Over Heating Protection :
Overheating protection is generally provided for generators above 1 MW.
Failure of the cooling system or by sustained overloads causes the rise in the temperature of the generator.
It is not practicable to provide overload protection by back up stator fault over current protection. Because back up over current protection is generally set for sensing fault currents and should not trip for overloads.
Electrical overcurrent relays cannot sense the winding temperature accurately because temperature rise depends upon I²Rt and also on cooling. Also, electrical protection cannot detect a cooling system failure.
Therefore embedded resistance detectors or thermocouples are provided in the slots among with the stator coils for large generators.
If temperature rises above safe value an alarm will give the indication.
Reverse Power Protection :
The turbine acts as a load on a synchronous motor and continues drawing the electrical power. During the motoring action of the generator, the power flows from the bus bars to the machine and the conditions in the three phases are balanced.
Normally the power taken by the synchronous motor is low of the order of 2 to 10% of the rated power. Power factor and current depends on excitation level.
Hence single element directional power relay (reverse power relay) is used to sense the direction of power flow.
The CTs for reverse power protection may be either at the neutral end or the Bus bar end of the generator winding,
The intentional time lag is provided in the reverse power protection so that the protection does not operate during system disturbances and power swings.
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