Seminar- Hvdc Technology and Short Circuit Contribution of HVDC Light

Seminar report on

HVDC TECHNOLOGY AND SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT

By Jijo Francis

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING FEDERAL INSTITUTE OF SCIENCE AND TECHNOLOGY (FISAT) MOOKKANNOOR P O, ANGAMALY-683577,

Affiliated to

MAHATMA GANDHI UNIVERSITY 2011

FEDERAL INSTITUTE OF SCIENCE AND TECHNOLOGY (FISAT) Mookkannoor Mookkannoor P O, Angamaly-683577. Affiliated to

MAHATMA GANDHI UNIVERSITY, Kottayam- 686560 DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

CERTIFICATE

This is to certify that this report entitled HVDC TECHNOLOGY AND SHORT “

CIRCUIT CONTRIBUTION OF HVDC LIGHT ” is a bonafide report of the seminar th

presented during 7

semester by JIJO FRANCIS (57164) in partial fulfillment of the

requirements for the award of the degree of Bachelor of Technology (B.Tech) in Electrical & Electronics Engineering during the academic year 2010-2011.

Head of the Department

Date: Place: Mookkannoor

ABSTRACT

The development of HVDC (High Voltage Direct Current) transmission system dates back to the 1930s when mercury arc rectifiers were invented. Since the 1960s, HVDC transmission system is now a mature technology and has played a vital part in both long distance transmission and in the interconnection of systems. Transmitting power at high voltage and in DC form instead of AC is a new technology proven to be economic and simple in operation which is HVDC transmission. HVDC transmission transmission systems, when installed, often form the backbone of an electric power system. They combine high reliability with a long useful life. An HVDC link a voids some of the disadvantages and limitations of AC transmission. HVDC transmission refers to that the AC power generated at a power plant is transformed into DC power before its transmission. At the inverter (receiving side), it is then transformed back into its original AC power and then supplied to each household. Such power po wer transmission method makes it possible to transmit electric power in an economic way. HVDC Light is the newly developed HVDC transmission technology, which is based on extruded DC cables and voltage source converters consisting of Insulated Gate Bipolar Transistors (IGBT’s) with high switching frequency. fre quency. It is a high voltage, direct current cu rrent transmission Technology i.e., Transmission up to 330MW and for DC voltage in the ± 150kV range. Under more strict environmental and economical constraints due to the deregulation, the HVDC Light provides the most promising solution to power transmission and distribution. The new system results in many application opportunities and new applications in turn bring up new issues of concern. One of the most concerned issues from customers is the contribution of HVDC Light to short circuit currents. The main reason for being interested in this issue is that the contribution of the HVDC Light to short circuit currents may have some significant impact on the ratings ratings for the circuit breakers in the existing AC systems. This paper presents a comprehensive investigation on one of the concerned issues, which is the contribution of HVDC Light to short circuit currents.

CONTENTS

Chapter 1

INTRODUCTION

1

Chapter 2

HVDC TECHNOLOGY

2

Chapter 3

HVDC LIGHT TECHNOLOGY

17

Chapter 4

SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT

24

Chapter 5

CONCLUSION

31

Chapter 6

REFERENCES

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1. INTRODUCTION The development of HVDC (High Voltage Direct Current) transmission system dates back to the 1930s when mercury arc rectifiers were invented. In 1941, the first HVDC transmission system contract for a commercial HVDC system was placed: 60MWwere to be supplied to the city of Berlin through an underground cable of 115 km in length. It was only in 1954 that the first HVDC (10MW) transmission system was commissioned in Gotland. Since the 1960s, HVDC transmission system is now a mature technology and has played a vital part in both long distance transmission and in the interconnection of systems.HVDC transmission systems, systems, when installed, often form the backbone b ackbone of an electric power system. They combine high reliability with a long useful life. Their core component is the power converter, which serves as the interface to the AC transmission system. The conversion from AC to DC, and vice versa, is achieved b y controllable electronic switches (valves) in a 3-phase bridge co nfiguration.

A new transmission and distribution technology, HVDC Light, makes it economically feasible to connect small scale, renewable power generation plants to the main AC grid. Vice versa, using the very same technology, techno logy, remote locations as islands, mining districts and drilling platforms can be supplied with power from the main grid, thereby eliminating the need for inefficient, polluting local generation such as diesel units. The voltage, frequency, active and reactive power can be controlled precisely and independently of each other. This technology also relies on a new type of underground cable which can replace overhead lines at no cost penalty. Equally important, HVDC Light has control capabilities that are not present or possible even in the most sophisticated AC.

Electrical & Electronics Engineering, FISAT

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2. HVDC TECHNOLOGY Electric power transmission was originally developed with direct current. A high-voltage, direct current (HVDC) electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long-distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other bene fits of direct current links are useful.

High voltage is used for electric power transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, higher voltage reduces the transmission power loss. The power lost as heat in the wires is proportional to the square of the current. So if a given power is transmitted at higher voltage and lower current, power loss in the wires is reduced. Power loss can also be reduced by b y reducing resistance, for example by increasing the diameter of the conductor, but larger conductors are heavier and more expensive.

