The reasons why a new transformer or a transformer after an overhaul should undergo an impact test before it is officially put into operation are as follows:

(1) Check the insulation of the transformer and its circuits for weaknesses or defects. When the no-load transformer is opened, operating overvoltage may occur. When the neutral point of the power system is not grounded or the arc suppression coil is grounded, the overvoltage amplitude can reach 4-4.5 times the phase voltage; when the neutral point is directly grounded, the overvoltage amplitude can reach 3 times the phase voltage. In order to check whether the insulation strength of the transformer can withstand the effects of full voltage or operating overvoltage, it is necessary to do a no-load full voltage impulse test before the transformer is put into operation. If the transformer and its circuit have insulation weaknesses, they will be exposed by operating overvoltage breakdown.

(2) Check whether the transformer differential protection malfunctions. When the no-load transformer is powered on, an excitation inrush current will be generated, which can reach 6-8 times the rated current. The magnetizing inrush current begins to decay quickly, generally it can be reduced to 0.25-0.5 times the rated current after 0.5-1s, but it takes a long time to complete the attenuation. It takes about a few seconds for small and medium transformers, and 10-20s for large transformers, so the magnetizing inrush current decays In the initial stage, the differential protection is often malfunctioning, causing the transformer to fail to be put into operation. Therefore, during the no-load impact closing, under the action of the magnetizing inrush current, the wiring, characteristics, and fixed value of the differential protection can be actually checked, and the evaluation and conclusion of whether the protection can be put into use can be made.

(3) Assess the mechanical strength of the transformer. Since the magnetizing inrush current produces a lot of electromotive force, in order to assess the mechanical strength of the transformer, a no-load impact test is required.

According to the regulations, the number of full-voltage no-load impact tests should be continuously impacted for 5 times when the new product is put in; after the overhaul, it should be impacted 3 times in a row. The interval between each impact is not less than 5min. Before operation, someone should be sent to the site to monitor the transformer and check whether the turn converter is abnormal. If there is any abnormality, stop the operation immediately.

After the first impact, continue to run for more than 10 minutes, and after the impact, wait for more than 5 minutes before proceeding to the next impact. As for why it is set to 5 times is stipulated in the regulations, the specific estimation is a conclusion drawn from the comprehensive consideration of mechanical strength, overvoltage, and magnetizing inrush current.