Page 144 - Practical Power System and Protective Relays Commissioning

Chapter 14

Gas Insulated System

Substations

14.1 INTRODUCTION
The gas insulated system (GIS) has been used since 1960, and consists of
tubes that contain SF 6 under a pressure of about 6 bars. This gas, under pres-
sure, provides very good insulation for high voltage and extra-high voltage
systems. Fig. 14.1 shows the GIS system components.
This system can be either a single three-phase enclosure, for example in
a high voltage (66 kV) system or a separate single phase enclosure, for
example in extra-high voltage (400 kV and 500 kV) systems, seen in
Fig. 14.2. The latter system is more expensive but safer and better for
maintenance.
A GIS consists of the following components in each bay or diameter.
Each will be discussed below:

Circuit breakers (CB).
Current transformers (CT) and voltage transformers (VT).
Disconnecting switch.
Earthing switch and high-speed earthing switch.
Busbars: feeder (bay) or diameter.
Cable sealing end or GIS bushing for transmission lines.

14.2 DISCONNECTING AND EARTHING SWITCHES
Both the disconnector switch and earthing switch are driven by a DC motor.
However, the earthing switch can be operated manually in the event of an
emergency.
The high-speed earthing switch is used to earth the system to discharge
the line from capacitive charge.
In most installations the disconnections and the earthing switch are in
one unit and have mechanical interlocking with each other, as seen in
Fig. 14.3.

Practical Power System and Protective Relays Commissioning.
DOI: https://doi.org/10.1016/B978-0-12-816858-5.00014-9 143
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