Torsional oscillations of unequally-loaded parallel identical turbine-generators

The phenomenon of torsional oscillations of closely coupled parallel identical turbine-generator (T-G) units under unequal loading conditions is studied, and the damping effect of an SVC system on the torsional oscillations of the T-G sets is investigated. An eigenvalue method is used for the studies, and the results are verified by detailed digital computer simulation, using the electromagnetic transients program. It is shown that, under unequal loading conditions, the corresponding torsional oscillations of parallel identical T-G sets are not in-phase. The phase deviation depends on the difference in the output powers of the generators and the mechanical damping torques. As a result of the phase-deviation, the SVC system which utilizes the speed(s) of one of the T-G sets as the torsional stabilizing signal(s) may provide inadequate damping or even cause further excitation of the torsional modes of the other T-G sets. Comparisons of the results with those obtained based on representing the T-G sets by the equivalent-generator model indicate that this model is not a valid representation of unequally loaded parallel identical T-G sets