ABB Review Special Report - ABB - ABB Group

145 km away and provides very littlevoltage support to the Eagle Pass area.Eagle Pass also has a 138-kV transmissionline that ties into Piedras Negrassubstation (operated by CommissionFederal Electricas, CFE) on the Mexicanside. This is used mainly in emergenciesto transfer load between power systems,but such transfers involve interruptingthe power as the CFE and AEP systemsare asynchronous (despite both being60 Hz). To overcome this disadvantage,and also solve problems arising fromincreasing demand, a better solutionwas sought.ABB’s response:voltage source convertersLoad flow studies demonstrated that theinstallation of a 36-MVAr voltage sourceconverter directly at the Eagle Pass substationwould provide years of respite.Installation of a VSC is ideal for weaksystems as the alternative, reactivesupport provided by shunt capacitors,decreases rapidly when the voltage isreduced. Extending the scenario, twoVSCs connected back-to-back wouldnot only supply the necessary reactivepower but also allow active powertransfer between the two power systems.A BtB scheme would enable the138-kV line between Eagle Pass andPiedras Negras to be continuously energizedand allow the instantaneous transferof active power from either system.2EaglePassAs commutation is driven by its internalcircuits, a VSC does not rely on the connectedAC system for its operation. Fullcontrol flexibility is achieved by usingpulse width modulation (PWM) to controlthe IGBT-based bridges. Furthermore,PWM provides unrestricted controlof both positive- and negative-sequencevoltages. Such control ensures reliableoperation of the BtB tie even when theconnected AC systems are unbalanced.In addition, the tie can energize, supplyand support an isolated load. In the caseof Eagle Pass, this allowed the uninterruptedsupply of power to local loadseven if connections to one of the surroundingnetworks were tripped. Bothsides of the tie can also be energizedfrom ‘across the border’, without anyswitching that could involve interruptionsof supply to consumers.The back-to-back installationA simplified one-line diagram of the BtBtie in Eagle Pass is shown in 2 .3VSCSingle-line diagram of back-tobacktie at Eagle PassVSCPiedrasNegrasThe BtB scheme consists of two36-MVA VSCs coupled to a common DCcapacitor bus. The VSCs are of the NPC(neutral point clamped) type, alsoknown as three-level converters. EachVSC is connected to a three-phase setof phase reactors, each connected toa conventional step-up transformeron its respective side of the BtB. Thelayout of the BtB installation is shownin 3 .BtB operating modesThe two VSCs of the BtB can be configuredfor a wide range of differentfunctions. At Eagle Pass, the main BtBoperating configurations are as follows:Voltage controlActive power controlIndependent operation of the twoVSCsContingency operation of the BtBVoltage controlIn this mode, both the AEP and CFEsystems are capable of independentvoltage control. The BtB provides therequired reactive power support onboth sides to maintain a pre-set voltage.Active power can be transferred fromEagle Pass back-to-back tieForeground: 138-kV equipment and harmonic filters. Middle: buildings with converters,controls and auxiliaries. Back: cooling towers for water-cooled IGBT convertersHaving the capability to control dynamicallyand simultaneously both activeand reactive power is unprecedentedfor BtB interconnections. This feature isan inherent characteristic of the VSC.Special ReportABB Review15