Abstract：Each power unit of a cascaded multilevel inverter (CMI) can be controlled independently without capacitor voltage sharing problems, which is suitable for distributed photovoltaic power generation systems. By combining quasi-Z-source (QZS) with CMI, quasi-Z-source cascaded multilevel inverter (QZS-CMI) can achieve boost and inverter functions in a single stage without setting a dead zone and thus improve system efficiency. A master-division control strategy and a multi-carrier phase-shifted PWM (MPSPWM) method are proposed to reduce the total harmonic distortion (THD) of QZS-CMI grid-connected current and improve the system's dynamic response. Moreover, aiming at the problem of over-modulation when the power of each module is seriously unbalanced, this paper proposes a third harmonic injection control. In the master-division control, the voltage outer-loop division controller uses PI control to achieve the stability of the DC-link voltage of each module. The grid-connected current master controller uses an improved deadbeat control (DBC), where the current deviation is corrected in a weighted form to fast-track the grid-connected current. Also, the MPSPWM prolongs the conduction time of two switches in the same bridge arm when the switching state changes. Accordingly, the shoot-through zero vector is inserted at the switching time to reduce the switching loss. However, suppose the system is over-modulated when the power is unbalanced. In that case, the third harmonic is injected into the modulation wave of the over-modulation module, and the reversed third harmonic is injected into the non-over-modulation modules. As a result, the modulation wave changes from a sine wave to a saddle wave. Thus, the system can operate normally without over-modulation. A three-module cascaded seven-level topology is used for simulation and experiments. In the steady state, each DC-link voltage of the three modules is stable at about 70 V, and the QZS-CMI output voltage amplitude is about 210 V with seven levels. After being filtered, the output voltage has no phase difference to the current, where its power factor approaches 1.0, and the sine wave is revealed well at 50 Hz. Using the MPSPWM method, the shoot-through zero vector in a switching cycle is divided into four parts and inserted at the switching time on average to reduce the switching frequency. The traditional PR control's THD of the inverter output current is 2.84%, and the improved DBC is decreased to 0.86%. When the photovoltaic cell's output power changes, the inverter's output current can reach a steady state again within 0.005 s. The improved DBC can effectively suppress the harmonics and improve the response speed of the output current. When the power is seriously unbalanced, the output current is distorted, with a THD of 5.69%. After using the third harmonic injection control, the THD is suppressed to 0.91%. The simulation and experiments concluded that: (1) The master-division control improves the overall power supply quality of the system. The improved DBC has a faster response speed than the traditional PR control, effectively suppressing the output current harmonics. (2) Using the proposed MPSPWM method, the switching frequency is effectively reduced, and the implementation scheme is simple. It is more suitable for QZS-CMI than simple boost modulation. (3) When the power is seriously unbalanced, the third harmonic injection control is adopted to ensure that each module photovoltaic cell is operating at the maximum power point and effectively prevent the occurrence of overmodulation.
[1] Peng Fangzheng.Z-source inverter[J]. IEEE Transa- ctions on Industry Applications, 2003, 39(2): 504-510. [2] Diao Naizhe, Sun Xianrui, Song Chonghui, et al.A multimodulation times SVPWM for dead-time effect elimination in three-level neutral point clamped converters[J]. IEEE Transactions on Industrial Electronics, 2020, 68(7): 5476-5485. [3] Ouni S, Narimani M, Cheng Zhongyuan, et al.A new postfault control method for CHB inverter to increase maximum output voltage[J]. IEEE Transactions on Industry Applications, 2020, 56(5): 5499-5510. [4] Guo Mingzhu, Liu Yushan, Ge Baoming, et al.Modeling and analysis of LC filter integrated quasi-Z source indirect matrix converter[J]. International Journal of Circuit Theory and Applications, 2020, 48(4): 567-586. [5] 徐杰, 徐天乐. 基于五电平控制的中点箝位单相逆变器的研究[J]. 太阳能, 2021(12): 69-74. Xu Jie, Xu Tianle.Research on NPC single-phase inverter based on five-level control[J]. Solar Energy, 2021(12): 69-74. [6] Li Xiangjun, Wang Lijun, Yan Ning, et al.Coopera- tive dispatch of distributed energy storage in distribution network with PV generation systems[J]. IEEE Transactions on Applied Superconductivity, 2021, 31(8): 0604304. [7] Zhou Yan, Liu Liming, Li Hui.A high-performance photovoltaic module-integrated converter (MIC) based on cascaded quasi-Z-source inverters (qZSI) using eGaN FETs[J]. IEEE Transactions on Power Electronics, 2013, 28(6): 2727-2738. [8] Liang Weihua, Liu Yushan, Ge Baoming, et al.Double-line-frequency ripple model, analysis, and impedance design for energy-stored single-phase quasi-Z-source photovoltaic system[J]. IEEE Transa- ctions on Industrial Electronics, 2018, 65(4): 3198-3209. [9] 张强, 吴延飞, 张保顺, 等. 