Rainer Patsch

PD source identification with novel discharge parameters using counterpropagation neural networks

Computer aided partial discharge (PD) source identification using different multidimensional discharge patterns is widely regarded as an important tool for insulation diagnosis. In this paper, a neural network (NN) approach to PD pattern classification is presented. The approach is based on applying variants of the counterpropagation NN architecture to the classification of PD patterns. These patterns are derived from physically related discharge parameters, different from those commonly used. It is shown that considerable improvements of the classification quality can be obtained when an extended counterpropagation network with a dynamically changing network topology is applied to patterns that employ the voltage difference between consecutive pulses instead of the phase of occurrence as the main discharge parameter. Furthermore, using a particular parameter vector that takes the correlation between consecutive discharges into account also allows to solve the rejection problem with this type of NN.

Electrical and water treeing: a chairman's view

The essentials of the reports on electrical and water treeing presented at the 3rd ICPADM are summarized. Starting from these findings and including generally accepted facts of the phenomena as well as results published recently, the corresponding model concepts of the two phenomena are described. It becomes evident that bond scission is the decisive point in both deterioration mechanisms. In the case of electrical treeing very high electric fields create hot electrons and/or high energetic photons that cleave molecular bonds and produce radicals. Small voids and tree channels are generated in which gas discharges occur. In the case of water treeing the precipitation of water droplets leads to a local mechanical overstressing of the neighboring polymer chains and finally to their disruption. Depending on the availability of foreign substances, e.g., oxygen, additional chemical reactions may occur and influence further development of the trees. >

Analyzing partial discharge pulse sequences-a new approach to investigate degradation phenomena

This paper presents a new method of partial discharge data analysis for the investigation of space charge and degradation phenomena in high voltage insulation systems. The basic principles of the pulse-sequence-analysis are illustrated and the characteristic differences with regard to standard procedures that have been established during the past few years are outlined. A description is given of an electronic partial discharge acquisition system, which is designed on a plug-in interface board for a standard personal computer to perform long term measurements of all the discharge parameters required for the succeeding PSA. Electrical treeing in polyethylene is used to demonstrate the usefulness of the PSA method. It is shown that information on space charge phenomena and the development of the local degradation can be obtained by considering correlations between consecutive discharge pulses.<<ETX>>

A physical model, describing the nature of partial discharge pulse sequences

A description is given of a straightforward physical model, which is based on only a few physically meaningful parameters and that allows to describe the PD behaviour resulting from specific insulation defects. It is demonstrated that computer simulations of PD pulse sequences on the basis of this model correspond closely to experimental results. The comparison of simulated sequences and experimental results suggests that the discharge activity is not mainly a stochastic process but consists of periods, with different prevailing processes, each showing a nearly deterministic behaviour.

Return voltage-a reliable diagnosis for water treed cables?

To characterize the state of deterioration of water treed cable insulation different methods are in use. The problems with all diagnostic tools are firstly the sensitivity and secondly the lack of selectivity. Except for very severely deteriorated cables, the commonly used signals like loss tangent or depolarization currents are not able to detect selectively water tree induced degradation. In some cases the methods are not even able to guarantee that the signal is not just the result of the sound insulating material itself. The return voltage and especially its nonlinear dependence on the magnitude of the poling voltage is claimed to be a good measure of the state of degradation of hv-cables. A survey of data collected in the field shows that this assumption is not always true if only one point of each return voltage curve (e.g. the maximum value) is used to calculate a diagnostic parameter. If, in contrast to common practice, the complete time dependence of the return voltage curve is considered, more meaningful and characteristic information about the state of deterioration due to water trees can be gained. This is documented by the results of field-experiments.

Return Voltage Measurements - a good tool for the diagnosis of paper-oil insulations

The condition of service aged paper-oil insulations can be assessed by measuring the dielectric response of the insulation. The paper deals with the interpretation of experimental results of return voltage measurements and discusses the relevance of different measurement conditions and the meaningfulness of the different diagnostic parameters chosen. Special emphasis is taken on the p-factor, a parameter that seems to be appropriate for all types of insulating materials.

p-factor, a meaningful parameter for the evaluation of return voltage measurements

The paper aims at a better understanding of the results of return voltage measurements of electric equipment whose insulation consists of two or more different dielectric materials, like in electric power equipment where often paper-oil insulations are used. The shape of the return voltage curves and the dependence on the experimental parameters are discussed and a characteristic parameter to describe the state of ageing of the insulation is proposed.

Pulse-sequence-analysis-chances to characterize defects

Partial discharge data are often used to get information about the ageing situation of high voltage equipment. Local defects generate characteristic sequences of partial discharges that also contain information about the critical regions in which the partial discharges occur. This information can be drawn efficiently from the sequence of the partial discharge events and especially the analysis of the sequence of the changes of the applied external voltage that are necessary to trigger the next discharge event. By means of the Pulse-Sequence-Analysis (PSA) more meaningful results can be drawn than from the commonly used phase-correlated collection and evaluation of partial discharge data. With regard to the total number of registered and evaluated discharge events, data collected from various defects in high voltage equipment show that the analysis of short homogeneous sequences is more meaningful than the analysis of the sequence of e.g. 5000 partial discharge events as a whole.

The interpretation of return voltage curves of paper-oil-cables

The paper reports on return voltage measurements performed on different 10 kV paper-oil insulated belted-type cables used in network systems with isolated neutral and with effective earthing of the neutral. The different cable design, e.g., the increased thickness of insulation between conductor and sheath for isolated systems, has a strong influence on the return voltage curves. Thus, a diagnostic method, which is exclusively based on single parameters such as the maximum or the initial slope of the return voltage curves, may lead to a misinterpretation. A more sophisticated evaluation method, which uses the time dependence of the complete return voltage curve is proposed in this paper.

The role of dielectrophoresis in the water treeing phenomenon

The water treeing phenomenon is considered from the viewpoint of how far dielectrophoresis can be the decisive phenomenon. In the model concept discussed, a locally increased electric field leads to a local increase of the concentration of water molecules. The consequent precipitation of liquid water produces water trees. A quantitative examination of the electric forces on molecular dipoles shows that, taking into account only the net flow of molecules, there is a much higher force on these dipoles than expected from previous model concepts. Consideration of the diffusion processes through curved boundaries shows that there is an additional factor in increasing the local concentration of molecules. The results of numerical computations show that for diffusion processes, only slight deviations from symmetry may lead to significant changes in the local concentrations of molecules. Computations of the electric field distribution around water trees (simulated by ellipsoids of rotation) indicate the directions of the movement of water molecules and show that the lines of constant electric field resemble the shape of the water trees.<<ETX>>