Evaluating the Voltage Transformer Thermal Stability during Magnetizing Current Inrush Caused by a Single-Line Ground Fault

The thermal stability of voltage transformers during magnetizing current inrush that occurs during a single-line ground fault in distribution networks with a resistively grounded neutral is evaluated. The studies are carried out for two circuit states: in a normal state and when there is an open-circuit fault in the neutral grounding resistor circuit. The process of developing and verifying computational models of non-grounded voltage transformers NOL-20 and grounded voltage transformers ZNOLP-20 based on mathematical models that take into account partial dynamic loops and the limiting hysteresis loop is described. To assess the voltage transformer thermal stability, the magnetizing current inrush Joule integral was used. The maximum permissible value of the Joule integral was assumed for a one-second short circuit of the secondary winding in accordance with GOST 1983 requirements. The influence of capacitive currents, transformer load and transient resistance at the fault location is analyzed. The design conditions under which the Joule integral reaches the maximum permissible value are determined. The maximum permissible durations of the magnetization current ferroresonance inrushes are presented. Recommendations on reducing the risks of voltage transformer damage are given.