Solving Turbine Governor Instability at Low-Load Conditions

During routine commissioning of a steam turbine load-sharing system, serious low-load frequency instabilities were discovered. These instabilities were causing undamped oscillations in power and frequency to escalate until protective relays tripped a generator offline. Root-cause investigation led to a robust solution and some rather startling revelations about the implications of electronic governor controls and small (micro) grids. There are basically two ways to form an electronic governor control loop with droop: 1) speed control with a MW droop; or 2) MW control with a speed droop. The analysis in this paper shows one method to be superior under low-load conditions. The results of this analysis have implications for the frequency stability of the power grid today. Microgrids, green energy, distributed generation, and isolated industrial plants can all be susceptible to this instability. The authors estimate that approximately 60% of today's generation is prone to destabilize the power system frequency under low-load conditions.