Modular multilevel converter (MMC)-based high voltage direct current (HVDC) systems have been increasingly researched and installed in recent years. During operation, systems may be subjected to various unbalanced conditions, the single-line to ground (SLG) fault is one of the most common unbalanced faults. When a SLG fault occurs in a three-phase alternating current (AC) system, the double-line frequency positive- and zero-sequence harmonic components will be produced besides the double-line frequency negative-sequence harmonic component. And if the control system cannot suppress these harmonic components, they can propagate into the direct current (DC) side of the HVDC system and cause ripples in DC-line voltage and current as well as other unwanted consequences. Among the proposed controllers in the published papers on this topic, rarely controllers can both effectively suppress the circulating current and completely eliminate ripples in the DC-line voltage and current while still keeping the AC-side current balanced under unbalanced grid conditions. Therefore, this paper first analyses the propagation of harmonic components in a two-terminal MMC-HVDC system under a SLG fault. Then, based on the mathematical analysis, a proposed controller including an inner current controller for controlling the negative- and positive-sequence components and a controller for suppressing the zero-sequence component is designed to eliminate harmonic propagation from the faulty three-phase AC system to the DC side of the HVDC system, resulting in suppressing ripples in DC-line voltage and current. The theoretical analysis and proposed controller are verified by a simulation model in PSCAD platform.

中文翻译：

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电网不平衡条件下MMC-HVDC系统的分析与控制

近年来，越来越多地研究和安装基于模块化多电平转换器 (MMC) 的高压直流 (HVDC) 系统。在运行过程中，系统可能会受到各种不平衡条件的影响，单线对地 (SLG) 故障是最常见的不平衡故障之一。三相交流系统发生SLG故障时，除双线频率负序谐波分量外，还会产生双线频率正序和零序谐波分量。如果控制系统无法抑制这些谐波分量，它们可能会传播到 HVDC 系统的直流 (DC) 侧，并导致直流线路电压和电流出现纹波以及其他不良后果。在有关该主题的已发表论文中提议的控制器中，很少有控制器既可以有效抑制环流，又可以完全消除直流线路电压和电流中的纹波，同时在不平衡电网条件下仍然保持交流侧电流平衡。因此，本文首先分析了SLG故障下两端MMC-HVDC系统中谐波分量的传播。然后，基于数学分析，提出的控制器包括用于控制负序和正序分量的内部电流控制器和用于抑制零序分量的控制器，以消除故障三相交流系统的谐波传播。到 HVDC 系统的直流侧，从而抑制直流线路电压和电流中的纹波。