The problem was solved in 1929 by aproposal to insert grading electrodesbetween the anode and cathode. Subsequentlypatented, this innovative solutioncan in some ways be considered asthe cornerstone of all later developmentwork on the high-voltage mercury-arcvalve. It was during this time that Dr.Uno Lamm, who led the work, earnedhis reputation as ‘the father of HVDC’.The Gotland linkThe time was now ripe for service trialsat higher powers. Together with theSwedish State Power Board, the companyset up, in 1945, a test station at Trollhättan,where there was a major powerplant that could provide energy. A 50-kmpower line was also made available.Analog simulator used in the design of the early HVDC transmission systemsTrials carried out over the followingyears led to the Swedish State PowerBoard placing, in 1950, an order forequipment for the world’s first HVDCtransmission link. This was to be builtbetween the island of Gotland in theBaltic Sea and the Swedish mainland.over which AC at ever-higher voltagesflowed. To bridge expanses of water,submarine cable was developed.Neither of these transmission media waswithout its problems, however. Specifically,they were caused by the reactivepower that oscillates between the capacitancesand inductances in the systems.As a result, power system plannersbegan once again to look at thepossibility of transmitting direct current.Back to DCWhat had held up high-voltage directcurrent transmission in the past was,first and foremost, the lack of reliableand economic valves that could convertHVAC into HVDC, and vice versa.The mercury-arc valve offered, for a longtime, the most promising line of development.Ever since the end of the 1920s,when the Swedish ASEA – a foundingcompany of <strong>ABB</strong> – began making staticconverters and mercury-arc valves for voltagesup to about 1000 V, the possibilityof developing valves for even higher voltageshad been continually investigated.This necessitated the study of new fieldsin which only a limited amount of existenttechnical experience could beapplied. In fact, for some years it wasdebated whether it would be possibleat all to find solutions to all the variousproblems. When HVDC transmissionfinally proved to be technically feasiblethere still remained uncertainty as towhether it could successfully competewith HVAC in the marketplace.Whereas rotating electrical machinesand transformers can be designed veryprecisely with the aid of mathematicallyformulatedphysicallaws, mercury-arcvalve designdepends to alarge degreeon knowledgeacquiredempirically.As a result, attempts to increasethe voltage in the mercury-vaporfilledtube by enlarging the gap betweenthe anode and cathode invariablyfailed.Following on this order, the companyintensified its development of the mercury-arcvalve and high-voltage DC cable,while also initiating design work onother components for the converter stations.Among the equipment that benefitedfrom the increased efforts weretransformers, reactors, switchgear andthe protection and control equipment.Even when HVDC transmissionfinally proved technically feasible,it was doubted for a long timewhether it could compete withHVAC in the marketplace.Only some of the existing AC systemtechnology could be applied to the newDC system. Completely new technologywas thereforenecessary.<strong>Special</strong>istsinLudvika,led byDr. ErichUhlmannandDr. HarryForsell, set about solving the many verycomplex problems involved. Subsequently,a concept was developed for the Gotlandsystem. This proved to be so successfulthat it has remained basically unchangedright down to the present time!Since Gotland is an island and thepower link was across water, it was62<strong>Special</strong> <strong>Report</strong><strong>ABB</strong> <strong>Review</strong>
During the 1960s several HVDC linkswere built: Konti-Skan between Swedenand Denmark, Sakuma in Japan (with50/60 Hz frequency converters), theNew Zealand link between the Southand the North Islands, the Italy – Sardinialink and the Vancouver Island linkin Canada.Early mercury-arc valve for HVDC transmissionalso necessary to manufacture a submarinecable that could carry DC. It wasseen that the ‘classic’ cable with massimpregnated paper insulation that hadbeen in use since 1895 for operationat 10 kV AC had potential for furtherdevelopment. Soon, this cable wasbeing developed for 100 kV DC!Finally, in 1954, after four years of innovativeendeavor, the Gotland HVDC transmissionlink, with a rating of 20 MW,200 A and 100 kV, went into operation.A new era of power transmission hadbegun.The original Gotland link was to see28 years of successful service beforebeing finally decommissioned in 1986.Two new links for higher powers havemeanwhile been built between theisland and the Swedish mainland, onein 1983 and the other in 1987.The largest mercury-arc valve HVDCtransmission link to be built by thecompany was the Pacific Intertie [1] inthe USA. Originally commissioned for1440 MW and later uprated to 1600 MWat ±400 kV, its northern terminal is sitedin The Dalles, Oregon, and its southernterminal at Sylmar, in the northern tipof the Los Angeles basin. This projectwas undertaken together with GeneralElectric, and started operating in 1970.In all, the company installed eight mercury-arcvalve based HVDC systemsfor a total power rating of 3400 MW.Although many of these projects havesince been replaced or upgraded withthyristor valves, some are still in operationtoday, after 30 to 35 years ofservice!Mercury-arc valves in the first Gotland link,1954The semiconductor ‘takeover’ beginsMercury-arc valve based HVDC hadcome a long way in a short time, but itwas a technology that still harboredsome weaknesses. One was the difficultyin predicting the behavior of thevalves themselves. As they could notalways absorb the reverse voltage, arcbacksoccurred. Also, mercury-arcvalves require regular maintenance,during which absolute cleanliness iscritical. A valve that avoided thesedrawbacks was needed.The invention of the thyristor in 1957had presented industry with a host ofnew opportunities, and HVDC transmissionwas soon seen as a promising areaEarly HVDC projectsThe early 1950s also saw the British andFrench power administrations planninga power transmission link across theEnglish Channel. High-voltage DCtransmission was chosen, and the companywon its second HVDC order – thistime a link for 160 MW.The success of these early projects generatedconsiderable worldwide interest.Gotland 1 extension, with the world’s first HVDC thyristor valves<strong>Special</strong> <strong>Report</strong><strong>ABB</strong> <strong>Review</strong>63