基于双自由度控制的直流微网中多DG负极接地故障环流抑制

Suppression of Circulating Current Caused by Multiple DG Negative Pole Ground Fault in DC Microgrid Based on Control With Two Degrees of Freedom

直流微网中的分布式发电单元（DG）将可再生能源转化为电能,并通过并联H桥DC/DC变流器将电能输送到直流母线,能够有效地解决能源危机和环境污染问题。但由于很多DG安装在室外,线路易于老化使绝缘变差,特别在雷雨天气时,DG侧容易发生接地故障,导致某些变流器内部流过很大的共地环流,这会触动变流器过流保护装置从而影响系统稳定性。针对直流微网中多DG负极接地故障造成的共地环流问题,利用双自由度控制策略对共地环流进行抑制,从而有效地提高系统的故障穿越能力,并且系统在正常运行和故障情况下无需控制策略切换,从而不影响经济成本。首先论证传统双极性调制方法不能抑制所述共地环流,然后提出双自由度控制策略,即通过共模控制来抑制共地环流,并且通过下垂控制来提高各变流器均流精度,同时施加惯性环节抑制电压跌落,最后通过仿真和实验结果表明各种工况下所提控制方法的优越性。

1： To effectively solve energy crisis and environmental pollution, DC microgrid grasps more and more attention, into which renewable energy is integrated mainly with parallel H-bridge DC/DC converters. But DGs are generally installed outdoor, and the lines are prone to age, resulting in poor insulation. Thus, there often appears DG ground fault, especially in thunderstorm weather, causing large common ground circulating current among some converters, triggering overcurrent protection of the converter and affecting system stability. Focusing on this problem, a control strategy with two degrees of freedom is proposed to suppress common ground circulating current, thereby increasing fault ride-through capability. The control strategy remains effective in both normal operation and faulty condition, hence the cost is not affected even under faults. Specifically, firstly it is proven that conventional bipolar modulation control cannot suppress the described circulating current. Then, a control method with two degrees of freedom is proposed, meaning that the common mode-based control can suppress the common ground circulating current, and droop-based control can improve current sharing precision among the converters. Inertial element is added to help voltage recovery. Finally, simulation and experimental results are presented to verify the effectiveness of the proposed method.