光伏并网系统故障二次谐波产生机理及其对变压器保护的影响

Second harmonic generation mechanism of a photovoltaic grid-connected system fault and its influence on transformer protection

光伏发电系统通过逆变器矢量控制并网,其短路暂态特性异于传统电源。电网发生故障时,光伏系统输出电流中可能含有二次谐波,接入弱电网时二次谐波的影响不可忽略。以光伏并网发电系统对称故障条件为例,首先分析基于锁相环的三相并网逆变器单频率输入双频率输出的现象产生机理。其次在综合考虑并网控制及系统低电压穿越策略切换的影响基础上,分析故障暂态过程中电流二次谐波产生机理并推导二次谐波解析表达式。指出在变压器区内故障时光伏并网系统产生的二次谐波可能威胁变压器差动保护的可靠性。最后基于PSCAD/EMTDC仿真验证理论分析的正确性,为后续光伏发电系统接入后的电网继电保护适应性分析提供参考。

A photovoltaic power generation system is controlled by an inverter vector,and its short-circuit transient characteristics are different from those of the traditional power supply.When the power grid fails,its output current may contain the second harmonic,and the influence of these second harmonics when connected to the weak grid cannot be ignored.Taking the symmetrical fault condition of a photovoltaic grid-connected power generation system as an example,this paper first analyzes the phenomenon of single-frequency input dual-frequency output of a three-phase grid-connected inverter based on phase-locked loop,and secondly considers grid-connected control and system low-voltage crossing.Based on the influence of policy switching,the mechanism of current second harmonic generation in the fault transient process is analyzed and the second harmonic analysis expression is derived.It is pointed out that the second harmonic generated by photovoltaic grid-connected system may threaten the reliability of transformer differential protection when the fault occurs in the transformer zone.Finally,based on the correctness of PSCAD/EMTDC simulation verification theory,it provides a reference for the adaptive analysis of grid relay protection after the subsequent PV system.