1. Generator protection Generator protection is very complex compared to protection for other elements of the power system. For example, when a turbo-alternator driven by steam turbine is tripped, the following must be done:. Steam supply to the turbine is stopped or bypassed. Firing of the boiler is stopped. Coal mills are stopped. Coal supply to the coal mills is stopped. Field circuit of the alternator is interrupted. Field coils are connected across a resistor to dissipate the stored energy. Alternator is kept running at a slow speed (few rpm) with the help of barring gear till it cools down uniformly, so as to avoid uneven expansions Putting back the alternator on line is rather a slow process because all the parameters (temperatures and pressures) have to be progressively built up. We have to keep in mind that a modern large turbo-alternator is a huge mass rotating at a very high speed (3000 rpm) in a very small air-gap. Thus, apart from the large electrical energies involved, there is tremendous amount of mechanical energy in the form of moment of inertia of the rotating mass and resultant forces on the shaft, the rotor body and the stator structure. It is said that running a large power station is like flying a supersonic jet aircraft ! without any forced landings or crashes!

2. Various Faults and Abnormal Operating Conditions

3. Stator Phase and Ground Faults Longitudinal Differential Scheme

5. Transverse Differential Protection (inter-turn faults)

6. Unbalanced Loading If the stator carries unbalanced currents, then it is the rotor, which is overheated. How long the generator can be allowed to run under unbalanced loading, depends upon the thermal withstand capacity of the machine, which in turn depends upon the type of cooling system adopted. If there is an unbalanced loading of the generator then the stator currents have a negative sequence component. An inverse type of over-current relay, which is fed with the negative sequence component of stator current, gives protection against unbalanced loading of the generator.

7. Over speeding

8. Loss of Excitation

9. Loss of Prime Mover