Wolfgang Pfeiffer

Surge immunity of electronic equipment

Only 7 of the sample with 20 video recorders passed the impulse test without any failure. Almost all failures observed were located in the primary part of the power supply. Typically the radio interference suppression capacitors failed and/or internal fuses were blowing. Although the damage is rather small and limited to a few components, a rather expensive repair is required in any case. Since the radio interference suppression capacitors are the weakest point in the insulation system, either their quality has to be improved or some series impedance has to be used for suppression of the transients.

Dielectric testing of solid insulation with respect to insulation coordination of low-voltage equipment

In real insulation systems with solid insulation, partial discharges usually occur far below the breakdown voltage. In the long run, these lead to the destruction of nearly all solid insulating materials. The question, How much time has passed until the partial discharges have destroyed the solid insulation?, is of little relevance in low-voltage equipment since the occurrence of partial discharge during normal use will, in general not allow a sufficient lifetime. Moreover, the results of such lifetime investigations are influenced by many parameters; therefore, statements with general validity cannot be made.

Influence of the ambient temperature on the impulse withstand voltage characteristics of insulation material surfaces under humidity conditions

Research work has demonstrated, that the impulse withstand voltage characteristics of insulation material surfaces are influenced by the relative humidity. As previous papers pointed out, the so called water adsorption test is suitable to determine the influence of relative humidity on the impulse withstand characteristics of insulation material surfaces. This test is performed according to a test procedure described in, which proposes an ambient temperature of /spl thetav/ = 25/spl deg/C. In order to get information about the influence of the temperature, we perform the water adsorption test at different temperature values within these investigations. All investigations presented in this paper have been carried out on an epoxy resin (FR-4) as well as on a melamine resin (MF1206) and on a polyester resin (GPO-2). Hereby, the influence of the relative humidity on the impulse withstand characteristics of these materials has been investigated at different ambient temperatures between /spl thetav/ = 20/spl deg/C and /spl thetav/ = 50/spl deg/C. The achieved test results demonstrate, that most of the investigated insulation materials show the poorest impulse withstand characteristics at an ambient temperature of /spl thetav/ = 30/spl deg/C, although the absolute humidity at /spl thetav/ = 30/spl deg/C is less than the absolute humidity at higher temperatures for the same value of the relative humidity.

Simulation of predischarge processes in SF/sub 6//N/sub 2/ mixtures stressed by very fast transient voltages

This paper deals with the numerical simulation of discharge processes in SF/sub 6//N/sub 2/ mixtures. We introduce the particle-in-cell (PIC) method using a Monte Carlo collision module (MCC) applied to a two dimensional model of an inhomogeneous gap configuration. Calculation of the electrical field and the spatial distribution of electrons and ions in pure SF/sub 6/ and in 10% SF/sub 6//N/sub 2/ gas mixtures for this configuration were carried out for an total time of 5 ns after applying the test voltage. Eight particle collision types including electron-ion impact and photoionisation have been considered.

About the PD-testing of insulation systems with high-frequency voltages

In the last decade, there has been an increasing interest in partial discharges (PD) at high frequency stress on insulation systems. At present, high-frequency working voltages exceeding 100 kHz are often used in low-voltage equipment and the frequencies are still increasing. Due to the switching in the power supplies, the voltage shapes are usually nonsinusoidal. The aspects of these nonsinusoidal voltages regarding the electrical stress are discussed by harmonic analysis. The difficulties of performing PD measurements at impulse voltages are shown and the alternative of measurements using sinusoidal voltages is discussed. Regarding the nonsinusoidal voltages and the increasing operating frequencies, test frequencies in the MHz range are of future interest. The requirements on the test equipment for these high-frequency tests are discussed. Finally, this paper describes how PD tests can be performed with respect to high-frequency stress and some examples are shown.

