The grounding transformer is a transformer intended solely for establishing a neutral connection point on an ungrounded power system. Then grounded with the Petersen-coil or the low resistance. This paper makes the depth analysis of the structure principle of the grounding transformer and its electromagnetic relation. Then a zigzag grounding transformer is modeled in MATLAB simulator with the single-phase 5 winding transformer. It could realize the phase-to-phase faults and interterm faults simulation of the grounding transformer' any point. The simulation results prove the analysis' correctness and validity, which provide the theoretical basis for the grounding transformer's protection device.