Practical || Stator Earth fault protection

#Generatorprotection #statorearthfault #EKguru Stator Earth fault protection of any Generator provided in the following ways. 95% Stator Protection: • Neutral Overvoltage Scheme • Overcurrent Scheme 100% Stator Protection: • 3rd Harmonic Under-Voltage Scheme • Neutral Injection Scheme 95% Stator Earth Fault Protection: Neutral Overvotage (59N) or Neutral Displacement Relay (64N): For generators with unit transformer (GT) and with high impedance grounding of the neutral, a neutral voltage relay with harmonic immunity and independent time delay is used. High impedance ground limits ground fault current to about 10A. • Conventional Neutral over voltage relay provides 90-95% stator winding coverage. • Last 5 to 10% near neutral is not covered. • Provided by a neutral voltage relay with harmonic restraint and adjustable time delay • The relay is normally set to operate at 5% of maximum neutral voltage with a time delay of 0.3 – 0.5 Second. With this voltage setting, it protects approximately 95% of the stator winding. • The relay is fed either from a neutral voltage transformer or from the broken delta winding of 3-phase voltage transformers on the generator line side. Overcurrent Protection (51N): This method is used for low impedance grounded machines. The relay 51N element detects over currents in neutral circuit. It coordinates with system ground fault protection for security and selectivity. In case of parallel operation of machines 67N protection is also used for directional ground fault detection. This relay is operated in case the zero sequence current or ground current flowing towards generator. Drawback of 95% Stator Protection: Undetected grounds in this region bypass grounding transformer, solidly grounding the machine. 100% Stator Earth Fault Protection: This protection also takes care about the fault occurring near to neutral. Such type of fault is critical as the fault near to neutral shunts the high resistance that saves the stator from large currents with an internal ground fault. 3rd Harmonic Undervoltage: Ground faults caused by mechanical damage may occur close to the generator neutral. Today there is a distinct trend towards providing ground fault protection for the entire stator winding (100% Stator Earth Fault Protection). The 100% stator earth fault scheme includes a 95% unit, which covers the stator winding from 5% of the neutral and a 3rd harmonic voltage measuring unit which protects the rest of the stator winding.