When a single-phase high-voltage fuse in a voltage transformer blows and causes grounding, the indication on the bus insulation monitoring device will change. If not carefully analyzed, this situation can often lead to incorrect judgments. However, by checking and comparing the three-phase relative-to-ground voltages, it is usually easy to distinguish between the two scenarios. The specific method for identification is as follows:
1. In the case of a single-phase ground fault, the normal phase-to-ground voltage increases, while the faulty phase-to-ground voltage decreases (it may drop to zero if fully grounded).
2. When a single-phase high-voltage fuse in the voltage transformer is blown, the phase-to-ground voltage will decrease but typically does not reach zero. There may still be an induced voltage present. At this point, the primary voltage applied to the transformer will be missing one phase, while the other two phases remain at normal voltage, with a 120° phase difference between them. This can result in a three times normal voltage zero-sequence component in the open delta winding of the transformer. As a result, a ground signal may also be triggered (since the setting value of the insulation monitoring relay is generally around 30V).
By analyzing and comparing these two situations, it is usually straightforward to identify which one is occurring, avoiding misjudgment. Understanding these differences helps in making accurate decisions during fault diagnosis. It’s important to always check all three phases and consider the behavior of the system under different fault conditions to ensure reliable operation and prevent unnecessary trips or maintenance actions.
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