Influence of humidity exposure time on dielectric properties of insulation material surfaces

The exact knowledge about the water adsorption characteristics of insulation materials is of significance for dimensioning of creepage distances with respect of flashover under humidity conditions. In order to describe each insulation material with respect to the water adsorption characteristics the insulation materials under test are classified into water adsorption groups. For this classification an appropriate test procedure is required. Hereby, however, two problems arise: The fundamental problem is involved with the preparation of the test samples especially the realisation of an appropriate electrode configuration. For this we will discuss two alternatives: The first one is using printed conductors, the second one is fixing the electrodes by application of high pressure upon the insulation material surface. Another problem is the duration of the test, because insulation materials, depending on surface quality and chemical compound, can change water adsorption characteristics during humidity exposure time. Therefore in this paper also the influence of humidity exposure time on the dielectric properties of the insulation material surface is discussed. Within the scope of this paper the investigated test specimen are consisting of three different resins (epoxy, polyester and polyimid resin) as well as a laminate consisting of presspan and polyester foil.

About the water adsorption characteristics of artificially degraded insulation material surfaces

In order to describe the water adsorption characteristics of insulation materials under humidity conditions, a test procedure called Water Adsorption Test has been introduced, which allows one to classify the various insulation materials into so-called water adsorption groups. This procedure is based on a test to be performed on new samples of insulation materials, and previous papers have shown results of the water adsorption test on such insulation materials. In this paper we investigate the influence of artificial degradation of insulation material surfaces on their water adsorption characteristics under humidity conditions. Hereby, the degradation has been performed by illumination of insulation material surfaces with an UV source. In this paper the investigated polymers consist of a glass fibre reinforced epoxy resin as well as a laminate consisting of presspan and polyester foil. We obtained the result, that the classification of the degraded laminate consisting of presspan and polyester foil does not differ from the classification of a new test sample. On the other hand, the water adsorption characteristics of the degraded fibre reinforced epoxy resin are worse in comparison to a new one.

Simulation of prebreakdown phenomena at a gas/solid interface in a 10% SF/sub 6//N/sub 2/ mixture stressed by very fast transient voltages

A two dimensional Particle-in-Cell (PIC) simulation of streamer propagation at a gas/solid interface is carried out for an inhomogeneous gap configuration. A Monte Carlo Collision module (MCC) is introduced into the usual PIC scheme to simulate the elastic, exciting, ionizing and attaching collisions between electrons and neutral gas molecules or the insulating surface. Photoionization phenomena are also considered using a probabilistic scheme.

Digital signal processing methods for interference recognition in partial discharge measurement-a comparison

Partial discharge (PD) testing is well known for the examination of the insulation characteristics of high voltage equipment. For low voltage components like relays, optocouples etc. PD testing is also of growing interest to ensure the quality of the products and the manufacturing process. The application for pieces testing of mass products in an industrial environment implies automatic interference recognition and short test cycle times. These features can be obtained using digital signal processing methods. In this paper, three different methods, the tolerance band method, the correlation factor analysis and the cross correlation analysis are introduced and their application to automatic PD testing is shown and discussed.

Simulation of Two-Dimensional Streamer Discharge in Uniform Field in Low SF 6 Content Mixtures

SF6 gas is an important insulated medium and is widely used in electric power industry for gas-insulated equipment. However, the environment impact made SF6 a greenhouse gas and be used under strict limitations. As a substitute to SF6, SF6/N2 gas mixture is a promising gas insulated medium. Though the simulations of streamer discharge have been carried out in SF6/N2 gas mixtures by many investigators, most of the researches are based on one-dimensional simulation. Using two-dimensional model, the formation and propagation of streamers in SF6/N2 gas mixtures, which contains 10% SF6, is simulated in the paper. The simulation is based on accurate numerical solution of Poissons equation with the continuity equations for electrons, negative and positive ions. Flux-corrected transport technique (FCT) is used for the solution of transport equation of charged particles under the action of the space-charge electric field. Photoionization in the gashas been taken into account. Streamers in the mixture have been found to be considerably different from those in SF6. The charged particles densities and electric field, which are important to describe the development of streamers and to estimate the insulated strength of the gas mixture, are presented in this paper.