Transformer protection relays safeguard transformers from internal and external faults by monitoring electrical and thermal parameters and triggering circuit breakers when abnormal conditions are detected.

Types of Transformer Protection Relays

1. Differential Relay (87)
   • Detects internal winding faults by comparing currents entering and leaving the transformer.
   • Requires matched current transformers (CTs) on all windings, with careful selection of CT ratio and accuracy to ensure proper operation .
2. Overcurrent Relay (50/51)
   • Protects against overloads and external short-circuit faults.
   • Trips when current exceeds preset limits, preventing overheating and insulation damage .
3. Buchholz Relay
   • Gas-actuated relay installed between the transformer tank and conservator.
   • Detects gas accumulation from internal faults or abnormal oil movement, providing early warning of internal issues .
4. Overheating/Thermal Protection
   • Monitors transformer temperature using sensors in the core and windings.
   • Prevents insulation damage due to excessive heat and triggers alarms or trips if limits are exceeded .
5. Over-Fluxing Protection (24)
   • Protects against excessive magnetic flux in the transformer core caused by voltage transients or overexcitation .
6. Pressure Relief and Restricted Earth Fault (REF/64) Protection
   • Prevents tank rupture during severe internal faults and provides sensitive earth fault detection within the transformer zone .

Wiring of Transformer Protection Relays

• Current Transformers (CTs)
  • CTs are connected in series with transformer windings to measure current.
  • Secondary windings of CTs feed the relay input circuits.
  • Proper CT ratio and polarity are critical for differential and overcurrent protection .
• Relay to Circuit Breaker Connection
  • Relays are wired to the trip coil of the circuit breaker.
  • When a fault is detected, the relay sends a trip signal to isolate the transformer immediately .
• Auxiliary Circuits
  • Include power supply for the relay, alarm contacts, and interlocks.
  • Some relays also interface with SCADA or monitoring systems for remote supervision .
• Buchholz Relay Wiring
  • Connected to alarm and trip circuits.
  • Gas accumulation triggers an alarm; severe faults trigger breaker trip .

Protection Scheme Considerations

• Small distribution transformers may only require fuses or overcurrent relays, while large power transformers need comprehensive protection including differential, Buchholz, overcurrent, earth fault, and temperature monitoring .
• Relay settings depend on transformer rating, criticality, and application.
• Proper coordination ensures fast fault clearance while minimizing unnecessary tripping .

Summary

Transformer protection relays act as intelligent safety devices, continuously monitoring electrical and thermal parameters. Correct wiring of CTs, relays, and trip circuits ensures rapid isolation of faults, protecting the transformer from damage, extending its life, and maintaining system reliability . Comprehensive protection schemes combine multiple relays to detect internal and external faults, overloading, overheating, and abnormal flux conditions.