Effects of non-stationary forced oscillation on electromechanical modes

The forced oscillation (FO) containing variable frequencies, known as non-stationary FO, has yet been well understood in power system engineers. This paper analyzes the effects of the non-stationary FO on electromechanical modes (EMs) in the range of 0.2 to 2.0 Hz. The non-stationary forced disturbance (FD) with such variable frequency bands is mathematically modeled. The injection of the non-stationary FD results in the non-stationary FO. A modified subspace-based state-space identification (so-called 4SID) method is applied to estimate the non-stationary frequencies and damping ratios of the EMs along with a moving window. Effects of the non-stationary FO on the EMs are compared to those of the conventional FO. Simulation results are verified in the future 14-machine Southeast Australian power system with converter controlled-based generations under various FO conditions.