Overheating of a synchronous generator may occur due to one of the following causes:

1. Overload
2. Failure of the ventilation or hydrogen cooling system
3. Shorted laminations in the stator iron
4. Core bolt insulation failures in the stator iron

Excessive overload is not likely since the prime mover rating is usually not much greater than the generator rating. There is the possibility of overload due to a high active power load coupled with high excitation. If the power factor is below the rating, this will give an alarm for high excitation. Failure of the cooling system is also likely to be detected by operator alarms,
The other failure, involving core failures and heating will develop slowly and must be detected by temperature measurements of some kind.

Temperature detection is often accomplished using embedded thermocouples in the stator winding slots, placing the thermocouples throughout the windings in several locations. Another measurement technique is to record the input and output cooling medium to note any marked changes in the readings. Smaller generators are often provided with “replica” type temperature estimating devices that use stator current in a heat storage enclosure to estimate actual machine temperature.

All of these devices are used to alarm the operator of possible serious problems. At unattended stations, the output of the temperature indicator may be used to shut down the unit.

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