The rotor of an alternator is wound by field winding. Any single earth fault occurring on the field winding or in the exciter circuit is not a big problem for the machine. But if more than one earth fault occurs, there may be a chance of short circuiting between the faulty points on the winding. This short circuited portion of the winding may cause unbalance magnetic field and subsequently mechanical damage may occur in the bearing of the machine due to unbalanced rotation. Hence it is always essential to detect the earth fault occurred on the rotor field winding circuit and to rectify it for normal operation of the machine. There are various methods available for detecting rotor earth fault of alternator or generator. But basic principle of all the methods is same and that is closing a relay circuit through the earth fault path. There are mainly three types of rotor earth fault protection scheme used for this purpose.
- Potentiometer method
- AC injection method
- DC injection method
Let us discuss the methods one by one.
Potentiometer Method of Rotor Earth Fault Protection in Alternator
The scheme is very simple. Here, one resistor of suitable value is connected across the field winding as well as across exciter. The resistor is centrally tapped and connected to the ground via a voltage sensitive relay. As it is seen in the figure below, any earth fault in the field winding as well as exciter circuit closes the relay circuit through earthed path. At the same time the voltage appears across the relay due to potentiometer action of the resistor.
This simple method of rotor earth fault protection of alternator has a big disadvantage. This arrangement can only sense the earth fault occurred in the any point except the center of the field winding. From the circuit it is also clear that in the case of earth fault on the center of the field circuit will not cause any voltage to be appeared across the relay. That means simple potentiometer methods of rotor earth fault protection, is blind to the faults at the center of the field winding. This difficulty can be minimized by using another tap on the resistor somewhere else from the center of the resistor via a push button. If this push button is pressed, the center tap is shift and the voltage will appear across the relay even in the event of central arc fault occurs on the field winding.
AC Injection Method of Rotor Earth Fault Protection in Alternator
Here, one voltage sensitive relay is connected at any point of the field and exciter circuit. Other terminal of the voltage sensitive relay is connected to the ground by a capacitor and secondary of one auxiliary transformer as shown in the figure below.
Here, if any earth fault occurs in the field winding or in the exciter circuit, the relay circuit gets closed via earthed path and hence secondary voltage of the auxiliary transformer will appear across the voltage sensitive relay and the relay will be operated. The main disadvantage of this system is, there would always be a chance of leakage current through the capacitors to the exciter and field circuit. This may cause unbalancing in magnetic field and hence mechanical stresses in the machine bearings.
Another disadvantage of this scheme is that as there is different source of voltage for operation of the relay, thus the protection of rotor is inactive when there is a failure of supply in the AC circuit of the scheme.
DC Injection Method of Rotor Earth Fault Protection in Alternator
The drawback of leakage current of AC injection method can be eliminated in DC Injection Method. Here, one terminal of DC voltage sensitive relay is connected with positive terminal of the exciter and another terminal of the relay is connected with the negative terminal of an external DC source. The external DC source is obtained by an auxiliary transformer with bridge rectifier. Here the positive terminal of bridge rectifier is grounded.
It is also seen from the figure below that at the event of any field earth fault or exciter earth fault, the positive potential of the external DC source will appear to the terminal of the relay which was connected to the positive terminal of the exciter. In this way the rectifier output voltage appears across the voltage relay and hence it is operated.