Showing papers by "John C. Fothergill published in 2016"

The role of bulk charge transport processes in electrical tree formation and breakdown mechanisms in epoxy resins

Abstract: Electrical treeing experiments have been conducted at different temperatures and levels of absorbed moisture in Araldite CY1311 epoxy resin samples above their glass transition temperature, i.e. when the resin was in a flexible state. The fractal dimension of the electrical trees obtained and the rate of tree growth were found to depend on the environmental factors: temperature and humidity. It has also been found that at certain levels of temperature and moisture absorbed in the samples, a transition occurs from electrical treeing degradation to breakdown by thermal runaway. Complementary investigations of the dielectric properties of the same epoxy resin system have revealed that a bulk quasi-dc (QDC) charge transport mechanism takes place above the glass transition temperature, and we show that the characteristic features of the dielectric response are related to the shape of the electrical treeing degradation and the transition to thermal breakdown. This is explained qualitatively through the effect of the bulk QDC charge transport process in modifying the local space charge electric field distribution.

Aging mechanisms of X2 metallized film capacitors in a high temperature and humidity environment

Abstract: Safety capacitors (usually denoted as X1, X2 or Y) are metallized film capacitors (MFC). Two kinds of capacitance loss mechanism typically occur in this metallized film structure: (1) caused by self-healing resulting in a very small electrode area loss; (2) caused by electrode oxidation by electrochemical corrosion under ac stress in a humid environment. This study focuses on the aging mechanism of X2 film capacitors working in high temperature and high humidity environments. Two types of X2 film capacitors have been stressed under an applied voltage of 270Vac at 85 ¼C, 78.68%RH. Capacitance and equivalent series resistance (ESR) were monitored during the experiment as parametric parameters that reflect the aging process. The aging mechanism including moisture ingress time were calculated and comprehensively analyzed with the capacitance change characteristics. It was found that capacitors with lower aluminum metallization has a better capacitance stability, the mean corrosion rate of Type2 (9.7% aluminum metallization) is more than 3 times of the Type1(6.9% aluminum metallization).The ingress time calculated by capacitance change and normalized ESR show reasonable agreement.

A comparison between PSA plots of partial discharges in needle voids and electrical trees

Abstract: The aim of this study was to investigate the difference in the pulse sequence analysis (PSA) plots of partial discharge (PD) data obtained from electrode-bound voids and electrical trees in epoxy resin samples. Electrical discharge tests were performed on a set of epoxy resin samples containing needle voids, which were obtained by a partial extraction of tungsten needle electrode. Two epoxy resin systems were used in this study, Araldite CY1301 and Araldite CY1311. The two resins have different dielectric properties, which allowed the effect of material conductivity on the discharge activity in the void to be investigated. Complementary electrical tree growth experiments were performed on a set of samples with a conventional needle-plane electrode configuration. The effects of the material properties, temperature and moisture absorbed in the dielectric samples were also investigated. The results show that additional material characterization is needed to supplement the PSA plots in order to achieve a reliable diagnostic tool.