Krzysztof Wieczorek

Acoustic investigations of long-rod insulators and their material properties

The paper presents a nondestructive ultrasonic method of investigating long-rod insulators used on overhead power lines and in substations. A correlation between the degree of degradation of porcelain C 120 and the ultrasonic wave propagation and attenuation parameters was established. The ultrasonic method was used to analyze the quality of groups of line and post insulators after many years of service. Also a new mechanoacoustic method of investigating the stages in the degradation of insulator porcelain was applied. As a result of the investigations carried out on samples of porcelain C 130 the successive stages in the degradation of ceramic insulators during their service life were determined.

Effect of shape of insulator housings and positions of composite insulators on ageing resistance under high voltage and precipitation

This paper outlines the results of investigations on the ageing deterioration of model composite insulators where the slope angles of their sheds equal 0, 10 and 30 degrees. It also describes how their vertical, horizontal and oblique working positions affect the manner of their surfaces moistening under conditions of simulated rain and fog water precipitation. It is demonstrated that how the surfaces of insulator sheds are moistened substantially affects time-dependant variations in leakage currents and the energy of electric discharges on the insulator surfaces. Proof is provided that deviations of insulators from their vertical working positions towards the horizontal lead to a growth in leakage currents of insulators. It was also found out that a steeper slope angle of the insulator shed substantially improves the ageing resistance of the devices operated at various working positions under conditions of high voltage and rainwater or fog precipitation.

Effect of voltage harmonics content on arrester diagnostic result

Diagnoses of HV arresters are usually based on an analysis of total leakage current harmonics. The fact that the harmonics content in the supply voltage can be a source of errors is seldom taken into account when routine diagnostic measurements are made. In this paper, the effect of harmonics in the voltage signal on the diagnostic result is analysed using Matlab computer simulations.

Influence of non-ceramic insulators' shed inclination on their long-term behaviour in cyclic rain and fog conditions

The paper presents the results of comparative tests of model composite insulators with housings made of HTV silicon rubber and sheds inclined at different angles (0deg, 10deg, 30deg). The main aim of the tests was to acquire experimental data for an assessment of the effect of the shed inclintion angle on the ageing degradation of composite insulators under high voltage in rain and fog conditions. The insulators were exposed to AC (50 Hz) voltage and water precipitation in a rain chamber and in a fog chamber. The tests showed clear differences in ageing times between insulators, leading to increased leakage current and surface electric discharges. It was found that the insulators with a large shed inclination angle (30deg) have a higher ageing resistance, particularly in rain conditions.

Influence of composite insulator operating position on the development of aging process under AC high voltage and water precipitation

Results of ageing tests of model composite insulators in rain and fog chambers are presented. The influence of different operating positions of the insulators: vertical, inclined and horizontal, on the development of leakage currents on their surfaces during the leakage tests is discussed. An important role of sheds inclination of the insulators in the leakage currents restriction is emphasized.

Influence of composite insulator inclination on its properties degradation in rain conditions

Results of ageing tests of composite insulators exposed to AC (50 Hz) high voltage and cyclic water precipitation in a rain chamber were presented. The subject of the investigation were composite insulators destined for 20 kV rated voltage with their housings made of HTV silicon rubber. The insulators were tested in horizontal, vertical and inclined position. During the tests, the leakage currents were recorded and surface hydrophobicity was estimated. Photo and video registration of the behavior of water drops on the tested surfaces were made. The following phenomena were described: non-uniform loss of insulator surface properties, merging of water drops which caused creation of conduction current paths, development of surface electric discharges. Essential influence of insulator operating position on the way of its surface wetting during rainfall and development of ageing process was described.

