The main relay protections for transformers include

Transformers are typically protected using a combination of differential, overcurrent, Buchholz, restricted earth fault, thermal, and pressure relays to detect internal and external faults, overloads, and abnormal operating conditions.Differential ProtectionDifferential relays are the primary protection for internal winding faults. They compare the current entering and leaving the transformer; any imbalance exceeding a set threshold indicates an internal fault, triggering a trip of the circuit breaker. This protection is highly sensitive and requires matched current transformers (CTs) on all windings to avoid nuisance tripping during normal operation or inrush currents .Overcurrent ProtectionOvercurrent relays protect transformers against overloads and external short circuits. They are often used as backup protection for differential relays. For smaller transformers, fuses may suffice, but larger transformers require overcurrent relays coordinated with circuit breakers to handle high fault currents and discriminate against secondary-side faults .Buchholz Relay (Gas Detection)A Buchholz relay is installed in oil-filled transformers above 500 kVA between the tank and conservator. It detects gas accumulation from internal faults or rapid oil surges from severe faults. Slow gas accumulation triggers an alarm, while sudden oil movement trips the breaker, preventing catastrophic failure .Restricted Earth Fault (REF) ProtectionREF relays provide sensitive earth fault protection within the transformer zone. They detect low-level ground faults that may not be sensed by standard overcurrent relays, offering enhanced protection for winding-to-ground faults .Thermal ProtectionThermal relays or sensors monitor winding and oil temperatures to prevent insulation damage from overheating. Fiber-optic or RTD sensors can measure hot-spot temperatures, triggering alarms or cooling systems to avoid transformer failure .Pressure and Over-Fluxing ProtectionPressure relief relays prevent tank rupture during severe internal faults, while over-fluxing protection safeguards against excessive magnetic flux density in the transformer core, which can cause overheating and insulation stress .Additional Protection MeasuresOil level and moisture monitoring: Detects leaks or contamination in the transformer oil.Breaker failure and lockout logic: Ensures backup tripping if the primary breaker fails.Fire and explosion mitigation: For indoor installations, less-flammable fluids and silica gel breathers reduce fire risk .Protection PhilosophyA robust transformer protection scheme is layered, combining electrical, mechanical, and thermal devices. Each fault type—internal winding faults, earth faults, overloads, or oil/gas anomalies—is assigned to a device capable of dependable detection and selective tripping, ensuring minimal damage and maintaining system stability . In practice, modern relays often integrate multiple protection functions in a single package, simplifying coordination and reducing CT burdens while providing comprehensive coverage for all credible fault scenarios .