基于桥臂电流控制的MMC改进电容电压均衡控制策略研究

Improved Capacitor Voltage Balancing Control Strategy for Modular Multilevel Converter With Arm Current Control

子模块是模块化多电平变换器(modular multilevel converter,MMC)交直流之间能量交互的缓冲环节,在稳态、暂态下子模块电容电压保持均衡对于MMC不间断运行至关重要。首先,分析电网故障对无变压器MMC内部动态特性以及对交直流侧电压电流的影响。其次,为了提升MMC电容电压控制性能,提出一种基于桥臂电流控制的MMC改进的四层结构子模块电容电压均衡控制策略,包括子模块全局电容电压平均值控制、相间电容电压平衡控制、上下桥臂电容电压平衡控制和桥臂内子模块电容电压平衡控制,由此得到内环桥臂电流控制的电流指令值。最后,通过仿真研究和样机实验验证所提控制策略的可行性和有效性。结果表明:所提的控制策略在交流电网不对称故障和直流极对极短路故障下均能保持MMC子模块电容电压的均衡,并有效地消除交流侧谐波电流和桥臂内部的正序、负序及零序交流环流。

Submodules(SMs) are the buffer link of power transmission between AC and DC grid for modular multilevel converter(MMC), which is very important to balance SMs capacitor voltages in steady and fault ride through period.Firstly, the influence of power grid fault on the internal dynamic characteristics of the transformerless MMC is investigated. Secondly, in order to improve the performance of the capacitor voltage control for MMC, this paper proposes an improved four layers capacitor voltage balancing control strategy with arm current control. The proposed four-layer capacitor voltage control consists of overall submodules voltage averaging control, leg-balancing control, arm-balancing control, and individual balancing control. The current reference value for the inner loop arm current control is subsequently generated by the voltage control outer loop. Finally, simulation and experimental results are provided to validate the feasibility and effectiveness of the proposed strategy. The results show that the proposed strategy in this article can well balance the capacitor voltages under ac and dc grid fault conditions, and effectively eliminate the harmonics of ac-side currents and the positive, negative and zero-sequence currents in circulating currents.