Torsional Interactions between Synchronous Generators and Long Transmission Lines

The dangers of subsynchronous resonance in series capacitor compensated transmission lines have led some utilities to consider the alternative of shunt capacitive compensation, since the electrical resonant frequencies in this case are usually considered to be supersynchronous. However, in heavily compensated or very long lines, at least one of the resonant frequencies may become subsynchronous. This paper examines the torsional interactions between electrical resonant frequencies (which can be supersynchronous or subsynchronous) and the mechanical oscillations of a generator connected to a long transmission line. Examples are given and the electrical damping is calculated in order to evaluate the risk of self-excitation occurring.