📘 Generator Protection – Complete Theory & Relay Applications
Generator is the most expensive and critical equipment in power system. Proper protection is essential to avoid severe damage and long outage.
🔹 1️⃣ Types of Generator Faults
(A) Stator Faults
- Phase-to-phase fault
- Phase-to-ground fault
- Inter-turn fault
(B) Rotor Faults
- Field winding earth fault
- Short circuit in rotor
(C) Abnormal Conditions
- Overload
- Unbalanced loading
- Loss of excitation
- Reverse power
- Overvoltage / Undervoltage
- Overfrequency / Underfrequency
🔹 2️⃣ Protection for Stator Faults
(1) Differential Protection
- Based on current difference principle
- Protects internal winding faults
- Very fast operation
(2) Stator Earth Fault Protection
- Neutral grounding resistor used
- Detects single line-to-ground fault
🔹 3️⃣ Rotor Protection
Field Earth Fault Relay
- Detects insulation failure in rotor
- DC sensitive relay used
🔹 4️⃣ Reverse Power Protection
When prime mover fails, generator acts as motor and draws power from grid.
- Directional power relay used
- Protects turbine
Reverse power setting ≈ 2–10% of rated power
🔹 5️⃣ Loss of Excitation Protection
- Generator becomes unstable
- Reactive power drawn from system
- Impedance relay used
🔹 6️⃣ Overcurrent & Overload Protection
- Time delay overcurrent relay
- Protects against sustained overload
🔹 7️⃣ Unbalanced Load Protection
Negative sequence current causes rotor heating.
I₂² × t = constant
- Negative sequence relay used
🔹 8️⃣ Overvoltage Protection
- Occurs during sudden load rejection
- Overvoltage relay trips generator
📊 Complete Generator Protection Scheme Includes
- Differential protection
- Stator earth fault
- Rotor earth fault
- Reverse power relay
- Loss of excitation relay
- Overcurrent relay
- Negative sequence relay
- Overvoltage relay
- Frequency protection
🎯 GATE & Interview Important Points
- Differential protection is primary protection
- Reverse power protects turbine
- Negative sequence protects rotor heating
- Loss of excitation detected using impedance relay
Generator Protection = Multi-layered Protection System
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