中压直流配电场景下混合型MMC过调制工况对运行稳定性的影响分析

Analysis of the Influence of Overmodulation Conditions on the Operational Stability of the Hybrid Modular Multilevel Converter in the Medium-Voltage DC Distribution Network Scenario

混合型模块化多电平换流器(hybrid MMC)可通过过调制运行实现中压直流(MVDC)配电网的直流故障穿越.现有研究未充分分析过调制工况下混合型MMC的运行稳定性,无法满足中压直流配电网的运行需求.为此,本文通过与正常工况进行对比,分析了过调制工况对混合型MMC运行稳定性的影响.首先,在电气与控制部分统一的dq坐标系下,构建考虑直流调制比的混合型MMC小信号模型,并将其与详细电磁暂态(EMT)仿真结果对比,验证所建立模型的准确性;其次,对比正常及过调制工况下的根轨迹,进而从控制参数可行域的角度分析了过调制工况对系统运行稳定性的影响;然后,基于主导模态特征根的虚部计算系统失稳时的振荡频率,并将其与相同控制参数下电磁暂态仿真结果进行对比,验证上述稳定性分析结果的正确性;最后,基于参与因子法计算各状态变量对于主导模态的参与程度,从而揭示对系统稳定性影响较大的关键状态变量.理论分析及仿真结果表明,混合型MMC的内部模态会随着交流及直流电流控制器参数的增大而逐渐趋于不稳定;相较于正常工况,过调制工况下混合型MMC控制参数可行域扩大,小信号稳定性提高.

The hybrid modular multilevel converter(hybrid MMC)can realize DC fault ride-through of the mediumvoltage DC(MVDC)distribution network through overmodulation operation.The existing research has not completely analyzed the operational stability of the hybrid MMC under overmodulation conditions,which cannot meet the operational requirements of the MVDC distribution network.Therefore,this study analyzes the influence of overmodulation conditions on the operational stability of the hybrid MMC by comparison with normal conditions.Firstly,under the unified dq coordinate system of electrical and control components,a small-signal model considering the DC modulation ratio of the hybrid MMC is constructed,and the accuracy of the established model is verified by comparison with the detailed electro-magnetic transient(EMT)simulation results.Secondly,the root-locus curves under normal and overmodulation conditions are compared,and the influence of overmodulation conditions on the operational stability of the system is analyzed from the perspective of the feasible regions of the control parameters.Thirdly,based on the imaginary component of the characteristic root of the dominant mode,the oscillation frequency is calculated when the system is unstable and compared with the result obtained from the EMT simulation with the same control parameters,and the accuracy of the above stability analysis results is verified.Finally,the participation degree of each state variable in the dominant mode is calculated based on the participation factor method,and the key state variables significantly affecting the stability of the system are revealed.Theoretical analysis and simulation results show that the internal modes of the hybrid MMC are unstable with increasing AC/DC controller parameters.Compared with those under normal conditions,the feasible ranges of the hybrid MMC control parameters are expanded,and the small-signal stability is improved under overmodulation conditions.