Protection of Generator

Common practice in most countries is to earth the generator neutral through a resistor. The current through the neutral point resistor will normally be 5 - 10 A when subjected to the rated phase to earth voltage. For generators with step-up transformer, a neutral point voltage relay with typical setting 5 % of generator phase voltage will provide earth-fault protection for 95 % of the stator winding. It also covers the generator bus, the low-voltage winding of the step-up transformer and other ancillaries galvanically interconnected with the stator winding. An earth-fault close to the neutral point of the generator will not give sufficient neutral point voltage to activate the neutral point voltage (95 %) relay. Generators which produce about 1 % or more third harmonic voltage under all service conditions, can have the entire stator winding, including the neutral point, protected using a scheme which combines the neutral point voltage function (95 % relay) and a third harmonic undervoltagefunction. Under normal service, typically 40 - 60 % of the generated third harmonic phase voltage will appear across the neutral point resistor and will activate the third harmonic voltage relay. If an earth-fault occurs close to the neutral point, the third harmonic voltage drops to a low value and the undervoltage relay operates. The relay must have a filter which prevents the fundamental frequency neutral point voltage from affecting the third harmonic voltage function.

The 100 % stator earth-fault protection can also be used for generators with unearthed neutral. The third harmonic voltage between the neutral point and earth will increase compared to generators with earthing resistor and the third harmonic under voltage (100 %) function can cover a larger part of the stator winding. However, in case of an earth-fault in the high voltage (HV) network, the zero-sequence voltage transferred to the stator circuit via the capacitance between the HV and LV windings of the step-up transformer increases compared to generators with neutral point resistor.The voltage setting or the time delay of the generator neutral point voltage (95 %) relay must then be increased to get selectivity against earth-fault relays on the HV side.The 100 % stator earth-fault protection can even be used for generators with neutral point tuned reactor. The third harmonic voltage between the neutral point and earth will increase, compared to generators with earthing resistor. The zero-sequence voltage transferred to the stator circuit in case of an earth-fault in the HV network will generally be so high that selectivity of the 95 % relay can not be obtained by a reasonable increase of the voltage setting.A generator bus voltage supervision relay is included to prevent unwanted operation of the third harmonic (100 %) relay when the generator is out of service and during running up/down. Alternatively, a stator current supervision relay is used to block the third harmonic voltage relay until the generator is in service.

For more detail, refer links:

http://www.scribd.com/doc/22672398/Users-Guide-RAGEK-100-and-95-Stator-Earth-fault-Protrection-for-Generators

http://www.scribd.com/doc/22672482/ABB-RAGEK-Generator-Protection

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