Circuit Breaker, Current And Voltage Transformers, Disconnect And Ground Switches For GIS

Circuit Breaker:

GIS uses essentially the same dead tank SF₆ puffer circuit breakers used in AIS. Instead of SF₆ -to-air as connections into the substation as a whole, the nozzles on the circuit breaker enclosure are directly connected to the adjacent GIS module.

Current Transformers:

CTs are inductive ring types installed either inside the GIS enclosure or outside the GIS enclosure. The GIS conductor is the single turn primary for the CT. CTs inside the enclosure must be shielded from the electric field produced by the high voltage conductor or high transient voltages can appear on the secondary through capacitive coupling. For CTs outside the enclosure, the enclosure itself must be provided with an insulating joint, and enclosure currents shunted around the CT. Both types of construction are in wide use.

Voltage Transformers:

VTs are inductive types with an iron core. The primary winding is supported on an insulating plastic film immersed in SF₆. The VT should have an electric field shield between the primary and secondary windings to prevent capacitive coupling of transient voltages. The VT is usually a sealed unit with a gas barrier insulator. The VT is either easily removable so the GIS can be high voltage tested without damaging the VT, or the VT is provided with a disconnect switch or removable link.

Disconnect Switches:

Disconnect switches have a moving contact that opens or closes a gap between stationary contacts when activated by an insulating operating rod that is itself moved by a sealed shaft coming through the enclosure wall. The stationary contacts have shields that provide the appropriate electric field distribution to avoid too high a surface stress. The moving contact velocity is relatively low (compared to a circuit breaker moving contact) and the disconnect switch can interrupt only low levels of capacitive current (for example, disconnecting a section of GIS bus) or small inductive currents (for example, transformer magnetizing current). Load break disconnect switches have been furnished in the past, but with improvements and cost reductions of circuit breakers, it is not practical to continue to furnish load break disconnect switches, and a circuit breaker should be used instead.

Related: GIS – Gas Insulated Substations

Ground Switches:

Ground switches have a moving contact that opens or closes a gap between the high voltage conductor and the enclosure. Sliding contacts with appropriate electric field shields are provided at the enclosure and the conductor. A “maintenance” ground switch is operated either manually or by the motor drive to close or open in several seconds and when fully closed to carry the rated short-circuit current for the specified time period (1 or 3 sec) without damage. A “fast-acting” ground switch has a high-speed drive, usually a spring, and contact materials that withstand arcing so it can be closed twice onto an energized conductor without significant damage to itself or adjacent parts. Fast-acting ground switches are frequently used at the connection point of the GIS to the rest of the electric power network, not only in case the connected line is energized, but also because the fast-acting ground switch is better able to handle discharge of trapped charge and breaking of capacitive or inductive coupled currents on the connected line.

Related: Construction and Service Life of GIS Substation

Ground switches are almost always provided with an insulating mount or an insulating bushing for the ground connection. In normal operation, the insulating element is bypassed with a bolted shunt to the GIS enclosure. During installation or maintenance, with the ground switch closed, the shunt can be removed and the ground switch used as a connection from test equipment to the GIS conductor. Voltage and current testing of the internal parts of the GIS can then be done without removing SF₆gas or opening the enclosure. A typical test is the measurement of contact resistance using two ground switches.

Source: ‘Electric Power Substations Engineering’ By Philip Bolin (Mitsubishi Electric Power)