OVERVOLTAGE AT THE TRANSFORMER WHEN DISCONNECTING CLOSE ASYMMETRICAL SHORT CIRCUITS

This article examines overvoltages at the inputs of high-voltage (HV) and low-voltage (LV) transformers rated at 110/6 kV and 110/10 kV, focusing on scenarios involving grounded and isolated neutrals during short circuits near the transformers. The study finds that with an isolated neutral, overvoltages resulting from a phase-to-ground short circuit reach the highest levels, as anticipated. However, the disconnection of all types of asymmetrical short circuits—whether with an isolated or grounded neutral—yields even greater, potentially excessive overvoltages. This occurs because the windings of undamaged transformer phases remain partially energized during disconnection, leading to significant currents being interrupted. The magnetic energy from these currents converts to electrical energy, resulting in substantial voltage increases, characterized as pulsed overvoltages lasting several microseconds. Implementing switches with shunt resistance can reduce these overvoltages considerably, though the remaining levels may still exceed acceptable thresholds. To mitigate the risk of such excessive overvoltages, installing surge arresters at the inputs of high-voltage transformers is recommended, ensuring that transformer input overvoltages remain within permissible limits.