On the Occurrence of Subsynchronous Torsional Oscillations of Gas Turbine Units in an Isolated Energy Area with an Industrial Load

The causes of the occurrence and development of random non-cyclic vibroactivity of gas turbine units (GTUs) in an isolated energy area with an industrial load have been analyzed. It has been found that torsional vibrations of the rotating GTU parts first occur, causing radial oscillation at frequencies close to the first natural frequency of rotating masses (11.8 Hz). This further leads to the manifestation of resonance phenomena in certain modes, a sharp increase in the GTU gear oscillation level, and the shutdown of all operating GTUs with the complete blackout of the energy area. The main cause of the occurrence of subsynchronous torsional oscillations of the GTUs is the load shedding by large asynchronous motors connected through thyristor variable frequency drives (VFDs). Load shedding leads to the VFD input current amplitude fluctuations, which, in turn, cause oscillations of the GTU load current and shaft torque. Field tests have shown that the modulation of the amplitude of the fundamental harmonic of the non-sinusoidal waveform during load shedding may reach 40 % of the rated value. The lists of top-priority and long-term organizational and technical measures are given, the implementation of which will prevent the occurrence of subsynchronous torsional oscillations of the GTUs and contribute to the reliable power supply to the energy area consumers.