Limitations of Kramers-Kronig transform for calculation of the DC conductance magnitude from dielectric measurements

The Kramers-Kronig (K-K) transform relates the real and imaginary parts of the complex susceptibility as a consequence of the principle of causality. It is a special case of the Hilbert transform and it is often used for estimation of the DC conductance from dielectric measurements. In this work, the practical limitations of a numerical implementation of the Kramers-Kronig transform was investigated in the case of materials that exhibit both DC conductance and quasi-DC (QDC) charge transport processes such as epoxy resins. The characteristic feature of a QDC process is that the real and imaginary parts of susceptibility (permittivity) follow fractional power law dependences with frequency with the low frequency exponent approaching -1. Dipolar relaxation in solids on the other hand has a lower frequency exponent <1. The computational procedure proposed by Jonscher for calculation of the K-K transform involves extrapolation and truncation of the data to low frequencies so that convergence of the integrals is ensured. The validity of the analysis is demonstrated by performing K-K transformation on real experimental data and on theoretical data generated using the Dissado-Hill function. It has been found that the algorithm works well for dielectric relaxation responses but it is apparent that it does not work in the case of a low frequency power law in which the low frequency exponent approaches -1, i.e. in the case of QDC responses. In this case convergence can only be guaranteed by extrapolating the low frequency power law over many decades towards zero frequency

Influence of water absorption in flexible epoxy resins on the space charge behaviour

The aim of the current work is to achieve a better understanding of the influence of water uptake in flexible epoxy resins on the space charge dynamics at high electric fields. The space charge behaviour was studied using pulsed electroacoustic (PEA) technique. The samples were prepared from Araldite CY1311, which is a bisphenol-A epoxy resin. This particular resin was chosen because its glass transition is 0°C and hence it is in a flexible state at room temperature. All samples were conditioned in containers with saturated salt solutions or de-ionised water so that various water uptake levels were obtained. It was found that the space charge dynamics was correlated with the amount of absorbed water in the samples and this is consistent with the dielectric measurements made on the same material where ion transport was identified as the main charge transport process from the observed QDC behaviour

Statistical Analysis of Partial Discharges from Electrical Trees Grown in a Flexible Epoxy Resin

Electrical treeing is a long-term degradation mechanism in polymeric insulation, which can lead to electrical failure of HV insulation systems. The rate at which trees grow across the insulation depends on the PD activity occurring within them and hence the detection of the onset of electrical treeing could be established by PD monitoring. In this paper, a statistical analysis of the partial discharges detected during the growth of trees in an epoxy resin will be reported. The aim of this work was to provide additional insight into the physical mechanisms that lead to the observed fluctuations in the partial discharge activity. The results demonstrate interesting correlations between a number of statistical parameters, such as average discharge magnitude and standard deviation in the partial discharge amplitudes. These correlations could also be related to physical parameters such as the applied voltage magnitude and the measured power dissipation due to the partial discharges occurring during tree growth. The implications of this work for deterministic methodologies for the simulation of tree growth as well as for condition monitoring using feature recognition strategies for the early detection of tree growth will be discussed

Influence of absorbed moisture on the dielectric properties of epoxy resins

The dielectric response of two bisphenol-A epoxy resin systems Araldite CY1301 (Tg ~ 50°C) and Araldite CY1311 (Tg ~0°C) was studied at different levels of absorbed moisture. The dielectric measurements were carried out over the frequency range 1 mHz to 100 kHz and the results were characterised in terms of dc bulk electrical conduction and dielectric processes. The characteristic parameters (frequency and magnitude) of all processes have been found to be moisture dependent. In both resins above the glass transition temperature, absorbed moisture was found to be implicated in the formation of a bulk quasi-dc dielectric response consistent with cluster formation of the absorbed water molecules

