Degradation Caused by Partial Discharges in Insulation Oil: A Comparison Between Mineral and Vegetable Oils

Abstract

Degradation of insulation is one of the main failure causes of transformers. Phenomena like thermal ageing of oil and paper, humidity, gases dissolution and occurrence of partial discharges (PD) can play a major role on this degradation. As a result, there is a progressive reduction on the insulation capability that is aggravated by the occurrence of transitory and transient overvoltage. For the insulation oils, several factors can contribute to its degradation, including effects of humidity and carbonization. Phenomena like PD are usually associated to failures in the solid insulation, like holes or gaps in paper or low adherence of varnishes, as though as high electric fields gradients, causing low intensity discharges through the oil. Although being a low intensity discharge, PD is a cyclic phenomenon, following the peaks of the alternate voltage. As result, the arcing can generate local carbonization and gases that remain dissolved on the oil, affecting properties like dielectric strength and permittivity. Although some characteristics of the vegetable oils are well known and are being continuously perfected, some parameters of behaviors are still unknown, as the cumulative effects of PD. This paper presents an introductory comparative study of the effects of PD on mineral and vegetable oils, analyzing parameters like dielectric strength, and chemical degradation. Tests were conducted on samples of mineral and vegetable oils, and evaluated after the occurrence of different numbers of discharges. Preliminary results indicate that although both oils are affected by these discharges, but mineral oils are more stable chemically.