交流谐波经MMC的传导机理及叠加特性研究

Study on conduction mechanism and superposition characteristics of AC harmonics via MMC

连接大量交流源、荷是基于模块化多电平换流器(MMC)的中压柔性直流配电网的未来应用场景之一。然而,非理想交流源、荷产生的谐波扰动会使MMC桥臂出现电压波动并激发环流,进而使桥臂电流畸变,导致直流侧出现纹波。为研究交流谐波经MMC的传导机理,通过平均开关函数和桥臂瞬时功率方程,推导了交流负荷谐波电流和交流系统背景谐波电压在MMC内部激发的桥臂电压波动及环流表达式,分析了其传导机理和规律。同时,分析了不同次交流谐波经MMC的传递叠加特性及其对直流侧纹波含量的影响。仿真结果验证了理论分析的正确性。

As one of the future applications,the medium-voltage flexible DC distribution network,which is based on Modular Multilevel Converter(MMC),will be used to connect to massive AC sources and loads.However,the harmonic disturbances caused by non-ideal AC sources and loads trigger the bridge arm voltage fluctuation and excite the circulation current.Furthermore,the circulation current distorts the bridge arm current and leads to the ripple on the DC side.To study the conduction mechanism of AC harmonics via MMC,this paper deduces the expressions of the circulation current and voltage fluctuation of the bridge arm excited by the harmonic current of AC load and the background harmonic voltage of AC system in MMC on the basis of the average switching function and the instantaneous power equation of the bridge arm,and analyzes the conduction mechanism and rules.Meanwhile,the superposition characteristics of different AC harmonics via MMC and its influences on DC side ripple contents are analyzed.The simulation results verify the theoretical analysis.