Switch Gear

 Switch Gear:

Switch apparatus is required for controlling and protecting the equipments during normal and abnormal conditions. Switch gear equipments Used in Substations generally Comprises of following, namely

1.   Insolators

2.   Air breakers

3.   Circuit breakers

4.   Fuses etc.

Insolators: In substations it is desired to disconnect a part of system for general maintenance and repair. This is accomplished by using an isolator.

An isolator is essentially a knife switch and is designed to open a circuit under NO-LOAD.

Circuit Brakers: A circuit braker is a protective device which make (or) break a circuit either manually (or) automatically under no-load, full-load and short-circuit conditions.

It performs the following functions

1.  To carry full load current continuously

2.   To open and close the circuit on no-load.

3.   To make and break the circuit for normal operations.

4.  To make and break the short-circuit currents of magnitude upto which it is designed for.

Fuses: A fuse is a short piece of metal, inserted in the circuit which melts when excessive current flows through it and thus breaks the circuit.

 In general, fuses may be classified in to two types, namely

1.  Low voltage Fuse                                      2. High voltage Fuse

Low voltage Fuse:

1.   Rewirable fuse      -        upto 500 Amps, 400volts

2.   HRC fuses             -           1600 Amp, 440 volts

High voltage fuse:

1.   Cartridge fuse      - 8700 Amps, 33kv.  200 Amp, 11kv.

2.   Metal clad fuses - 10000 Amps, 66kv.

3.   Liquid type fuses - 5000 Amps, 66kv.

Indicating and metering equipment

Ammeters. Voltmeter, power factor meters, wattmeters, kWh meter, kvArh meters are installed in the substations to control and maintain a watch over the currents flowing in the circuits and over the power loads.

Protective Relays

A protective relay is a device which detects the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system.

In substations the following protective relays are mainly used, namely

1.  Over Current Relays: provides protection mainly against phase-to-phase faults and overloading.

2.  Earth Fault Relays: provides protection against earth fault only

3. Buchholz relay: provides protection against all kinds of incipient faults i.e., insulation failure of windings, core heating, fall of oil level due to leaky joints etc.

4. Differential Relay: provides protection against earth and phase