Life estimation and relaxation behaviour of low density polyethylene (LDPE) used for electrical insulation

Abstract

Life estimation and dielectric characterization of polymers are considered important owing to widespread use of polymeric insulation worldwide in power cables for electric power transmission. Low density polyethylene (LDPE) is one of the most widely used insulation polymers derived from ethylene. Hence it is important for manufacturers and utilities alike to evaluate the ageing and dielectric response of LDPE. Conventional constant-stress accelerated ageing tests for life estimation are time consuming, cumbersome and suffer from the disadvantage that some samples may not fail even after a long time. This necessitates censoring the data for life estimation, increasing the complexity of data analysis and leaving some amount of uncertainty due to censoring. We present a simpler yet robust procedure i.e. damage equalization method for estimating the life of power equipment with Step-stress breakdown tests. Inverse power law model is used in conjunction with Miner’s cumulative damage theory. The approach is compared with existing cumbersome, time-consuming techniques of life estimation and found to be matching well. Several types of breakdown experiments were conducted for the samples, with different step-size of step-stress accelerated ageing tests, at different temperatures. Life under ac and dc fields has been dealt with for several ages. However, polarity reversal is a distinct operating condition which is an essential part of a conventional, line commutated converter based high voltage dc system, complicating the life analysis. In literature, various empirical or phenomenological models have been proposed for life estimation under combined stresses. Attempts were also made to correlate ageing with space charges, with partial success. In this work we have examined existing methods and their practical limitations and a simple empirical method is proposed that can be used effectively. Inverse power law is one of the most commonly used empirical models for ageing under one type of stress. It is examined here, for its applicability under combined dc and polarity reversal conditions. Carefully designed experiments indicate that, changing frequency of polarity reversal results in change in slope of v-t characteristics on log-log scale. Therefore, the power law parameters which are generally assumed as constant for a given set of experimental conditions tend to depend on frequency of reversals. We have proposed modification in conventional inverse power law for combined stress application. Further, electric conduction in polymers is a complex phenomenon and differs considerably at high electric fields where phenomena like space charge formation and charge injection from the electrodes come into the picture. This work investigates the relaxation behavior of low density polyethylene (LDPE) films at high electric fields (> 10 kV/mm) and under different temperatures. A clear distinction is made between the short term Debye-type exponential behavior and long term power law behavior. Apart from the variation in the power law exponent for absorption current with increasing field, anomalies were found in both types of relaxation currents at high electric fields and temperatures. Evidence of the polarization current anomaly was correlated with space charge behavior of samples. Interestingly, the space charge evolution under step voltage application was observed to be of oscillatory, wherein, the charge trapping inside the dielectric bulk showed an oscillatory profile. The experimental factors which could contribute to anomalous relaxation behavior have also been thoroughly investigated and reported, which seem to have not been paid attention until now.