Zinc oxide surge arrester elements suitable for immersion in SF6 are supported by an insulating cylinder inside a GIS enclosure section to make a surge arrester for overvoltage. Because the GIS conductors are inside in a grounded metal enclosure, the only way for lightning impulse voltages to enter is through the connections of the GIS to the rest of the electrical system.
Cable and direct transformer connections are not subject to lightning strikes, so only at SF6-to-air bushing connections is lightning a concern. Air-insulated surge arresters in parallel with the SF6-to-air bushings usually provide adequate protection of the GIS from lightning impulse voltages at a much lower cost than SF6 insulated arresters.
Switching surges are seldom a concern in GIS because with SF6 insulation the withstand voltages for switching surges are not much less than the lightning impulse voltage withstand. In AIS there is a significant decrease in withstand voltage for switching surges than for lightning impulse because the longer time span of the switching surge allows time for the discharge to completely bridge the long insulation distances in air.
In the GIS, the short insulation distances can be bridged in the short time span of a lightning impulse so the longer time span of a switching surge does not significantly decrease the breakdown voltage. Insulation coordination studies usually show there is no need for surge arresters in a GIS; however, many users specify surge arresters at transformers and cable connections as the most conservative approach.
For ease of operation and convenience in wiring the GIS back to the substation control room, a local control cabinet (LCC) is provided for each circuit breaker position. The control and power wires for all the operating mechanisms, auxiliary switches, alarms, heaters, CTs, and VTs are brought from the GIS equipment modules to the LCC using shielded multi-conductor control cables.
In addition to providing terminals for all the GIS wiring, the LCC has a mimic diagram of the part of the GIS being controlled. Associated with the mimic diagram are control switches and position indicators for the circuit breaker and switches. Annunciation of alarms is also usually provided in the LCC. Electrical interlocking and some other control functions can be conveniently implemented in the LCC.
Although the LCC is an extra expense, with no equivalent in the typical AIS, it is so well established and popular that attemptsto eliminate it to reduce cost have not succeeded. The LCC does have the advantage of providing a very clear division of responsibility between the GIS manufacturer and user in terms of scope of equipment supply.
Switching and circuit breaker operation in a GIS produces internal surge voltages with a very fast rise time on the order of nanoseconds and a peak voltage level of about 2 per unit. These “very fast transient overvoltage’s” are not a problem inside the GIS because the duration of this type of surge voltage is very short — much shorter than the lightning impulse voltage.
However, a portion of the VFTO will emerge from the inside of the GIS at any place where there is a discontinuity of the metal enclosure — for example, at insulating enclosure joints for external CTs or at the SF6-to-air bushings.
The resulting “transient ground rise voltage” on the outside of the enclosure may cause some small sparks across the insulating enclosure joint or to adjacent grounded parts. These may alarm nearby personnel but are not harmful to a person because the energy content is very low.
However, if these VFT voltages enter the control wires, they could cause faulty operation of control devices. Solid-state controls can be particularly affected. The solution is thorough shielding and grounding of the control wires.
For this reason, in a GIS, the control cable shield should be grounded at both the equipment and the LCC ends using either coaxial ground bushings or short connections to the cabinet walls at the location where the control cable first enters the cabinet.
Source: ‘Electric Power Substations Engineering’