Circuit Breakers Both AIS and GIS use basically the same type of dead tank SF6 puffer circuit breaker. However, with GIS systems, Instead of SF6-to-air bushings mounted on the circuit breaker enclosure, the GIS circuit breaker is directly connected to the adjacent GIS module.
Current Transformers for GIS ApplicationsCurrent transformers are devices which are installed insubstations for the purpose of transforming the high-current from the conductor into standardized, equivalent values, which are then used for purposes of metering, measuring, and protection. Installation Location - Current transformers installed in aGIS system are of the inductive-ring type, and can be installed either inside or outside the GIS enclosure. Metering and protection - are performed on the secondary winding side of the transformer. The GIS conductor is the single-turn primary for the current transformer unit.
Considerations for Current TransformersFor CTs installed on the inside of the enclosure - they must be shielded from the electric field produced by the high-voltage conductor, otherwise high transient voltages can appear on these secondary winding through capacitive coupling. For CTs mounted on the outside of the enclosure - the enclosure itself must have an insulating joint, and enclosure currents shuntedaround the CT. Both of these types of GIS construction are widely used.
Digital Output Signals from CTs - Analog RelaysAdvanced types of CTs have been developed to save space and reduce the cost of GIS, by removing the magnetic core or Rowgowski coil. The output signals of these types of CT units are at a low level, allowing for the immediate conversion by an enclosure-mounted device, into a digital signal. Once the signal is digital, It can be transmitted over long distances via wire or fiber optic cabling, to the control or protective relays. One issue though is, that many protective relays being used by utilities do not presently accept a digital input even though the relay may be converting the conventional analog signal to digital, prior to processing.
Voltage Transformers Voltage transformers (VTs) used in GIS applications are the inductive type with an iron core. The VT is usually a sealed unit with a gas barrier insulator. Installation Option 1 - the VT is easily removable so the GIS can be high-voltage tested without damaging the unit.Installation Option 2 - the VT is provided with a disconnect switch or removable conductor link. Primary winding - is supported on an insulating plastic film immersed in SF6.To prevent capacitive coupling of transient voltages - the VT should possess an electric field shield between the primary and secondary windings.