Installation, Operation and Interlocks For GIS

Gas Insulated Substations 420kV

Installation:

The GIS is usually installed on a monolithic concrete pad or the floor of a building. It is most often rigidly attached by bolting and/or welding the GIS support frames to embedded steel plates or beams. Chemical drill anchors can also be used.

Expansion drill anchors are not recommended because dynamic loads may loosen expansion anchors when the circuit breaker operates.

Large GIS installations may need bus expansion joints between various sections of the GIS to adjust to the fit-up in the field and, in some cases, provide for thermal expansion of the GIS. The GIS modules are shipped in the largest practical assemblies.

At the lower voltage level, two or more circuit breaker positions can be delivered fully assembled. The physical assembly of the GIS modules to each other using the bolted flanged enclosure joints and sliding conductor contacts goes very quickly.

More time is used for evacuation of air from gas compartments that have been opened, filling with SF6 gas, and control system wiring. The field tests are then done. For a high voltage GIS shipped as many separate modules, installation and testing takes about two weeks per circuit breaker position.

Lower voltage systems shipped as complete bays, and mostly factory-wired, can be installed more quickly.

Operation and Interlocks:

Operation of a GIS in terms of providing monitoring, control, and protection of the power system as a whole is the same as for an AIS except that internal faults are not self-clearing so reclosing should not be used for faults internal to the GIS.

Special care should be taken for disconnect and ground switch operation because if these are opened with load current flowing, or closed into the load or fault current, the arcing between the switch moving and stationary contacts will usually cause a phase-to-phase fault in three-phase enclosure GIS or to a phase-to-ground fault in single-phase enclosure GIS.

The internal fault will cause severe damage to the GIS. A GIS switch cannot be as easily or quickly replaced as an AIS switch. There will also be a pressure rise in the GIS gas compartment as the arc heats the gas.

In extreme cases, the internal arc will cause a rupture disk to operate or may even cause a burn-through of the enclosure. The resulting release of hot, decomposed SF6 gas may cause serious injury to nearby personnel.

For both the sake of the GIS and the safety of personnel, secure interlocks are provided so that the circuit breaker must be open before an associated disconnect switch can be opened or closed, and the disconnect switch must be open before the associated ground switch can be closed or opened.

Source: ‘Electric Power Substations Engineering’