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A fault could interrupt service and must be detected immediately.
Your generator protection must have a way to detect generator faults. A fault is any unwanted current flow in an electrical system. Faults can cause all kinds of problems for your generators, including current loss, interruption of power delivery, and damage to the generator due to overheating.
There are three things your generator protection needs to do when your system encounters a fault. First, the protection system has to detect generator faults. Next, the circuit in which the fault occurs has to be tripped. Finally, the specific physical location of the fault has to be found so that the fault can be repaired.

How to Detect Generator Faults: Multiphase Faults and Differential Protection

One of the best ways to pick up multiphase faults on any machine, generators included, is by using differential protection. This is a fairly simple implementation of protective relaying.
A simple circuit that can be used to detect faults on any protected equipment.
The circuit consists of:

• Two current transformers (CTs), one on either side of the protected equipment. We’ll call these A and B.
• A relay between the two CTs.

Here's how this circuit can detect generator faults: the logic is that the current going into the generator should be the same as the current coming from the generator. The relay is tripped if the difference between the current passing through A and B grows too large. You want to make sure that you use a percentage differential relay here to mitigate error that could be caused by non-identical CT properties. In an ideal world you would have ideal CTs, and they would have the exact same properties. Real world manufacturing processes try to get as close to this as possible, but your CTs will always have some kind of tiny differences. So instead of the relay tripping when the difference is a hard value, it trips when the difference is a percentage away on either side of where the current value should be. The problem now is that you know the general area of where the fault is, but do not have an exact location.

Locating the Fault

So you know how to detect generator faults, and based on where the relay tripped, you have a rough idea of the location of the fault. What you need to know now is the exact location of the fault so that the fault can be repaired.
But how do you go about finding a fault? As usual, before starting work, make sure that the section of the system you are going to be working on is de-energize. It’s time to troubleshoot! The first thing that you can do is look for any visible signs of damage. Any kind of blackness as though something burned is a good indicator of a fault location (or at the very least something that needs repairs). If the fault occurred very recently you’ll probably be able to smell it too. The next thing you can do is test the circuit with your trusty multimeter. Start at one end and work your way through the circuit. If you can, get your hands on the circuit diagram and find out what the multimeter is supposed to read if the circuit is operating properly. If you can’t get at the circuit for one reason or another, you can try a time-domain reflectometer. These send a pulse down the wire and pick up changes in impedance, which reflects the signal back to the device. This will tell you how far down the line your fault is.

Closing

So that wraps it up! Hopefully now you know a little bit more about how to detect generator faults and how to locate a fault once it’s been detected. 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!


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