An important part of generator protection is that generators need to be grounded. And because they need to be grounded, generators are susceptible to ground faults. This article is going to talk about general case ground faults on a generator and the appropriate use of generator protection, and then talk about the specific case of a ground fault on the generator field coils.

Ground Fault Generator Protection

One of the most important things to note when protecting against ground faults is that the higher the magnitude of the grounding impedance, the smaller the magnitude of your fault current will be. This makes it difficult to detect weaker faults with high resistance grounding. For example, a differential relay might not pick up a single phase to ground fault because the current magnitude change is not enough to cause the relay to trip.
With high resistance grounding, you need a relay on the grounded neutral to catch ground faults that are single phase to ground, since the relay on the neutral doesn’t care about load current. However, when the magnitude of your ground resistance is high enough, it becomes difficult to differentiate between low magnitude fault currents and harmonics.
An important thing to remember is that all of the relays in your generator protection system need to be coordinated in order to function properly. The last thing you want is for a ground fault detected on a relatively unimportant piece of equipment, like a table saw plugged into the wall, to kill the power to your whole building! You can learn more about coordination here.
The following image is an example of a unit-type grounding system.

This is a fairly common grounding configuration for generators. There is a high resistance ground at the generator, connected to the system through a distribution transformer. The resistor and the relay are grounded in parallel on the grounding transformer.
Now that we’ve talked about generator protection for ground faults in a general case, I’m going to talk specifically about generator protection in the case of field grounds.

Field Ground Generator Protection

A field ground is exactly what it sounds like: a ground on the field of your generator. A single phase to ground fault on the field of a synchronous machine should in most cases produce no immediate damaging effect. Don’t let this fool you though: the field ground fault still needs to be cut off. A single phase to ground fault on a field is dangerous because a second ground fault could short part of the field winding, causing damaging vibrations. A field ground fault must be detected and removed quickly and efficiently.
The following is a circuit that could be used for field ground generator protection.

In this circuit, the relay uses a voltage divider consisting of two linear resistors and one non-linear resistor. The resistance value of the non-linear resistor varies with applied voltage. When the field becomes grounded, voltage develops at the point between the two resistors and the ground. The magnitude of this voltage depends on the exciter voltage of the generator, and the location of the ground. The maximum voltage value is obtained if the field is grounded at either end of the winding.
There will be a point on the field winding where a ground fault will produce no voltage between the middle point and ground. This “null point” is the point on the field winding where there is an electrical balance between the two field winding resistances and the two relay resistances. The non-linear resistor varies the location of this null point so that a ground can be detected at any point in the field winding.
Finally, you have a pushbutton connected across a portion of the R2 resistor. This permits a manual check for possible ground faults at the center of the winding. This is so that if the generator is base loaded and will not experience periodic excitation variations, you can still check for ground faults.
 

Closing

Hopefully now you have a better understanding of how to use grounding and relays to provide generator protection against ground faults. If you have any more questions, feel free to shoot me an e-mail at cole@jmkengineering.com. You can also sign up for the Sparky Resource newsletter below, which goes out every Sunday. It’s got all kinds of cool stuff in it, possibly including answers to questions you might have after reading this article!
As always, thanks for reading!