Loss of Excitation Protection :
Otherwise, if the system cannot provide adequate reactive power support for the induction generator mode of operation, then synchronism is lost.
The change is a gradual one and if the field is tripped by accident, an alarm can be used to alert the operator.
However, if the field is not quickly restored, then the unit should be shut down.
Reduction or loss of excitation to the field winding is an abnormality rather than a fault. If the field winding is completely lost, then in principle, the synchronous generator will try to mimic an induction generator.
This mode of operation is possible provided that the power system to which the generator is connected is strong enough to provide necessary reactive power support.
Recall that an induction generator has no field winding and hence it cannot generate reactive power.
If adequate reactive power support is not available then the generator will have to be shut down.
It is likely that field winding will be accidentally shut off and usually, loss of synchronism will require appreciable time to take effect.
Hence, it is preferable to first raise an alarm for the operator to restore the field, failing which, the generator has to be shut down.
A consequence of reduced excitation may not appear to be dramatic, but it can lead to end-region over-heating in turbo-alternators.
Hence, this abnormality has to be detected and an alarm has to be raised for the operator. The ultimate measure would be to shut down the generator.
The protection system for synchronous generators should detect reduced or loss of excitation condition, raise an alarm and if the abnormality persists, trip the generator.
Rotor earth fault protection:
This force can excessive pressure on the bearings and consequent failure or even displacement of rotor sufficient to cause fouling of the stator. The overheating in the rotor can cause deformation of the winding which could lead to the development of short circuits.
Two main methods are used for detecting earth faults in the rotor circuit. In the first method, a high resistance potentiometer is connected to the earth across the Rotor circuit the center point of which is connected to the earth.
Thorough a sensitivity relay (see above Fig. The relay will respond to earth faults occurring over most of the rotor circuit.)
There is, however, a blind spot at the center point of the field winding which is at the same potential as the mid-point of the potentiometer. This blind spot can be examined by arranging a tapping switch which when operated shifts the earth point from the mid-point of the potentiometer to a point a little to one side.
The tapping switch can be mounted on the control panel and the check operation performed at suitable intervals.
An alternative method that avoids a blind spot consists of biasing the field circuit relative to earth by means of a simple transformer rectifier unit as shown
In above Fig. This is connected between the positive bar of the field system and earth through a high-resistance relay. The positive bar of the field system is biased some 30 V negative to earth and therefore the remaining portions of the field circuit are proportionally more negative. A fault occurring at any point in the field system will apply a voltage to the relay which is sufficient to cause operation, the fault current being limited to a low value by the resistance of the relay. It is usual to connect the relay to give an alarm.
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