Explain circuit breaker Tripping circuit in electrical power station

When any section of power system or equipment needs to be protected, a primary of current transformer is connected in series with that.

While secondary of current transformer (CT) is connected to current coil of the relay.  If the relay is two quantity relay, secondary of potential transformer (PT) is also  connected to potential coil of the relay.

When fault occurs, a current flowing through that section of power system or equipment increases to a very high value.  Fault current also flows through the CT primary and accordingly increases the CT secondary current that further increases relay coil current.

Thus relay contacts get closed under the influence of such high fault current.  Consequently trip circuit of circuit breaker get closed and current starts flowing from battery through trip coil.  Once trip coil of circuit breaker is energized, it activates the circuit breaker opening  mechanism and open the breaker contacts.

This is how faulty section of power system or equipment gets isolated from healthy one.  Another important device in trip circuit is an auxiliary switch (52-a) which is mechanically coupled with operating mechanism of circuit breaker.

It is ON when the circuit breaker is ON and OFF when the circuit breaker is OFF.  As auxiliary switch is in trip circuit hence when it opens, it breaks the current in trip circuit. Once the current in trip circuit interrupts, the relay contacts comes to normal position.

The purpose of auxiliary switch is that breaking of trip circuit takes place at auxiliary switch contacts and hence possible arcing due to current interruption across relay contacts is eliminated.

Also a circuit breaker tripping takes a time ranging from 1 to 5 cycles. A trip coil is not designed for energizing it continuously once the breaker trips.

It is possible that auxiliary relay contact gets locked due to some internal mechanism failure, hence a continuous current flows through the trip coil if auxiliary switch (52-a) is not provided in the trip circuit.

Many other functions such as annunciations, alarms, interlocks, are simultaneously performed through multi contact auxiliary switch when the relay operates.  Auxiliary switch is generally placed in control cabinet of circuit breaker.