Generator Protection

A generator is subjected to electrical traces imposed on the insulation of the machine, mechanical forces acting on the various parts of the machine, and temperature rises. These are the main factors which make protection necessary for the generator or alternator. Even when properly used, a machine in its perfect running condition does not only maintain its specified rated performance for many years, but it does also repeatedly withstand certain excess of over load. Hence, preventive measures must be taken against overloads and abnormal conditions of the machine so that it can serve safely. Despite of sound, efficient design, construction, operation, and preventive means of protection, the risk of that fault cannot be completely eliminated from any machine. The devices used ingenerator protection, ensure the fault, made dead as quickly as possible. An electrical generator can be subjected to either internal fault or external fault or both. The generators are normally connected to an electrical power system, hence any fault occurred in the power system should also be cleared from the generator as soon as possible otherwise it may create permanent damage in the generator. The number and variety of faults occur in generator, are huge. That is why generator or alternator is protected with several protective schemes. The generator protection is of both discriminative and non-discriminative type. Great care is to be taken in coordinating the systems used and the settings adopted, so that the sensitive, selective and discriminative generator protection scheme is achieved.

Types of Generator Protection

The various forms of protection applied to the generator can be categorized into two manners,

1. Protective relays to detect faults occurring outside the generator.
2. Protective relays to detect faults occurring inside the generator.

Other than protective relays, associated directly with the generator and its associated transformer, there are lightning arrestors, over speed safe guards, oil flow devises and temperature measuring devises for shaft bearing, stator winding, transformer winding and transformer oil etc. Some of these protective arrangement are of non-trip type i.e. they only generate alarm during abnormalities. But the other protective schemes ultimately operate master tripping relay of the generator. This should be noted that no protective relay can prevent fault, it only indicates and minimises the duration of the fault to prevent high temperature rise in the generator otherwise there may be permanent damage in it. It is desirable to avoid any undue tresses in the generator, and for that it is usual practice to install surge capacitor or surge divertor or both to reduce the effects of lightning and other voltage surges on the machine.