GENERATOR COOLANT FAILURE PROTECTION
Some of the larger generators have stator windings that are cooled by water circulating through the windings. If the flow of cooling water is impeded or stopped for some reason, the winding will quickly overheat. The cooling water is circulated by water pumps, which are backed up by standby pumps.
Flow detectors placed in the water line monitor the adequacy of the coolant flow. If the flow is reduced for any reason, these detectors can start the standby pump. If this restores the normal flow, no further action is taken, but if not restored within I minute, the turbine and generator are tripped sequentially, starting with the turbine and then the generator.
WHAT IS GENERATOR PROTECTION?
A generator is subjected to electrical stresses imposed on the insulation of the machine, mechanical forces acting on the various parts of the machine, and temperature rise. These are the main factors that make protection necessary for the generator or alternator. Even when properly used, a machine in its perfect running condition not only maintains its specified rated performance for many years, but it also repeatedly withstands certain excess of overload.
Preventive measures must be taken against overloads and abnormal conditions of the machine so that it can serve safely. Even ensuring an efficient design, construction, operation, and preventive means of protection – the risk of a fault cannot be completely eliminated from any machine. The devices used in generator protection, ensure that when a fault arises, it is eliminated as quickly as possible.
An electrical generator can be subjected to either an 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 to the generator.
The number and variety of faults that occur in a generator are huge. That is why the generator or alternator is protected with several protective schemes. Generator protection is of both discriminative and non-discriminative types. Great care is to be taken in coordinating the systems used and the settings adopted to ensure that a 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,
- Protective relays to detect faults occurring outside the generator.
- 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 safeguards, oil flow devises, temperature measuring devices for shaft bearing, stator winding, transformer winding and transformer oil, etc. Some of these protective arrangements are of non-trip type i.e. they only generate alarm during abnormalities.
But the other protective schemes ultimately operate the master tripping relay of the generator. This should be noted that no protective relay can prevent fault, it only indicates and minimizes the duration of the fault to prevent the high-temperature rise in the generator otherwise there may be permanent damage to it.
It is desirable to avoid any undue tresses in the generator, and for that, it is usual practice to install a surge capacitor or surge diverter or both to reduce the effects of lightning and other voltage surges on the machine. The protection schemes usually applied to the generator are discussed here below in brief.
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