一种拓展模块化多电平变换器的光伏系统稳定运行范围的谐波补偿调制策略

A Harmonic Compensation Modulation Strategy for Extending Steady Operation Range of Modular Multilevel Converter-Based Photovoltaic System

由于阴影遮挡、损坏等问题,基于模块化多电平变换器的光伏系统中存在光伏(PV)输出功率不平衡的现象,会使高输出功率PV对应的子模块过调制。谐波补偿调制策略是解决该问题的一种常见方案,通过向高功率PV的子模块中注入谐波而避免子模块过调制,低功率PV的子模块(称为正常子模块)则需要补偿数量相同、相位相反的谐波,使桥臂内的谐波相抵消而不会造成桥臂电压和电流畸变。然而,在PV功率严重不平衡时,为了避免高功率PV的子模块过调制,现有谐波补偿调制策略需要注入的谐波过大,会超过正常子模块的补偿范围,使正常子模块出现过调制,导致桥臂电压和电流畸变,影响系统的稳定运行。该文提出一种新型谐波补偿调制策略,在高功率PV的子模块不发生过调制的前提下避免正常子模块过调制,保证系统的稳定运行。同时,该文对不同谐波补偿调制策略稳定运行范围进行了定性和定量评估,表明所提谐波补偿调制策略能在更多PV功率不平衡工况下稳定运行。通过仿真与实验验证谐波补偿调制策略在模块化多电平变换器光伏系统中的可行性与理论分析的正确性。

Harmonic compensation modulation(HCM)strategies are commonly used solutions for photovoltaic(PV)power imbalance conditions of a modular multilevel converter-based photovoltaic system(MMC-PVS).The maximum equivalent fundamental-frequency modulation index is extended to 1.273,and the system’s stable operation range is expanded.According to the principle of HCM strategy,harmonics are injected into submodules(SMs)with high-power PVs to prevent over-modulation,and the rest of SMs corresponding to low-power PVs(referred as normal SMs)compensate for the same amount of opposite harmonics to avoid distortions on phase arm voltages and currents.However,existing HCM strategies may lead to over-modulation on normal SMs and distortions on phase arm voltages and currents when facing severe PV power imbalance.This paper proposes a novel HCM strategy to prevent the over-modulation on SMs and normal SMs simultaneously.The principle of HCM strategy is introduced,and an HCM strategy for expanding the system’s stable operation range is proposed.Then,relationships between the injected harmonic amplitude and equivalent fundamental-frequency modulation index in over-modulation SM are analyzed.On this basis,operation constraints of HCM strategies in MMC-PVS are derived as the criteria of system stable operation.Combined with the concepts of stable operation space(SOS)and stable operation factor(SOF),stable operation ranges of different HCM strategies are evaluated qualitatively and quantitatively.The results show that the SOF of the proposed HCS strategy is improved by up to 71.57%,which is 4.61 times that of the traditional sinusoidal pulse width modulation(SPWM)strategy and significantly larger than those of most existing HCM strategies.When PV power is slightly imbalanced,the proposed HCM strategy and existing HCM strategies achieve smooth transitions under various working conditions with negligible current distortion.Under severe PV power imbalance,the proposed HCM strategy maintains system stability,while other HCM strategies exhibit current distortions.The HCM strategies can normally operate when the number of SMs of each phase arm is 3 and 4,respectively.Compared with the existing HCM strategies,the proposed HCM strategy extends the stable operation range of MMC-PVS,avoiding over-modulation on normal SMs and SMs with high power PVs simultaneously.The HCM strategies maintain the normal operation of MMC-PVS with odd or even numbers of SMs in each phase arm under PV power imbalance conditions.The proposed HCM strategy can also be applied in cascaded H-bridge converter-based PV systems with the capability of stable operation range extension.