含主动限流控制的MMC-HVDC电网直流短路故障电流解析计算

Analytical Calculation of DC Short-Circuit Fault Current of Modular Multi-Level Converter-HVDC Grid with Active Current Limiting Control

传统的MMC-HVDC电网直流短路故障电流计算一般采用等效RLC电路方法。但是该方法没有考虑主动限流控制(ACLC)对等效电路RLC参数的影响,因此无法直接用于含ACLC的MMC-HVDC电网的直流短路故障电流的解析计算问题。该文通过将ACLC的控制效果等效为虚拟阻抗产生的压降,建立了体现ACLC的控制影响的等效RLC电路,以包含电流变化率限流环节的典型ACLC算法为例,推导了直流双极短路故障条件下ACLC的控制参数与虚拟阻抗的映射关系以及相应的直流故障电流解析表达式。最后,以基于含ACLC的半桥型四端MMC-HVDC电网为例,建立PSCAD/EMTDC仿真模型,对采用ACLC的直流双极短路故障电流解析计算结果进行了验证。结果表明:故障后10ms内直流短路故障电流的PSCAD仿真结果与所提出的含ACLC的等效RLC电路解析计算结果的平均误差在8.29%以内,最大误差在10.99%。因此,使用所提出的方法具有工程实用性。

The traditional short-circuit fault current analytical calculation of modular multi-level converter based on light voltage direct current(MMC-HVDC)grid generally uses equivalent RLC circuit method.However,this method does not consider the influence of active current limiting control(ACLC)on the RLC parameters of the equivalent circuit,thus it can not be directly applied to the analytical calculation for MMC-HVDC grid with an ACLC.To fill this gap,this paper proposes an analytical calculation method for DC short-circuit fault current of MMC-HVDC grid with an ACLC by establishing the relationship between the control effect and RLC parameters in the equivalent RLC circuit.Firstly,the control principle of the typical ACLC is analyzed,concluding that the essence of ACLC’s principle is to reduce the outlet DC voltages of converter stations by dynamically reducing the inputting number of sub-modules of the converters.Secondly,by mapping the voltage drop resulted from ACLC to the voltage drop generated by the introduced virtual impedance in the equivalent RLC circuit,the DC short-circuit fault loop can be equivalent to the RLC circuit with the virtual impedance within 10 ms after the pole to pole short-circuit fault occurs.Then,the virtual impedance value can be determined according to the parameters of the current limiting controller,and the analytical solution of the DC short-circuit fault current can be found according to the known RLC parameters and the derived virtual impedance value.Thirdly,taking the four-terminal MMC-HVDC grid with the ACLC with current limiting link of current change rate as an example,the equivalent RLC circuit with the virtual impedance is derived according to the relationship between the control parameters of the ACLC and the virtual impedance,thus,the analytical solution of the DC short-circuit fault current is obtained.Finally,taking the four-terminal MMC-HVDC grid based on half bridge as an example,the PSCAD/EMTDC simulation model is established to validate the proposed method.The calculation results of DC short-circuit fault current of MMC-HVDC grid with ACLC by the proposed method arecompared with the electromagnetic transient simulation results of DC short-circuit fault current under different fault points and different parameters by PSCAD/EMTDC model.According to the simulation analysis results,the following conclusions are obtained:(1)By mapping the DC voltage drop at the outlet of the converter station with the ACLC to the voltage drop generated by the equivalent virtual impedance,the pole to pole short-circuit fault circuit can be equivalent to the RLC discharge circuit with virtual impedance,thus the equivalent RLC circuit model with ACLC characteristics can be derived.(2)Using the virtual inductance mapping method(Experiment 1),the average relative error of DC fault current within 10 ms is below 3.31%under different DC short-circuit fault scenarios.Using the virtual impedance mapping method(Experiment 2),the average relative error of fault current within 10 ms is below 8.29%under different DC short-circuit fault scenarios.Thus,the calculation method proposed in this paper has good engineering practicability.(3)But,when analyzing the equivalent RLC circuit with virtual impedance,the influence of the dynamic characteristics of circulating current suppressor on the equivalent virtual impedance is ignored.Therefore,the influence of circulating current suppressor on fault current calculation method can be considered in future work to further improve the calculation accuracy of the DC fault current by the proposed method.(4)Although this paper uses specific ACLC algorithm of current limiting link type with current change rate to show the idea of the proposed method,this method can be extended to analyze the short-circuit fault current calculation problem of other types of ACLC algorithms.