ABB Review Special Report - ABB - ABB Group

of application. As part of its activities inthe semiconductor field, the companyhad continued to work at developinghigh-voltage thyristor valves as an alternativeto the mercury-arc type. In thespring of 1967, one of the mercury-arcvalves used in the Gotland HVDC linkwas replaced with a thyristor valve. Itwas the first time anywhere that thiskind of valve had been taken into commercialoperation for HVDC transmission.After a trial of just one year, theSwedish State Power Board ordered acomplete thyristor valve group for eachconverter station, at the same time increasingthe transmission capacity by50 percent.Around the same time, tests were carriedout on the Gotland submarine cable,which had been operating withoutany problems at 100 kV, to see if itsvoltage could be increased to 150 kV –the level needed to transmit the higherpower. The tests showed that it could,and this cable was subsequently operatedat an electrical stress of 28 kV/mm,which is still the worldwide benchmarkfor large HVDC cable projects today.The new valve groups were connectedin series with the two existing mercuryarcvalve groups, thereby increasingthe transmission voltage from 100 to150 kV. This higher-rated system wastaken into service in the spring of 1970– another world’s ‘first’ for the Gotlandtransmission link.With the advent of thyristor valves itbecame possible to simplify the converterstations, and semiconductors havebeen used in all subsequent HVDClinks. Other companies now began toenter thefield.BrownBoveri(BBC) –which latermergedwith ASEAto formABB – teamed up with Siemens andAEG in the mid-1970s to build the1920-MW Cahora Bassa HVDC linkbetween Mozambique and South Africa.The same group then went on to buildthe 2000-MW Nelson River 2 link inCanada. This was the first project toemploy water-cooled HVDC valves.The late 1970s also saw the completionof new projects. These were the Skagerraklink between Norway and Denmark,Inga-Shaba in the Congo, and the CUProject in the USA.The Pacific Intertie was also extendedtwice in the 1980s, each time with thyristorconverters, to raise its capacity to3100 MW at ±500 kV. (ABB is currentlyupgrading the Sylmar terminal by replacingthe converters and control system.)Itaipu – the new benchmarkThe contract for the largest of all HVDCtransmission schemes to date, the6300-MW Itaipu HVDC link in Brazil,was awarded to the ASEA-PROMONconsortiumThe scale and complexity of theItaipu project presented a considerablechallenge, and it can beconsidered as the start of themodern HVDC era.in 1979.This projectwas completedandput intooperation inseveralstages between1984 and 1987. It plays a key rolein the Brazilian power scheme, supplyinga large portion of the electricity forthe city of São Paulo.The scale and technical complexity ofthe Itaipu project presented a considerablechallenge, and it can be consideredas the start of the modern HVDC era.The experience gained in the course ofits completion has been in no small wayresponsible for the many HVDC ordersawarded to ABB in the years since.After Itaipu, the most challenging HVDCproject was undoubtedly the 2000-MWQuébec – New England link. This wasthe first large multi-terminal HVDCtransmission system to be built anywherein the world.HVDC cables have kept paceAs the converter station ratings increased,so too did the powers and voltagelevels for which the HVDC cableshad to be built.The most powerful HVDC submarinecables to date are rated 600 MW at450 kV. The longest of these are the230-km cable for the Baltic Cable linkbetween Sweden and Germany, andthe 260-km cable for the SwePol linkbetween Sweden and Poland.Foz do Iguaçu converter station with the Itaipu 12,600-MW power station in the backgroundHVDC todayThe majority of HVDC converter stationsbuilt today are still based on the principlesthat made the original Gotland linksuch a success back in 1954. Station de-64Special ReportABB Review