Flexible Power Decoupling Control for QAB Converters in Cascaded Wind Power Systems by Coordinate Transformation

The cascaded H‐bridge (CHB) converter is a promising topology for medium‐voltage and high‐power offshore wind turbines. Recently, the quad‐active‐bridge (QAB) converter has been used to reduce the required DC‐link capacitors by transferring and mitigating the low‐frequency pulsation power. However, the existing power control schemes transfer all the low‐frequency pulsation power to the QAB side, increasing current stress on the QAB converter. This article proposes a flexible power decoupling control for the QAB converter using a proposed coordinate transformation method. This scheme can easily decouple the instantaneous power into two parts: the constant DC power delivered to the grid‐side inverter and the low‐frequency pulsation power canceled inside the QAB converter, both of which can be independently controlled. By flexibly adjusting the DC‐link reference voltages and designing appropriate regulators for the decoupling voltage loops, the low‐frequency pulsation power flowing into the QAB converter can be precisely regulated, meaning the ripple voltage of the DC‐link can be flexibly controlled as required. This method is also beneficial for reducing the current stress (peak and RMS) of the QAB converter. Simulation and experimental results are presented to validate the feasibility of the proposed flexible power decoupling control scheme.