Why must 5 impulse tests be carried out before the transformer is put into operation?

The reasons for the impact test of new transformers or transformers after overhaul before they are 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 pulled apart, it is possible to generate operating overvoltages. 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 check whether the insulation strength of the transformer can withstand the effect 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 circuits have insulation weaknesses, they will be exposed to operating overvoltage breakdown.

(2) Check whether the differential protection of the transformer is malfunctioning. When the no-load transformer is powered on, an inrush current will be generated, and its value can reach 6-8 times the rated current. The magnetizing inrush current starts to attenuate 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 for all the attenuation to complete, about a few seconds for small and medium-sized transformers, and 10-20s for large transformers, so the magnetizing inrush current attenuates In the initial stage, the differential protection is often misactivated, causing the transformer to be unable to be put into operation. Therefore, when the no-load impact closing, under the action of the excitation inrush current, the wiring, characteristics and setting 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) Check the mechanical strength of the transformer. Since the excitation inrush current generates a large amount of electric power, in order to evaluate the mechanical strength of the transformer, 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 new products are put into operation; after overhaul, it should be continuously impacted for 3 times. The interval between each impact is not less than 5min. Before operation, someone should be sent to the site to monitor the transformer to check whether there is any abnormality in the transformer. If there is any abnormality, the operation should be stopped immediately.

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

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