Generator protection is an important part of having a reliable power delivery service. Electrical systems are complex, and the failure of a generator can mean loss of critical plant systems.
A fault is any unwanted electrical current flow. Faults can cause all kinds of problems for your generator. For example, if the fault is a short circuit across the generator windings, then the winding could heat up and become damaged.
If the generator cannot supply enough power to its loads, it will not necessarily shutdown automatically. What it will do is overheat, and maybe fall out of synchronization: both of these conditions can threaten to destroy your generator.
Overheating can be caused by a number of things, such as the generator being overloaded, a winding insulation breakdown due to a fault, or insufficient bearing oil lubrication. Overheating a generator decreases its operational life, and can outright destroy the generator if the problem isn’t solved quickly enough.
Typically, a generator will be forced to run above normal operating speeds when there is a loss of load on the generator circuit. The loss of load increases the generator voltage which in turn over excites the field and increases generator speed.
Motoring is what happens when the prime mover is not supplying enough power to the generator. The system is forced to make up for these losses by supplying real power to the prime mover, which causes the generator to act like a motor. Real power flows into the generator instead of out of it, and reactive power flows both into and out of the generator.
The biggest problem with unbalanced current operation is that it causes current to flow through the neutral, increasing the power loss in your lines. This can be caused by unbalanced loads, or a fault, and once again, can result in overheating.