SUDHIR KUMAR SRIVASTAV
Additional General Manager-RAPDRP
NTPC LIMITED, NEW DELHI
Rotor Earth Fault Protection
Rotor winding of a Generator is completely isolated from earth. So earth fault of rotor winding at single point is not at all dangerous. As the rotor winding of a Generator is completely isolated from earth, there will not be any return path for flow of fault current in case of single rotor earth fault and earth fault current will be zero.
But in case of second earth fault, there will be huge fault current depending on fault resistance. This is because return path for flow of fault current will be through first fault point.
Heavy fault current may burn the conductor, causing sever damage to rotor. If a large portion of winding is shorted, the field flux may be highly unbalanced causing flux concentration at one pole & flux dispensation at other pole. So the attractive forces, which is proportional to the square of the flux density, may be stronger at one pole & weaker at other pole. This unbalance force may cause high vibration and may damage the rotor. If the rotor is sufficiently displaced due to unbalance force and foul with stator, It may also damage stator,.
Rotor E/F protection is done through measurement of Insulation resistance between rotor winding & earth point.
Alarm may operate below 5.5 K insulation resistance between rotor winding & earth point & trip may operate below 2.2 K insulation resistance between rotor winding & earth point.
Case Study:
We have observed rotor earth fault alarm at times, but actually there was no earth fault in rotor. While investigations it was found that following are the reason for false alarm
1. Mixing of AC voltage with DC voltage of protection system, causing mal-operation of protective relay.
2. Detachment of carbon brush from rotor winding centre point (High resistance centre point earthing of rotor winding through carbon brush were used for rotor E/F protection system).
3. Failure of DC protection voltage causing mal-operation of protective relay.
Online checks:
1. Take two high resistance voltmeters. Connect one voltmeter between positive end of rotor winding to earth and second voltmeter between negative end of rotor winding to earth and hold for few minutes. Measure the voltage in both the meters. Initially there will be difference of reading in two meters, but after some time i.e. after stabilising, the readings will be same & equal to half of applied field voltage. This is the indication of healthy system. If there is any E/F, the voltage between two meters will be different even after stabilizing time.
2. Check DC protection voltage for any mixing with AC voltage.
3. Monitor healthiness of DC protection voltage.
4. Check the carbon position of high resistance mid-point earthing.
