ABB Review Special Report - ABB - ABB Group

8400 kVCircuit of dynamic load balancer in Sellindge substation (33 kV, –80/+170 MVAr)25 kV 25 kV84 MVAr2x42 MVArCatenaryFeedererated by the power systems, the loadbalancer generates a (pure) negativephasesequence current, (I LB ), as shownin 7 . This current balances exactly thenegative-phase sequence current fromthe load (I -LOAD in 6 ).The load balancer in the Sellindge substation8 is optimized to handle a loadconnected between the C and A phases.Load balancing theory says that, tobalance a purely active load, a capacitorhas to be connected between phasesA and B and a reactor betweenphases B and C. The traction load alsohas a reactive part, which likewise hasto be balanced. In this substation, notonly the asymmetry is compensated butalso the power factor. This is achievedby inserting a capacitor between phasesC and A.RedundancyHigh availability is required, so all criticalcomponents are redundant: A completefourth redundant phase has beenadded in the main circuit. All the phasesneed to be as independent of eachother as possible.3rd5th7thTCRTCRStandbyphase33 kVlogic circuits, measuring transformers,relay protection devices and coolingsystem. Each of the connections to thesubstation busbars has a circuit-breakerand disconnector inserted in it. Filterscan be connected to or disconnectedfrom the fourth interphase to turn itinto either an inductive or a capacitivebranch.Two independent control systems acton the three-phase system, while thethyristor firing and logic circuits actdirectly on each interphase. The controlsystems are strictly segregated, asare the valve-firing logic circuits andthe overall protection system. If aninterphase fails, the control systemtrips it and automatically substitutesthe standby unit.The thyristor valves make use of a newtype of thyristor – a bidirectional devicewith two antiparallel thyristors on acommon silicon wafer. This halves thenumber of units needed in the valves.The thyristor is a 5-inch device with acurrent-handling capability of about2000 A(rms).The Dafang project is a classic exampleof a transmission capacity upgrade providingmuch-needed power to a fastgrowingarea, in this case the regionaround Beijing. The project was completedin the extremely short timeof nine months and brings existing,remotely generated power to an areawhere it is urgently needed.The case of Eagle Pass shows the possibilitiesoffered by new technologiesable to combine advanced FACTS propertieswith network interconnectioncapability. The latest developments insemiconductor and control technologyhave made this possible. Thanks to thisback-to-back tie, existing transmissionfacilities can be utilized to a muchgreater extent than before.Finally, the Channel Tunnel rail linkillustrates well the flexibility of FACTSdevices by showing how they can alsobe used to solve the problems createdby new, sophisticated types of load.The unbalance caused by new tractionloads, for example, can be mitigated,and downgrading of the electricity supplyfor other users avoided, by meansof the described solid-state solutions.These examples show that FACTS deviceswill be used on a much widerscale in the future as grid performancebecomes an even more important factor.Having better grid controllability willallow utilities to reduce investment inthe transmission lines themselves. ABBis currently exploring ways in whichFACTS devices can be combined withreal-time information and informationtechnologies in order to move themeven closer to their physical limits.These requirements have resulted ina unique plant layout and design forthe control and protection. There arefour fully independent ‘interphases’(an assembly of components connectedbetween two phases). Each interphasefeatures an independent set of filters,reactors, thyristor valves, thyristor firingSummary and outlookThe importance of improving grid performanceis growing for economical aswell as environmental reasons. FACTSdevices have established themselvesas the most suitable solutions for increasingpower transmission capabilityand stability.Rolf GrünbaumÅke PeterssonBjörn ThorvaldssonABB Power TechnologiesSE-721 64 VästeråsSwedenrolf.grunbaum@se.abb.com18Special ReportABB Review