Introduction Protection of transformers Protection of generators Protection of transmission and distribution

lines Bus protection

A part of the system protected by a certain protective scheme is called as protective zone or zone of protection.

The entire part of a power system is covered by several protective zones and no part of the system is left unprotected.

The protective zones are shown in figure.Overlapping of zones is done to ensure safety of each

element in the system.

Primary Protection –This is the first protection to act. This is responsible to protect power system element from all types of faults .If primary protection fails, then back up protection comes into action and removes the faulty part from healthy system.

Backup Protection- This protection comes into action when primary protection fails.

If due to some reason (such as fault in CT,PT,trip circuit etc.) the primary protection fails to operate, there should be additional protection to take care of fault otherwise the fault will not be cleared and may damage the equipments .

The maintenance and testing work is carried on primary protection system. Then primary protection is not operative due to maintenance work. Then back up protection acts as main protection.

If fault occurs beyond C region ,Circuit breaker of C region operates as primary protection.

In the meanwhile relays at B and A may also start operating.They ar given time lag.If breaker at C does not operate then after 0.6 seconds breaker at B

will operate and act as backup protection.Breaker at A will operate if breaker at B fails to operate.In this way back up protection is provided.

When earth current flows through earth return path, the fault is called earth fault. The earth faults are frequent. So earth fault protection is important.

Residual Earth Fault Relay: When earth fault does not take place, the sum of three

secondary currents of CT is zero i.e. IR +IY+IB =0 The sum of all these currents is called residual current.

When earth fault develops, the sum of three currents is not zero, the residual current flows through earth fault relay.

This protection is also called unrestricted earth fault protection.

It consists of a domed vessel. There are two mercury switches. The upper mercury switch is attached to a float. The lower mercury switch is attached to a flap and the mechanism is hinged as shown in figure.

Alarm contacts are brought to upper mercury switch and trip contacts are brought to lower mercury switch.

The upper mercury switch operates during incipient(less sever faults like overloads) faults and the lower mercury switch operates in case of severe faults.

When the fault current flows through earth return path, the fault is called earth fault. The earth faults are frequent. So earth fault protection is important.

Residual earth fault relay-When earth fault does not take place, the sum of the three

currents of CT is zero.IR+IY+IB=0The sum of these currents is called residual current. When

earth fault develops, the sum of three currents is not zero, the residual current flows through earth fault relay.

This protection is also called unrestricted earth fault protection. In other scheme, the relay is connected in the neutral wire of the transformer.

RESIDUAL EARTH FAULT RELAY

RELAY IN NEUTRAL WIRE

This scheme is used for multi turn alternators.

For multi turn alternators, the winding of each phase is divided into two equal sections (S1 and S2) as shown in figure.

Two identical CTs are connected in respective windings. The CT secondary are connected in phase opposition and a relay R is connected in CT secondary loop.

Distance protection scheme uses distance relay. The relay operation is based on the impedance (or distance)between the relay and point of fault. This scheme of protection does not require any pilot wire.

The voltage element of impedance receives supply from PT secondary and current element receives supply from CT secondary.

Bus bar Protection by Over Current Relays