Minimum Flashover Voltage of Iced Insulators

Experimental studies were performed in a 4.8×2.8×3.5 m3 cold room with a HV transformer of 124 kVA, 120 kV. Atmospheric ice was accreted on a vertical string of three types of insulators: a classic glass insulator and two long-rod synthetic insulators. The results obtained show that flashover occurs at the lowest voltage with hard rime formed at -12°C with a density of 0.87. With hard rime it is found that the minimum flashover voltage of the glass insulator used is about 40% of that of a wet insulator. The minimum flashover voltage measured without applied voltage during the icing period is higher than that obtained with voltage applied during icing. Except for very light icing conditions, the length of icicles is a less accurate parameter for the measurement of the severity of the icing than the thickness of ice accreted on a monitoring conductor. For clean iced insulators, it is found that the minmum flashover voltage decreases with increasing thickness of ice up to 2 cm. In addition, the results obtained show that the minimum flashover voltage is linearly proportional to the arc path on iced insulators, i.e. the sum of the spacings and the leakage distance of one clean insulator. It is shown that the effect of the thickness of ice on precontaminated iced insulators, decreases with increasing degree of the contamination.