Why Must 5 Shock Tests Be Carried Out Before The Transformer Is Put Into Operation?

The reasons for the impact test of a new transformer or overhauled transformer before it is formally put into operation are as follows:

(1) Check the insulation of the transformer and its circuit for weaknesses or defects. When the no-load transformer is pulled apart, operating overvoltage may be generated. 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, the overvoltage amplitude can reach 3 times the phase voltage. In order to test whether the transformer insulation strength can withstand the effect of full voltage or operating overvoltage, it is necessary to do no-load full voltage shock test before the transformer is put into operation. If the transformer and its circuit have insulation weaknesses, they will be exposed by the operating overvoltage breakdown.
(2) Check whether the transformer differential protection is misoperated. When the electric power is put into the no-load transformer, the excitation inrush current will be generated, and its value can reach 6-8 times the rated current. Excitation inrush current begins to decay faster, generally by O.5-1s can be reduced to 0.25-0.5 times the rated current, but all attenuation is completed for a long time, small and medium-sized transformers about a few seconds, large transformers can reach 10-20s, so the initial attenuation of excitation inrush current, often make the differential protection misoperation, resulting in transformer can not be put into. Therefore, when no load impact closing, under the action of the excitation inrush current, the connection, characteristics and setting value of the differential protection can be actually checked, and the evaluation and conclusion of whether the protection can be put in can be made
(3) Check the mechanical strength of the transformer. Because the excitation inrush current generates a lot of electric power, in order to check the mechanical strength of the transformer, it is necessary to do no-load impact test.

According to the regulations, the number of full-voltage no-load impact test, new product input, should be continuous impact 5 times; After the overhaul is put into operation, it should be impacted for 3 consecutive times. Each impact interval time is not less than 5min, before the operation should be sent to the site to monitor the transformer, check the turn converter has no difference, if there is an anomaly should immediately stop the operation.

Keep running for more than 10 minutes after the first impact and wait for more than 5 minutes before the next impact.

As for why the specific estimate set to 5 times is the specification is the conclusion of the comprehensive consideration of mechanical strength overvoltage excitation inrush current