Effect of performing DC tests before dielectric frequency response on a MV stator

In this paper the effect of performing a direct current (DC) test before a dielectric frequency response measurement is analyzed. Tests were performed on two pairs of twin machines of 6600 V and 2300 V. First a DFR test was performed to be used as reference measurements. Immediately after, a DC voltage was applied for 2000s in order to measure Polarization Current. Different DC voltages were used in each case in order to consider the effect of different voltage levels.Then, a new DFR sweep was performed. These last measurements were compared with the reference responses and conclusions were obtained. In all cases, regardless the DC voltage level, performing a DC test before a DFR affected the measurement, thus when this test is performed on a medium voltage rotating machine, it should be done before all DC tests in order to obtain a more accurate measurement.