October 11, 2004

Trans-Rupter II^® Fills a “Tall Order” for Improved Transformer Protection.

Background

A large utility in the southwest U.S. contracted with S&C’s Power Systems Services Division to study ways to improve their system reliability. One concern was a substation with four 138/13.8-kV transformers.

The existing transformer protection scheme used the line-terminal circuit breakers and bus breakers to trip the entire high-voltage bus after a fault. The motor-operated air-break disconnect switch on the primary side of each transformer would then open, isolating its associated transformer. Only after the system operator located the faulted transformer could the disconnect switches on the unfaulted transformers be closed, followed by the line-terminal and bus breakers.

No room for Circuit-Switchers

By installing individual transformer protective devices, a secondary fault or a faulted transformer could be quickly isolated without affecting the rest of the transmission line or neighboring transformers, dramatically decreasing outage time and increasing power reliability by reducing the number of customers out of service.

The utility had previous experience with S&C’s Series 2000 Circuit-Switchers and was interested in using them in this application. But space constraints made this impractical. The disconnect switches, sensors, buswork, and dead-ending insulators were installed on an elaborate overhead structure. There was no room between the transformers and the structure for Circuit-Switchers with vertical interrupters. And there was no clearance underneath the structure for Circuit-Switchers with horizontal interrupters.

What did they do?

S&C proposed using Trans-Rupter II® Transformer Protectors, mounted 30 feet up on the existing structure. Trans-Rupter II has no mechanical interphase linkage, allowing for more flexibility in its installation. The pole-units are electrically linked, thus allowing mounting at almost any height. The lightweight pole-units could be easily accommodated by the existing structure.

Since the 30-foot mounting height would make use of the manual charging tool impractical, op-tional motor operators were specified, which allow the pole-units to be closed and charged from ground level. The existing disconnect switches were retained to provide visible air-gap isolation when the Trans-Rupter II pole-units are open, and also pick up transformer magnetizing inrush current after the pole-units have been closed and charged. A specially modified S&C LS-1 Switch Operator was furnished for each disconnect switch, wired to permit the Trans-Rupter II Transformer Protector with motor operators and disconnect switch to be controlled from the same location with sequenced operation.

Installation was quick and easy

Channels were installed on the overhead structure to support the Trans-Rupter II pole-units and the insulators for the conductors connecting the transformers to the pole-units. The installation crew used slings attached to the overhead structure to hoist the pole-units into place . . . no crane was required. The existing vertical operating pipe for the disconnect switches was retained, making installation of the LS-1 Switch Operators simple. All four Trans-Rupter IIs were installed in the allotted two-day outage . . . with time to spare! Click here to view a simplified one-line diagram of the substation.

Results

The utility is more than satisfied with their Trans-Rupter IIs. The upgrade project cost was well within the utility’s original budget, and the work was completed ahead of schedule.