Transformer Protection

Transformers are normally protected against short circuits and overheating. Transformer protections justifiable in different installations will vary widely depending upon the size of the equipment, importance as a unit part of the Distribution system, and its voltage rating. Small distribution Transformers maybe protected by fuses while larger ones in feeder circuit or at injection substations may be equipped with various types of protective equipment varying from long-time overcurrent to high speed differential protection, standby earth fault relay, bucholz gas relay, etc.

Relay Coordination

This exercise aids selectivity of operation in an interconnected system. The discrimination using overcurrent protection is achieved in these ways:

• Time Graded System : Based on the time of operation of relays

• Current Graded System: Adopted where the fault current varies appreciably with location.

• Time Current Graded System: This type of grading is achieved using inverse time overcurrent relay and the most widely used is the IDMT relay.

Lightening Protection

Lightening strikes can cause damage to poles, failure to Transformer, puncturing insulators or underground cables and blowing of fuses. Damage may also occur to meters or appliances in the premises of the Transformer.

In order to protect the transformer and other elements of the network, the schemes usually employed include:

• Installation of Lightening Arresters
• Grounding of all the circuits
• Aerial earthing of substation