The reasons for conducting impact tests on a new transformer or a transformer that has undergone overhaul before it is officially put into operation are as follows:
(1) Check whether there are any weaknesses or defects in the insulation of the transformer and its circuit. When an unloaded transformer is disconnected, an operating overvoltage may occur. When the neutral point of the power system is ungrounded or grounded through an arc suppression coil, the amplitude of the overvoltage can reach 4 to 4.5 times the phase voltage; when the neutral point is directly grounded, the amplitude of the overvoltage can reach 3 times the phase voltage. In order to check whether the insulation strength of the transformer can withstand the full voltage or the operating overvoltage, an unloaded full voltage impact test needs to be carried out before the transformer is put into operation. If there are any insulation weaknesses in the transformer and its circuit, it will be broken down by the operating overvoltage and exposed.
(2) Check whether the differential protection of the transformer malfunctions. When an unloaded transformer is energized, an exciting inrush current will be generated, and its value can reach 6 to 8 times the rated current. The exciting inrush current decays relatively quickly at the beginning. Generally, it can be reduced to 0.25 to 0.5 times the rated current within 0.5 to 1 second, but the total decay time is relatively long. For small and medium-sized transformers, it takes about a few seconds, and for large transformers, it can reach 10 to 20 seconds. Therefore, in the initial stage of the decay of the exciting inrush current, the differential protection often malfunctions, resulting in the transformer not being able to be put into operation. Therefore, during the no-load impact closing, under the action of the exciting inrush current, the wiring, characteristics, and setting value of the differential protection can be actually checked, and an evaluation and conclusion on whether the protection can be put into operation can be made.
(3) Assess the mechanical strength of the transformer. Since a large electrodynamic force is generated by the exciting inrush current, in order to assess the mechanical strength of the transformer, a no-load impact test is required. According to the regulations, for new products, the number of full voltage no-load impact tests should be 5 consecutive impacts; for transformers after overhaul, 3 consecutive impacts should be carried out.
The interval between each impact should be no less than 5 minutes. Before the operation, personnel should be sent to the site to monitor the transformer and check whether there are any abnormal conditions in the transformer. If there are any abnormalities, the operation should be stopped immediately. As for why it is set to 5 times, it is specified in the regulations. Specifically, it is estimated to be a conclusion drawn by comprehensively considering the mechanical strength, overvoltage, and exciting inrush current.
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