Ungrounded neutral system

Most of the 6–10 kV high-voltage power grids in our country, as well as some 35 kV grids, operate with an ungrounded neutral point. Its main feature is that when a single-phase ground fault occurs in the system, the voltage levels and phases between the various phases remain unchanged; the balance of the three-phase system is not disrupted, allowing it to continue operating for a short period of time. However, to prevent the fault from spreading and causing inter-phase short circuits ； Or in the case of single-phase arc grounding, it can cause resonance in the system, leading to overvoltage and system failure; it is specified that the system shall not operate with a fault present for more than 2 hours, as operating for such a long time with a fault poses significant challenges to production and scheduling. When a single phase experiences a single-phase ground fault, there is no zero-sequence current in that phase; zero-sequence currents are generated in the other two phases. The zero-sequence currents from all the healthy phases flow toward the system and then from there to the fault location. Please refer to the figure below for details

The attached diagram from the previous answer to the question; the diagram was inserted for the first time in that response

In a neutral-ungrounded system with single-phase grounding, there is a leakage current in the grounded phase. However, since the neutral point is not grounded, no circuit can be formed; as a result, the equipment can still operate normally. Yet, the voltage of the other two phases rises to the line voltage of 380V, increasing the risk. Therefore, per regulations, such operation must not exceed two hours.