CHARLES ISIADINSO

GENERATOR CIRCUIT BREAKER

(GCB)

A circuit breaker is an electronic switch designed to isolate a circuit

in the case of overload or short circuit. The GCB typically installed

between the generator and the main generator step-up (GSU)

transformer, electricity leaves the generator and enters the

generator circuit breaker via isolated-phase bus (device used to

conduct electricity because of the magnitude of the power being

transmitted). Its purpose is to create a breakpoint in the system and

is typically used to protect the overall system in the event of

overload or any other electrical fault.

The GCB works by automatically interrupting current flow at the

onset of a fault. GCBs have two contact points, a physical

connection (made up of two fixed and mobile units) and a gaseous

connection, which is usually sulfur hexafluoride (SF6) gas because

it is twice as resistant to the flow of electricity as air. Power stations

usually transfer electricity at very high voltages (between 72000V

and 1200000V), at these voltages if contacts are simply separated;

an electrical arc forms bridging the gap created and continuing the

flow of electricity. This is counterproductive as the main point of

breaking the connection is to stop flow of electricity. To solve this

problem, the temperature conditions and distance between the

contacts has to be controlled. To achieve effective control of the

arc, the separation takes place in an arc chamber filled with SF6

gas. The physical connection is designed with two parts, a

permanent contact (located close to the chamber walls) and arc

contact (located at the center of the chamber and specifically

designed to withstand electrical arcs); the gaps between the

contacts are also filled with SF6 gas.

To carry out a break, the permanent contacts are separated leaving

only the arc contacts. This concentrates the flow of electricity in

the arc contacts. The arc contacts are then separated and a

controlled electrical arc is formed between the arc contacts. The

CHARLES ISIADINSO

electrical arc causes rapid rise in the temperature and pressure of

the SF6 gas. The SF6 molecules explode and lose their insulating

qualities. The rise in pressure causes high-pressure gas from the

center of the chamber to move to the low-pressure part of the

chamber (usually the back), the gas moves taking the arc with it,

and this extinguishes the arc and completes the circuit break. The

SF6 quickly cools, recombines, and regains it insulating capacity.

The entire break is completed in 50 milliseconds.

REFERENCE:

1. "Generator Circuit-breakers (GCB)." Generator Circuit

Breakers. ABB., n.d. Web. [Accessed17 June 2014].

2. "Circuit Breaker." Wikipedia. Wikimedia Foundation, 06 July

2014. Web. [Accessed 17 June 2014].