High voltages cannot easily be used for lighting and motors, and so transmission-level voltages must be reduced to values compatible with end-use equipment. Transformers are used to change the voltage voltag e level in alternating current (AC) transmission circuits. The competition between the direct current (DC) of Th omas Edison and the AC of Nikola Tesla and George Westinghouse was known as the War of Currents, with AC becoming dominant. Practical manipulation of DC voltages became possible with the development of high power electronic devices such as mercury arc valves and, more recently, semiconductor devices such as thyristors, th yristors, insulated-gate bipolar transistors (IGBTs), (IGBTs), high power MOSFETs and gate turn-off thyristors (GTOs).


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DC transmission now became practical when long d istances were to be covered or where cables were required. The development of o f HVDC (High Voltage Direct Current) transmission system dates back to the 1930s when mercury arc rectifiers were invented. HVDC transmission systems, when installed, often form the backbone of an electric power system. They combine high reliability with a long useful life. Their core component is the power converter, which serves as the interface to the AC transmission system. system. The conversion from AC to DC, and vice versa, is achieved by b y controllable electronic switches (valves) in a 3-phase bridge configuration. An HVDC link avoids some of the disadvantages and limitations of AC transmission and has the following advantages:

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No technical limit to the length of a submarine cable connection.

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No requirement that the linked systems run in synchronism.

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No increase to the short circuit capacity imposed on AC switchgear.

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Immunity from impedance, phase angle, frequency or vo ltage fluctuations.

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Preserves independent management of frequency and generator control.

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Improves both the AC system’s system’s stability and, therefore, improves the internal power carrying capacity, by modulation of power in response to frequency, power swing or line

rating.

2.1 NEED FOR DC TRANSMISSION

The losses in DC transmission are lower. The level of losses is designed into a transmission system and is regulated by the size of conductor selected. DC and ac conductors, either as overhead transmission lines or submarine cables can have lower losses but at higher expense since the larger cross-sectional area will generally result in lower losses but cost more.


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When converters are used for dc transmission in preference to ac transmission, it is generally by economic choice driven by one of the following reasons :

1. An overhead dc transmission line with with its towers can be designed to be less costly costly per unit of length than an equivalent eq uivalent ac. line designed to transmit the same level of electric power. However the dc converter stations at each end are more costly than the terminating stations stations of an ac line and so there is a breakeven breakeven distance above which the total cost of dc transmission is less than its ac transmission alternative. The dc transmission line can have a lower visual profile than an equivalent ac line and so contributes to a lower environmental impact. There are other environmental advantages to a dc transmission line through the electric and magnetic fields being dc instead of ac.

2. If transmission transmission is by submarine or underground cable, the breakeven distance is much less than overhead transmission. It is not practical to consider ac cable systems exceeding 50 km but dc cable transmission systems are in service whose length is in the hundreds of kilometers and even distances of 600 km or greater have been considered feasible.

3. Some ac electric power systems systems are not synchronized to neighboring networks networks even though their physical distances between them is qu ite small. This occurs in Japan where half the country is a 60 Hz network and the other o ther is a 50 Hz system. It is physically impossible to connect the two together by direct ac methods in order to exchange electric power between them. However, if a dc converter station is located in each system with an interconnecting dc link between them, it is possible to transfer the required power flow even though the ac systems so connected remain asynchronous.


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2.2 ADVANTAGES OF HVDC OVER AC TRANSMISSION:

The advantage of HVDC is the ability to transmit large amounts of power over long distances with lower capital costs and with lower losses than AC. Depending on voltage level and construction details, losses are quoted as about 3% per 1,000 km.High-voltage km. High-voltage direct current transmission allows efficient use of energy sources remote from load centers. In a number of applications HVDC is more effective than AC transmission. Examples include:

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Undersea cables, where high capacitance causes additional AC losses. (e.g., 250 km Baltic Cable between Sweden and Germany the 600 km Nor Ned cable between Norway and the Netherlands, and 290 km Bass link between the Australian mainland and Tasmania)

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Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps', for example, in remote areas

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Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install

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Power transmission and stabilization between unsynchronized AC distribution systems

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Connecting a remote generating plant to the distribution grid, for example Nelson River Bipole

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Stabilizing a predominantly AC power-grid, without increasing prospective short circuit current

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Reducing line cost. HVDC needs fewer condu ctors as there is no need to support multiple phases. Also, thinner conductors can be u sed since HVDC does not suffer from the skin effect

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Facilitate power transmission between different countries that use AC at differing voltages and/or frequencies

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Synchronize AC produced by renewable energy sources


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Long undersea / underground high voltage cables have a high electrical capacitance, since the conductors are surrounded by a relatively thin layer of insulation and a metal sheath while the extensive length of the cable multiplies the area between the conductors. The geometry is that of a long co-axial co -axial capacitor. Where alternating current is used for cable transmission, this capacitance appears in parallel with load. Additional c urrent must flow in the cable to charge the cable capacitance, which generates additional losses in the conductors of the cable. Additionally, there is a dielectric loss component in the material of the cable insulation, which consumes power. When, however, direct current is used, the cable capacitance is charged only when the cable is first energized or when the voltage is chan ged; there is no steady-state additional current required. For a long AC undersea un dersea cable, the entire current-carrying capacity of the conductor could be used to supply the charging current alone.

