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The first high voltage DC transmission line began operation in Sweden in 1946. Since that time, there has been a proliferation of DC transmission lines, operating at higher and ever higher voltages and powers. By BRYAN MAHER The Story of Electrical Energy, Pt.10 AST month, we looked at some of the early DC transmission lines which were developed to overcome the problems of AC electrical power at very high voltages. High voltage DC transmission lines became possible through the development of high power mercury arc valves. These were continually improved upon and developed by the ASEA Company of Sweden until as late as 1971. The original HVDC line from the Swedish mainland to Gotland Island used two parallel compound anodes in each mercury arc valve. Further development by the ASEA Company L 82 SILICON CHIP at their Ludvika laboratory produced the 4-anode high voltage mercury valve pictured in Fig, 1. This controlled diode, unveiled in 1958, was capable of much greater current, though it was more complex in construction and circuitry. Further research into simpler single anode mercury valves of even greater current carrying ability proceeded at the rebuilt Trollhattan laboratory. As you can imagine though, making electrical measurements on these very high voltage valves presented a host of problems. To perform the detailed voltage, current and timing tests on a bridge circuit of these valves, the scientists and engineers used oscilloscopes with their entire circuitry, case and power supply elevated to the HVDC potential. A motor generator set was specially Fig.1 (above): the world's first 4anode HVDC mercury arc valve being tested in 1958 at ASEA's Ludvika Laboratory. The four separate anode units can be clearly seen. This complex unit was effectively one controlled diode. adapted with the generator body and shaft isolated from ground by insulated mountings and shaft couplings. While this generator powered the oscilloscope, the researchers remained safe at ground potential and operated the CRO via long insulating shafts. A general view of this test setup is shown in Fig.2. These early mercury arc valves used air cooling but later higher power units used water cooling. NZ inter-island link The North and South Islands of New Zealand, originally had separate electricity supplies. Then in 1965, the two islands were joined electrically by a 125kV DC submarine cable across Cook Strait. Laid in water no more than 200 metres deep, this single-core steel armoured cable carries DC power in either direction as required. Thus excess load in either North or South Island can be supplied from the other. For this most important power link, the ASEA company installed converter bridges with mercury arc valves, each consisting of four parallel anodes with up to 10 intermediate electrodes. The photo of Fig.3 shows the original 1965 6-diode bridge at the North Island terminal. Later, the system was upgraded so that each station consisted of two bridges in series, with the centre connection earthed. In this form, the link operates at ± 250kV, at powers of up to 600 megawatts. For this level of power, the current is a maximum of 1200 amps, so high that water cooling of the valves is necessary. Thus ended the previous prevalence of air cooling for converter sets. Since the coolant is passing through equipment operating as high as 250kV above earth, this technique requires purification of the circulating water and long insulating hoses of teflon or polythene. Sweden to Denmark link A world first was celebrated in 1965 when two different countries were joined electrically by a HVDC submarine cable. Sweden and Denmark were connected by a 240kV DC cable, 86km long, between the two landfall points at Konti and Skan. This 250MW link, installed in 1965, used very large mercury valves, one of which is shown partly dissembled in Fig.4. Note here the grating-like Fig.2: high voltage testing on single anode mercury arc valves in progress at ASEA's Trollhatten Laboratory in 1962. In order to monitor the valves, the oscilloscopes were operated at the HVDC potential and were separately powered by an insulated generator. intermediate electrodes. These allow passage of the dense electron stream from the mercury pool at the bottom to the final anode at the top. As discussed last month, these intermediate electrodes, each connected in turn to ascending steps on a voltage divider, distribute the high voltage potential gradient in the cutoff mode. Thus, the valve does not flash over when in the high potential state during each negative half cycle. Fig.5 shows the Danish terminal as it appeared in 1965. As the benefits of HVDC submarine supply links became evident, many countries contracted with ASEA for such installations. In 1967, the island of Sardinia was connected to mainland Italy by a 200kV DC submarine cable 116km long. This 200MW link was at the time the world's longest. Again, a seawater return path was used. Harmonic suppression Inevitably, in any AC to DC conversion using rectifiers, harmonics are introduced into the system. So mercury arc rectifiers operating at high voltage produce harmonics of the AC mains frequency (50 or 60Hz) on a large scale. For 6-phase AC drive to a full wave group of 12 valves, the harmonics present are given by the expression (6n ± 1) where n is an integer. Thus, the harmonics produced will be the MAY1991 83 ferent frequencies; 50Hz in one , 60Hz in the other. Thus, a very short HVDC link can be used solely as a means of paralleling on multi-frequency systems. 