ABSTRACT: The subsynchronous interaction between the wind farm’s grid-side converter and the seriescompensated transmission line can trigger subsynchronous oscillations (SSO). The wind power system oscillation frequency depends on the actual operational conditions. This paper proposes a repetitive-PI control strategy to mitigate oscillations under various conditions. The repetitive-PI control offers fast dynamic performance and high steady-state accuracy, which can eliminate steady-state errors and improve the system’s robustness. Firstly, the repetitive-PI composite controller is derived based on the frequency domain expansion of the repetition controller. Then, the stability of the repetitive-PI composite controller is analyzed, and its main parameters of it are designed. Finally, to demonstrate the efficacy of the proposed suppression technique for SSO, simulations are conducted under different wind speeds, series compensation levels, and numbers of operating wind turbines. A battery energy storage system is utilized in the DC link between RSC and GSC of the DFIG to improve the stability of the system under transient conditions. The simulation work is carried out using MATLAB/SIMULINK.
INTRODUCTION
With the increasing demand for electricity, there is an urgent requirement to transmit large amounts of power over long distances from remote regions to load centers. Series compensation technology is frequently utilized in transmission systems to enhance transmission capacity costefficiently. However, recent research has shown that using series compensation can lead to oscillations in wind turbine generator systems. In fact, in 2009, a wind farm based on a doubly-fed induction generator (DFIG) connected to the grid via a series compensation line experienced a large-scale wind power oscillation in South Texas, causing equipment damage. Similar oscillation events have also been reported in the Guyuan wind farm and Oklahoma Gas & Electric. Previous studies have demonstrated that subsynchronous oscillation is triggered by the interaction between doublyfed induction generators and the series compensation line. As a result, subsynchronous oscillation is also known as subsynchronous control interaction (SSCI). When a disturbance occurs in a doubly-fed wind turbine series-compensated grid-connected system, the resonant currents in the system generate corresponding subsynchronous currents in the rotor-side converter. These subsynchronous currents cause current waveform distortion and phase shift. The rotor-side controller detects this change and adjusts the inverter output voltage, changing the rotor current. If the output voltage increases the rotor current, the oscillation of the resonant current will intensify, resulting in system oscillation and subsynchronous control interactions. The wind speed, series compensation, and the actual number of operating turbines all led to oscillations. If SSCI is not mitigated in time, it can lead to power system oscillations and even breakdowns. Therefore, investigating measures to solve SSCI in different scenarios is crucial to ensuring the stable operation of the wind power system. The SSO suppression strategies of wind power gridconnected systems can be divided into machine-side controls and grid-side controls according to their different locations. The machine-side controls include transformer control parameter optimization and additional damping control. The grid-side control includes changing the operation mode of the power grid and installing flexible AC transmission system (FACTS) devices. The influence of TCSC and fixed series compensation on subsynchronous resonance is studied separately and pointed out that TCSC has a certain SSO suppression effect compared to fixed series compensation. Furthermore, a unified powerflow controller (UPFC) with a novel damping control based on the modal control theory is proposed to mitigate SSCI in a series-compensated wind farm. A different method to mitigate SSCI in wind farms with series-compensated lines is proposed. This approach proposes a wide-band damping control strategy based on MMC-STATCOM. The generation-side controls primarily involve adjusting PI parameters or implementing advanced control techniques. The use of a non-dominated sorting genetic algorithm is proposed to optimize the PI parameters of the wind power system. This approach is aimed at improving system damping performance and mitigating oscillations. The proportional gain of the RSC control loop is reduced to suppress SSCI. Advanced nonlinear controllers have also been proposed to replace PI. To mitigate subsynchronous control interaction (SSCI) in DFIG-based wind farms, The use of active disturbance rejection control (ADRC) strategy is proposed. This approach adopts ADRC to add the system’s damping over the subsynchronous frequency range. This incorporates notch filters into the inner loop control of the rotor-side converter (RSC) in DFIG to interrupt the propagation of subsynchronous oscillations and suppress them. Furthermore, adding damping control on the side of the converter can improve wind power system damping and stability. However, the above-mentioned SSCI damping approaches have certain drawbacks. Wind power systems are complex and highly nonlinear multivariable systems that operate under varying and unpredictable conditions. As a result, adjusting the PI parameters to mitigate SSCI may have a limited effect. Moreover, adding FACTS devices would increase costs, making wind farms less economically viable. The SSCI oscillation frequency varies with the system operating parameters, making selecting appropriate filtering parameters and phase shift link parameters difficult in practical scenarios. This further complicates the system design and implementation.

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