The cable capacitance issue limits the length a nd power carrying capacity of AC cables. DC cables have no such limitation, and are essentially bound by only Ohm's Law. Although some DC leakage current continues to flow through the dielectric insulators, this is very small compared to the cable rating and much less than with AC transmission cables. HVDC can carry more power per conductor because, for a given power rating, the constant voltage in a DC line is the same as the peak voltage in an AC line. The power delivered in an AC system is defined by the root mean square (RMS) of an AC voltage, but RMS is only about 71% of the peak voltage. The peak voltage of AC determines the actual insulation thickness and conductor spacing. Because DC operates at a constant maximum voltage, this allows existing transmission line corridors with equally sized conductors and insulation to carry more power into an area of high power consumption than AC, which can lower costs.


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Because, HVDC allows power transmission between unsynchronized AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and separate would not similarly affect a DC link, and the power p ower flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly commanded, and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone.

2.3 DISADVANTAGES:

The disadvantages of HVDC are in conversion, c onversion, switching, control, availability and maintenance..HVDC is less reliable and has lower availability than AC systems, mainly due to the extra conversion equipment. Single pole systems have availability of about 98.5%, with about a third of the downtime unscheduled due to faults. Fault redundant bipole systems provide high availability for 50% of the link capacity, but availability of the full capacity is about 97% to 98%.

The required static inverters are expensive and have limited overload capacity. At smaller transmission distances the losses in the static inverters may be bigger than in an AC transmission line. The cost of the inverters may not be offset by reductions in line construction cost and lower line loss. With two exceptions, all former mercury rectifiers worldwide have been dismantled or replaced by thyristor units. Pole 1 of the th e HVDC scheme between the North and South Islands of New Zealand still uses mercury arc rectifiers, as does Pole 1 of the Vancouver Vancou ver Island link in Canada. Both are a re currently being replaced – replaced – in in New Zealand by a new thyristor pole and in Canada by a three-phase AC link. In contrast to AC systems, realizing multi-terminal systems is complex, as is expanding existing schemes to multi-terminal systems.


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Controlling power flow in a multi-terminal DC system requires good communication between all the terminals; power flow must be actively regulated by the inverter control system instead of the inherent impedance and phase p hase angle properties of the transmission line. Multi-terminal lines are rare. Another example is the S ardinia-mainland Italy link which was modified in 1989 to also provide power to the island of Corsica. High voltage DC circuit breakers are difficult to b uild because some mechanism must be included in the circuit breaker to force current to zero, otherwise arcing and contact wear would be too great to allow reliable switching. Operating a HVDC scheme requires many spare parts to be kept, often exclusively ex clusively for one system as HVDC systems are less standardized than AC systems and technology technolog y changes faster.

2.4 RECTIFYING AND INVERTING:

2.4.1 Components

Most of the HVDC systems in operation today are based on Line-Commutated Converters. Early static systems used mercury arc rectifiers, which were unreliable. Two HVDC systems using mercury arc rectifiers are still in service (As of 2008). The thyristor valve was first used in HVDC systems in the 1960s. The thyristor is a solidstate semiconductor device similar to the diode, b ut with an extra control terminal that is used to switch the device on at a particular instant during the AC cycle. c ycle. The insulated-gate bipolar transistor (IGBT) is now also used, forming a Voltage Sourced Converter, and offers simpler control, reduced harmonics and reduced valve cost. Because the voltages in HVDC systems, up to 800 kV in some cases, exceed the breakdown voltages of the semiconductor devices, HVDC converters are built using large numbers of semiconductors in series. The low-voltage control circuits used to switch the thyristors on and off need to be isolated from the high voltages present on the transmission lines.


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This is usually done optically. In a h ybrid control system, the low-voltage control electronics sends light pulses along optical fibers to the high-side high-side control electronics. Another system, called direct light triggering , dispenses with the high-side electronics, instead using light pulses from the control electronics to switch light-triggered thyristors. A complete switching element is commonly referred to as a valve, valve, irrespective of its construction.

2.4.2 Rectifying & Inverting Systems

Rectification and inversion use essentially the same machinery. Man y substations (Converter Stations) Stations) are set up in such a way that they can act as a s both rectifiers and inverters. At the AC end a set of transformers, often three physically separated single-phase transformers, isolate the station from the AC supply, to provide a lo cal earth, and to ensure the correct eventual DC voltage. The output of these transformers is then connected to a bridge rectifier formed by a number of valves. The basic configuration uses six valves, connecting each of the three phases to each of the two DC rails. However, with a phase change only every sixty degrees, considerable harmonics remain on the DC rails.

An enhancement of this configuration uses 12 valves (often known as a twelve-pulse system). The AC is split into two separate three phase supplies before transformation. One of the sets of supplies is then configured to have a star secondary, the other a delta secondary, establishing a thirty degree phase difference b etween the two sets of three phases. With twelve valves connecting each of the two sets of three phases to the two DC rails, there is a phase change every 30 degrees, and harmonics are considerably reduced.

In addition to the conversion transformers and valve -sets, various passive resistive and reactive components help filter harmonics out of the DC rails.


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2.5 CONFIGURATIONS OF HVDC SYSTEM:

2.5.1 Monopole And Earth Return

In a common configuration, called monopole, one of the terminals of the rectifier is connected to earth ground. The other terminal, at a potential high above or below ground, is connected to a transmission line. The earthed terminal may be connected to the corresponding connection at the inverting station by means of a second conductor. If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations.

Figure 1: Block diagram of a monopole system with earth return

Therefore it is a type of single wire earth return. The issues surrounding earth-return current include: 

Electrochemical corrosion of long buried metal objects such as pipelines.

