Sunday, July 10, 2011

Rotor Earth Fault Protection

Generator Protection Part 10
Rotor Earth Fault Protection
       The rotor circuit of the alternator is not earthed and d.c. voltage is imposed on it. And hence single ground fault in rotor does not cause circulating current to flow through the rotor circuit. Hence single ground fault in rotor does not cause any damage to it. But single ground fault causes an increase in the stress to ground at other points in the field winding when voltage are induced in the rotor due to transients. Thus the probability of second ground fault increases.
       If the second ground fault occurs then part of the rotor winding is bypassed and the currents in the remaining portion increases abruptly. This cause the unbalance of rotor circuit and hence the mechanical and thermal stresses on the rotor. Due to this, rotor may get damaged. Sometimes damage of bearings and bending of rotor shaft take place due to the vibrations. Hence the rotor must be protected against earth fault.
        Method 1 : In this method a high resistance is connected across the rotor circuit. It is provided with centre tap and the centre tap point is connected to the ground through a sensitive earth fault relay as shown in the Fig.1.
Fig. 1 Rotor earth fault protection

       Except the centre point, the earth fault relay detects the earth faults for most of the rotor circuit. Thus most of the rotor winding part is protected against the earth faults.
       Method 2 : The modern method of providing earth fault protection includes d.c. injection or a.c. injection. The scheme is shown in the Fig. 2.
Fig. 2 Rotor earth fault protection

       A small d.c. power supply is connected to the field circuit. A fault detecting sensitive relay and the resistance are also connected in series with the circuit. This high resistance limits the current through the circuit.
       A fault at any point on the field circuit will pass a current of sufficient magnitude through the relay to cause its operation. The d.c. supply is preferred and simple to use and it has no problem of the leakage currents. In case of a.c. injection, the high resistance is replaced by a capacitor.
       The earth fault relays are instantaneous in operation and are connected to an alarm circuit for indication and to take the proper action. This is because, a single ground fault does not require an immediate action of isolating the generator.

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