Types of Rotor Earth Fault in a Generator:

Rotor Earth Fault occurs in a rotor of a Generator in either two ways they are

1. Single Rotor Earth fault
2. Double Rotor Earth fault

Let’s see how these faults occur and what is their impact on the generator parts?

1. Single Rotor Earth fault

The field winding of a synchronous machine is usually floating with respect to ground. This means it is not grounded at all. So a single earth fault does not draw any fault current as there is no closed path to flow. The Single earth fault currents which are negligible amounts are limited by Neutral impedance.

What is the impact of Single earth fault on the rotor?

A single earth fault is not dangerous as there is no flow of current but it helps to develop the second earth fault quickly by stressing the insulation in portions of the windings of rotor.

It may be allowed to keep on and in fact, some machines have been allowed to continue operating in this condition for lengthy periods.

Note: In this case, we can run the generator until a convenient repair schedule is possible, but during this period it is made sure that the generator should not vibrate abnormally. If we detect any unusual vibration then the unit should trip immediately by operating the generator main and field circuit breakers.

2. Double Rotor Earth fault

The second earth fault is significantly dangerous as it creates a path to the flow of currents. The double earth fault current is only limited by the resistance of the fault path which is rather low and leads to very high currents.

These high-value currents flow through the rotor windings and develop forces. And hence cause extensive damage to the field conductors and rotor core. Sometimes the insulation gets damaged and the machine needs to take out of service immediately.

The best way to prevent this from occurring is to detect the first ground, thereby preventing a more serious chain of events.

What is the impact of Double earth fault on the rotor?

 If more than one earth fault occurs, a short circuit between the faulty points on the winding occurs. This may cause an unbalance magnetic field and mechanical damage may occur in the bearing of the machine.

The double earth fault here acts like a Phase to phase fault of the generator but the impact is not that much severe as the short circuit power is comparatively less in case of double earthed rotor faults.

And due to variation in stator currents causes additional voltages induced in the rotor, which led to arcing between poles and rotor body. The voltage surge is strong enough to damage the inter-turn insulation of rotor poles. Inter turn faults rarely occur on rotor windings, but should they do so it is very probable that they will develop rapidly and become earth faults.

In addition, the magnetic field then produced by the rotor might no longer be sinusoidally distributed around the periphery and it might also be asymmetric, the flux densities at corresponding points on the poles being different.

This could have serious consequences because the attractive forces existing between the poles and the stator core could be unequal and vibrations could be set up during each revolution of the rotor. This behavior could cause severe physical damage, particularly to components such as bearings. 

We can’t prevent the occurrence of rotor earth faults as the rotor is continuously revolving it is prone to earth faults always but certainly, we can provide a Rotor Earth Fault Protection-64R to a generator to prevent the damage occurring to a rotor as a result of various earth faults.