Review of space charge measurements in dielectrics

In the last two decades, a significant effort has been directed to understand the internal charge distributions in dielectrics. This has resulted in the development of a number of methods which give detailed information about space charge distributions. This review presents the evolution of several experimental approaches for the determination of spatial charge and field distributions in dielectrics, including the nondestructive methods of direct probing such as the thermal-pulse methods and the methods using pressure waves that propagate through the charged sample. Detailed information is also provided on the principles used in each technique, typical resolution, limitations and advantages associated with each method. The applications of the methods to several insulating materials charged with various mechanisms are described. The paper concludes with a discussion on a number of quite interesting techniques which seem most promising     

Mechanism of high field electroluminesence and determination of the space charge limited field in polymeric dielectrics

The mechanism of electroluminescence under inhomogeneous field conditions is investigated, as is the use of electroluminescence for determination of the space charge limited field. The latter investigation indicates that if the volume of polymer within the space charge limited field region is too small, the photon flux can be too low to permit accurate determination of the space charge limited field. Further, no means is available to assure that the photon flux is sufficient to assure accuracy.     

In Memoriam [Robert W. Thomas (1937-2008)]

Recounts the life and career of Robert W. Thomas, a Fellow of the IEEE, who passed away earlier this year.     

Electrically active centers of capture of charge carriers and electret effect in polymeric dielectrics and mica

The results of an experimental investigation of the electret effect and electrically active centers of capture in polymeric dielectrics modified by radiation and subjected to deformation (X-ray radiation of electric gas barrier discharge using the EGBD technique), as well as mica after δCo⁶⁰ radiation and heat treatment, have been presented. It has been shown that electrically active centers of capture in polymeric electrets are polar C-H bonds and, in mica crystals, these centers are radiation defects of K⁺-OH⁻ ionic crystalline sublattice. It was shown that the process of the formation of polymeric and mica electrets is described by the wave-current model of polarization.     

Demonstrating a threshold for trapped space charge accumulation in solid dielectrics under DC field

This paper presents a discussion of the concept of threshold for trapped space charge accumulation in solid dielectrics submitted to a DC field. The starting point is the fact that it is often possible to define a critical field separating an ohmic type of conduction from a nonlinear regime in the current-voltage characteristic of solid dielectrics. In the space charge limited conduction theory, this critical field corresponds to the onset of space charge accumulation. However, other conduction processes, such as hopping conduction for example, can also explain nonlinearity in the current-voltage characteristic, which does not involve space charge. It is proposed to check for the existence of a critical field for space charge accumulation using complementary techniques, i.e., space charge detection and electroluminescence techniques. Polyethylene, polyester and polycarbonate were investigated as being representative of three different families of polymers. It is shown that similar values of thresholds are found for a given material using the three above-mentioned techniques, lending support to the physical explanation of a threshold for trapped space charge accumulation.     

Electric field and space charge measurements in thick powerr cable isulation - [Feature Article]

This paper describes the evolution of electric field and space charge in high voltage AC and DC cable polymeric insulation, followed using the thermal step method.     

Phase-resolved pulsed electro-acoustic technique to detect space charge in solid dielectrics subjected to AC voltage

The PEA technique has been modified to study the space charge development in solid dielectrics subjected to AC electric field. Narrow (5 ns) electrical pulses are applied at various phase angles of the AC waveform. Special software, developed to precisely synchronize the pulse generator with the high voltage supply, applied the narrow pulses at 0deg phase angle and then in steps of 10deg till 360deg. By processing the PEA data at various phase angles of the AC waveform, without resorting to complex mathematical analysis, the electric field at which charges are injected into the polymeric insulation was determined. The phase resolved PEA technique can also provide the dynamics of space charge development under AC fields     

Electret effect and charge-carrier mobility in polymeric and mica dielectrics

We measure charge-carrier mobility μ _e,p in polymeric and mica electrets by using a new procedure for forming radiation conductivity gradient through an electret’s thickness; the experimental findings are presented in the paper.     

Novel numerical methods for measuring distributions of space charge and electric field in solid dielectrics with deconvolution algorithm

The deconvolution algorithm for measuring the distribution of space charge under DC by the pressure wave propagation (PWP) method is studied in this paper. A new Fredholm integral equation of the first kind, including a space charge distribution without a partial differential operator is presented. Numerical methods based on Tikhonov regularization for solving this integral equation and the original PWP equation are studied. Numerical simulation is studied for the effect of signal-to-noise ratio (SNR), and comparison with other algorithms is discussed. The numerical solution of an electric field distribution from measurements of a LDPE specimen is obtained successfully.     

Electric charge transport and trapping in dielectrics deduced from isothermal and nonisothermal measurements

By sequential use of the isothermal charging, the isothermal discharging, the final thermally stimulated discharge current and the final isothermal discharging current techniques, the charge dynamics in highly insulating materials may be investigated. The method is demonstrated for polyethylene terephthalate. The injected charge for a field of 20 MV m/sup -/1 and polarization temperatures up to 110/spl deg/C is almost totally trapped in the material and is released during the heating of the sample at 180/spl deg/C for a sufficiently long time. A significant current at high temperatures, about 90/spl deg/C above the poling temperature, was observed proving that it originates from charge detrapping. The final thermally stimulated discharge current peaks shift to higher temperature when the polarization temperature increases, and are characterized by activation energies in the range from 1.03 to 1.56 eV. They allowed the identification of the glass transition around 114/spl deg/C. The relaxation time of the trapped charge, at 180/spl deg/C, was determined to be about 3780 s, explaining the very good stability of trapped charge.     

