Why should the impact test be done before the transformer is put into operation

When the no-load transformer is opened, there may be an operating overvoltage. When the neutral point of the power system is not grounded or grounded through the arc suppression coil, the overvoltage amplitude can reach 4-4.5 times the phase voltage; when the neutral point is directly grounded, it can reach 3 times the phase voltage, in order to check whether the insulation strength of the transformer can withstand the impact of full voltage or operating overvoltage, an impact test is required.
When a live transformer is put into a no-load transformer, an exciting inrush current will be generated, its value may exceed the rated current, and the decay time is long, even up to tens of seconds. Due to the large electromotive force generated by the inrush current, in order to assess the mechanical strength of each part of the transformer, an impact closing test is required, that is, several times of closing at the rated voltage.
Operating overvoltage may occur when the no-load transformer is switched off. For ungrounded windings, this voltage can reach 4 times the phase voltage; for neutral point directly grounded windings, this voltage can still reach 2 times the phase voltage. In order to assess whether the insulation strength of the transformer can bear or not, a breaking test must be done. Since there may be quite a large excitation inrush current when closing, in order to check whether the excitation inrush current will cause the relay protection to malfunction, it is necessary to perform several impulse closing tests.
This is to replace the lightning impulse test with the operating overvoltage test. There is a lightning impulse generator in the transformer factory, but it is impossible to have it on the installation site. However, transformers do experience lightning impulses and operating overvoltage impulses during operation. This is the insulation performance index that the transformer must be able to meet.
There are strict indicators when doing lightning strikes in transformer manufacturers, such as full wave, cut wave and how long it takes. But it is impossible to have such strict and precise control on the spot. Moreover, in many cases, the multiple of the operating overvoltage often does not reach the multiple of the lightning impulse. Therefore, it is compensated by increasing the number of closing times. When closing the switch at the moment when the voltage crosses zero, the overvoltage multiple is the highest.
After each impact closing test, check whether the transformer has abnormal sound or abnormal shape. Before the new transformer was put into operation, it was closed 5 times without load, and the interval between each time was not less than 5 minutes, so that the transformer could restore the insulation. The number of transformers after overhaul can be 3 times.

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