Abstract：The maximum modulation index of the cascaded H-bridge photovoltaic grid-connected inverter controlled by the conventional sinusoidal modulation wave injection strategy (MWIS) is 1, so the working range is small. When the power imbalance is severe, the MWIS will have the problem of over-modulation, leading to the distortion of the grid-connected current. As a result, the output current cannot meet the grid-connected standard, thereby reducing the harvesting ability of photovoltaic energy. In the MWIS, the fine-tuning signal of the modulation signal of each H-bridge module is a part of the modulation wave signal. This paper proposes a modified modulation wave injection strategy (MMWIS). The sinusoidal modulation wave fine-tuning signal in the MWIS can be replaced with a part of the difference between the square wave signal in the same phase as the inductor current and the common modulation wave signal, thereby improving the active power transmission range of the system. The implementation procedures of the MMWIS are as follows. First, the PI controller in the total voltage outer loop calculates the amplitude signal of the grid-connected current. The current inner loop calculates the common modulation signal of each H-bridge unit, and the voltage produced by this modulation signal represents the multi-level voltage on the AC side of the cascaded H-bridge inverter. Second, the remainder of the common modulation wave signal subtracted from the square wave signal (in the same phase as the ac current) is multiplied by the injection coefficient output by the voltage balancing PI controller of each H-bridge unit. The modification signal of each H-bridge unit can be obtained to replace the sine wave modification signal in the MWIS. Third, the low-power cells may be over-modulated around the zero-crossing point due to the large opposite injected modification signals. During the over-modulation period of the low-power modules, the fine-tuning amounts of all modules must be multiplied by the same coefficient to solve the over-modulation problem. At the same time, this common coefficient ensures that the sum of the AC voltages produced by the modulation wave of each H-bridge unit is equal to that produced by the common modulation signal. The simulation and experimental results are as follows. First, the MMWIS can achieve a smooth transition between various working conditions, and the inductor current distortion is small. Second, when the power imbalance is large, compared with conventional methods such as the sinusoidal modulation wave injection strategy and third harmonic compensation strategy, the MMWIS can still maintain high grid-connected current quality. Third, the DC voltage fluctuation of the high-power module is small under the control of MMWIS. Because the peak value of the modulation wave reaches 1 only when the modulation ratio is equal to 1.27 in this method, the instantaneous power fluctuation of the high-power H-bridge unit is small. The conclusions are as follows. The MMWIS can further improve the ability of high-power modules to transmit active power. The AC side fundamental voltage and current of the high-power module are in the same phase when the peak value of the modulation wave reaches 1.27, thereby transmitting the maximum active power. In addition, the MMWIS can overcome the problem of grid-connected current distortion caused by over-modulation in some areas due to the superposition of too many reverse adjustment quantities for low-power modules.
李金玉, 陈杰, 龚春英, 何清. 一种提高级联H桥逆变器功率不平衡运行能力的控制策略[J]. 电工技术学报, 2023, 38(10): 2731-2743.
Li Jinyu, Chen Jie, Gong Chunying, He Qing. A Control Strategy to Improve Working Ability of Cascaded H-Bridge Inverter Under Power Imbalance. Transactions of China Electrotechnical Society, 2023, 38(10): 2731-2743.
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2023, Vol. 38 
