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Particles influence both AC and DC voltage breakdown in insulating oil. Larger and conducting particles decrease the breakdown voltage most. For both DC and AC the ratio of breakdown voltage for clean oil to oil with non-conducting particles is nearly the same. For conducting particles the ratio for DC is generally larger than for AC. Particle concentration and motion is influenced by DC and AC stress...
It is shown that a direct voltage component is obtained when measuring the surface field strength of a polyethylene sample which is tested under an alternating voltage stress. So far this effect cannot be explained, but it is assumed that it is connected rather with solid-electrode interface phenomena than with solid-bulk parameters.
This paper summarizes the progress of an extensive study of the ac voltage endurance, extending from a single cycle to more than a year, for cast silica-filled epoxy resin, with recessed silver paint electrodes and cast-in aluminum electrodes, giving a nearly uniform electric field. Although the specimens were prepared to avoid gas cavities, and partial discharges were usually undetected, the ac strength...
Accelerated aging tests on high-voltage solid insulation can be correctly interpreted with a convenient technique recently developed for the analysis of Weibull statistical data. Examples illustrate the significance of this technique in making valid comparisons of expected service lives on the basis of such tests.
Strong electromagnetic fields exist in the region of the windings of large electrical machines. The structure which supports the windings must be capable of taking overloads arising from abnormal operating conditions. Glass fibre reinforced plastics (GRP) are nonmagnetic, electrically insulating and have high mechanical strengths over a wide range of temperature. Insulating components for supporting...
Epoxy resin has been widely used as the insulating materials of various electrical apparatuses due to its' excellent mechanical and electrical properties.
The work described attempts to define the steady state d.c. resistivity and governing parameters of materials for H.V.D.C. extruded cables. With high density polyethylene steady state was never achieved in tests of several days' duration. With EPR, polypropylene and cross-linked polyethylene steady state was achieved within twenty hours at low stresses (< 10 MV/m) and at temperatures above 50°C...
Many different materials have been used for slot liners in turbine-generator rotors. Until the early 1950's, composites using flake mica were in widespread use. These were replaced by the introduction of glass reinforced plastics whose use continues to predominate today. More recently, however, high temperature organic materials have been developed which are suitable for slot armor construction, thus...
For a better understanding of the basic mechanisms of electrical treeing in polymer insulations the dependence of the DC-conductivity of polyethylene (with and without additives) on temperature and field strength was examined. Here we found that all additives, and especially acetophenone which is a favorite additive against treeing, substantially increase the conductivity.
The installation in northern regions of new power plants and facilities presents certain unique problems due to the very low ambient temperatures. Many electrical insulating materials commonly used become glass like under extreme cold conditions. Insulating oils such as those used in cables and transformers present the risk of becoming highly viscous or solid at low temperatures. In oil impregnated...
The partial discharge properties of the cable model insulated with the cryogenic liquid impregnated tape were studied. As we confirmed that the breakdown strength of the paper-like single sheet sample in liquid was dependent on the permittivity ratio of liquid to fiber, the partial discharge inception stress in the butt gap of the cable was analysed using the permittivity ratio of the liquid to the...
Tests are described on lapped taped cable models impregnated with liquid nitrogen to investigate discharge activity and insulation life. A number of polymer materials are discussed with an emphasis on high density polyethylene. For all the polymers a life expectancy-stress relationship of the form t En = Constant is obtained. The exponent “n” is seen to be strongly dependent on the pressure of the...
1. Increasing test temperatures have been shown to be both beneficial and adverse to life depending upon the system on test. An increasing test frequency can give rise to increasing electrode temperature. This effect on linearity of frequency acceleration will be different for different insulation systems. Clearly, a controlled and constant electrode temperature is desirable in the absence of data...
The generation and transmission of electric power using equipment operating at cryogenic temperatures has been considered to be economically and technically feasible within ten to fifteen years. Cryogenic generators have the ad vantage of smaller size yielding greater generated power per unit volume. Superconducting or cryoresistive underground cables can transmit more power and with less losses than...
UHV cables such as 750 KV and 1000 KV cables will be desinged at high electrical working stresses because the insulation thickness is restricted by dimensional limitations due to manufacturing, transportation and installation conditions. Therefore the insulation material for UHV cables must have high electrical strength, low dielectric loss and stable dielectric properties under high electrical stress.
A synthesis is presented of the possible types of insulation for DC cables. These are: a. impregnated paper insulation, either non-pressure assisted or pressure assisted; b. lapped synthetic insulation, either liquid-filled or gas-filled; c. lapped composite insulation; d. extruded synthetic insulation, e. compressed gas insulation.
The influences of insulating paper formation on the dielectric and tensile breakdown of oil-impregnated paper are discussed. It is concluded that Kraft insulating paper formation, which is measured by Beta-Radiography method, is classified roughly into three types: Type I, Type II and Type III, and that the insulating paper of Type I is most appropriate for the oil-impreganted Kraft paper for OF cable...
Equations used to describe aging phenomena in extruded dielectric materials are reviewed and some new ideas are presented. Attempts are made to predict cable performance, and to see if cable materials have characteristic failure temperatures when subjected to thermal and electrical stress. The results are unsatisfactory, and highlight the simplistic treatment inherent in the starting equations. Also,...
To insure good dielectric performance of a high voltage cable splice, electrical stresses at key locations need to be controlled and kept at a level where corona discharges will not occur during normal operation. The cable -splice interface, in general, is the weakest dielectric path. Where a multipiece concept is used, the interface between cable adapter and splice housing is also a potentially weak...
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