Investigation into pulse sequence analysis of PD features due to electrical tree growth in epoxy resin

Electrical trees developed using point-plane samples have been investigated under three different voltage conditions: AC, AC with positive DC bias, and AC with negative DC bias. Visual observations mainly indicate two types of electrical tree progression from initiation to breakdown: “forward and backward” (FB) trees and "forward" (F) trees. FB trees can be observed in AC tests, while F trees occur in AC with DC bias tests. The difference between AC with negative DC bias and AC with positive DC bias is the growth of a rapid long branch prior to breakdown under negative DC bias conditions. Based on the pulse sequence analysis (PSA) technique applied to the PD data associated with electrical tree growth, the findings confirm that PSA curves under different voltage tests have different regions and PSA features can be indicators of tree growth

The impact of DC bias on electrical tree growth characteristics in epoxy resin samples

The effect of DC bias on electrical tree growth characteristics in epoxy resin samples is investigated, using three waveforms types: AC, AC with positive DC bias, and AC with negative DC bias. Point-to-plane samples were used. AC tests resulted in 62% and 48% longer average time to breakdown than positive and negative DC biased tests respectively. The negative DC bias test had 14% longer average time to breakdown than positive DC bias test. It is suggested that this is due to space charge injection modifying the field at the tree tip. 4 stages of distinct tree growth are identified in AC tests compared to 3 stages in DC bias tests. In particular, the phenomena of trees growing in the ‘reverse direction’ (from the planar to the point electrode) observed in the latter stages of AC tests, is not seen in the DC tests reported here. This is thought to be due to the peak field magnitudes involved in each case

DC electrical tree growth in epoxy resin and the influence of the size of inceptive AC trees

Electrical tree propagation is a precursor to dielectric failure in high voltage polymeric insulation. Tree growth has been widely studied under AC conditions, but its behavior under DC is not well understood. The aim of this work was to examine the importance of polarity and initiating defect size on DC tree propagation. Electrical tree propagation in epoxy resin under constant DC voltages of +60 kV and −60 kV was measured in samples with classical needle-plane electrodes, but with small initial trees (< 100 μm) incepted under lower AC voltages before the DC tests. Experimental results showed strong polarity dependence. In either polarity, the length of the initial AC tree had a major influence on the inception of the subsequent DC tree. The effect was attributed to the defect being influential if it is larger than the space charge injection region. As a consequence, there is a critical defect size that will accelerate failure of DC insulation dependent on space charge injection behavior of the polymer/electrode system in question — a critical finding for high voltage asset management

The Impact of Interfaces and Space Charge Formation on Breakdown Strength of Epoxy Resin

The re-emergence of HVDC necessitates re-evaluation of dielectric materials. Epoxy resin is employed as a dielectric material for joints and terminations in HV electrical systems up to about 75 kV, and higher ratings are required. Under DC however, space charge accumulation is a problem especially where interfaces are encountered. In this study, epoxy-epoxy laminates were fabricated and tested for space charge formation, using the PEA method, and breakdown strength under AC and DC were investigated. It was found that negative charges are accumulated at the layer interface and the breakdown strength under DC conditions is about 39% and 35% higher than that under AC condition for single and double layer samples respectively

Tracking and fracturing in epoxy resin due to partial discharges in artificial voids

A void geometry has been developed to generate partial discharge activity and initiate electrical tracking degradation in laboratory insulation systems, providing an alternative to the needle-plane configuration. Reproducible voids were successfully manufactured in epoxy resin at the surface of a rod electrode, creating an enhanced electric field within the voids. Partial discharge measurements have been used to understand the role of discharges in the progression of degradation in the void sample. Fractures formed due to mechanical stresses are observed and investigated by venting the void. Finite element analysis is used to explain damage patterns seen in the epoxy resin samples including a broad region of tracking formation at an associated interface