Mechanoacoustic research method of degradation processes in electroceramics materials

The paper presents the concept of mechanoacoustic testing of degradation processes of ceramic materials, which is applied to diagnose high voltage insulators in operation. The method depends on the application of slowly increasing compressive loading acting on the sample, with simultaneous recording of acoustic emission (AE) descriptors. The process of loading is continued to the destruction or stopped at different stresses, and the samples are subjected to microscopic examination. Microscopic analysis of the samples enables determining the effects of stress action. The results were compared with the images of similar materials obtained from the insulators after different periods of operation. On this basis, there were distinguished consecutive stages of ageing of electrotechnical porcelain materials and corresponding effects of their structure degradation. Using slowly increasing quasi-static compressive loading in a relatively short-term mechanoacoustic test makes it possible to get results similar to those of long lasting degradation effects in operated electroinsulating objects. Using this method there is possible to describe the factors which affect short and long-term mechanical strength of the tested materials. There is also possible assessment of the validity and application of the theories explaining the strength of porcelains. There was presented research of the influence of the mullite phase on the short- and long-term mechanical strength of electrotechnical porcelain of different types.

Capacitance changes in degraded metal oxide varistors

ZnO based ceramics show a highly nonlinear current-voltage (I-V) characteristic due to intergranular layers at the boundaries of ZnO grains. This research indicates that the junctions formed on the border between ZnO grains and the intergranular phase are responsible for the creation of potential barriers which control the flow of current through the bulk of such a ceramic. Having polycrystalline nature and a large number of intergranular barriers, and so a high energy absorption capability, ZnO ceramics are used as active components in overvoltage protection devices. However, as a result of charge transport the height of the barriers decreases and varistor degradation occurs. The degradation seems to affect mainly the prebreakdown region of the I-V curve and it results in increased leakage current. This paper presents a part of research aimed at assessing the effect of ageing on the capacitance characteristic of ZnO varistors.

Influence of the Shape of Composite Insulators Housing on their Aging Resistance in presence of High DC Voltage and Clean Fog

The increased interest in transmission of electricity by using high DC voltage forced the need to examine the ageing resistance of silicone rubber insulators exposed to high DC voltage. Under operating conditions, in the presence of a high DC voltage due to electrostatic phenomena is observed even a fourfold increase in the accumulation of contaminants on the surface of the insulator compared to the high AC voltage. Lack of crossing zero values by current and voltage curves favors to the maintenance of surface discharges concentrated especially in the presence of water. In the paper tests results of the model composite insulators with different inclination of upper surface sheds in the clean fog chamber under high DC voltage is described. The aim of the research is to recognition of degradation ageing process of composite insulators electric properties and the phenomena of leakage current development. Laboratory test results allow to inference about the influence of the shape and design parameters of sheds on the behavior of the silicone composite insulators in operation. (). Streszczenie. W artykule przedstawiono wyniki rozkładów napięcia na połączonych w łańcuch izolatorach: kompozytowym na 110 kV i pojedynczym izolatorze kołpakowym pokrytym warstwą silikonu RTV. Zasymulowano zmianę właściwości powierzchniowych izolatorów w czasie eksploatacji. Stwierdzono, że dołączenie do izolatora kompozytowego niewielkiego izolatora ceramicznego, powleczonego silikonem RTV, w istotny sposób łagodzi występującą na izolatorze kompozytowym nierównomierność rozkładu napięcia. Wpływ kształtu osłon izolatorów kompozytowych na ich odporność starzeniową w obecności wysokiego napięcia stałego i czystej mgły

Steep-front impulse voltage in diagnostic studies of composite insulators

Laboratory studies of composite insulators by means of steep-front impulse voltage tests are presented. The tests were performed on four groups of insulators, i.e. on newly manufactured insulators, on insulators removed from service, on insulators earlier exposed to mechanical fatigue test and on insulators with intentionally introduced technological defects. Extensive results of these investigations indicate that application of steep-front impulses of 4 kV/ns provides a better diagnostic efficiency as compared with the standard testing by means of impulses with steepness of 1 kV/ns. It is shown that the application of 4 kV/ns impulses do not cause any damage to healthy insulators, while defected insulators are identified reliably. A new methodology allowing to assess the status of internal insulation in insulators removed from service is proposed based on the performed studies.