Ground faults in networks with an isolated neutral are not switched off by relay line protections due to low capacitive currents. It is recommended to disconnect the line with a earth fault as quickly as possible, but sometimes it is difficult to find and localize the damage. To analyze the emergency mode of single-phase ground fault in networks with an isolated neutral, the phase coordinate method for calculating phase voltages is used. At the point of damage, the values of the phase-to-phase voltages remain unchanged, and the phase voltages change. Real research and analysis of electrical network modes of 6-35 kV with an isolated neutral is rather time-consuming task. Therefore, the calculation of phase voltages at the substation during an earth fault is carried out through the transition resistance on the line based on the phase coordinate method. The authors developed a virtual model of studied power transmission line for analyzing asymmetric modes of three-phase networks and checking the operability of the method in the MATLAB Simulink simulation package. The simulation results showed that there was no discrepancy between the voltage and current values of the original model in phase coordinates and the Simulink model. As a result of the research, the authors have found that the phase coordinate method helps calculate emergency modes with the overlay of several component accidents taking into account load currents. The model in phase coordinates will allow analyzing relay protection operation at high speed under different operating modes of the electric power system.