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Underwater earth-return electrodes in seawater may produce chlorine or otherwise affect water chemistry.

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An unbalanced current path may result in a net magnetic field, which can affect magnetic navigational compasses for ships passing over an und erwater cable.


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These effects can be eliminated with installation of a metallic return conductor between the two ends of the monopolar transmission line. Since on e terminal of the converters is connected to earth, the return conductor need not be insulated for the full transmission voltage which makes it less costly than the high-voltage conductor.

Use of a metallic return conductor is decided based on economic, technical and environmental factors. Modern monopolar systems for pure overhead lines carry typically 1,500 MW. If underground or underwater cables are used, the typical value is 600 MW. Most monopolar systems are designed for future bipolar ex pansion. Transmission line towers may be designed to carry two conductors, even if only one is used initially for the monopole transmission system. The second conductor is either un used or used as electrode line or connected in parallel with the other (as in case of Baltic-Cable).

2.5.2 Bipolar

In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Since these con ductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor.

Figure 2: Block diagram of a bipolar system that also has an earth return.


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However, there are a number of advantages to bipolar transmission which can make it the attractive option. 

Under normal load, negligible earth-current flows, as in the case of monopolar transmission with a metallic earth-return. This reduces earth return loss and environmental effects.

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When a fault develops in a line, with earth return electrodes installed at each end of the line, approximately half the rated power can continue to flow using the earth as a return path, operating in monopolar mode.

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Since for a given total power rating each conductor of a bipolar line carries only o nly half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating.

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In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to b e transmitted even if one line is damaged.

A bipolar system may also be installed with a metallic earth return conductor. Bipolar systems may carry as much as 3,200 MW at voltages of +/-600 kV. Submarine Sub marine cable installations initially commissioned as a monopole may be up graded with additional cables and operated as a bipole.

2.5.3 Back to Back

A back-to-back station (or B2B for short) is a plant in which both static inverters and rectifiers are in the same area, usually in the same building.


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The length of the direct current cu rrent line is kept as short as possible. HVDC back -to-back stations are used for:

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Coupling of electricity mains of different frequency (as in Japan; and the GCC interconnection between UAE [50 Hz] and Saudi Arabia [60 Hz] under construction in ±2009 – 2011). 2011).

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Coupling two networks of the same nominal frequency but no fixed phase relationship (as until 1995/96 in Etzenricht, Dürnrohr, Vienna, and the Vyborg HVDC scheme).

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Different frequency and phase number (for example, ex ample, as a replacement for traction current converter plants).

The DC voltage in the intermediate circuit can be selected freely at HVDC back-to-back stations because of the short conductor length. The DC voltage is as low as possible, pos sible, in order to build a small valve hall and to avoid series connections of valves. For this reason at HVDC back-to-back stations valves with the highest av ailable current rating are used.

2.6 SYSTEMS WITH TRANSMISSION LINES

The most common configuration of an HVDC link is two inverter/rectifier stations connected by an overhead power line. This is also a configuration commonly used in connecting unsynchronized grids, in long-haul power transmission, and in undersea cables. Multi-terminal HVDC links, connecting more than two points, are rare. Th e configuration of multiple terminals can be series, parallel, or hybrid (a mix ture of series and parallel).


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Parallel configuration tends to be used for large c apacity stations, and series for lower capacity stations. An example is the 2,000 MW Quebec - New England Transmission system opened in 1992, which is c urrently the largest multi-terminal HVDC system in the world.

2.7 CORONA DISCHARGE

Corona discharge is the creation of ions in air by the presence of a strong electric field. Electrons are torn from neutral air, and either the positive ions or the electrons are attracted to the conductor, while the charged particles drift. This effect can cause considerable power loss, create audible and radio-frequency interference, generate toxic compounds such as oxides of nitrogen and ozone, and bring forth arcing. Both AC and DC transmission lines can generate coronas, in the former case in the form of oscillating particles, in the latter a constant wind. Due to the space charge formed around the conductors, an HVDC system may have about half the loss per unit length of a high voltage AC system carrying the same amount of power. With monopolar transmission the choice of polarity of the energized conductor leads to a degree of control over the corona discharge.

In particular, the polarity of the ions emitted can be controlled, which may ma y have an environmental impact on particulate condensation. (particles of different polarities have a different mean-free path.) Negative coronas generate considerably more ozone than positive coronas, and generate it further downwind of the power line, creating the potential for health effects. The use of a positive voltage will reduce the ozone impacts of monopole HVDC power lines.

2.8 AREAS FOR DEVELOPMENT IN HVDC CONVERTERS

The thyristor as the key component of a converter bridge continues to be developed so that its voltage and current rating is increasing.


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Gate-turn-off thyristors (GTOs) and insulated gate bipole transistors (IGBTs) are required for the voltage source converter (VSC) converter bridge configuration. It is the VSC converter bridge which is being applied app lied in new developments . Its special properties include the ability to independently control real and reactive power at the connection bus to the ac system. Reactive power can be either capacitive or inductive and can be controlled to quickly change from one to the other.

