The purpose of impact test of power transformer before commissioning
New put into operation of the transformer, in addition to the handover test standards to do some necessary tests and protection, quadratic surface test, before the formal input, usually have to do the no-load full voltage switching impact test.
Why do impact tests?
1. Check the insulation of the transformer and its circuit for any weakness or defect
An operating overvoltage may occur when the no-load transformer is pulled open. When the neutral point of the power system is not grounded or grounded by 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 insulation strength of the transformer can withstand the full voltage or the operation overvoltage, it is necessary to do the no-load full voltage impact test before the transformer is put into operation. If there is insulation weakness in the transformer and its circuit, it will be exposed by operating overvoltage breakdown.
2. Check whether the transformer differential protection is misoperated
When the live transformer is put into no-load transformer, the inrush current will be generated, and its value can reach 6 ~ 8 times of rated current. Generally, it can be reduced to 0.25 ~ 0.5 times of rated current after 0.5 ~ 1s. However, it takes a long time to complete all the attenuation. Small and medium-sized transformers take about a few seconds, while large transformers can reach 10 ~ 20 seconds. Therefore, under the action of magnetizing inrush current, the connection, characteristics and fixed value of differential protection can be practically checked, and the evaluation and conclusion of whether the protection can be put into operation can be made.
3. Check the mechanical strength of transformer
Since inrush current generates a large amount of electrical power, in order to evaluate the mechanical strength of transformer, it is necessary to do no-load impact test.
Five shocks in general?
The Times of no-load impact test of full voltage should be five times before the new product is put into operation. Because the closing Angle is different at each closing time, the corresponding inrush current is also different, sometimes large, sometimes small, generally requires the no-load closing five times to comprehensively test the insulation, mechanical strength and differential protection of the transformer.
What does exciter inrush current look like?
Characteristics of inrush current:
1. An aperiodic component containing a large number of components tends to skew the current to one side of the time axis, usually with one phase opposite to the other two
2. Contains a large number of higher harmonics and maximizes the second harmonic component
3. There are discontinuity angles between inrush waveforms
4. Inrush current has a large value at the initial stage, which can reach 6~8 times of rated current, and then gradually attenuates.
What are the causes of transformer leakage?