2-way power transfer In all HVDC links, power can be transferred either way as desired (note: this operation is very different from the case of DC power interchanged between distant paralleled motor-generator sets, where we reverse the current direction for power to flow the other way). HVDC lines are unique in that the valves (mercury arc or solid state) at either end are fundamentally diodes, so they must always pass current in the same direction. By manipulating the phase triggering of the converters at either end of link, it is possible to arrange for power to flow in either direction. End of the mercury arc era Fig.3: the New Zealand inter-island link in 1965. Six diodes which constitute one 3-phase converting bridge are visible. Each diode consists of four parallel anodes, each with multiple intermediate electrodes. In its final form, this installation operates at ±250kV and can supply up to 600 megawatts in either direction. Notice that even though the valves operate at very high potentials, they are water cooled. 5th and 7th; 11th and 13th; 17th and 19th and so on. To avoid this problem, most HVDC links include filters to reduce all harmonics up to the 25th (ie, up to 1500Hz for a 60Hz AC mains input). Synchronising problems There is another point about AC powerlines which needs to be mentioned. Every AC transmission which joins areas fed by different power stations is, by the nature of AC, a synchronous link between those two power stations. Often this is desirable but it sometimes becomes a disadvantage. Should a small power transfer be desired between two large generating systems, a weak synchronous connection will not do. In times of trouble in one system, its frequency may fall slightly until corrected. During that 84 SILICON CHIP time interval, the weak interconnector will be called upon to carry very large synchronising currents in attempts to hold up the frequency of the troubled system. This overload would certainly trip off the interconnector on overcurrent, just when it is most needed. By contrast, HVDC power links do not need or carry frequency, timing or phase angle information. A DC line can only carry amperes of steady current. This fundamental asynchronous nature of HVDC links can be most useful. Cases do occur where the frequency stability of one generating system is unreliable, yet power needs to be transmitted to or from another grid which is far more stable. Here, asynchronous transfer is the only possible way. Again, the two AC systems to be joined may normally operate on dif- Development continued at the ASEA laboratories to produce larger and more efficient single anode mercury arc valves. In fact, ASEA had a 1000 amp monster mercury arc rectifier under development in 1971 but it did not go into production as solidstate high power thyristors had by then become available. Throughout the mercury arc valve era, ASEA avoided publishing any details of the valve's internal construction. Even the patent application showed nothing to indicate how they were made. No other company in the world achieved such development in mercury arc inverters. First solid state HVDC link The first HVDC thyristor installation in the world was a 50kV ZODA rectifier/inverter group. This was installed in the Vastervik converter station which was one end of the Gotland line. As a first step, an existing mercury arc valve was removed and replaced by one group of series parallel thyristors (ie, series strings of thyristors , with the strings then connected in parallel to carry the high currents). Fig.6 shows the trial installation which was in service from 1967-1969. After 15,000 hours of successful operation of the pilot solid state valve, the entire Gotland link was upgraded. A new solid state bridge was connected in series with the high voltage Fig.4: this photo gives so.m e idea of the size of the mercury arc valves from the 1965 Konti-Skan HVDC installation. Fully assembled, these valves operated at 250kV and 1000 amps. top terminal of the existing mercury arc valves. In the new addition, each diode was a series parallel group of silicon thyristors. This new bridge, supplied by its own transformer, developed 50kV DC. This, added to the 100kV of the existing original mercury arc valves, gave a total output voltage of 150kV. The original submarine cable was retained as its insulation was