A voltage source converter as in inverter do es not require an active ac voltage source to commutate into as does the conventional line commutated converter. The VSC inverter can generate an ac three phase voltage and supply electricity to a load as the only source of power. It does require harmonic filtering, harmonic cancel lation or pulse width modulation to provide an acceptable ac voltage wave shape. Two applications are now available for the voltage source converter. The first is for low voltage dc converters applied to dc distribution systems. The first application of a dc distribution system in 1997 was developed in Sweden and known as “HVDC Light”. Light”. Other applications for a dc distribution system may be:

1. In a dc feeder to remote or isolated loads, particularly if underwater or un derground cable is necessary. 2. For a collector system of a wind farm where cable delivery and optimum and individual speed control of the wind turbines is desired for pe ak turbine efficiency.

The second immediate application for the VSC converter bridges is in back-to-back configuration. The back-to-back VSC link is the ultimate transmission and power flow controller. It can control and reverse power flow easily, and control reactive power independently on each side. With a suitable control system, it can control power to enhance and preserve ac system synchronism, and act as a rapid phase angle power flow regulator with 360 degree range of control.


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There is considerable flexibility in the configuration of the VSC converter bridges. Many two level converter bridges can be assembled with appropriate harmonic cancellation properties in order to generate acceptable ac system voltage wave shapes. Another option is to use multilevel converter bridges to provide harmonic cancellation. Additionally, both two level and multilevel converter bridges can utilize pulse width modulation to eliminate low order harmonics. With pulse width modulation, h igh pass filters may still be required since PWM adds to the higher order harmonics.

As VSC converter bridge technology develops for higher dc voltage applications, it will be possible to eliminate converter transformers. This is possible possible with the low voltage applications in use today. It is expected the exciting developments in power electronics will continue to provide exciting new configurations and applications for HVDC converters.


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3. HVDC LIGHT TECHNOLOGY A new transmission and distribution technology, HVDC Light, makes it economically feasible to connect small-scale, renewable power generation plants to the main AC grid. Vice versa, using the very same technology, techno logy, remote locations as islands, mining districts and drilling platforms can be supplied with power from the main grid, thereby eliminating the need for inefficient, polluting local generation such as diesel units. The voltage, frequency, active and reactive power can be controlled precisely and independently of each other. This technology also relies on a new type of underground cable which can replace overhead lines at no cost penalty. Equally important, HVDC Light has control capabilities that are not present or possible even in the most sophisticated AC systems. As its name implies, HVDC Light is a dc transmission technolog y. However, it is different from the classic HVDC technology used in a large number of transmission schemes. Classic HVDC technology is mostly used for large point -to-point transmissions, often over vast distances across land or under water. It requires fast communications channels between the two stations, and there must be large rotating units - generators or synchronous condensers - present in the AC networks at both ends of the transmission.

HVDC Light consists of only two elements: a con verter station and a pair of ground cables. The converters are voltage source converters, VSC’s. The outputs from the VSC’s are determined by the control system, which does not require any communications links between the different converter stations. converter stations. Also, they don’t need to rely on the AC network’s ability to keep the voltage and frequency stable. These feature make it possible to connect the converters to the points bests suited for the ac system as a whole.


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The converter station is designed for a power range of 1-100 MW and for a dc voltage in the 10-100 kV range. One such station occupies an area of less than 250 sq. meters (2 700 sq. ft), and consists of ust a few elements: two con tainers for the converters and the control system, three small AC air-core reactors, a simple harmonics filter and some coo ling fans.

The converters are using a set of six valves, two for each phase, equipped with high power transistors, IGBT (Insulated Gate Bipolar Transistor). The valves are controlled by a computerized control system by pulse width modulation, PWM. Since the IGBTs can be switched on or off at will, the output voltages and currents on the AC side can be controlled con trolled precisely.

The control system automatically adjusts the voltage, frequen cy and flow of active and reactive power according to the needs of the AC system. The PWM technology has been tried and tested for two decades in switched power supplies for electronic equipment as computers. Due to the new, high power IGBTs, the PWM technology can now be used for high power applications as electric power transmission

.HVDC Light can be used with regular overhead transmission lines, but it reaches its full potential when used with a new kind of dc cable. The new HVDC Light cable is an extruded, single-pole cable. The easiest way of laying this cable is by plowing. Handling the cable is easy, despite its large power-carrying capacity. It has a specific we ight of just over 1 kg/m. Contrary to the case with AC transmission; distance is not the factor that determines the line voltage. The only limit is the cost of the line losses, which may be lowered by choosing a cable with a conductor with a larger cross section. Thus, the cost of a pair of dc cables is linear with distance.


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A dc cable connection could be more cost efficient than even a medium distance AC overhead line, or local generating ge nerating units such as diesel generators. The conv erter stations can be used in different grid configurations. A single station can connect a dc load or generating unit, such as a photo-voltaic power plant, with an AC grid.

Two converter stations and a pair of cables make a point-to point dc transmission with AC connections at each end. Three or more converter stations make up a dc grid that can be connected to one or more points in the AC grid or to different AC grids. The dc grids can be radial with multi-drop converters, meshed or a combination of both. In other words, they can be configured, changed and expanded in much the same way AC grids are.

3.1 HVDC LIGHT INSTALLATION

HVDC light system mainly consists of transformers, converter units, phase reactors and filters.

Figure 4: HVDC Light transmission System


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The transformers are used to step-up/step-down voltages and the converters units converts AC to DC and vice versa. HVDC cables are used to carry currents and the filters are used for filtering unwanted signals. 3.2 HVDC LIGHT CHARACTERISCTICS

An HVDC Light converter is easy to control. co ntrol. The performance during steady state and transient operation makes it very attractive for the system planner as w ell as for the project developer. The benefits are technical, economical, environmental as well as operational.

