What is Transformer Protection? Theory, Protection Relays & Types

The transformer is heart of power system. Power transformer is a major equipment in power system. It requires highly reliable protective devices. The protective scheme depends on size of transformer. The rating of transformers used in transmission and distribution systems range from few kVA to several hundred MVA. For small transformers, simple fuse is sufficient for protection. For medium capacity transformers over current relays are used. For large transformers differential protection scheme is used.

Abnormalities and Faults of Transformer

The faults occurring in power transformer are earth faults, phase to phase faults, inter turn faults, over heating because of overloading, core heating. The most common type of faults are; earth faults, inter turn faults.

Through fault: It is a fault that lies outside the protective zone of the transformer hut fed through the transformer.

Table 1 shows abnormal conditions; the protection necessary for power transformer.

Table 1

Abnormal Condition/Fault	Protection (Relay) Employed
Over load	(i) Thermal overload relays. (ii) Temperature relays.
Earth faults	(i) Differential protection. (ii) Earth fault relay
Through faults	(i) Graded time lag over current relay. (ii) HRC fuses.
Incipient faults below oil level resulting into decomposition of oil.	(i) Buchholz relay i.e. gas actuated relay. (ii) Pressure relief valve.
Large internal faults : Phase to phase, phase to ground, below oil level.	Buchholz relay and circuit breaker.
Faults in tap changer.	(i) Percentage differential protection. (ii) High speed over current relay.
High voltage surges due to lightning and switching.	(i) Horn gaps. (ii) Lightning arrestor

For other transformers the protections provided are as per Table 2.

Table 2

Abnormal Condition/Fault	Protection (Relay) Employed
Small distribution transformer.	HV fuses for earth fault protection and phase fault protection.
Transformers in important locations ratings 500 kVA above	(i) Restricted earth fault protection. (ii) Over current protection. (iii) Buchholz relay

Information Needed for Protective Scheme Selection for Transformer

While selecting protection scheme for transformer the following in format ion is necessary.

1. kVA rating.
2. Whether it is indoor or outdoor.
3. Voltage ratio.
4. Connections of winding.
5. Percentage reactance.
6. Neutral point earthing, value of resistance.
7. Conservator is provided or not.
8. Value of system earthing resistance.
9. Details of connection of CTs and relays.
10. Fault level at transformer terminals.
11. Network diagram showing position of transformer.