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With the increasing power ratings of large wind turbines, medium-voltage power conversion is drawing more and more attention because it generally offers higher power density and higher efficiency with reduced current levels. This paper evaluates the practical performance of a medium-voltage, fault-tolerant modular high-power conversion system based on cascaded modular converters. To avoid many large capacitor banks, the low-frequency voltage/power ripple in each cell has been attenuated through balancing the power flow between the H-bridge rectifier and the three-phase inverter by adding a frequency-adaptive quasiresonant controller in the control loops. This can improve the system reliability and reduce the system cost and volume. In the experimental evaluation, an unexpected envelope oscillation issue was observed at the grid-side inverters. It was found that the above issue is caused by the discrepancy of the crystal oscillators on different digital signal processor boards. Then, a carrier-wave phase-locked loop has been proposed to eliminate the envelope oscillation issue. Experimental results of a downscaled three-phase five-level modular wind energy conversion system show that the dc-link voltage ripple can be eliminated without affecting the grid current quality.
Zhang et al. (Fri,) studied this question.