POWER SIGNAL BASED STABILIZERS AND THEIR INTERACTION WITH TORSIONAL OSCILLATIONS OF LARGE TURBO-GENERATORS
Niamat, Mohammed Yaqzan
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A power system stabilizer is employed in a power system to damp the low-frequency electromechanical oscillations of the system. For this purpose, most power-utilities employ speed as the input signal to the stabilizer. However, such speed signal based stabilizers, when used with large turbo-generators, may destabilize the torsional modes of oscillations of the turbo-generator shaft and, under certain adverse operating conditions, this destabilization may be high enough to cause instability of the uncompensated or series capacitor compensated power systems. It has been suggested that a stabilizer deriving its input from electrical power or a combination of electrical and mechanical powers rather than speed may not cause this problem of torsional interaction. The main purpose of this thesis is to investigate the possibility of torsional interaction between a power signal based stabilizer and an uncompensated or series capacitor compensated power system. Both the eigenvalues and the frequency response methods are employed to investigate this problem for the small perturbation case. The results are compared with that of a speed signal based stabilizer. The studies conducted in this thesis reveal that in the case of the uncompensated power system the amount of torsional interaction between a power signal based stabilizer. and the system is almost negligible. However, in the case of the. compensated power system, this torsional interaction can be under certain operating conditions high enough to cause system instability. The investigations show that the torsional mode, the frequency of which lies in the close neighbourhood of the electrical subsynchronous network mode frequency, is the one which is most affected in this case.