Protection of Transformer• A: Electrical Protection:

Differential Protection HT & LT Over Current Protection Restricted Earth Fault Protection Standby Earth Fault Protection

• B: Mechanical Protection: Buchholz Protection Pressure Relief Device Explosion Vent Oil Surge Relay

• C: Thermal Protection: OT/WT Protection Overload Trimming Protection

• D: Lightning Protection: Lightning Arrestors Spark gaps

Transformer Differential Protection Merz-Prize 95% Differential

Protection HT/LT CTs connected

differentially CTs on Star side connected in delta

and vice-versa CT mismatch compensated by

slope setting In-rush currents taken care by

Harmonic Restraint Unit Stable for through faults High set unit takes care heavy un-

cleared through faults High Speed, reliable, dedicated but

expensive protection Not very sensitive for earth faults

closer to neutral end

Transformer Over Current Protection Simple Over Current Relays on

HT & LT. HT Over current isolates

transformer. LT Over current relives load

and the transformer remains energized from HT

Can mal-operate during in-rush and for through faults if not set properly.

Unstable for through faults. Backup protection for un-

cleared feeder faults. Moderate Speed, dependable

inexpensive protection

Transformer-Restricted & Standby Earth Fault Protection

• Restricted Earth fault Protection: Over Current relay connected

across the summation of 3 and Neutral CT

Very sensitive, dedicated, reliable, expensive protection for Star windings.

• Stand by Earth Fault Protection: Over Current relay connected on

the neutral CT Back up for un-cleared feeder

earth faults– In-expensive and simple protection

Transformer-Mechanical Protection• 1. Buchholz Protection:

Located on the oil pipe connecting the main tank with conservator above the tank

Gas evolution principle Alarm and trip at different levels

of gas evolution Very sensitive, inexpensive,

reliable for incipient evolving faults

• 2. Pressure Relief Device: Relives the hydraulic pressure

developed inside due to faults Slow protection

• 3. Oil Surge Relay: Operates due to Oil surges caused

by internal faults

Transformer- Thermal Protection- Operates due to thermal stresses

developed due to overload.- Can detect inadequate cooling.- The temperatures of the Oil

/winding simulated through image coil Can detect inadequate cooling.

- Protects the transformer from thermal overloads

- Relieves the load fully or partially

- Starts/stops the oil pumps and fans

Transformer -Lightning Protection• Lightning Arrestors &

Spark Gaps: Protects from lightning

and switching over voltages

Spark gaps for small and non-vital transformers

Lightning arrestors for large and vital transformers