Sub-synchronous oscillation (SSO) caused by direct-drive permanent magnet synchronous generators (PMSGs) occurs frequently in Xinjiang Uygur Autonomous Region, China. As a new type of SSO, the mechanism of power oscillation triggered by PMSGs has not been clarified yet. In this study, a small-signal analysis method was used to investigate the response process of the grid-side converter (GSC) controller and a phase-locked loop (PLL) to harmonics. Components with the same frequency of input disturbance signal and an output signal which generated by response process of controller superpose at the outlet of PMSGs could be amplified by positive feedback. Amplified harmonics will cause sustained power oscillation in sub-synchronous frequency. A criterion for judging dangerous frequencies of SSO was deduced based on phase relation between input and output signals, and the form of output signal was predicted under the condition of certain parameters setting. The theoretical analysis is then validated by the simulation results and the on-site data measured by the phasor measurement unit. Furthermore, the impacts of GSC and PLL parameters on SSO were discussed. This study provides the theoretical basis for SSO mechanism and judgement method in practical engineering. Nomenclature u GSC's voltage v equivalent grid voltage s Laplace operator θ feedback angle of PLL θ c angle of GSC's frame U t0 amplitude of grid-side voltage C capacitor in converter's DC link L, L g equivalent inductance of GSC and grid K pp , K ip parameters of PLL's PI controller K ip , K ii parameters of GSC's PI controller Superscriptŝ value on the α-axis Subscripts ref reference value α reference frame of GSC dq global synchronous coordinates of the system 0 steady-state value
Eigen-analysis is widely used in the studies of power system oscillation and small-signal stability. However, it may give inaccurate analyses on subsynchronous oscillation (SSO) when nonlinearity is not negligible. In this paper, a nonlinear analytical approach based on the describing function and generalized Nyquist criterion is proposed to analyze the characteristics of SSO with wind farms. The paper first presents describing function-based model reduction considering key nonlinear elements involved in SSO, and then uses a generalized Nyquist criterion for accurate estimation of SSO amplitude and frequency. The results are verified by time-domain simulations on a detailed model with different scenarios considering variations of the system condition and controller parameters.
Subsynchronous oscillation (SSO) phenomena caused by interactions between controllers of Doubly-fed induction generators (DFIGs) and series capacitor compensators has been well recognized. However, SSO can also happen on a DFIG-based wind farm without series compensation as reported in a recent real event. This paper firstly analyzes the recorded voltage waveform during this SSO event. Then an equivalent model of the system including the wind farm, static synchronous compensator (STATCOM) and the power grid was established. The paper derives the subsynchronous frequency response of the DFIG converter together with the STATCOM in detail and gives the criterion for SSO to occur, which is verified by both time domain simulation and spectrum analysis. The methodology adopted in this paper can be generalized to analyze the causes of a type of SSO that involves DFIGs and STATCOM but not series compensation. The paper also provides practical suggestions on adjustments of control parameters with the DFIG and STATCOM to prevent this type of SSO.
This paper presents an active disturbance rejection control (ADRC) method to mitigate subsynchronous resonance (SSR) in a doubly-fed induction generator (DFIG)-based wind farm connected to series compensated transmission lines. First, the frequency of the SSR and its influencing factors are analyzed, and the limitation of the traditional damping controller based on phase compensation is discussed. Then, a damping controller is designed using the ADRC method, which is adapted to the uncertainty SSR frequency of wind farms. The stator current of the DFIG that contain apparent SSR frequency is adopted as the input signals of the ADRC damping controller, and the outputs are added to the current inner loop of the rotor side converter (RSC), which has fast response speed. The performances of the ADRC damping controller are tested in different compensation levels, numbers of DFIG in wind farm, and parameters of the converter control. The simulation results indicate that the ADRC damping controller is superior to the traditional damping controller for mitigating the SSR because it can automatically estimate and compensate the uncertainty disturbances.
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