performed with reference to [12]. The test samples are kept in a vessel with deionized boiling water with 1 kg / m3 NaCl for 42 hours.
Table 9 Possible protection modes in low-voltage systems.
7.1 Selection of Uc 6.3 Routine tests The maximum continuous operating voltage Uc of an arrester has to be selected with respect to the power frequency voltages which can occure in the low-voltage system. Maximum system voltage has to be considered and possible temporary overvoltages in the network.
Routine tests are carried out on every arrester or parts of it (e. g. on the MO-resistors) in order to ascertain that the product meets the requirements of the design specification. The test method and the pass criteria are declared by the manufacturer.
Uc shall be equal or higher than the maximum power frequency voltage Ucs occuring in the system.
All above mentioned MO-surge arresters for low-voltage application made by ABB are tested to 100 % in the routine test. On each arrester the reference voltage Uref is measured at the declared reference current Iref. Additional the arresters are checked to be free of internal partial discharges or contact noise.
Uc ≥ Ucs
13
The temporary overvoltage withstand capability UT of the arrester has to be higher than the temporary overvoltage UTOV coming from the system.
In cases when the insulation withstand capability is lower than the values given in Table 4, or the overvoltage should be limited to a particular value to protect sensitive equipment in a special application, then the voltage protection level Up has to be calculated case by case.
UT > UTOV If a transformer failure occures in a solidly earthed MV system, a temporary overvoltage can result in a UTOV of up to 1200 V in the LV system. It may be impossible to find surge arresters providing acceptable protection. In such cases surge arresters have to be used which have safe overload conditions.
As a general rule the voltage protection level Up of the arrester and the maximum allowed impulse voltage at the point of protection should have a safety margin of at least 20 %. An important parameter to characterize a surge arrester is the ratio between the voltage protection level Up and the maximum continuous operating voltage Uc. This ratio Up / Uc depends on the technology used and, in case of MO-surge arresters, on the diameter of the MO-resistors and the nominal current In. For MO-surge arresters available today on the market typical values of Up / Uc are in the range from 3 to 5. The lower the ratio Up / Uc of an arrester, the higher the provided protection level against overvoltages. Good engineering design is required when arresters are connected in parallel with coordinated arresters.
Considering an upper tolerance in the system voltages of 10 %, see chapter 3.1 and [7] the continuous operating voltage of the arrester should be chosen to
Uc ≥ 1,1 × UN for arresters to be connected line-to-line and
Uc ≥ 1,1 × UN / √3 for arresters line-to-neutral or line-toearth.
7.3 Selection of the energy capability
As standard values (preferred values) for the system voltages 220/380 V, 230/400 V, 240/415 V (Table 3) the following values for Uc are proposed for outdoor application on overhead lines:
The energy capability of arresters is in principle defined by the nominal discharge current In and the impulse current Iimp for class I arresters or Imax for class II arresters. According Table 6 the arrester has to be chosen with respect to the place of installation and the expected stresses or surges. For class II arresters typical values for the nominal current are In = 5 kA or 10 kA. As long as there is no fixed relation given between the nominal current In and the maximum discharge current Imax, both values have to be specified.
Uc = 280 V for the protection phase to neutral and neutral to earth (TT and TN systems) Uc = 440 V for the protection phase to neutral and neutral to earth (IT system) Uc = 440 V for protection phase to phase (TT, TN, IT systems)
From lightning statistics [5] it is known that about 95 % of the lightning currents have peak values of up to 14 kA, and 5 % up to 80 kA. Considering that in distribution and low-voltage systems a direct lightning will struck not only one phase, but all three (due to the short distance between the phases), and that the current from the lightning will travel in both directions of the line, the lightning current can be divided by 6 (as a first approximation). With this we result for the 95 % value at 2,3 kA, and for the 5 % value at 13 kA as a peak value for one phase (e. g. one arrester).
MO-surge arresters with the above given Uc values will cover almost all possible temporary overvoltages in the low-voltage network with sufficient safety margin, providing in the same time a good protection ratio Up / Uc.
7.2 Selection of Up The purpose of surge arresters is to protect an installation or a specific piece of electrical equipment against overvoltages. Overvoltages can destroy the insulation of the installation or connected electrical equipment like transformers, cables, motors, etc., and they can lead to malfunction or destruction of connected electronic equipment.
