Effect of Corona on Nonceramic Insulator Housing Materials

Introduction: Corona discharges can be a significant threat to the integrity of nonceramic insulators (NCI) due to the organic nature of the housing material [1]. For transmission class insulators (69 kV and above), corona can be problem not only in contaminated but in clean environments as well. From worldwide experience it has been identified that a majority of NCI applications are under relatively clean or lig htly contaminated conditions. On NCI, corona can be present locally for long periods of time due to inadequate hardware design, damaged hardware, and deficient interfaces due to improper design and/or manufacturing. Field inspections on 230 kV and 500 kV insulators confirm the existence of corona even under relatively dry and clean conditions [2]. In practical high-voltage systems it is difficult to avoid corona in the field, especially under wet and contaminated conditions [3, 4]. Hence, knowledge of the corona discharge magnitude and damage threshold of the housing material is essential. Corona is responsible for multiple effects such as mechanical, due to the impingement of charged particles, ultraviolet (UV) radiation, and liberation of ozone. Highly oxidizing and hydrated species of nitrogen oxides are generated which on dissolving in moisture leads to the formation of acids [5, 6]. Any cracks in the housing can expose the fiberglass core to moisture. The exposed rod can fail by tracking, erosion or brittle fracture [7, 8, 9], leading to premature failure. Hence, there is a necessity to periodically inspect NCI and replace degraded insulators in a timely manner.