In the power grid system, in addition to conducting some necessary tests, protection and secondary tests according to the handover test standards, the transformers newly put into operation usually need to undergo no-load full voltage impulse tests before being put into operation. This is done for two purposes.
First of all, when the no-load transformer is disconnected without power, it is possible to generate operating overvoltage. When the neutral point of the power system is ungrounded or grounded through the arc suppression coil, the overvoltage amplitude can reach 4~4.5 times of the phase voltage. When the neutral point is directly grounded, the phase voltage can reach 3 times. Therefore, in order to check whether the insulation strength of the transformer can withstand full voltage or switching over-voltage, impact test is required.
Secondly, under the condition of electrification, the excitation inrush current will be generated when the no-load transformer is put into operation. Its value can reach 6~8 times of the rated current. Since the excitation current generates a large amount of electric power, impact test is required to assess the mechanical strength of the transformer and whether the initial period of excitation inrush current attenuation can cause maloperation of the relay protection device.
How many times does the impact test usually need to be done?
Generally, before the first operation of the newly installed transformer, the impulse closing shall be conducted for 5 times under the rated voltage to comprehensively detect the insulation, mechanical strength and differential protection of the transformer.
As the closing angle of each closing time is different, the corresponding inrush current is also different, sometimes large, sometimes small. The duration after the first impact should not be less than 10 minutes, and the interval between each time should be more than 5 minutes. A complete impact test generally takes 1 hour.
If the transformer is overhauled, the main transformer shall be impacted for 3 times.