Comparing this values with the technical data of the arresters ( Table 8), an arrester with In = 5 kA (covering 95 % of the events), and Imax = 25 kA (covering the very rare 5 % values) is fully complying with the occuring stresses.
The protection level Up of the arrester has to be below the voltage withstand capability of the equipment to be protected. The required impulse withstand voltages for the four insulation categories are given in Table 4. For category IV (fixed outdoor installation, for instance from the LV-bushing of a MV / LV transformer via an overhead line to a building) 6000 V impulse withstand is required. Comparing this value with the voltage protection level Up = 1800 V of a MO-surge arrester with In = 10 kA, for instance type LOVOS with Uc = 440 V (Table 8), shows the excellent protection of the insulation provided by the arrester.
Therefore, as a standard type for outdoor application on overhead lines an ABB arrester of type LOVOS-5 is proposed with
In = 5 kA and Imax = 25 kA. If higher stresses from lightnings are expected, or for regions with very high isokeraunic level, an ABB arrester of type LOVOS-10 is proposed with
In = 10 kA and Imax = 40 kA.
However, it has to be considered, that the distance between the arrester and the equipment to be protected (e.g. a transformer or a meter in the building) has a great influence on the overvoltage occuring at the equipment to be protected. This is known as the protective distance of the arrester [1]. As a rule of thumb it can be said, that the arrester should be installed as close as possible to the equipment to be protected.
Wherever an arrester is used to protect equipment which can store energy, as for instance capacitor-banks, cables, inductances in filters, etc. the maximum energy stored in these elements should be used to determine the right arrester. 14
8 Coordination of surge arresters The energy capability of MO-arresters can be increased by connecting MO-resistors in parallel [13]. Using identical u-i- characteristics of the MO-resistors, an even current sharing (and energy sharing) can be reached. This is possible due to an exact classification of the MO-resistors during the routine test of the MO-resistors. It is possible to connect two or more MO-resistors in parallel internally in a MO-surge arrester, or connect two or more MO-surge arresters in parallel thereby increasing the energy capability of the devise. In the latter case the MO-surge arresters have to be installed close to one another to avoid decoupling effects.
Consumer protection
Installation protection
i
Public energy supply
An other possibility of coordinating MO-surge arresters is shown in Figure 15. Three MO-surge arresters with slightly different u-i-characteristics are coordinated in such a way that the arrester A1 has the highest energy capability and the lowest voltage protection level Up, arrester A2 has a lower energy capability than A1 but a higher Up, and arrester A3 has again a lower energy capability and a higher Up than A2.
M A1
Incoming current surge
A2
A3
Distribution box
Terminal box
Junction box
Energy-consumer
Figure 15 Components and borders in an EMP-protection system with coordinated MO-surge arresters. A1, A2, A3, see Figure 16.
A3
In Figure 16 the u-i-characteristics of the arresters A1 to A3 are given. Under the same overvoltage stress from incoming surge arrester A1 will conduct most of the current to earth at the entrance of the installation, where as the arresters A2 and A3 will recieve a much lower stress, keeping the occuring overvoltage in the whole installation on a low level.
A2
A1 -1
1,3 p.u.
-2
1,2
Ures Up ( In )
Figure 17 illustrates the protection principle. The given values in Figure 17 are measured results from a realized installation in a civil defence construction in Switzerland [14]. Starting with an injected impulse current of approximately 23 kA (28 / 50 µs) at the entrance of the installation close to arrester A1, in the first distribution box a current 0,4 kA was measured, in the second distribution box a current of 0,08 kA and in the junction box almost no current. The residual voltage Ures was in the whole installation kept below 1,6 kV. This example shows the effectiveness of a protection concept with several steps in an energy supply, realized with MO-surge arresters with coordinated u-i-characteristics.