quite adequate for this increased voltage. Thus, the upgraded Gotland link in 1970 was capable of carrying 30 megawatts instead of the original 20 megawatt rating. However, by 1983 the electrical load on Gotland Island had so increased that a completely new HVDC system was installed. A new submarine cable, 90km long and weighing 3000 tonnes, was manufactured by ASEA Kabel of Stockholm, to a rating of 150kV. Together with the new converter stations built at each end, this cable supplied the full 130MW load of Gotland. The original cable and converter equipment and the old power station on Fig.5: this picture shows the Danish terminal of the Konti-Skan HVDC link to Sweden. The AC equipment is in the foreground while the DC converters and harmonic filters are in the background at right. MAY 1991 85 Fig.6: the world's first HVDC solid state thyristor valve group is shown at right in this picture as part of a trial installation at Gotland. It was run for 15,000 hours at 100kV and 200 amps. the island are held as reserves. The new solid state AC/DC converters shown in Fig. 7 are huge in comparison to the original Vastervik plant, now 37-years old, but still operable. Large numbers of series paral- Fig.7: the 150kV 130 megawatt HVDC installation for the Gotland Mk.3 connection. Suspended from the ceiling are the quadruple solid state valves which make up the · inverter/converter bridge. lel connected silicon thyristors carry up to 1000 amps and withstand 150kV. In 1987, to cope with further increased load, a second similar installation and cable were installed, doubling the power capacity. Suspended converters Fig.8: HVDC links are continuing to grow in size and power. This is the Konti-Skan-2 installation which was brought into operation in mid-1989. It operates at 285kV and supplies 300 megawatts. 86 SILICON CHIP Because of the multiplicity of connected components, the physical mounting of solid state HVDC converters is a critical point of design. By 1983, ASEA had installed systems in many countries, including North and South America. As some locations are subject to earth tremors, volcanic activity and other ground instability, an earthquake-proof converter mounting method was devised. Here a strong reinforced concrete building is constructed, then the complete HVDC converter units are suspended from the ceiling by tension insulators. This means that the electronic system can remain relatively stationary even though the ground and building may move laterally and vertically during earthquakes. Suspended converters have survived some violent earthquakes on the American continent, though in those disasters even powerlines, bridges and other buildings have collapsed. The first suspended HVDC convert- ers were the 1983 Gotland Mk.3 units. In the photo of Fig.7, this method of mounting is clearly seen, the bottom 1000A SUBMARINE CABLE 3-PHASE TRANSFORMER CENTRE EARTH - - - + - , T O WATER ..,. - 3-PHASE TRANSFORMER 3-PHASE GRID SKAGERRAK CHANNEL 127km WIDE - - 50Dm DEEP 25DkV CONVERTER/ INVERTER -250kV 1000A 3-PHASE TRANSFORMER SUBMARINE CABLE DENMARK Fig.9: the Skagerrak HVDC link between Denmark and Norway effectively uses two submarine cables in series to supply 500 megawatts at ±250kV. of the units hanging about two metres above the floor. World's longest DC link In 1977, Norway and Denmark were connected by the (then) world's longest submarine power link. From Tjele 3-PHASE GRID NORWAY in Denmark, two HVDC submarine cables were laid across the Skagerrak channel to Norway. Each cable is single core, steel-armoured and rated at 250kV. To carry the 500 megawatt load, these cables effectively operate in series, giving an equivalent of 500kV at lO00A. To ease the cable insulation problem, the converter stations at the ends of the line each consist of two 250kV bridges in series, grounded at their centre, as shown in Fig.9. The task of manufacturing transporting and laying one 130km length of submarine cable weighing 6000 tonnes was immense. For this purpose, a specially built ship, the M.V. Skagerrak, was equipped with a huge deck-mounted motor driven turntable (Fig.10). A loading/unloading gantry gently eases the heavy cable onto the turntable when loading and off when laying at sea. Because the cable was in one piece, no cable joining at sea was necessary and it could be voltage tested before the voyage began. In 1989, a completely new HVDC submarine link, Konti-Skan 2, was completed, carrying 300MW either way between Sweden and Denmark. With this facility, the Swedish State Power Board (Vattenfall) can exchange power with the Danish power system (Elsam) and the huge central European grid. Acknowledgement Special thanks and acknowledgements to ABB Australia and Sweden for supplying historic photographs and data; and to ABB Review, ASEA Journal and Action. SC Fig.10 (left): built especially to lay HVDC submarine cables, this ship carries 6000 tonnes of cable in one piece, 130 kilometres long. MAY 1991 87