The most advantageous are the following: • Independent control of active and reactive power • Feeding of power into passive networks (i.e. network without any generation) • Power quality control • Modular compact design, factory pre-tested pre-tested • Short Short delivery times • Re-locatable/Leasable Re-locatable/Leasable • Unmanned operation • Robust against grid alterations

3.1.1 Control Of Active & Reactive Power

The control makes it possible to create any phase angle or amplitude, which can be done almost instantly. This offers the possibility to control both active and rea ctive power independently. As a consequence, no reactive power compensation equipment is needed at the station, only an AC-filter is installed.


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While the transmitted active power is kept constant the reactive power controller can automatically control the voltage in the AC-network. Reactive power generation and consumption of an HVDC Light converter can be used for compensating the needs of the connected network within the rating of o f a converter. As the rating of the converters is based on maximum currents and voltages the reactive power capabilities of a converter can be traded against the active power capability.

3.1.4 Robust Against Grid Alterations

The fact that a Light converter con verter can feed power into a passive network makes it very robust and can easily accommodate alterations in the AC-grid to where it is conne cted. This is a very valuable property in a deregulated electricity market where AC-network conditions in the future will change more frequently frequentl y than in a regulated market.

3.2 THE CABLE SYSTEM

The HVDC Light extruded cable is the outcome of a comprehensive development program, where space charge accumulation, resistivity and electrical breakdown strength were identified as the most important material properties when selecting the insulation system. The selected material gives cables with high mechanical strength, high flexibility and low weight. Extruded HVDC Light cables systems in bipolar configuration hav e both technical and environmental advantages. The cables are small yet robust and can be installed by plowing, making the installation fast and economical.


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3.3 APPLICATIONS

3.3.1 Overhead Lines

In general, it is getting increasingly difficult to build ove rhead lines. Overhead lines change the landscape, and the construction of new lines is often met by public resentment and political resistance. People are often concerned about the possible health hazards of living close to overhead lines. In addition, a right-of-way for a high voltage line occupants valuable land. The process of obtaining permissions for building new overhead lines is also becoming time-consuming and expensive. Laying an underground cable is a much easier process than building an overhead line. A cable doesn’t change the landscape and it doesn’t need a wide right-of-way. right-of-way.

Cables are rarely met with any public opposition, o pposition, and the electromagnetic field from a dc cable pair is very low, and also a static field. Usually, the process of obtaining the rights for laying an underground cable is much easier, quicker and cheaper than for an overhead line. A pair of HVDC Light cables can be plowed into the ground. Despite their large power capacity, they can be put in place with the same equipment as ordinary, AC high voltage distribution cables. Thus, HVDC Light is ideally suited for feeding po wer into growing metropolitan areas from a suburban substation.

3.3.2 Replacing Local Generation

Remote locations often need local generation if they are situated far away from an AC grid. The distance to the grid makes it technically or economically unfeasible to connect the area to the main grid. Such remote re mote locations may be islands, mining areas, gas and oil fields or drilling platforms. Sometimes the local generators use gas turbines, but diesel ge nerators are much more common. An HVDC Light cable connection could be a better choice than building a local power plant based on fossil fuels. The environmental gains would be substantial, since the power supplied via the d c cables will be transmitted from efficient power plants in the main AC grid.


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Also, the pollution and noise produced when the diesel fuel is transported will be completely eliminated by an HVDC line, as the need for frequent maintenance of the diesels. Since the cost of building an HVDC Light line is a linear function of o f the distance, a break-even might be reached for as short distances as 50 - 60 kilometers.

3.3.3 Connecting Remote Power Grids

Renewable power sources are often built from scratch, beginning on a small scale and gradually expanded. Wind turbine farm is the typical case, but this is also true for photovoltaic power generation. These power sources are usually located where the conditions are particularly favorable, often far away from the main AC network. At the beginning, such a slowly expanding energy resource cannot supply a remote community with enough power. An HVDC Light link could be an ideal solution in such cases. First, the link could supply the community with power po wer from the main AC grid, eliminating the need n eed for local generation. The HVDC Light link could also supply the wind turbine farm with reactive power for the generators, and keeping the power frequency stable. When the power output from the wind generators grows as more units are added, they may supply the community with a substantial share of its po wer needs. When the output exceeds the needs of the community, communit y, the power flow on the HVDC Light link is reversed automatically, and the surplus power is transmitted to the main AC grid. 3.3.4 Asynchronous Links

Two AC grids, adjacent to each other but running asynchronously with respect to each other, cannot exchange any power between each other. If there is a surplus of generating capacity in one of the grids it cannot be utilized in the other grid. Each of the networks must have its own capacity of peak power generation, usually in the form of older, inefficient fuel fossil plants, or diesel or gas turbine units. Thus, peak power generation is often a source of substantial pollution, and their fuel economy is frequently bad. A DC link, connecting two such networks, can be used for combining the generation capacities of both networks. Cheap surplus power from one network can replace peak power generation in the other. This will result in both reduced pollution lev els and increased fuel economy. econom y. The power exchange between the networks is also very easy to measure accurately.