基于三相电流型五电平整流器的空间矢量脉宽调制算法[J]. 电工技术学报, 2020, 35(24): 5134-5141. Zhang Qiang, Wu Yanfei, Zhang Baoshun, et al.Space vector pulse width modulation strategy based on three-phase five-level current source rectifier[J]. Transactions of China Electrotechnical Society, 2020, 35(24): 5134-5141. [10] Liu Yushan, Abu-Rub H, Ge Baoming, et al.Phase- shifted pulse-width-amplitude modulation for quasi- Z-source cascade multilevel inverter based PV power system[C]//2013 IEEE Energy Conversion Congress and Exposition, Denver, CO, USA, 2013: 94-100. [11] 姜翼展, 张经纬, 何凤有, 等. 一种开关损耗优化的Z源逆变器调制策略[J]. 电工技术学报, 2023, 38(16): 4312-4323. Jiang Yizhan, Zhang Jingwei, He Fengyou, et al.A switching loss optimization modulation strategy for Z-source inverter[J]. Transactions of China Elec- trotechnical Society, 2023, 38(16): 4312-4323. [12] 李媛, 彭方正. Z源/准Z源逆变器在光伏并网系统中的电容电压恒压控制策略[J]. 电工技术学报, 2011, 26(5): 62-69. Li Yuan, Peng Fangzheng.Constant capacitor voltage control strategy for Z-source/quasi-Z-source inverter in grid-connected photovoltaic systems[J]. Transa- ctions of China Electrotechnical Society, 2011, 26(5): 62-69. [13] 方番, 李媛, 肖先勇, 等. 储能型准Z源逆变器的有限集模型预测控制策略[J]. 中国电机工程学报, 2019, 39(7): 2133-2144. Fang Fan, Li Yuan, Xiao Xianyong, et al.An finite control set-model predictive control for energy-stored quasi-Z-source inverters[J]. Proceedings of the CSEE, 2019, 39(7): 2133-2144. [14] 王春霖, 赵涛, 徐友, 等. 基于模型预测控制的级联式模块化多电平换流器控制策略[J]. 电气自动化, 2023, 45(5): 53-56. Wang Chunlin, Zhao Tao, Xu You, et al.Cascaded modular multilevel converter control strategy based on model predictive control[J]. Electrical Automation, 2023, 45(5): 53-56. [15] 吴春, 杨佳磊, 陈强. 准Z源逆变器级联模型预测控制[J]. 中国电机工程学报, 2021, 41(12): 4286-4296. Wu Chun, Yang Jialei, Chen Qiang.Sequential-model predictive control for quasi-Z-source inverter[J]. Proceedings of the CSEE, 2021, 41(12): 4286-4296. [16] 黄海宏, 颜碧琛, 王海欣. 并联级联H桥电源电流跟踪优化与环流抑制的模型预测控制[J]. 电工技术学报, 2023, 38(16): 4376-4390. Huang Haihong, Yan Bichen, Wang Haixin.Model predictive control of current tracking optimization and circulating current suppression of parallel cascaded H-bridge power supply[J]. Transactions of China Electrotechnical Society, 2023, 38(16): 4376-4390. [17] Liu Yushan, Ge Baoming, Abu-Rub H, et al.An effective control method for quasi-Z-source cascade multilevel inverter-based grid-Tie single-phase photo- voltaic power system[J]. IEEE Transactions on Industrial Informatics, 2014, 10(1): 399-407. [18] Zhao Tiefu, Wang Gangyao, Bhattacharya S, et al.Voltage and power balance control for a cascaded H-bridge converter-based solid-state transformer[J]. IEEE Transactions on Power Electronics, 2013, 28(4): 1523-1532. [19] 杨浩, 杨黎晖, 陈延明, 等. 基于扰动补偿的LCL型并网逆变器无差拍预测电流控制策略[J]. 中国电机工程学报, 2023, 43(21): 8411-8423. Yang Hao, Yang Lihui, Chen Yanming, et al.Deadbeat predictive current control strategy based on disturbance compensation for grid connected inverter with LCL filter[J]. Proceedings of the CSEE, 2023, 43(21): 8411-8423. [20] 谷鑫, 鲁金月, 王志强, 等. 基于无差拍电流预测控制的永磁同步电机谐波电流抑制策略[J]. 电工技术学报, 2022, 37(24): 6345-6356. Gu Xin, Lu Jinyue, Wang Zhiqiang, et al.Harmonic current suppression strategy for permanent magnet synchronous motor based on deadbeat current predi- ction control[J]. Transactions of China Electro- technical Society, 2022, 37(24): 6345-6356. [21] Ramachandran N, Murugan S, Vinayagam C, et al.Real-time implementation of a seven-level multilevel DC link inverter for solar PV system during partial shading[J]. Electric Power Components and Systems, 2023, 51(18): 2125-2134. [22] 王书征, 赵剑锋, 姚晓君, 等. 级联型光伏并网逆变器在光照不均匀条件下的功率平衡控制[J]. 电工技术学报, 2013, 28(12): 251-261. Wang Shuzheng, Zhao Jianfeng, Yao Xiaojun, et al.Power balanced controlling of cascaded inverter for grid-connected photovoltaic systems under unequal irradiance conditions[J]. Transactions of China Electrotechnical Society, 2013, 28(12): 251-261. [23] Eskandari A, Javadian V, Iman-Eini H, et al.Stable operation of grid connected cascaded H-bridge inverter under unbalanced insolation conditions[C]// 2013 3rd International Conference on Electric Power and Energy Conversion Systems, Istanbul, Turkey, 2013: 1-6. [24] Lewicki A, Odeh I C, Morawiec M.Space vector pulsewidth modulation strategy for multilevel cascaded H-bridge inverter with DC-link voltage balancing ability[J]. IEEE Transactions on Industrial Electronics, 2022, 70(2): 1161-1170. [25] 李金玉, 陈杰, 龚春英, 等. 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, et al.A control strategy to improve working ability of cascaded H-bridge inverter under power imbalance[J]. Transa- ctions of China Electrotechnical Society, 2023, 38(10): 2731-2743.
2025, Vol. 40 
