The rotor of the generator is normally unearthed i.e. remain isolated from the earth and therefore the single fault due to insulation breakdown will not rise the fault current. A single fault will not affect the rotor, but if the fault occurs, continues then it will damage the field winding of the generator. For a large generator, the rotor earth fault protection system is used for the protection of the field winding.
When the one earth fault occurs in the rotor then it is not necessary that the system is completely trip, only the relay indicates that the fault has occurred. So that the generator should be taken out of service at leisure. The methods of rotor earth fault protection are explained below.
Methods of Rotor Earth Fault Protection
Rotor Earth Fault Protection By Using High Resistance
In this method, a high resistance is connected across the field winding of the rotor.The midpoint of the resistor is grounded through a sensitive relay. When the fault occurs the relay detect the fault and send the tripping command to the breaker.
The major disadvantage of such type of system it that it can detect the fault for most of the rotor circuit except the rotor centre point. This difficulty can be overcome by shifting the tap on the resistor from centre to somewhere else. Thus, the relay can detect the midpoint fault of the rotor.
In this method, alternating current is injected into the field winding circuit and ground along with a sensitive overvoltage relay and a current limiting capacitor. A single earth fault in the rotor will complete the circuit comprises the alternating current source, sensitive relay and earth fault. Thus, the earth fault is sensed by the relay.
The major disadvantage of such type of system is the leakage current that flows through the capacitor. This current unbalanced the magnetic field and increase the stress on the magnetic bearing. Another disadvantage of alternating current is that the relay cannot pick up the current that normally flow through the capacitance to the ground. Thus, the care must be taken to avoid resonance between the capacitance and the relay inductance.
The problem of the AC injection system can be overcome by using the DC injection method. This method is simple and has no problem of leakage currents. The one terminal of the sensitive relay is connected to the exciter, and the other terminal is connected to the negative terminal of the DC source. The positive terminal of the DC source is grounded. When the earth fault occurs, the fault current will complete the circuit path, and the fault is sensed by the relay.