The main transformer differential protection trip is typically triggered by several possible causes. First, a short circuit fault may have occurred in the main transformer or its bushing lead-out lines. Second, there could be a malfunction in the secondary protection circuit. Third, the current transformer might be either open-circuited or short-circuited. Lastly, an internal fault within the transformer itself can also lead to a differential protection trip.
When dealing with such a situation, the first step is to visually inspect the external bushings and lead-out lines of the main transformer for any signs of damage or abnormalities. If no issues are found during this inspection, but there is a known DC grounding hazard or the system has been operating with DC grounding, it's important to check for a potential two-point grounding fault in the DC system. If detected, the fault should be immediately resolved before re-energizing the transformer.
If the previous step does not reveal any DC ground faults, but voltage is detected across both ends of the outlet intermediate relay coil and the differential relay’s grounding has returned, it may indicate a short circuit in the differential trip circuit or the secondary protection wiring. In such cases, the short-circuit point must be located and repaired as soon as possible before restoring power.
Next, it’s essential to check the high-voltage and low-voltage current transformers for any open circuits or poor terminal contacts. Any identified issues should be addressed promptly to ensure safe operation and resume power transmission to the transformer.
If all the above checks fail to identify any faults, the most likely cause is an internal fault within the transformer. In this case, the transformer should be taken offline for further testing. If the fault is determined to be in the lead wires, they should be replaced immediately to avoid further damage.
Additionally, if both the differential protection and gas protection systems trip simultaneously, it is a strong indication of an internal fault. In such cases, the heavy gas protection action procedure should be followed to handle the situation safely and effectively.
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