When there are more than two ground points of the transformer core for some reason, a closed loop will be formed and a circulating current will be generated. This is the so-called multi-point ground fault of the iron core. It can cause partial short-circuit and overheating of the iron core, and even major failures such as partial burning of the iron core.
At present, the iron cores of large and medium-sized transformers manufactured in China are led to the outside of the tank through a bushing and grounded. During normal operation of a power transformer, there is an electric field around it, and metal components such as iron cores and clamps are in the electric field. If the iron core is not reliably grounded, charging and discharging will occur and the insulation will be damaged. Therefore, the iron core must have a little reliable grounding. If another ground point appears for the iron core for some reason, forming a closed loop, there will be circulating current on the normally grounded lead. On the one hand, the core will be locally short-circuited and overheated. In severe cases, the core will be partially burned; on the other hand, Due to the circulating current generated by the normal ground wire of the iron core, local overheating of the transformer may also cause discharge faults.
Judgment of iron core multi-point ground fault
1. Measure the insulation resistance of the iron core: If the insulationresistance is zero or very low, it indicates that there may be an iron core ground fault.
2. Monitor the transformer for grounding caused by the circulating current to the iron core or the clamp through the small bushing. It should be monitored whether there is circulating current in the grounding wire. If so, the transformer should be stopped and the insulation resistance of the iron core should be measured.
3. Gas chromatographic analysis: Gas chromatographic analysis of the gas content in oil is also the most effective method for discovering the grounding of transformer cores.