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Electrical field distribution in transformer oil

Developments in Kerr electro-optic measurement technology at the laboratory have made possible high resolution electric field measurements in nonpolar liquids. The electric field distribution in transformer oil has been studied at service stress levels in both the transient and steady state. The field distributions observed are well described by a one-dimensional quantitative model (with measurable parameters) based on charge carrier generation through dissociation of a species of ion pairs in the liquid bulk. As two-dimensional measurements are also possible, an engineering tool now exists which can be used, for example, in the study of geometries relevant for converter transformer design. >

Dielectric spectroscopy for diagnosis of water tree deterioration in XLPE cables

An HV dielectric spectroscopy system has been developed for diagnostics of water tree deteriorated extruded medium voltage cables. The technique is based on the measurement of nonlinear dielectric response in the frequency domain. Todays commercially available systems are capable of resolving low loss and small variations of permittivity as a function of frequency and voltage. Experience from more than 200 field measurements was combined with laboratory investigations. Small samples were used in an accelerated aging test to elucidate the correlation between water tree growth and dielectric response. Furthermore, field aged cables were investigated in the laboratory. It has been shown that the dielectric response of water tree deteriorated crosslinked polyethylene (XLPE) cables can be recognized and classified into different types of responses related to the aging status and breakdown strength. The influence of termination and artifacts such as surface currents was investigated. The measurement method enables us to separate the response of the cable from the influence of accessories. Finally, two different field studies of the implementation of the diagnostic method are presented. The field studies show that the fault rate decreased significantly when replacement strategy was based on the diagnostic criteria formulated.

Extraction of high frequency power cable characteristics from S-parameter measurements

A technique is developed for extraction of the wave propagation properties of power cables from S-parameter measurements. The method extracts the complex propagation constant and the characteristic impedance, as well as the LCRG telegraphers equation parameters. The extraction process is developed after clarifying the effect of the connection between the measurement port and the power cable. It is concluded that treating the connection solely as a characteristic impedance change could lead to considerable errors in the parameter extraction. Furthermore, the method corrects for electrical lengths, which are not accounted for by the standard network analyzer calibration. The extraction is demonstrated for a medium voltage cross linked polyethylene (XLPE) cable over the frequency range 300 kHz to 300 MHz. The results are compared to a time domain short pulse propagation method for cable characterization. Both measurement methods are evaluated against a cable model.

Measurement technique for high frequency characterization of semiconducting materials in extruded cables

Knowledge on the dependence of wave propagation characteristics on material properties and cable design is important in establishing diagnostic methods for cable insulation. In this study, a high frequency measurement technique to characterize the semi-conducting screens in medium voltage cross-linked polyethylene (XLPE) cables has been developed. The frequency ranges from 30 kHz to 500 MHz. The influence of the experimental set-up, sample preparation methods, pressure and temperature are investigated. A dielectric function is developed for the semiconducting screens and this is incorporated into a high frequency model for the cable. The propagation characteristics obtained from the high frequency cable model are compared with those obtained from measurements made on the same cables.

Modeling the Effect of Ionic Dissociation on Charge Transport in Transformer Oil

Transformer oil is one of the most important and widely used of all dielectric liquids. Significant research into the insulating properties of transformer oil has been carried out. Much of this work has focused on the development of streamers, and how they can result in the electrical breakdown of the oil. Due to the complex nature of transformer oil no universally accepted model describing streamer initiation and propagation has emerged. In this paper, a model is developed which explores the role played by electric field dependent ionic dissociation in the initiation and propagation of streamers in transformer oil. This model couples the electrodynamics of charge transport to thermal enhancement in transformer oil. Using the COMSOL Multiphysics software application and the finite element method, the model is solved to show that ionic dissociation alone will not lead to a high enough temperature rise to initiate a streamer in transformer oil.

Observation of space charge dynamics in air under DC electric fields

Even under corona-free conditions insulating structures (materials) may be charged in DC fields. This is due to the fact that air has a finite ionic conductivity. The finite conductivity of air results from charges generated in the air due to background radioactivity and cosmic radiation. Typical ion concentrations on ground are 107 to 1010 1/m3 which leads to a conductivity ranging from 10-16 to 10-13 S/m. An experiment has been conducted to measure order of magnitude, time scales, and boundary influences of air ionic motion under DC electric fields. The experimental setup consisted of two concentric cylinders of metallic mesh. The radius of the inner cylinder was 0.5 m and that of the outer cylinder 2 m. The current was measured as a function of time after a potential was applied to the inner cylinder. The background ion concentration was monitored by an ion counter. The results were analyzed and compared with simulations using an ion-drift model considering small cluster ions of positive and negative polarity. The measurements have been found in good agreement with the model.

Modeling and Measurements of Electric Fields in Composite Oil/Cellulose Insulation

In this paper we report on basic studies of the electric fields under DC stress in a composite insulation consisting of transformer oil and oil-impregnated pressboard, typical for HVDC converter transformers. The time dependent electrical fields in our model geometry are measured using electro-optic Kerr method, and theoretically studied with an ion drift-diffusion conduction model. The measured and calculated electrical fields are compared and analyzed.

Modelling of partial discharge spectra measured with variable applied frequency

Variable Frequency Phase Resolved Partial Discharge Analysis (VF-PRPDA) is a newly developed method for partial discharge measurements. In this method the frequency of the applied voltage is variable and not fixed at the power frequency as in the standard PRPDA implementation. In this paper, simulations illuminate the frequency dependence of PD of a discharging cavity embedded in an otherwise homogeneous insulation. It is shown how the frequency dependence is influenced by the conductivity of the bulk insulation, the surface conductivity of the cavity walls and the statistical time lag. The results of simulations are compared with result of measurements on an asphalt-mica insulated machine stator bar.

Harmonic content in the partial discharge current measured with dielectric spectroscopy

Phase resolved partial discharge analysis and dielectric spectroscopy measurements have been performed at 50 Hz and variable voltage on two artificial defects that have different average charge phase distributions. Influence of partial discharges on permittivity and loss-factor is deduced to the phase position of the partial discharge activity. Symmetry properties between positive and negative charge distributions have been related to the harmonic content of the partial discharge current.

High frequency characterization of the semi-conducting screens of medium voltage XLPE cables

In this study, a high frequency measurement technique to characterize the semi-conducting materials of the medium voltage XLPE cables in the frequency range of 30 kHz to 500 MHz has been developed. The influence of the experimental set-up and the sample preparation methods are investigated. A dielectric response model is then developed for the semiconducting materials and this is incorporated into a model for the whole cable. The propagation characteristics obtained from the cable model are then compared with those obtained from measurements carried out on the actual XLPE cables.