An isolated modular multi-level AC/AC converter with high-frequency link and pulse flip circuit for fractional frequency transmission system application

Based on the modular topology and the traditional voltage-sourced converter (VSC) model, this paper presents an isolated modular multi-level AC/AC converter with high-frequency (HF) link and pulse flip circuit for fractional frequency transmission system (FFTS) application. The proposed converter uses the HF transformer to realize electrical isolation, omitting the industrial frequency (IF) transformer with large volume and heavy weight. In the primary side of the HF transformer, the power transmission is realized by the rectifier and the inverter. In the secondary side of the HF transformer, the double H-bridges are used to realize the natural freewheeling of the leakage current, which solves the problem of voltage overshoot. On the basis of carrier phase-shifted pulse width modulation (CPS-PWM), an improved modulation strategy is proposed to realize the HF output voltage of the inverter. It has the advantages of easy expansion, no circulation current, good output voltage quality, low cost and no need of offshore converter station, so it has great potential in the field of offshore wind power grid connection. The working principle, natural freewheeling modes, modulation method and overall system control scheme of the proposed topology are introduced in detail, and the feasibility of the proposed topology is verified by simulation in MATLAB/Simulink environment and test through APPSIM platform.