When the transformer is first produced, hipot tester has not been tested in harsh environments for a long time. The power supply with hipot tester rated voltage and frequency is applied for testing. The voltage between winding turns, layers, and sections are not enough to reach the breakdown voltage of the dielectric defect. Discharge and breakdown at insulation defects, the no-load current and no-load power consumption of transformers with hidden insulation faults, and similar transformers with good insulation performance are not much different, so hipot tester is difficult to find these hidden dangers.
The induction hipot tester applies a voltage more than twice the rated voltage to the transformer, which can establish a higher and more concentrated field strength at the longitudinal insulation defect. The voltages between turns, layers and segments of the windings reach and exceed the breakdown voltage at dielectric defects. The induction withstand voltage test applies a frequency above twice the rated frequency to the transformer, and a higher frequency can greatly reduce the breakdown voltage of the solid dielectric, making insulation defects more likely to be broken down; the applied voltage specified in the induction withstand voltage test. The action time can also ensure the breakdown of insulation defects, so the induction withstands voltage test can reliably detect the quality of the longitudinal insulation performance of the transformer.
The reason why the frequency of applying power to the transformer in the induction withstand voltage test is more than 2 times the rated frequency is because the excitation current I of the transformer – the characteristic curve of the main magnetic flux amplitude Фm. Generally, the design is close to the bending saturation part at the rated frequency and rated voltage. Because the main magnetic flux Фm is determined by the applied voltage U when the power frequency is unchanged.
U ——Applied power supply voltage, V △Фm
E ——The induced electromotive force of the energized winding, V
f —— Frequency of applied power supply, Hz
So add a voltage △i more than twice the rated voltage to the transformer.
ATO Hipot tester will inevitably lead to serious saturation of the iron core, and the main magnetic flux Фm will increase by △Фm.
The exciting current I will increase sharply, causing the transformer to heat up and burn out. In order to make the iron core of the transformer not saturated when the voltage is more than 2 times, hipot tester is necessary to increase the frequency of the power supply to more than 2 times the frequency.
In the induction hipot tester, add more than 2 times the voltage and more than 2 times the frequency of the primary side of the transformer. The main magnetic flux of the transformer will induce the induced electromotive force E1 and E2 on the primary side and the secondary side at the same time, and they are respectively under their rated working conditions. More than 2 times, the induction withstand voltage test can test the longitudinal insulation performance of the main and auxiliary windings at the same time. Of course, we can also test from the secondary side of the transformer as needed, but the applied voltage should be more than twice the no-load voltage under the rated working state of the transformer, and the frequency is also more than twice the rated frequency.