Dielectric Withstand Voltage Tests and Test Conversion Factors for Oil-Filled Transformers

Dielectric withstand voltage tests to verify oil-filled transformer insulation performance are in most cases carried out using two to three types of tests from lightning impulse (LI), switching impulse (SI) and power frequency (PF) withstand voltage tests based on economical factors and past achievements. Test conversion factors (TCF) are used to evaluate whether the tests cover overvoltages not directly dealt with in tests and the omitted tests. In this paper, insulation characteristics for LI, SI, and PF (AC) are first organized including the recent ones. Several sets of TCF are next calculated based on these characteristics, compared with TCF in present standards, and representative values are then derived. Finally, the dielectric withstand voltage tests specified in present standards are compared using the representative TCF, and the AC withstand voltage test was found to be the severest test overall.