Case Study 731-1008 - S&C Electric Company

TRANS-RUPTER II®TRANSFORMER PROTECTORTrans-Rupter II Fills a “Tall Order” for ImprovedTransformer ProtectionBackgroundA large utility in the southwest U.S. contracted with S&C’sPower Systems Services Division to study ways to improvetheir system reliability. One concern was a substation withfour 138/13.8-kV transformers.The existing transformer protection scheme used the lineterminalcircuit breakers and bus breakers to trip the entirehigh-voltage bus after a fault. The motor-operated air-breakdisconnect switch on the primary side of each transformerwould then open, isolating its associated transformer. Onlyafter the system operator located the faulted transformercould the disconnect switches on the unfaulted transformersbe reclosed, followed by the line-terminal and busbreakers.No room for Circuit-SwitchersBy installing individual transformer protective devices, asecondary fault or a faulted transformer could be quicklyisolated without affecting the rest of the transmissionline or neighboring transformers, dramatically decreasingoutage time and increasing power reliability by reducing thenumber of customers out of service.Two of the four new 138-kV Trans-Rupter II Transformer Protectors are visible, installed on the structure above the transformers they protect.The disconnect switches—now motor-operated by LS-1 Switch Operators—are installed above the Trans-Rupter II pole-units.S&C ELECTRIC COMPANYSpecialists in Electric Power Switching and ProtectionCase Study 731-1008October 25, 2004New Publication ©2004

The utility had previous experience withS&C’s Series 2000 Circuit-Switchers and wasinterested in using them in this application.But space constraints made this impractical.The disconnect switches, sensors, buswork,and dead-ending insulators were installed onan elaborate overhead structure. There was noroom between the transformers and the structurefor Circuit-Switchers with vertical interrupters.And there was no clearance underneath thestructure for Circuit-Switchers with horizontalinterrupters.What did they do?S&C proposed using Trans-Rupter IITransformer Protectors, mounted 30 feet up onthe existing structure! Trans-Rupter II offersextraordinary application flexibility. There’sno mechanical linkage. The pole-units areelectrically linked . . . thus allowing mountingat almost any height. The lightweight pole-unitscould be easily accommodated by the existingstructure.Since the 30-foot mounting height would makeuse of the manual charging tool impractical,optional motor operators were specified, whichallow the pole-units to be closed and chargedfrom ground level. The existing disconnectswitches were retained to provide visible air-gapisolation when the Trans-Rupter II pole-units areopen, and also pick up transformer magnetizinginrush current after the pole-units have been closed andcharged. A specially modified S&C LS-1 Switch Operatorwas furnished for each disconnect switch, wired to providesequenced operation of the Trans-Rupter II TransformerProtector motor operators and the disconnect switch.=138-kVTransmissionLine138-kVTransmissionLineTrans-Rupter IIModel EX withMotor OperatorsCircuit BreakerCircuit Breaker87Circuit BreakerSingle-line diagram of the four-transformer substation.V34.5-kV distribution line to mixedresidential/commercial load87N.O.(3)8787To low-side circuit breakersand 34.5-kV distribution linesCircuit Breakerstructure to hoist the pole-units into place . . . no cranewas required. The existing vertical operating pipe for thedisconnect switches was retained, making installation ofthe LS-1 Switch Operators simple. All four Trans-RupterIIs were installed in the allotted two-day outage . . . withtime to spare!Circuit BreakerV138-kVTransmission LineInstallation was quick and easyChannels were installed on the overhead structure to supportthe Trans-Rupter II pole-units and the insulators for theconductors connecting the transformers to the pole-units.The installation crew used slings attached to the overheadResultsThe utility is more than satisfied with their Trans-RupterIIs. The upgrade project cost was well within the utility’soriginal budget, and the work was completed ahead ofschedule.