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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. The time-domain simulation results and impedance modelbased analysis demonstrate that the proposed repetitive-PI control strategy enhances the system's damping performance and improves its disturbance resistance.INDEX TERMS Subsynchronous oscillation, wind power system, series-compensated transmission line, repetitive-PI control.

As the continuous consumption of non-renewable energy leads to resource shortages, distributed PV generation technology has received widespread attention. The DC microgrid is an effective way to connect distributed PV generation. However, DC microgrids have numerous challenges, such as the absence of zero current crossing point and high fault current rising rate. In the paper, a bi-directional DC solid-state circuit breaker based on flipped Γ-source is proposed to overcome some of the challenges in DC microgrid. The topology uses the mutual inductance current generated by the transformer to force the SCR naturally commutates off. Which can rapidly interrupt and isolate the faulty part, and at the same time, there will be no circulating current impact on the source side. The diode bridges allow the response to faults on either the source or load side with only a single controlled switch. Therefore, the topology is simplified while enabling it to obtain the capability of bidirectional operation protection. Finally, using the simulation software PSCAD/EMTDC and experimental platform verifies the effectiveness of the topology in this paper.

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