Understanding a Type of Forced Oscillation Caused by Steam-Turbine Governors

Abstract

This letter studies the mechanism of a type of forced power system oscillation induced by imperfect nonlinearity compensation with a steam-turbine governing system. The nonlinear valve control characteristic of a steam turbine is usually compensated by its inverse characteristic performed by its governor for an overall linear characteristic of the governing system. In reality, a governing system may remain nonlinear due to imperfect compensation, which can result in a limit cycle forcing the generator and even an extensive neighboring area of the power grid to oscillate at a low frequency. A compensation imperfection index is proposed to evaluate the residual nonlinearity and discover how a limit cycle can occur and cause forced oscillation of a steam-turbine generator. Time-domain simulation on a real forced oscillation event caused by a steam-turbine governor validates the finding of the paper and supports the fact that opening the governor control loop of the oscillation-source generator can effectively eliminate this type of forced oscillation.