An Interharmonic Phasor and Frequency Estimator for Subsynchronous Oscillation Identification and Monitoring

Recently, subsynchronous oscillations (SSOs) have occurred frequently due to the interaction between wind farm controllers and transmission networks. When an SSO occurs, subsynchronous and supersynchronous interharmonics are present in a voltage/current signal. Because SSOs are serious threats to power system safety and stability, it is important to study subsynchronous and supersynchronous interharmonic phasor and frequency estimators for SSO identification and monitoring (thus for mitigation equipment operation). The systematic errors of the Taylor-Fourier multifrequency model-based parameter estimator are analyzed theoretically. It is found that the key to high estimation accuracy is to select interharmonic and fundamental model frequencies as accurately as possible. To this end, the three-point interpolated discrete Fourier transform and an iteration scheme are used to select the initial model frequencies and modify them iteratively. Simulation tests show that the interharmonic total vector error (TVE) and frequency error (FE) of the proposed method are always below 0.6% and 25 mHz, respectively. The fundamental TVE and FE limits in the IEEE standard can also be fully met, and the computation time can meet the requirements of high reporting rate phasor measurement units. In addition, the current samples recorded in an SSO event are used to demonstrate the real benefits of the proposed method.