小电流接地系统单相接地故障选相的传统判据是建立在电网对地参数严格对称的条件下形成的，由于电网不对称度的普遍存在，当电网发生不完全接地故障时，特别是经高阻接地时，传统判据失效。详细分析并给出电网发生不完全接地故障后的零序电压轨迹，根据零序电压变化的特征将平面坐标系分成若干扇区，每个扇区与电网参数“不平衡矢量和”相对应。
The traditional criterion of phase selection for single-phase ground fault in small current grounding system is based on the condition that the parameters of power grid are strictly symmetrical to the ground. Because of the general existence of power grid asymmetry, when the power grid has incomplete ground fault, especially through high resistance grounding, the traditional criterion is ineffective. The zero sequence voltage trace after incomplete grounding fault is analyzed and given in detail. According to the characteristics of zero sequence voltage change, the plane coordinate system is divided into several sectors, each sector corresponds to the "unbalanced vector sum" of grid parameters.
研究发现，零序不平衡电压处在的扇区不同，随着过渡电导的增加，零序电压的幅值和相位变化均出现不同的特征，可根据零序电压轨迹变化的规律特征，实现单相不完全接地故障相的判别。理论及仿真分析表明，所提方法可有效解决不对称电网发生不完全接地故障相的识别。
It is found that with the increase of the transition conductance, the amplitude and phase of the zero sequence voltage have different characteristics. According to the regular characteristics of the change of the zero sequence voltage trajectory, the phase identification of the single-phase incomplete ground fault can be realized. Theoretical and simulation analysis shows that the proposed method can effectively solve the problem of identifying the phase of incomplete grounding fault in asymmetric power grid.