High-Voltage Protective Equipment

Gang-Operated, Air-Break Disconnect Switch Picture The first device encountered in a substation is typically a disconnect switch.

The most commonly used switch in small to medium substations is a gang-operated, air-break disconnect switch. "Gang-operated" because the three separate switches for each phase are operated as a group from a single control; "air-break" because the switch operates in air rather than in another medium, such as oil.

The purpose of this switch is to disconnect the substation from the incoming line, not to disconnect the transformer from the load. It is like a large safety switch with no load breaking capability. It can only break, or "interrupt" the relatively small "magnetizing current" of the substation transformer. (This is the small amount of current needed to set up the magnetic field in the transformer core.) A substation must first be disconnected from its secondary or load side before the primary or high voltage side can be disconnected using the disconnect switch. High Voltage Power Fuses Picture

The next device encountered in a substation are the high voltage power fuses. Depending on the line voltage, they may be up to six feet long. These fuses stop the flow of current in the event of an internal fault or short-circuit in the transformer. Overloads due to faults or short circuits on the distribution side of the substation are prevented by low voltage protective equipment.

Transmission level circuit breakers or circuit switchers are some of the last devices found in a substation. They are utilized when there is a need to remotely switch the incoming or outgoing transmission circuits in a substation. They also may be used in place of high voltage power fuses.

Circuit Switches Picture  Oil Insulated Circuit Breaker Picture

High voltage circuit breakers historically have been designed with a set of contacts immersed in a tank filled with insulating oil. When activated the contacts open or close thereby allowing current to flow or stop flowing. The insulating oil serves to quench the arc generated by the opening of the contacts and keep electricity from flowing. Circuit switchers and many of today's circuit breakers use an insulating gas (commonly SF-6 gas) in place of oil. The advantage here is the elimination of a tank and risk of oil spillage along with improved arc quenching.

Insulating Gas Circuit Breaker Picture  Insulating Gas Circuit Switches Picture