TABLE OF CONTENTS During operation, a turbine rotor can be subjected to a series of operational phenomena that can cause its condition to deteriorate. The anticipated problems, stress, torque, erosion, etc. are known. Their magnitude can be predicted, designed for, and guarded against.
A sudden short circuit of a generator when running fully excited has a profound influence on the mechanical design of both the generator and the turbine. The forces developed in the generator windings (which are proportional to the square of the field current) are transmitted back to the turbine rotor as an arresting torque. The magnitude of the torque developed in the rotor is a function of three separate factors of the electrical system:
The instant in the electric cycle at which the short occurs
The generator load and the excitation of the generator immediately before the short
The extent of the short circuit, i.e., whether 1, 2, or 3 phases are involved, and whether the fault occurs close to the machine terminals or more remotely in the network
Once a fault has occurred, large forces act on the generator, trying to arrest its rotation. Now, if one end of the turbine generator shaft is given an angular deflection relative to the other, the shaft will deflect by some small angular displacement (see Fig. 9.6.1). On being released by the generator rotor breaking free of the arresting magnetic force, the flywheel effects of the individual rotors will cause them to...
