Dielectric failure of electronic voltage regulator due to interaction with a power transformer during switching

This paper will investigate the effect of resonance on the power electronic devices. As a case study, a capacitive-ladder-based electronic voltage regulator has been chosen for the study and experimental test. Firstly, an extensive simulation has been performed in PSCAD software to check the capability of the electronic voltage regulator during different faults and transients originating from the inductive load side. The simulation proved that the system would not fail for the short circuit fault and the voltage regulator could perform its tasks. For the experimental side of the results, an extensive theoretical analysis has been performed to make sure that there will be no resonance in the selected rating for the test, and therefore capacitor value has been chosen to be far away from the resonance region. For the experiment test, the voltage regulator is in series with a main supply transformer with a rating of 168kV-13.8kV, and the voltage regulator provides 10% of the LV side voltage to maintain the load voltage in an acceptable range. To isolate the electronic voltage regulator from HV side, the voltage regulator has been connected through a series transformer. When connecting the electronic voltage regulator to a standalone transformer and increasing the voltage gradually to perform some tests to verify lack of the resonance, a flashover happened to the thyristors in the voltage regulator and some capacitors became faulty, resulting in a loss in the capacitance value of more than 50%. Further investigation has been done, and it proved that at a specific voltage level, a resonance will be amplified between the capacitor and the regulated transformer which causes the system to experience a massive transient voltage. This led us to the conclusion that the parasitic capacitances and internal circuit of the transformer could push the system to the resonance region, even for a short period of time, and it is proven that the power electronic device such as thyristors cannot handle the surge current claimed in their datasheet. The paper will include simulation results and electric stress evaluation.