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4. SHORT CIRCUIT CONTRIBUTION OF HVDC LIGHT The HVDC Light transmission system mainly consists of two cables and t wo converter stations. Each converter station is composed of a voltage source converter (VSC) built up with IGBTs, phase reactors, ac filters and transformer. By using pulse width modulation (PWM), the amplitude and phase angle (even the frequency) of the converter AC output voltage can be adjusted simultaneously. Since the AC side voltage holds two degrees of control freedom, independent active and reactive power control can be realized. Regarding the active power control, the feedback control loop can be formulized such that either tracks the predetermined active ac tive power order, or tracks the given DC voltage reference. This gives two different control modes, i.e., active power control mode (Pctrl) and DC voltage control mode (Udc ctrl). If one station is selected to control the power, namely, in Pctrl mode, the other station should set to control the DC voltage, namely, in Udc ctrl mode. Regarding the reactive power control, the feedback control loop can be formulized such that it either tracks the predetermined reactive po wer order, or tracks the given AC voltage reference. This also gives two control modes, i.e., reactive power control mode (Qctrl) and AC voltage control mode (Uac ctrl). The two control modes can be chosen freely as desired in each station. Under the normal operation condition, the VSC can be seen as a voltage source. However, under abnormal operation conditions, for instance, during an ac short-circuit fault, the VSC may be seen as a current source, as the current capacity of o f the VSC is limited and controllable. 4.1 INVESTIGATION OF SHORT CIRCUIT CURRENTS

4.1.1 Studied AC System

The studied AC system has a mixture structure in radial and mesh connection. It includes high, medium and low voltage vo ltage buses. The AC transmission lines are modeled with p -link. The loads are constant current loads. Three types of fault, namely, the close-in fault; the near-by fault and the distant fault, are applied app lied at bus A, B and C, respectively. A 3-ph close-in fault results in a voltage reduction of almost 100%, whereas a 3-ph 3 -ph near-by fault and distant fault result in v oltage reduction on CCP bus of about 80% and 20%, respectively. In the following discussion, the short circuit ratio (SCR) is defined as the short circuit capacity of the AC system observed at CCP divided with the power rating of the converter.


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Figure 5: SLD of studied AC system

4.1.2 The Impact of Strength of AC Networks

The possible maximum relative short circuit current increment (∆Imax) is determined by the short circuit ratio (SCR). Supposing that the ∆Imax is defined as (1), it is found that the ∆Imax is inversely in proportional to the SCR as the solid curve shown in Figure 6.


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Figure 6: Characteristic showing the impact of AC network strength.

where, Isc is the short-circuit current of the original AC system alone at a 3-ph fault and I SC_HVDC_L , is the short-circuit current of the AC system with converter station connected and in operation at the same fault. fault. It should be noticed that the solid solid curve in Figure 6 is valid if there is no tap-changer, or the tap-change is at the position corresponding to the nominal winding ratio. If there is a tap changer chan ger in transformer, the AC network will observe a different current although the maximum current of the converter is a fixed value. Therefore, Th erefore, the maximum possible short circuit current increment is in the boundary defined by the two dashed curves. AC networks with SCR equal to 1.85, 3.14 and 12 have been simulated and the results are also shown in figure 6 with black dots.

Different control modes and different operation points may chan ge the short circuit current contribution from the VSC. However, it will not be higher than the ∆Imax. For instance, the short circuit current contribution from the VSC will not exceed 12% if the SCR is 10 and voltage tap-change range is ± 20%.

4.1.3. The Impact of Control Modes

The current is mainly limited by the impedances o f transmission lines and transformers when a short circuit occurs. Since the impedance of lines and transformers is dominated b y the inductive impedance, the short circuit current is mainly consisted of reactive current.


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Because of that, the choice of different control modes in respect of the active power control does not give any impact to the short circuit current. Therefore, the following discussion will focus on the choice between the control modes Qctrl and Uac ctrl.

It is important to notice that the change of short circuit current and the variation of bus voltages usually go hand in hand. The increase of short circuit current, namely, the increase of short circuit capacity, will improve the voltage stability and minimize the reduction of bus voltage due to faults. Inversely, the reduction of short circuit current may leads to voltage instability and voltage collapse during faults, in particular in weak AC systems. With Uac ctrl control mode, the reactive current generation will be automatically increased when the AC voltage decreases. Therefore, the Uacctrl control mode prov ides the possibility of improving the voltage stability and minimizing the reduction of bus voltage due to faults. On contrast, with Qctrl control mode it has the potential po tential risk of getting voltage instability or voltage collapse during faults if the AC system is weak and no control protection action is taken. One way to avoid this potential po tential risk is that the control is automatically switched to Uac ctrl if the AC voltage is detected out of the specified range (Umin~Umax, for instance, 0.9 ~1.1 per-unit). The other way is that the maximum value for the current order should be decreased with the AC voltage decreasing d ecreasing during faults. If the current from the VSC is reduced, its contribution to the short circuit current will also be reduced. Therefore, with Qctrl control mode the contribution of VSC to the short circuit current is almost neglectable independent of operation points, or load level. It will then be only onl y interesting to discuss the Uac ctrl control mode in respect of different operation points.

