Torsional Oscillations of Unequally-Loaded Parallel Identical Turbine-Generators

This paper provides a detailed analytical investigation of the torsional phenomenon of closely coupled parallel identical T-G sets, under unequal loading conditions. The impact of important factors, i.e., 1) load difference of generators, 2) mechanical damping torque components and 3) series capacitor compensation level on the torsional modes of unequally loaded parallel T-G set, are examined. The studies are conducted on a system composed of two parallel identical T-G sets. An eigenvalue method is used for the studies and the analytical results are verified by detailed digital computer simulation of the system, using the EMTP. The T-G sets also represented by the so called EGM, and whenever applicable, the study results are compared with those obtained from the EGM. The studies indicate that the corresponding torsional modes of the parallel T-G sets under unequal loading conditions, are not in-phase. The phase-deviation depends on: 1) the load-difference of the generators and 2) the mechanical damping torque components. Among the mechanical damping coefficients, i.e., mutual-damping coefficient, absolute-speed self-damping coefficient and speed-deviation, self-damping coefficient, the first two are constants and the third one is load dependent. The mechanical damping torque components corresponding to the speed-deviation self-damping coefficients have a noticeable impact on the phase-deviation. This paper also reports the results of an investigation in which an SVC system is used to mitigate the torsional oscillatory modes of the unequally loaded T-G sets.