29-05-2011, 10:04 PM
12-PULSE HVDC TRNSMISSION SYSTEM BY USING IGBT VALUES
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ABSTRACT
In this project an attempt has been made to analyze the effect of Using IGBTs rather than semiconductor devices such as Transistors, SCRs in the converter stations for HVDC transmission.
The main parts of any basic HVDC transmission system are converter stations transmission line and DC switch gear and protection equipment. Till now the main devices that are used in converter stations for the purpose of rectification and inversion are SCRs. But now a day’s IGBTs are replacing SCRs in some of the applications due to its advantages when compared to SCRs.
IGBT is a new development in the area of POWER MOSFET Technology. This device combines into it the advantages of both MOSFET and BJT. SO an IGBT has high input impedance and low ON state power loss and also it is free from second break down problem. Though IGBTs are somewhat more expensive, yet they are becoming popular in the application of converter stations in HVDC transmission systems because of low gate drive requirements. At present IGBTs are available up to 6600V, 1600A and there is a rapid growth in IGBT technology. In future IGBTs will eventually push out TRANSISTORS, SCRs from most of the applications.
This project work “12 – PULSE HVDC TRANSMISSION BY USING IGBT VALUES “Simulation is done by demo version PSIM software. All observations and results along with graphs are presented.
Software Requirements:
PSIM SOFTWARE
CHAPTER - 1
INTRODUCTION
1.1 INTRODUCTION TO HVDC
The industrial growth of a nation requires increased consumption Of energy, particularly electrical energy. This has led to increase in the Generation and transmission facilities to meet the increasing demand. In U.S.A. till the early seventies, the demand doubled every ten years. In Developing countries, like India, the demand doubles every seven years, Which requires considerable investment in electric power sector.
The imperative of supplying energy at reasonable costs coupled With the depleting reserves of non-renewable energy sources has led to The establishment of remoter generating stations- predominantly fossil fuel Tired thermal stations as pit head. Environment al considerations Also sometimes dictate the sitting of power stations at remote locations. Large hydro stations are invariably a t distances of hundreds of Kilometers from load centers. The need to economize on costly Investments in generation reserves, sharing of benefits in utilizing
Variability in generation mixes and load patters have given rise to Interconnection of neighboring systems and development of large power grids.
Remotes generation and system interconnections lead to a search for efficient power transmission at increasing power Levels. The increase in voltage levels is not always feasible. The problems of AC transmission Particularly in a long distance transmission, has led to the development of DC transmission. However, as generation and utilization power remain at alternating current, the DC transmission requires conversion
at two ends, from AC to DC the sending end and back to AC at the receiving end. This conversion is done at converter stations rectifier station at the sending end and inverter station at the receiving en d The converters are static-using high power thyristors connected in series to give the required voltage ratings. The physical process of conversions is such that facilitating power reversal.
The HVDC transmission made a modest beginning in 1954 when a 100 kV, 20 MW dc page link was established between Swedish mainland and the island of Gotland. Until 1970, the converter stations utilized mercury arc valves for rectification. The successful use of thyristors for power control in industrial devices encouraged its adoption in HVDC converters by development of high power semiconductor devices. The largest device rating is now in the range of 11 kV, 3000A. The highest transmission voltage reached is ± 600kV.
1.2 HISTORY OF HVDC TRANSMISSION
An early method of high-voltage DC transmission was developed by the Swiss engineer Rene Theory, This system used series-connected motor-generator sets to increase voltage. Each set was insulated fromground and driven by insulated shafts from a prime mover. The line was
operated in constant current mode, with up to 5000 volts on each machine, some machines having double commutators to reduce the voltage on each commutator. An early example of this system was installed in 1889 in Italy by the Society Acquedotto de Ferrari-Galliera.This system transmitted 630 kW at 14 kV DC over a distance of 120 km. Other Thury systems operating at up to 100 kV DC operated up until the 1930s, but the rotating machinery required high maintenance and had high energy loss. Various other electromechanical devices were tested during the first half of the 20th century with little commercial success.
The grid controlled mercury arc valves became available for power transmission during the period 1920 to 1940. In 1941 a 60 MW, ± 200 kV, 115 km buried cable page link was designed for the city of Berlin u s in gmercury arc valves (Elbe-project) , but owing to the collapse of the German government in 1945 the project was never completed. The nominal justification for the project was that, during wartime, a buried cable would be less conspicuous as a bombing target.
The equipment was moved to the soviet union and was put into service there.
CHAPTER- 2
DESCRIPTION OF HVDC TRANSMISSION SYSTEM
2.1 TYPES OF DC LINKS
2.1.1 Monopolar DC Link
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 mayor may not 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. 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. Underwater earth -return electrodes in seawater may produce chlorine or otherwise affect water chemistry. An unbalanced current path may result in a net magnetic field, which can affect magnetic navigational compasses for ships passing over an underwater cable. These effects can be eliminated with installation of a metallic return conductor between the two ends of the monopolar transmission line.
Since one 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 1500 MW. If underground cables are used, the typical value is 600 MW.
Most monopolar systems are designed for future bipolar expansion. If overhead power transmission lines are used, the used electricity pylons are often designed to carry two conductors and in many cases they do also. The second conductor is either unused, used as electrode line or permanently parallelized with the other (as in case of Baltic cable).
2.1.2 Bipolar Dc link
In bipolar transmission a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor.
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, minimizing earth return loss and environmental effects.
When a fault develops in a line, with earth return electrodes installed at each end of the line, current can continue flow using the earth as a return path, operating in monopolar mode.
Since for a given power rating bipolar lines carry only half the current of monopolar lines, the cost of the second conductor is reduced compared to a monopolar line of the same rating. In very adverse terrain, the second conductor may be carried on an independent set of transmission towers, so that some power may continue to be transmitted even if one line is damaged.
