ABB Review Special Report - ABB - ABB Group

1The Dafang 500-kV series capacitorsThe main protective devices used areZnO varistors and circuit-breakers. Thefirst is to limit the voltage across thecapacitor and is supplemented by aforced-triggered spark gap to handleexcess current during a fault sequence.The circuit-breakers connect and disconnectthe series capacitors as required.They are also needed to extinguishthe spark gap, as it is not selfextinguishing.The capacitors are rated for operationduring normal, steady-state grid conditionsas well as for severe system contingencies,such as loss of one of thetwo parallel 500-kV lines. In such acase, the capacitor of the line remainingin service must be able to take the fullload of both lines for a certain amountof time. This was, in fact, one of thereasons for installing the series capacitorsin the first place – to ensure thesafe transfer of power to the Beijingarea even with a line down.ABB was contracted to do this, and installedtwo series capacitors (each rated372 MVAr, 500 kV) in the middle of eachline of a 300-km twin-circuit corridorbetween Datong and Fangshan 1 . Theycame on stream in June, 2001, a merenine months after the contract wasawarded and some 3 to 6 months fasterthan for similar installations.A series capacitor acts to reduce thetransfer reactance of the power line atpower frequency (50 Hz) and suppliesreactive power to the circuit at the sametime. The benefits of this are:Increased angular stability. Theremust always be a certain differencebetween the voltage phase angles ateither end of the power line to enabletransmission. The phase angledifference increases with powertransmission and the series capacitorkeeps the angular difference withinsafe limits, ie it ensures that the angulardifference does not increase somuch that it could jeopardize theangular stability.Improved voltage stability of thecorridor.Optimized power sharing betweenparallel circuits. Without series capacitors,the line with the least powertransmission capacity would saturatefirst and no additional power couldbe fed into the system, despite thefact that the other line still has capacityto spare. The series capacitorsredistribute power between the linesfor better overall utilization of thesystem.The series capacitors are fully integratedin the power system and benefit fromits control, protection and supervisorycapability. They are fully insulated toground.Eagle Pass Back-to-Back (BtB) LightSVC Light technology 1) has successfullysolved power quality problems in severalprojects undertaken by ABB. Basedon a common platform of voltagesource converters (VSC), SVC Light alsoprovides solutions for power conditioningapplications in transmission systems.The Eagle Pass tie is a good exampleof a project in which the VSC platformis configured as back-to-back HVDC,although functionally with priority givento voltage support with the dual SVCLight systems.Most important in this respect is the factthat installation of active power transfercapability, using HVDC Light across acertain distance or in a back-to-backconfiguration, will provide both bidirectionalactive power and dynamic reactivepower support simultaneously.Thus, strong voltage support is readilyavailable along with the steady-statepower transfer.The Eagle Pass substation (operated byAmerican Electric Power, AEP) is locatedin a remote part of Texas, on theMexican border, and is connected to theTexas transmission system through two138-kV transmission lines. The nearestsignificant generating station is located1) SVC Light is a product name for an IGBTbasedstatic synchronous compensator fromABB.14Special ReportABB Review