1,1 1,0 0,9 0,8 0,7 102
2
103
5
2
104
5
I n ( A3 )
I n ( A2 )
2
3
105 A 2
5
I n ( A1 - 2 )
I n ( A1 - 1 ) I
Figure 16 Coordination of u-i-characteristics of different dimensioned MO-surge arresters. The used arresters in this coordination concept are (Table 8): A1-2: POLIM-R...2N with In = 20 kA; Up / Uc = 3,1; E` = 24 kJ / kVUc A1-1: POLIM-R...1N with In = 10 kA; Up / Uc = 3,1; E` = 12 kJ / kVUc A2: MVR...-5 with In = 5 kA; Up / Uc = 3,5; E` = 3,0 kJ / kVUc A3: MR...ZS with In = 1 kA; Up / Uc = 4,25; E` = 0,6 kJ / kVUc The arresters of type POLIM-R and MVR are standard arresters producted by ABB. The type MR...ZS was specially developed according to existing special requirements for the mentioned coordination concept.
Buildings and structures equipped with lightning protection systems (LPS), as for instance franklin rods, need special protection measures. It is generally assumed that a direct lightning hits the LPS, and that part of the lightning current is transferred into the structure or the building. In such cases the concept of Lightning Protection Zones (LPZ) has to be considered. The LPZ concept is described for instance in [15] and various other publications. The concept of LPZ requires that surge arresters are installed, whenever an electrical line crosses the boundary between two zones. These surge arresters have to be well coordinated to effectively reduce the lightning threat down to the surge withstand capability of the equipment to be protected. For this LPZ concepts, which are realized inside buildings, different types of arresters are used, as spark gaps, gas discharge tubes, varistors (MO-resistors), diodes, and combinations of these. The principle is the same as mentioned above, the energy content of the surge has to be reduced step-by-step with cascaded surge protective devices. The whole structure is subdivided into a series of LPZs, thus successively reducing the interference level from the primary lightning threat down to the basic immunity of the electronic equipment.
A1 - 2
A2
A2
A3
Terminal box
First distribution box
Second distribution box
Junction box
L = 15 m
i1
L = 24 m
i2
22 kA 1,45 kV
i2 Ures2
0,4 kA 1,5 kV
L=3m
i4
i3 Ures2
Ures1
i1 Ures1
L = 18 m
Ures3
i3 Ures3
0,08 kA 1,1 kV
U5 Ures4
i4 Ures4
0 kA 1,4 kV U5
1,6 kV
Figure 17 Current distribution in a NEMP-protected civil defence construction. Measured values of i and U. Arresters A1-2 to A3 see Figures 15 and 16. The injected current (incoming surge) had a peak value of approximately 23 kA.
15
9 MO-surge arresters for d. c. systems
10 Installation of surge arresters National requirements and regulations apply to the installation of surge arresters. Surge arresters for outdoor application are in most cases out of reach. The IP degree of the surge arresters depends on the accessories used.
MO-surge arresters without spark gaps are especially suitable for application in low-voltage d. c. systems, because they do not conduct any follow current like spark gaps. Due to the extreme nonlinear u-i-characteristic of the MO-resistors the current after limiting the overvoltage is immediately again in the range of less than 1 mA. It is not necessary to extinguish any d. c. current arc.
ABB offers a variety of accessories for different methods of installation, including fully insulated connections. For details please refer to ABB.
Tests for surge arresters for d. c. application are under discussion in working group 5 of IEC SC 37A. For the time being the type tests for a. c. application apply. In special cases the purchaser should contact the manufacturer for clarification.
As a general rule the surge arresters should be installed avoiding constant mechanical stresses on the terminals. One terminal should be connected with a flexible lead. It is not important whether this is the voltage side or the earth side, though normally, the earth connection is flexible.
As pointed out in chapter 6.2 it is very important to ensure that the MOresistors used in surge arresters for d. c. application are tested in the accelerated ageing test with d. c. voltage. MO-resistors for a. c. application are not generally long term stable under d. c. voltage stress.
As long as MO-surge arresters have a symmetrical characteristic it is, from the electrical point of view, not important which terminal is connected to the voltage and which to the earth.
Most of the d. c. current networks are railway networks. Arresters for use in railway networks are described in [2]. Other examples of d. c. applications are in power electronics, chemical industry and data transmission.
For optimal protection, the arrester should be installed as close as possible to the equipment to be protected, with connections as short and straight as possible.
D.C. voltage can be subjected to superimposed voltage peaks (i. e. from commutation of converter stations) and may have strong voltage fluctuations, presenting a difficult determination of continuous operating voltage Uc. The selection of MO-surge arresters for d. c. railway application is given in [2]. For other d. c. applications the user should contact the manufacturer for selecting the right surge arrester. Table 10 gives the main electrical data of ABB MO-surge arresters for use in d. c. systems.
