考虑电网内阻影响的孤岛检测研究

Research on Islanding Detection Considering the Effect of Grid Impedance

在基于电流幅值扰动的孤岛检测过程中,电网未断电时,由于电网内阻的存在,公共耦合点(PCC)电压会随着电流扰动而波动,可能造成检测误判。为此,本文采用间歇性正反向电流幅值扰动,利用逆变输出电流和PCC电压的相关系数判断孤岛,并研究电网内阻对孤岛检测的影响。通过对符合IEEE Std 929—2000标准孤岛检测电路模型的理论分析,分别得到了在电网断电孤岛运行和电网未断电且考虑电网内阻时,PCC电压与扰动电流之间的关系,同时得到了两种状态下相关系数计算值及其与电网内阻的关系,为相关系数保护阈值的选取提供了理论依据。实验结果显示,方法可在规定时间内检测出孤岛,验证了理论分析的正确性。本文提出的孤岛检测方法克服了电网内阻致使PCC电压波动的影响,检测结果不依赖于电压额定值,且在PCC电压不超出过欠压保护范围时即可检测出孤岛,检测可靠性高。

In the process of islanding detection using current disturbance, the grid impedance will lead to the fluctuation of PCC voltage when the grid is in normal operation, which may cause faulty judgment. Aiming at this issue, a periodical up/down current disturbance is adopted in this paper and the correlation factor(Cf) between the output current and the PCC voltage is defined as the islanding detection index, while the effect of grid impedance on detection result is considered at the same time. Through theoretical analysis to the equivalent circuit, the exact effect of current perturbation on PCC voltage is obtained without and with grid, respectively. The value of Cf in each state is mathematically calculated and obtained as well, particularly the relationship between Cf and the grid impedance, which provides a reasonable reference for the choice of correlation factor protection threshold(Cf_th). The theoretical conclusion is confirmed with the prototype experiment. The result shows that this island-detection method rationally considers the grid impedance during current disturbance and the detection result has no relation to the nominal voltage. The islanding condition can be reliably distinguished when the PCC voltage is still within the protection range.