A molecular model to evaluate the impact of aging on space chargesin polymer dielectrics [comment and reply]

For original paper see J.P. Crine, ibid., vol.4, pp.487-95, 1997. The original paper by Crine provides further consideration and experimental data in support of the already well-known model for the electrical/thermal/mechanical aging of polymeric insulation proposed by the same author some years ago. This paper comments on certain aspects of Crine's model. A reply by Crine is included     

Optical, electrical and electromechanical measurement methodologiesof field, charge and polarization in dielectrics

A selective review is presented of the use of optical, electrical, and electromechanical measurement methodologies of the electric field, charge, conductivity, and permittivity in dielectrics. Kerr electro-optic measurements are presented to demonstrate how volume charge distributions can distort the electric field distribution significantly and how the field and charge distributions depend on dielectric and electrode materials and geometry, and on voltage magnitude, polarity, and time duration. A new class of interdigital dielectrometry sensors is reviewed which from one side can measure profiles in dielectric permittivity and conductivity and related physical properties such as moisture content. Electromechanical devices such as the absolute charge sensor, a flow loop, and the couette charger (cc) with a rotating cylindrical electrode are reviewed for their applications in liquid flow electrification measurements     

Space charge in dielectrics. Energy storage and transfer dynamicsfrom atomistic to macroscopic scale

The consequence of the coupling of a charged particle with a polarization field is the formation of a quasi-particle called the `polaron'. The degree of `localization' of such charge depends on the nature of this coupling. In this way, trapping at an atomic scale is described as resulting from the evolution of a coupling involving successively: the electronic polarization field, the infrared polarization field, and the quasi-static ionic polarization field. The internal energy stored in the surrounding medium polarized by the charge is high because the charge is so well localized. The maximum of this energy is obtained for the quasi-static polarization field; its magnitude is of the order of 5 to 10 eV per trapped charge. This work addresses the physics of aging and of the breakdown process on the basis of an unsustainable increase in local internal energy within the material, due to charge trapping-the polarization around a trapped charge increases the local energy; the relaxation of the material lattice then follows a rapid detrapping of charges from their sites, releasing the local excess site region energy into the material. Such a release executes transient virtual work on the material, producing macroscopic dielectric damage, and when critical, unstable conditions are achieved in the time domain, this is followed by electrical breakdown. This interpretation of electrical material breakdown is related to bulk breakdown as observed in thin film laminate structures (~μm) and surface flashover as seen in large structures (~mm to cm)     

Smart electrical energy networks [review of Power Electronics in Smart Electrical Energy Networks (Strzelecki, R. and Benysek, G.; 2008)] [Book News]

The major aim of this book, a monograph written by an international team of authors, is to present the features, solutions, and applications of the power electronics arrangements likely to be useful in smart electrical energy networks. The book is organized into 12 chapters and includes the following topics: principles of electrical power control; an overview of power electronics converters and controls; quality problems in smart networks; high frequency AC power distribution platforms; integration of distributed generation with electrical power systems; active power quality controllers; and grid integration of wind energy systems, among other topics. The book includes numerous practical examples and very good illustrations that highlight the discussed topics. Each chapter begins with an introduction and is supplemented with a summary and list of references at the end. The book can be used a s a reference for advanced senior-level courses.     

Feature article - Polymeric HVDC cable design and space charge accumulation. Part 2: insulation interfaces

The electric field distribution of cable insulation systems under HVDC can be affected significantly at interfaces due to space charge build-up. In this article, the second part of a three-article series, face and space charge accumulation are analyzed first in terms of macroscopic physics, then through approximate mathematical models that will be used to fit experimental data obtained for model cables having two insulation layers and constituting cylindrical interfaces.     

Electric field calculations for needle-plane geometry and space charge in polyethylene

Electric fields in polymeric insulation are calculated with the boundary element method (BEM) in the presence and absence of space charge. A 3-D version of the BEM software is employed to determine the profiles of divergent electric fields produced by metallic and semicon needles embedded in the polymeric insulation. It is shown that the value of the electric field in the insulation decreases rapidly and within 5 /spl mu/m from the tip it is less than 50% of its initial value. The BEM can also provide 3D profiles of the electric field distributions in the presence of space charge.     

Time-resolved space charge and electroluminescence measurements in polyethylene under ac stress

Time-resolved space charge (SC) and electroluminescence (EL) measurements are carried out on polyethylene films stressed under ac voltage at industrial frequency to probe injection, trapping and recombination of charge. SC is measured by pulsed electro-acoustic (PEA) technique at each zero voltage cross-over point of the ac voltage, thereby getting rid of the capacitive charge. EL is detected using photon counting techniques with a time resolution down to 50 /spl mu/s. Both SC and EL exhibit a threshold response as a function of the applied voltage. Numerical values of these thresholds are in good correspondence for SC and EL measurements in every tested material. Observations are consistent with a model of bipolar injection with an unbalanced situation in positive and negative charge behavior. It is also shown that different polyethylene materials behave differently relative to the field above which the charge is detected, providing thereby a way to compare the space charge behavior of those materials under ac field.     

Correlation between space charge distribution and water-treelocation in aged XLPE cable

The correlation between charge distribution in an aged crosslinked polyethylene (XLPE) cable and the location of water trees has been studied. The space charge radial distributions at different angle in the insulating wall of a service-aged XLPE coaxial cable, induced by application of a DC stress, were measured using the pulsed electroacoustic method. Furthermore, a cross section of sliced insulating wall was observed with an optical microscope to compare with the radial space charge distribution. The cable under investigation was electrically aged in service at 6.6 kV AC for 25 years in wet conditions. Water trees were found at the position in which space charge was observed. On the other hand, no water tree was found at the position in which the space charge was not observed. Judging from these results, the space charge measurement seems to be useful for the diagnosis of water trees in aged cables