Abstract: After grounding transformer and small resistance grounding, 10 kV medium voltage cabinet introduces power neutral point and zero sequence grounding point. In this paper, the mathematical calculation and vector analysis of each phase voltage after singlephase grounding of the medium voltage cabinet are carried out. With the increase of grounding resistance, the fault phase voltage decreases, the non fault leading phase voltage first increases and then decreases, and the non fault lagging phase voltage always increases. The calculation results are applied to the zero sequence voltage transformer with 4TV connection, and the analysis conclusion is consistent with the actual voltage change. After clarifying the mechanism of abnormal conditions, take correct corrective actions and eliminate defects in time.