The switching impulse test is applied to confirm the withstand of the transformer's insulation against excessive voltages occuring during switching. During the switching impulse voltage test, the insulation between windings and between winding and earth and withstand between different terminals is
checked.
The switching impulse voltage is generated in conventional impulse voltage generators at the laboratories. The polarity of the voltage is negative and the voltage waveform should normally be T1 / Td / T2 20/200/500 S /figure 12.2) according to IEC 60076-3. Due to over-saturation of the core during the switching impulse test, a few low amplitude, reverse polarity (e.g. positive) impulses are applied after each test impulse in order to reset the transformer core to it's starting condition (demagnetised). By this way, the next impulse voltage waveform is applied.
The tap position of the transformer during test is determined according to test conditions
The on-off impulse voltages are applied to each high-voltage terminal sequentially. Meanwhile, the neutral terminal is earthed. The windings which are not under test are left open (earthed at one point). This connection is similar to the induced voltage test connection. The voltgaes distribution on the
winding is linear like the induced voltage test and the voltage amplitudes at the un-impulsed windings are induced according to the turn ratio. Meanwhile, necessary arrangements should be made since the voltage between phases will be 1,5 times the phase-neutral voltage.
The test circuit connections of three-phase transformers depend on; the structure of the core (three or five-legged), the voltage level between phases and the open or closed state of the delta winding (if any). At first, a voltage with 50 % decresed value is used at the tests, then impulse voltages at full values and at numbers given in standards are used. The peak value of the voltage is measured. The change of the voltage waveform and winding current are measured with a special measuring instrument and recorded. The negativities in the transformer during the test are determined by comparing the voltage and current oscillograms. The sudden collapses of the voltage (surges) and abnormal sounds show deformation of the insulation in the transfomer. The deformation of the voltage waveform and increase in noise due to magnetic saturation of the core should not be considered as fault.