When a fault occurs in a power network, only through fault current in a short time and single-phase grounding fault occurs in a neutral point through a small resistance grounded power grid, high-sensitivity zero-sequence protection judges and removes the fault line for a short time. The grounding transformer only works during the period from the grounding fault to the zero-sequence protection action of the fault line removing the fault line, and the neutral-point grounding resistance and grounding transformer will be turned on. Zero-sequence circuit with over-IR= (U is the system voltage, R1 is the neutral grounding resistance, R2 is the additional resistance of the grounding fault circuit).
According to the above analysis, the operation characteristics of grounding transformer are long time and short time overload. In short, the grounding transformer is manufactured artificially as a neutral point to connect the grounding resistance. When the grounding fault occurs in the system, the positive sequence negative sequence current has a high impedance and the zero sequence current has a low impedance, which makes the grounding protection operate reliably.