Bipolar charge transport (BCT) models have been used for the past few decades to simulate and analyze charge transport and its associated phenomena in polymeric insulation materials. Whilst in this method primarily, electronic charge carriers are used, recent studies show that there has been a high interest in the addition of ionic charge carriers and their physical processes in the model. Among the byproducts in the crosslinked XLPE, acetophenone is speculated to have deep electron traps and therefore can contribute to the electrical conduction processes in XLPE insulation, leading to charge and field dynamics in XLPE insulation. In this paper, a new model is attempted considering acetophenone for modelling the ions originating from acetophenone molecules, which demonstrates the charge and field dynamics in the insulation. Using the BCT model, combining with the electronic species, the impact of the presence of the ions is discussed. The ionic charge carrier and the neutral molecule are modelled based on Marcus theory. The results reported in this paper draw attention to the charge and field distribution for different ionic concentrations, the effect of thermal gradient and mobilities of the ions. A comparison between the experimental conductivity values from various literature and the proposed model has been done.

The results of breakdown voltages in liquid silicone rubber (LSiR) insulation under stepped polarity reversal voltages at a uniform temperature in a needle-plane electrode system are presented in this article. In the presence of defects such as sharp conductor projections or protrusions in the cable insulation stressed under polarity reversal direct current (dc) voltages, the electric field distribution is far more complex due to nonlinear insulation conductivity and time-dependent field distribution. The electric field distribution and space-charge accumulation are intricate functions of factors such as geometry and nonlinear conduction. In this article, a two-dimensional (2-D) axisymmetric model of a needle-plane geometry is developed using the finite-element method to compute the time-dependent field, space charge, and current density distribution under polarity reversal dc, which apparently did not get much attention until now. Interesting results on the effect of polarity transition time and applied polarity time step on the abovementioned distributions are addressed and presented. The pronounced effects of nonlinear conductivity on the time-dependent field distribution are also demonstrated. Furthermore, from the breakdown experiments conducted on tip-plane electrode system for different tip radii and applied time steps, the results put forward a rational and practical estimate of the breakdown field using the proposed model.

The surface voltage of an insulator assumes importance for understanding tracking and flashover characteristics. Until now, determination of surface voltage of insulating surfaces is left to simulation software and such simulations are left unverified experimentally, leading to uncertainty of the results, which may sometimes be erratic with improper choice of boundary or initial conditions or inaccurate material properties, nonlinearities etc. The paper presents a novel method, using which the surface voltage of any insulating surface such as insulator strings or transformer bushings can be measured. Unlike a conducting surface, the measurement of insulating surface voltage using voltage dividers would be erratic as the measuring arrangement itself will vary the voltage division due to the local unknown impedance between the insulated surface and hv conductor. Recently a method for measurement of surface voltage of an insulated conductor using cylindrical strips was reported. In this paper, based on analytical derivations, a more generalized method of measurement, applicable to any point on an insulating surface of arbitrary geometry is proposed using circular-disc strips. The proposed experimental method is applied to insulator strings and transformer bushings and validated by simulation of the entire systems. The simulation and experimental results are in excellent agreement.

Surface voltage measurement of an insulated overhead conductor assumes importance for understanding corona, radio interference voltage and for design purposes. Surface voltage measurement of a bare overhead conductor is quite simple as a voltage divider can be directly connected. However, there are challenges in the measurement of surface voltage on an insulating surface of an insulated covered conductor using voltage dividers. In case of ac measurements, the capacitance parameter dominates in deciding the potential distribution. In this article, it is demonstrated that the measuring system itself will alter the voltage division between the capacitance of the cylindrical insulated conductor and that of surrounding air acting as a capacitive divider. The voltage distribution is shown to be drastically influenced by the measuring system capacitance. In view of these issues, a novel experimental method is proposed for the measurement of surface voltage of an overhead insulated covered conductor. In general, the method can be applied to any cylindrical insulated conductor and for calibration of sensor-based measurements. The proposed empirical method is tested by a validated simulation of the entire system and it is proven analytically. The radial voltage gradients are then estimated from measured surface voltages. The analytical, experimental, and simulation results are in close conformity.

Electrical treeing is considered to be one of the most important physical evidence of insulation damage in HV cables, caused due to the high field enhancement at the tip of these defects in the insulation. In this paper, breakdown experiments are conducted using needle-tip electrode configuration under stepped DC stress profile. Further, the authors present an estimation of electric field and space charge distributions in needle tip-plane system based on FEM. Nonlinear conduction is incorporated in the computation by using a semi-empirical equation of field and temperature-dependent conductivity. The results show the interesting aspects of nonlinear conductivity on the electric field and space charge distribution. Further, the effect of temperature dependence on field distribution at the needle-tip is also presented. Furthermore, from the breakdown experiments, the electric field at the tip is estimated for different tip radii and the results give a reasonable and realistic estimate of breakdown tip-field, using the proposed model.

This paper presents a modified Damage Equalization Method with statistical features for life estimation of dielectrics. The paper postulates damage as a random variable, which resulted in establishing a relationship between the Weibull scale, shape parameters and the endurance coefficient `n' of inverse power law. By the modified DEM (m-DEM), for the first time, a range of values of n for each material is proposed, which is useful for design engineers. Until now, the endurance coefficient of inverse power law and strength constant were treated as constants. Here, it is shown that these coefficients do have statistical features or variations. Experiments are conducted on oil impregnated paper and LDPE samples using accelerated step-stresses for life estimation. The results of the estimation of endurance coefficient are compared with existing methods and reasonable conclusions are drawn in favor of the proposed method.

