• Generator protection :1. Generator grounding2. Excitation system3. Generator stator thermal protection

Generator grounding

• High resistance grounding• Distributive transformer• High resistance across the secondary

Excitation system• An electric generator or electric motor consists of a rotor spinning in a magnetic

field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation.

Generator stator thermal protection

• In case of overload, thermal protection is required for generator stator windings.

• Temperature sensors1. Resistance temperature detectors2. thermocouples

Generator stator fault protection

• Generator faults can cause sever damage to insulatin,windings and the core.

• The differential relay (87G) is commonly used as primary protection for phase fault of generator stator windings. This function is mostly completely selective and can be used with very short tripping times. Differential relays will detect three phase fault, phase to phase fault and double phase to ground faults. Differential relays will not detect turn to turn faults in the same phase since there is no difference in the current entering and leaving the phase winding.

Application and forms of differential relays

• one that operates when the vector difference of two or more similar electrical quantities exceeds a predetermined amount.

Differential relay

• Under normal operating conditions

• When a short circuit develops between the two CTs:

Ground fault protection

• Differential relays will not provide ground fault protection on high impedance grounded machines where primary fault current levels are limited to 3-25 A.

• Time delay overvoltage relay

Generator rotor field protection

• Field circuit of a generator is ungrounded system.

• A portion of the field of the winding will be short circuited, producing unbalanced air gap fluxes

• Rotor vibrations• A voltage relay is used to detect overvoltage in

the field winding produced by a ground fault.

Generator loss of field

• When a synchronous generator loses excitation it will over speed and operate as an induction generator.

• It will continue to supply some power to the system and will receive its excitation from the system in the form of VAR.

• During this condition, the stator currents will be increased and, since the generator has lost synchronism, there can be high levels of current induced in the rotor that can cause dangerous overheating of the stator windings and the rotor within a very short time.

• The most widely applied methodfor loss of field protection (40) is the use of distance relays to sense the variation of impedance as viewed from the generator terminals.

Loss of synchronism

• When a generator loses synchronism, the resulting high peak currents and off-frequency operation cause winding stresses, pulsating torques, and mechanical resonances that are potentially damaging to the generator and turbine generator shaft (IEEE, 1995).

• The conventional method for loss of synchronism protection (78) is an impedance relay that analyzes the variation in apparent impedance as viewed at the terminals of the system element.

• Over excitation• Motoring• Over voltage

Motoring or(reverse power)• Reverse power generally describes a condition where the prime

mover of a generator is not supplying sufficient torque to keep the generator rotor spinning at the same frequency as the grid to which the generator is connected. In other words, the generator has actually become a motor and is drawing current from the grid to which is connected and is supplying torque to the prime mover which is supposed to be supplying torque to the generator.

• Reverse power protection (32) is a power relay set to look into the machine and is, therefore, used on most units. Although listed along with generator protection functions, reverse power protection is actually designed for the protection of the steam turbine.

Over voltage

• Generator overvoltage may occur without necessarily exceeding the V/Hz limits of the machine.

• Upon load rejection, the speed may increase and cause a proportional rise in voltage. Under this condition on a V/Hz basis, the overexcitation may not be excessive but the sustained voltage magnitude may be above permissible limits. Overvoltage conditions can also occur due to voltage regulator failure. Protection for generator overvoltage is provided with an overvoltage relay

Abnormal frequencies

Generator breaker failure protection system