A bipolar system may also be installed with a metallic earth return conductor.
Bipolar systems carry as much as 3000MW at voltages of ±533kV. Submarine cable installations initially commissioned as a monopole may be upgraded with additional cables and operated as a bipole.
2.1.3 Homopolar
Homopolar page link has two or more conductors all having the same polarity (usually negative) and always operated with ground or metallic return.
Because of the desirability of operating a DC page link without a ground return, bipolar links are most com manly used. Homopolar page link has the advantage of reduced insulation costs. But the disadvantages of earth return outweigh the advantages. Incidentally,the cona effects in a DC line are substantially less with the negative polarity of the conductor as compared to the positive polarity.
2.1.4 Systems with transmission lines
The most common configuration of an HVDC page link is a station-to-station link, where two inverter/ rectifier stations are connected by means of a dedicated HVDC link. This is also a configuration commonly used in connecting unsynchronised grids, in long-haul power transmission, and in undersea cables.
Multi-terminal HVDC links, connecting more than two points, are rare. The configuration of multiple terminals can be series, parallel, or hybrid mixture of series and parallel). Parallel configuration tends to be used for large capacity stations, and series for lower capacity stations. An example is the 2000 MW Quebec-New England transmission system opened in 1992, which is currently the largest multi-terminal HVDC system in the world.
2.1.5 Tripole - current-modulating control
Current modulation of DC transmission lines is useful for converting existing AC transmission lines to HVDC. Two of the three circuit conductors are operated as a bipole. The third conductor is used as a parallel monopole, equipped with reversing valves (or parallel valves connected in reverse polarity). The parallel monopole periodically relieves current from one pole or the other, switching polarity over a span of several minutes. The bipole conductors would be loaded to either 1.37 or 0.37 of their thermal limit,with the parallel monopole always carrying ± 1 times its thermal limit current. The combined RMS heating effect is as if each of the conductors is always carrying 1.0 of its rated current. This allows heavier currents to be carried by the bipole conductors, and full use of the installed third conductor for energy transmission. . High currents can be circulated through the line conductors even when load demand is low. Combined with the higher average power possible with a DC transmission line for the same line-to-ground voltage, a conversion of an existing AC hence couldn’t allow up to 80% more power to a point-to-point transmission requires two converter station The system operates current, of a pole converter or a conductor results in only a small loss of capacity and no earth-return current, reliability of this scheme would be high. No time would be lost in switching if a conductor broke. The valves would inherently have an emergency overload rating in bipole mode. This would possibly allow great increase in power transmission with significant effect in congested transmission systems, where consequences of a single line failure limit the allowed loading of other parallel transmission lines. While capital costs are higher than for a bipole conversion operating at the same voltage class, the extra power capability reduces incremental cost per megawatt. Depending on transmission line physical configuration, replacement of insulators may be required to achieve the highest power rating, to insure proper line-to-line clearance distances.
As of 2005 no tri-pole conversions are in operation, although a transmission line in India has been converted to bipole HVDC.
2.2 CONVERTER STATION
The major components of a HVDC transmission system are converter station where conversions from AC to DC (rectifier station) and from DC to AC (inverter station) are performed.
rectifier and inverter station can be reversed (resulting in power reversals) by suitable converter control.
12 Pulse converter Transf arrner Srrioolrvnq reactors DC Fi He-is Tunnel AC Filters HP AC Fil ters
Fig: 2.4 schematic diagram of typical HVDC converter station
A typical converter station with two twelve-pulse converter units per pole is shown in figure 2,4 .The various components of a converter station are discussed below.
CHAPTER – 3
IGBT VALVES
IGBT VALVES
HVDC converters are an assembly of valves, which have the property of conducting in the forward direction and blocking in the reverse direction. The term 'valve', carried over from the mercury arc , is applied even now for IGBT valves which are made up of series and parallel connection of many IGBT cells or devices.
The major problem with the mercury arc valves is the occurrence arc backs or backfires which results in the destruction of the rectifying property of the valves. Arc backs are random phenomena which result in failure to block in the reverse direction .Although the incidence of arc backs can be reduced by carefully controlling the factors that influence them, complete elimination is impossible and the valve is also increased. Furthermore, arc backs are non self clearing and result in line to line faults which stress transformer windings and nodes in the valve. The maintenance requirements for the valves go up and lead to poor reliability.
Thyristor valves which were developed in the late sixties have eliminated all these problems. They have now completely displaced mercury arc valves in HVDC transmission.
Thyristors that constitute the valves are also not perfect devices. The major problem is that their ratings cannot be exceeded even for short durations. However, there is continuing development in the field of power semiconductors which has brought down the cost while improving the reliability. IGBT valves are the new development in the power semiconductor technology and now these are replacing the Thyristor valves
3.1 IGBTs
IGBT is a new development in area of power MOSFET technology. an IGBT combines the advantages of BJT and MOSFETs. An IGBT has high input impedance, like MOSFETs, and low on-state conduction
losses, like BJTs. However there is no second breakdown problem, as with BJTs. By chip design and structure, the equivalent drain to source resistance Rds is controlled to behave like that of a BJT. IGBT is also i known as metal oxide insulated gate transistor (MOSIGT), conductively-modulated field effect transistor (COMFET) or gain- modulated FET (GEMFET). It was also initially called insulated gate transistor (IGT). The "first-generation" devices of the 1980s and early '90s were relatively slow in switching, and prone to failure through such
modes as latch up and secondary break down. Second-generation devices were much improved, and the current third-generation ones are even better, with speed rivaling MOSFETs, and excellent ruggedness and tolerance of overloads