Influence of the temperature on the dielectric properties of epoxy resins

Electrical degradation processes in epoxy resins, such as electrical treeing, were found to be dependent on the temperature at which the experiments were carried out. Therefore, it is of considerable research interest to study the influence of temperature on the dielectric properties of the polymers and to relate the effect of temperature on these properties to the possible electrical degradation mechanisms. In this work, the dielectric properties of two different epoxy resin systems have been characterized via dielectric spectroscopy. The epoxy resins used were bisphenol-A epoxy resins Araldite CY1301 and Araldite CY1311, the later being a modified version of the former with added plasticizer. The CY1301 samples were tested below and above their glass transition temperature, while the CY1311 were tested well above it. Both epoxy systems possess similar behaviour above the glass transition temperature, e.g. in a flexible state, which can be characterized as a low frequency dispersion (LFD). On the other hand, it was found that below the glass transition temperature CY1301 samples have almost “flat” dielectric response in the frequency range considered. The influence of possible interfacial features on the measured results is discussed

Partial Discharge Patterns in Conducting and Non-Conducting Electrical Trees

Previous observations on electrical tree growth in epoxy resins has shown that different types of tree growth structure, electrically conducting and non-conducting, can occur dependent on the state, glassy or flexible, of the epoxy resin. In this current study, the partial discharge characteristics were characterized experimentally at a temperature of 20°C within two different epoxy resins systems having glass transition temperatures of 0°C and 50°C. The partial discharge activity (determined from apparent charge measurements) was characterized in terms of φ~q~n patterns using statistical tools. The aim was to compare the apparent charge measurements obtained from conducting and non-conducting electrical tree structures with computer simulations of the partial discharge activity in both conducting and non-conducting electrical trees. The results show that there is a significant relationship between the local extent of the partial discharge phenomena, as determined by the conductivity of the tree channels, and the apparent charge, as shown by the experimental and simulated partial discharge patterns. The implications of this work for partial discharge detection as well as for condition monitoring in real insulating systems are discussed

PD pattern recognition using ANFIS

An application of an adaptive neuro-fuzzy inference system (ANFIS) has been investigated for partial discharge (PD) pattern recognition. The proposed classifier was used to discriminate between PD patterns occurring in internal voids. Three different void shapes were considered in this work, namely flat, square and narrow. Initially, the input feature vector used for classification was based on 15 statistical parameters. The discrimination capabilities of each feature were assessed by applying discriminant analysis. This analysis suggested that some of the features possess much higher discriminatory power than the others. As a result, a simplified classifier with reduced feature vector has been obtained. The results demonstrate the importance in identifying and removing redundancy in the input feature vector for reliable PD identification

Pulse sequence analysis on PD data from electrical trees in flexible epoxy resins

Pulse Sequence Analysis (PSA) was carried on PD data from electrical trees grown in flexible epoxy resins. The samples used for the electrical tree experiments were conditioned in environments with different relative humidities in the range 15-100% prior to the electrical tests with the corresponding moisture content in the samples between 0.1 and 6.9%. The electrical treeing experiments were carried out at different temperatures in the range 20-70°C. The details of PD dynamics during the electrical tree growth have been found to change significantly with temperature and absorbed moisture. In this paper, it is shown that PSA could be successfully used to discriminate between PD data from electrical trees with different shapes and runaway tree growth

The Role of Bulk Charge Transport Processes in Electrical Tree Formation and Breakdown Mechanisms in Epoxy Resins

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

Plasma Treated Low-Density Polyethylene Nanocomposites: Investigation of Partial Discharge and Breakdown Strength

Polymeric insulating materials have been widely used in high voltage equipment, particularly in power cables, as insulating material, due to their excellent performances. This study investigates the significance of atmospheric pressure plasma (APP) treatment on silicon dioxide (SiO2) nanoparticles in enhancing the partial discharge (PD) resistance and breakdown strength of low-density polyethylene (LDPE) nanocomposites. The duration of plasma treatment was manipulated for 15 and 30 minutes to identify the effects of treatment duration on their dielectric properties. The loading of fillers was varied into 1, 3, and 5 wt% to identify the promising formulations. The results exhibited that the dielectric properties of LDPE nanocomposites have improved as the SiO2 nanoparticles were treated, anticipated due to the surface activation via functionalizing hydroxyl group on the fillers as ultimate oxidation agent, resulting in reduced size of agglomerated clusters. The PD resistance and breakdown strength have increased up to 47% and 70% of the unfilled samples, respectively, as the SiO2 nanoparticles were treated using plasma. Plasma treatment was found to be an alternative technique for improving the filler-polymer surface interaction, at once promising the better dielectric properties of LDPE nanocomposites, potentially prolonging the lifetime of the insulating materials