In this paper, electric field and space charge accumulation have been investigated for needle-plane system (defect) under DC voltage conditions. Prolate spheroidal electrode system, believed to be closer to needle-plane system, has been used for solving the governing differential equations, numerically, for space charge and electric field distributions. Unlike past works in which space charge at needle tip was assumed either qualitatively or quantitatively, in this work, space charge formation is estimated using nonlinearity of material properties alone. A comparison with previous methods based on concentric spherical electrode approximations reveals that the results are different for prolate spheroidal system at different nonlinearities. Interesting results on the role of nonlinear conductivity on the DC electric field and space charge accumulation at needle-tip are presented. Furthermore, needle tip-plane breakdown experiments are conducted and the results suggest a reasonable and realistic estimate of tip-field at breakdown, using the proposed model.

Recently, in medium voltage distribution systems, overhead conductors are replaced by insulation covered conductors owing to their profound advantages. The insulated overhead conductor shares a boundary between the dielectric insulation used and the air insulation. Because of the higher insulation resistance of the air when compared with any polymeric insulation, most of the voltage drop will be on the dielectric-air interface (i.e. on the surface of the dielectric). The main goal of this work is to investigate the surface potential of the XLPE covered conductor accurately at different positions using a lab designed voltage divider under applied alternating voltages. Furthermore, 2D simulations have been done to estimate the surface potential of an insulation covered conductor. Interesting results on the surface voltage with applied voltages and different positions in the insulation are presented.

Recently, insulated covered overhead conductors are gaining momentum instead of bare overhead conductor due to several advantages associated with them like improved power system reliability and other environmental aspects. Under normal conditions, the insulation surface of the covered conductor will be affected by different stresses like electrical, mechanical, environmental etc. Covered conductors are not subjected to confined electric field as in underground cables. However, there will be leakage currents, both insulation and surface due to the above-mentioned stresses which will cause degradation of the insulation of the covered conductor. In this paper, surface voltage measurements have been done on an insulated covered conductor. Simulation of the insulated covered conductor using equivalent electrical model of the covered conductor has also been done and the results of the simulated values are found to be in close conformity with the experimental data. Using the equivalent circuit, the effect of surface resistances on the surface voltages and surface leakage currents have been simulated. Interesting results on the role of surface resistances are presented. It is shown here that a lower surface resistance may cause a higher stress to the surface insulation of a covered conductor.

In high voltage insulation system, the insulation will be undergoing various stresses and the life of the insulation will be different from uniform fields when stressed by a divergent field. In this paper, dc breakdown experiments at different step stresses are conducted using needle-plane and plane-plane electrode systems. Using the data from the breakdown tests, assessment of life of the insulating material can be done by estimating the voltage endurance coefficient (n) and accumulated damage. Stepped-stress damage equalization method (SSDEM) is used for estimating the life of the insulating material used. Interesting results on the role of uniform and non-uniform field on endurance coefficient is reported which in turn shows the effect of divergent and uniform stress on the life of the insulation. Furthermore, in this study, a numerical model is developed to estimate the electric field around the tip of the needle electrode using a semi-empirical equation of nonlinear conduction under steady state DC. Spherical geometry system is used for solving the differential equations numerically. The results show the developed numerical model can predict the Poisson's field when conduction is field dependent.

The converter transformers are reported to fail generally on the thyristor-valve side, where complex alternating voltage waveforms superimposed with dc voltages would occur. The effect of these waveforms on the failure and endurance of the valve side winding insulation of the transformers is not yet understood, which is investigated here. In this paper, the voltage waveforms occurring in a converter transformer are obtained through PSCAD/EMTDC for a typical double pole ±500 kV, 1000 MW DC-link HVDC transmission system based on the CIGRE benchmark system. The waveforms are then amplified using a High Voltage amplifier for experimental breakdown investigations. Stepped-stress damage equalization method has been used for obtaining the lifetime characteristics from failure data. A comparison with life curve under pure sinusoidal and actual alternating voltages with superimposed dc and harmonic voltage has been made to understand why the converter transformers generally failed on the valve side. The results first time give fundamental reasons for the possible failure of the converter transformers and put forth important design considerations.

In this paper, dc electrical treeing breakdown experiments are conducted using needle-plane system. Electrical treeing test can be used for the assessment of life estimation of the insulating material by estimating the voltage endurance coefficient and accumulated damage. Damage equalization method (DEM) is used for estimating the life of the dielectric material used in HVDC cables. Breakdown experiments are conducted with progressive voltage steps of different step size. Interesting new results on the role of different step size on breakdown voltage is reported which in turn shows the effect of space charge injection and accumulation. The results from the space charge distribution in a plane-plane geometry justify the authors’ results on the effect step size on dc breakdown in needle-plane geometry. The experimental results obtained are compared with data available in the literature for ac electrical treeing tests for different rate of progressive voltage rise.

The failures caused by short circuits are one of the causes of transformer outages. The short circuit currents induce excessive forces in the transformer windings which result in winding deformations affecting the mechanical and electrical characteristics of the winding. In the present work, a 722VA transformer winding is considered for the short circuit force and displacement analyses. The winding is subjected to short circuit current and the respective displacement of the winding is simulated and compared with the measurement. By using Finite element analysis, change in equivalent circuit parameters due to displacement is calculated and corresponding change in sweep frequency response is measured using SFRA. An attempt has been made to predict the displacement profile from change in resonant frequency in case of displaced conditions.