Coupling Phenomenon of Torsional Modes

The level of coupling of the oscillatory mode A to the oscillatory mode B is a measure of tendency of mode B to interact and affect the frequency and/or the amplitude and/ or the damping of mode A. Although the coupling phenomenon of the low frequency (0.1-2.0 Hz) oscillatory modes of power systems has been investigated, however, the coupling phenomenon of the torsional modes of a T-G set has not attracted enough attention. This stems from the fact that the torsional response of an unsynchronized T-G set with zero mechanical damping coefficients, can be expressed in terms of "decoupled" modes. Furthermore, in spite of the lack of mathematical justification, the tendency exists to use the concept of either "decoupled" or "loosely-coupled" torsional modes even for modelling synchronized T-G sets. This investigation shows that depending on the operating point and the system parameters, the coupling phenomenon can have a noticeable impact on the torsional oscillations of a T-G set. In some studies based on all-mode models, the effect of mode coupling has been surfaced as the instability of one torsional mode while attempt is made to mitigate the other mode(s). This sort of torsional instability has been blamed on inadequate conditioning of the control signals or poor design of the control system.