ABB Review Special Report - ABB - ABB Group

Channel Tunnel rail linkWhen the high-speed electrified railwayline between London and the ChannelTunnel to France is finished in 2007 itwill be possible to travel between Londonand Paris in just over two hours,at a maximum speed of 300 km/h. Therailway power system is designed forpower ratings in the range of 10 MWand which fluctuate (rapid accelerationand retardation). The traction feedingsystem that was supplied by ABB is amodern 50-Hz, 2 ž 25-kV supply incorporatingan autotransformer scheme tokeep the voltage drop along the tractionlines low. Power step-down from thegrid is direct, via transformers connectedbetween two phases 5 .5Power feeding system for the Channel Tunnel rail link between Englandand France. Singlewell substation with two single-phase static var compensators,each rated 25 kV, –5/+40 MVAr25 kV 25 kV 45 MVAr 40 MVArTCR 3rd 5th 7thTCR3rd5th7thSVCCatenaryFeeder400 kVSVCs for the three traction feedingpointsA major feature of this power system isthe static VAr compensator (SVC) support,the primary purpose of which is tobalance the unsymmetrical load and tosupport the railway voltage in the caseof a feeder station trip – when two sectionshave to be fed from one station.The second purpose of the SVCs is toensure a low tariff for the active powerby maintaining unity power factor duringnormal operation.Thirdly, the SVCs mitigate harmonicpollution by filtering out the harmonicsfrom the traction load. This is importantas strict limits apply to the traction system’scontribution to the harmonic levelat the supergrid connection points.The SVCs for voltage support only areconnected on the traction side of the interconnectingpower transformers. The45 MVAr 40 MVArsupergrid transformers for the tractionsupply have two series-connected medium-voltagewindings, each with its midpointgrounded. This results in twovoltages, 180 degrees apart, betweenthe winding terminals and ground. TheSVCs are connected across these windings;consequently, there are identicalsingle-phase SVCs connected feeder toground and catenary to ground.The traction load of up to 120 MW isconnected between two phases. Withoutcompensation, this would result inan approximately 2 % negative phasesequence voltage. To counteract theunbalanced load, a load balancer (anasymmetrically controlled SVC) hasbeen installed in the Sellindge substation.This has a three-phase connectionto the grid.The load balancer transfers active powerbetween the phases in order to createa balanced load (as seen by the supergrid).A brief explanation of how theload balancing works is given in thefollowing.Load currentWhen the load is connected betweentwo phases (B & C) only, the tractioncurrent can be expressed by two phasevectors, one representing the positivesequence and the other the negativesequence 6 . The summation of the twovectors is the resulting current (currentin phase A is zero and currents in phaseB and C are of equal magnitude, but ofopposite phase). Note that the vectoramplitudes are not truly representative.To compensate the negative sequenceand thus balance the current to be gen-6 Phase-sequence components of the load current7 Load current balancingIcIcIcIcIbIcIbIa+ Ia I=LOADI LOAD IaIa+ =I +LOAD I -LOAD ILBILB +I LOADIbIbIbIcIbSpecial ReportABB Review17