Abstract：The novel cascaded wind power converter with isolated quad-active-bridge (QAB) converters has the advantages of high voltages and modular structures. Furthermore, it can directly eliminate low-frequency voltage pulsations and significantly reduce DC-bus capacitances under instantaneous pulsating power aggregation and transmission. Therefore, the proposed cascaded wind power converter has a broad application prospect in 10 MW to 20 MW offshore wind power systems. However, the input-winding currents of the QAB converter are biased with instantaneous pulsating power transmission, resulting in worsened current stress and losses within the QAB converter. This research focuses on the causes of the input-winding current bias and the approach to thoroughly eliminating the input-winding current bias. The main work is given as follows. (1) Based on the decoupled equivalent circuit model of the QAB converter and the employed single-phase-shifted (SPS) modulation, the factors contributing to winding current bias and the pulsating trends of the winding current amplitude are analyzed. When the QAB converter adopts the SPS modulation strategy to transmit instantaneous pulsating power, the primary leakage inductance volt-second is imbalanced due to the unequal duty cycle of the input winding voltage's positive and negative polarities. The input winding current shows a downward bias. Although the equivalent resistance of the QAB converter can mitigate part of the winding bias by consuming the remnant of the primary leakage inductance, the effect of this approach is limited, and the downward bias of the input winding current is still significant. (2) An improved phase-shifted modulation strategy is proposed. The proposed optimized modulation strategy maintains the external phase-shifted angle necessary for transmitting instantaneous pulsating power while introducing an internal phase-shifted angle to balance the input winding voltage. By ensuring equal duty ratios for the positive and negative polarities of the input winding voltage, a balanced volt-second product in the primary leakage inductances can be achieved, thereby thoroughly eliminating the input-winding current bias. (3) Utilizing Matlab/Simulink, a 10 kV/20 MW wind turbine simulation model is constructed. The proposed improved phase-shifted modulation can eliminate the input-winding current bias. In contrast, the conventional SPS modulation yields a bias of approximately 600 A. The results validate the accuracy of the theoretical analysis and the feasibility of the proposed optimized phase-shifted modulation strategy. Additionally, a 2 kW low-power experimental platform is constructed. When the QAB converter operates in the rated state, the losses and current stress (peak) of the QAB converter are 94.3 W and 6 A with the conventional SPS modulation, while only 83.1 W and 5 A with the proposed optimized phase-shifted modulation. The losses and current stress (peak) can be decreased by about 11.9% and 16.7%, respectively, which optimizes the operation characteristics of the cascaded wind power converter.
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