In a modular multilevel converter-based high-voltage direct current (MMC-HVDC) system, the dc fault ride through (FRT) control is an effective way to deal with a dc-side pole-to-ground (PTG) fault. However, the setting of FRT duration brings potential hazards: 1) MMC will face the risk of “secondary short circuit” if FRT duration is short; 2) ac grid may have power angle stability issue if FRT duration is long. To avoid these hazards and provide theoretical guidance for the FRT duration setting, transient behaviour of dc fault current during the PTG FRT stage is explored in this work. Firstly, challenges of the transient analysis are summarised as non-linearity and high-order issues. In light of this, numerical and Hilbert-Huang Transformation methods are introduced to evaluate the non-linearity issue. It is found that the path of MMC from dc current to dc internal voltage is weakly non-linear. Hence, a linear transient model is built to analyse the dc fault current. By participation factor analysis, the order of the proposed model is further reduced, so that an analytical expression of the dc fault current is approximately derived. Based on the analytical expression, regularities and mechanism of dc fault current are fully revealed. Application of the transient analysis to the setting of FRT duration is elaborated in detail. Finally, the PSCAD/EMTDC simulation verifies the validity of the proposed model and analytical expression.

中文翻译：

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穿越直流侧极地故障时半桥/全桥混合MMC的建模与分析

在基于模块化多电平转换器的高压直流 (MMC-HVDC) 系统中，直流故障穿越 (FRT) 控制是处理直流侧极对地 (PTG) 故障的有效方法。但是，FRT时长的设置带来了潜在的危害：1）如果FRT时长短，MMC将面临“二次短路”的风险；2) 如果 FRT 持续时间较长，交流电网可能会出现功角稳定性问题。为了避免这些危险并为 FRT 持续时间设置提供理论指导，本文探讨了 PTG FRT 阶段直流故障电流的瞬态行为。首先，瞬态分析的挑战概括为非线性和高阶问题。鉴于此，引入数值和希尔伯特-黄变换方法来评估非线性问题。发现MMC从直流电流到直流内部电压的路径是弱非线性的。因此，建立了一个线性瞬态模型来分析直流故障电流。通过参与因子分析，进一步降低所提出模型的阶数，从而近似推导出直流故障电流的解析表达式。根据解析表达式，充分揭示了直流故障电流的规律和机理。详细阐述了瞬态分析在 FRT 持续时间设置中的应用。最后，PSCAD/EMTDC仿真验证了所提模型和解析表达式的有效性。从而近似推导出直流故障电流的解析表达式。根据解析表达式，充分揭示了直流故障电流的规律和机理。详细阐述了瞬态分析在 FRT 持续时间设置中的应用。最后，PSCAD/EMTDC仿真验证了所提模型和解析表达式的有效性。从而近似推导出直流故障电流的解析表达式。根据解析表达式，充分揭示了直流故障电流的规律和机理。详细阐述了瞬态分析在 FRT 持续时间设置中的应用。最后，PSCAD/EMTDC仿真验证了所提模型和解析表达式的有效性。