Abstract In modular multilevel converter-based photovoltaic systems (MMC-PVSs), the PV power imbalance caused by shadowing leads to differences among modulation signals of sub-modules (SMs) and further changes the characteristics of the high-frequency common mode voltage (CMV) of the MMC-PVS AC port. Thus, the existing high-frequency CMV suppression methods can not eliminate the high-frequency CMV of MMC-PVS. This paper analyzes the elimination condition of the high-frequency CMV. That is, the sum of the upper arm voltages of three phases is always the same as that of the lower arm voltages of three phases at any time. According to the control loop, the sum of pulse widths of all SMs in the three phases’ upper arm and lower arm are equal, illustrating that zero-CMV is feasible from the aspect of the control loop. Thus, the key is to shape the arm voltages using the modulation method to satisfy the zero-CMV condition. Then, the principle of SM modulation signal difference caused by PV power imbalance is revealed and illustrated: a high PV module power results in a high SM modulation index and a high risk of over-modulation. This paper proposes a nose-to-tail pulse arrangement modulation to eliminate the high-frequency CMV of MMC-PVS under PV power imbalance conditions. The basic principle of nose-to-tail pulse arrangement modulation is to arrange the SM pulse voltages in sequence; namely, the rising edge time of the current SM pulse voltage is aligned with the falling edge time of the previous SM pulse voltage. Accordingly, the shapes of the sums of the three-phase upper arm voltages and the three-phase lower arm voltages are the same during one switching period, and the condition of zero-CMV is satisfied. Compared with the sawtooth carrier, the rising time and falling time of each SM are determined by the duty cycle of the SM during each switching period to obtain the rising and falling edges of the SM pulse voltage. Then, according to the derived logical expression, the SM switching function can be generated. With the generated switching function of each SM, high-frequency CMV elimination can be achieved on the premise of SM capacitor voltage balance. The simulation and experimental results are as follows. When the system modulation index m is set as 0.8, the proposed nose-to-tail pulse arrangement modulation can eliminate the high-frequency CMV on both the PV power balance and imbalance conditions via arranging the SM pulse voltages with the given orders, maintaining the SM capacitor voltage balance. From the simulation and experimental results of m=0.9, it can be concluded that the proposed method can operate under m larger than 0.866 under both PV power balance and imbalance conditions on the premises of SM capacitor voltage balance and system stability.
Pan Yao,Sun Xiaofeng,Cai Yao等. A Nose-to-Tail Pulse Arrangement Modulation for Suppressing Common Mode Voltage of Modular Multilevel Converter Photovoltaic Systems[J]. Transactions of China Electrotechnical Society, 2025, 40(4): 1221-1235.
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2025, Vol. 40 
