LCC-MMC型混合直流送端交流系统故障时直流电流的暂态过程解析

电网换相换流器和模块化多电平换流器(LCC-MMC)型混合直流输电解决了传统直流受端的换相失败问题，目前葛洲坝—上海直流系统正在进行受端柔性直流化改造的方案论证，而焦点在于送端交流系统故障引起直流电流快速下降的故障穿越问题。为此，首先根据送端交流系统故障时的系统等值电路得到其拉氏运算电路，基于回路电流法通过拉氏反变换求得直流电流的暂态过程，并分析了暂态电流的衰减分量及振荡分量。在PSCAD/EMTDC仿真平台上建立了葛洲坝—上海直流的电磁暂态仿真模型，仿真结果验证了分析的正确性。进一步地，忽略暂态电流的振荡分量，得到了直流电流及其过零时间的近似解析表达式。最后，利用解析表达式分析了交流电压跌落程度、平波电抗器和控制策略对直流电流过零时间的影响。所提方法可为LCC-MMC型混合直流输电的送端交流系统保护定值整定及平波电抗器参数的选取提供依据。

Analysis of DC current transient process under AC system fault at LCC-MMC hybrid HVDC sending end

LCC-MMC(Line Commutated Converter-Modular Multilevel Converter) hybrid HVDC transmission solves the problem of commutation failure at traditional DC receiving end. At present, the project demonstration of receiving end flexibility in Gezhouba-Shanghai HVDC system has been carried out, and the focus is on the problem of fault ride-through problem which causes rapid decrease of DC current under the AC fault at the sending end. Therefore, firstly, the Laplace arithmetic circuit is obtained according to the system equivalent circuit of the AC fault at the sending end. Based on the loop current method, the transient process of DC current is obtained by Laplace inverse transformation, and the attenuation and oscillation components of the transient current are analyzed. The electromagnetic transient simulation model of Gezhouba-Shanghai HVDC system is established in PSCAD/EMTDC, and simulative results verify the correctness of analysis. Furthermore, the approximate analytical expressions of DC current and its zero-crossing time are obtained by neglecting the oscillation component of transient current. Finally, the influence of AC voltage dropping degree, smoothing reactor and control strategy on DC current zero-crossing time is analyzed by analytic expression. The proposed method can provide the basis for protection value setting of sending end AC system and the selection of smoothing reactor parameters in LCC-MMC hybrid HVDC system.