4.1.4 The Impact of Operation Points

As it has been discussed, the maximum max imum possible short circuit increment (∆Imax) due to HVDC Light is determined by the SCR. SC R. It will occur if the VSC is operating at z ero active power, namely, it is operating as an SVC or STATCOM. Figure 7 shows the characteristic of short circuit current contribution versus the load level. The two dashed curves are the result by taking into account the transformer winding ratio variation due to the tapchanger.AC networks with SCR equal to 3.14 has been simulated. For different load levels the observed short circuit currents, during a 3 -phase close fault, are marked with black dots in Figure 7. It should be noted that the short circuit current would be also reduced if the current order is also limited with the Uac ctrl. The black dot with a circle in Fig. 4 shows the result when the current order is limited to 35% of the rated current during du ring the AC fault.


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4.1.5 The Impact of Fault Type and Location

If the fault current is evaluated in per unit u nit with the base value equal to the 3-phase fault current at the corresponding fault location and without HVDC Light connected, it turns out that the impact of the fault location seems to be insignificant. Under the same load and operation condition, the 1-ph fault current is usually smaller than the 3-ph fault current. This is because the average voltage reduction is smaller for 1-phase fault, thereby the required reactive power generation is smaller during a 1-phase fault. In addition, the VSC only generates balanced 3-phase currents, even if the AC bus voltage v oltage is unbalanced due to 1-phase faults. As an example, Figure 8 shows 1-phase and 3-phase fault currents at different locations (bus B and bus C in Figure 5) under the same operation condition (SCR=3.14, P=-0.8 and Uac ctrl). Currents in plot (a) and (b) have one base value, and currents in plot (c) and (d) have another base value. Plot (b) shows that the peak value is slightly higher than 1, which means the short circuit current with HVDC Light is slightly higher than that without the HVDC Light for the same fault. It should be noticed that when a close-in short-circuit fault occurs the connected converter conv erter station will only feed the fault current. This implies that the current during the fault in the rest AC lines will be the same as the original AC network alone. In other words, the close-in fault isolates the HVDC Light terminal from the AC network. If it is the circuit breakers in the AC network n etwork to be mainly concerned, this type t ype of fault will be less significant. This is why that the performed studies do no t focus on this type of faults.


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Figure 8: Different fault currents in per unit of the corresponding 3-phase fault current without HVDC Light. (a): 1-ph close fault current. (b): 3-ph close fault current. (c): 1-ph distant fault current. (d): 3-ph distant fault current.


Figure 9: AC voltage and fault current with different control strategies. (a): AC voltage measured at CCP. (b): Case 1 – 1 – with with Uac ctrl and no change on current order limit. (c): Case 2 – 2 – with with Uac ctrl and current orde limit depending on voltage. (d): Case 3 – 3 – with with Qctrl and current order limit depending on voltage.

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4.1.6 Line Current during Faults

It is seen that the contribution from the HVDC Light makes the difference between the current of health lines and faulted lines larger, which may have a positive p ositive impact in distinguishing the faulted and health line. When a short circuit occur in the AC network, the sudden AC bus voltage variation may result in over current to the converter due to the measurement and control delay. As soon as the over current in the converter is detected, the protection will trigger a temporary blocking of converter.. It is obvious that the transient and steady state current contribution from the HVDC Light is different. Nevertheless, it should be noted that usually the circuit breakers do not react to the over current spontaneously, and it often has a delay time of about 60 ~100 ms. Therefore, Th erefore, it is the steady state current during the fault that should be considered.


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5. CONCLUSION From detailed analysis it is seen that HVDC system is used for long distance transmission and its more reliable and best method for power transmission when compared to ac po wer transmission. A comprehensive investigation on the issue regarding the contribution of HVDC Light to short circuit current has also been performed. The studies lead to the following conclusions; The HVDC Light, in contrast to the conventional HVDC which does not contribute an y short circuit current, may contribute some short circuit current. The possible maximum short circuit current contribution is determined by the SCR. It is inversely in proportional to the SCR and it occurs when the transmission system is operating at z ero active power. Hence, it is comparable to the STATCOM as long as the maximum short circuit current contribution is concerned. The amount of contribution depends on control modes, operation points and control strategies. With the reactive power control mode, the short circuit current contribution will be limited due to the current order limit decreasing with the voltage. With the AC voltage control mode, the short circuit current contribution will be increased with the decreasing of active power, p ower, if the current order limit is not changed. If the current order limit is decreasing with voltage, the short circuit current con tribution will be small even if the load level is low. The contribution to the short circuit current is irrelevant to the fault location if the fault current is evaluated in pe r unit with the base value equal to the 3 phase fault current at the corresponding fault location and without HVDC Light connected. Under the same load and operation condition, the 1-phase fault current is usually smaller than the 3-phase fault current. Finally, it should be noticed that in associated with higher short-circuit current the voltage stability and performance is likely to be improved. If the HVDC Light contributes a higher short-circuit current, the voltage dip du e to distant fault is possibly reduced and thereby the connected electricity consumers may suffer less from disturbances.


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6. REFERENCES

1.) DC Transmission based on voltage source con verters, Gunnar Asplund, Kjell Eriksson and Kjell Svesson,1997. 2.) The ABCs of HVDC transmission technologies, IEEE Power and Energy Magazine,2006. 3.) A course in Electrical Power, J.B. Gup ta. 4.) On the Short Circuit Current Contribution of HVDC Light, IEEE , Y. Jiang-Hafner, M. Hyttinen, and B. Paajarvi. 5.) Wikipedia & other web resources.


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Seminar- Hvdc Technology and Short Circuit Contribution of HVDC Light

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