Arrester Type Test class II for d. c. Systems POLIM-H...ND POLIM-R...1ND POLIM-R...2ND MVR...5 MVR...10 Test class I for d. c. systems
POLIM-R...1ND POLIM-R...2ND
In
Up / U c ,d. c. .
8/20 µs kA 20 10 20 5 10
2,7 2,4 2,4 2,8 2,9
In 8/20 µs kA
Up / U c ,d. c.
10 20
2,4 2,4
Imax
Energy Capability
8/20 µs kA
kJ/kVUc d.c.
50 50 100 15 32
6,0 6,0 12,0 2,4 3,6 Iimp (10/350 µs)
Ipeak kA 10 20
Charge Q As 5 10
Table 10 Electrical main data of the ABB MO-surge arresters for the application in d.c. networks. The type POLIM-H...ND is mechanically a very strong arrester, especially used in railway systems. The maximum continuous voltage Uc for the types POLIM-R...ND ranges from 140 V to 1000 V, offering a large variety of applications.
16
Bibliography [1]
Application Guidelines Overvoltage Protection Dimensioning, testing and application of metal oxide surge arresters in medium voltage networks, 3rd revised edition July 1999. ABB High Voltage Technologies Ltd. Wettingen / Switzerland
[2]
Application Guidelines Overvoltage Protection Dimensioning, testing and application of metal oxide surge arresters in railway facilities, 1st edition June 2000. ABB High Voltage Technologies Ltd. Wettingen / Switzerland
[3]
J. Huse; Compact Course: Lightning Surge Protection in Low Voltage Electric Power Distribution Systems, Including Consumers Installations and Equipment. V International Symposium on Lightning Protection,Sao Paulo – Brazil, May 17th – 21st 1999
[4]
C. Mirra, A. Porrini, A. Ardito, C.A. Nucci; Lightning Overvoltages in Low Voltage Networks. 14th CIRED Conference, Birmingham, U.K., June 1997
[5]
Joint CIRED/CIGRE Working Group 05: Protection of MV and LV networks against lightning. Part I: Basic Information. 14th CIRED Conference, Birmingham, U.K., June 1997
[6]
International Standard IEC 60664-1, Edition 1.1 (2000-04); Insulation coordination for equipment within low-voltage systems – Part 1: Principles, requirements and tests
[7]
CENELEC publication HD 472 S1 (1988); Nominal voltages for low voltage public electricity supply systems
[8]
International Standard IEC 60364-4-442 (1993-03); Electrical installations of buildings – Part 4: Protection for safety – Section 442: Protection of low-voltage installations against faults between high-voltage systems and earth
[9]
International Standard IEC 61643-1, First edition, 1998-02; Surge protective devices connected to lowvoltage power distribution systems- Part 1: Performance requirements and testing methods
[10]
F. Greuter, R. Perkins, M. Rossinelli, F. Schmückle; The metal-oxide resistor – at the heart of modern surge arresters. ABB Review 1/89
[11]
International standard IEC 60099-4: Surge arresters – Part 4 : Metal-oxide surge arresters without gaps for a. c. systems
[12]
Amendment 2 to IEC 60099-4; IEC TC 37/231/CDV
[13]
B. Richter, W. Schmidt, K. Tanner; Protection against high energy surges with MO-surge arresters: a new concept for low voltage systems. 10th International Zurich Symposium on electromagnetic compatibility 1993. Paper 70K5, pages 383 to 388.
[14]
K. Tanner, P. Bertholet, B. Richter, W. Schmidt; NEMP-Protection in the Energy-Supply of Civil Defence Constructions. Federal Office of Civil Defence, Material Division. CH-Bern, August 1992.
[15]
P. Hasse, P. Zahlmann, J. Wiesinger, W. Zischank; Principle for an advanced coordination of surge protective devices in low voltage systems. 22nd ICLP 1994, Budapest, paper R5-04.
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[email protected]
Creation LOPO ZH Printed in Switzerland 2001-06
CHHOS / AR 3610 E
ABB High Voltage Technologies Ltd Division Surge Arresters Jurastrasse 45 CH-5430 Wettingen 1 Switzerland Tel.: ++41 56 / 205 29 11 Fax: ++41 56 / 205 55 70
