Partial discharge endurance life of polymer insulating materials at high temperature

This paper describes the partial discharge (PD) characteristics and V‐t characteristics of several insulating materials, such as liquid crystal polymer, epoxy resins, polyimide, and polyethylene‐terephthalate (PET) with a needle‐plane electrode system at elevated temperature. The life index n for an applied electric field less than 20 kV/mm is found to be about 9 and that for more than 20 kV/mm is found to be less than 4. The PD characteristics of the insulating materials varied. These V‐t characteristics due to PD degradation were similar to those at room temperature except for the absolute life time. © 1998 Scripta Technica, Electr Eng Jpn, 126(1): 15–22, 1999     

A new test method for internal partial discharge resistance of insulating materials

A new test method for internal PD resistance of insulating materials has been studied intensively by a task force at CIGRE for over ten years. Recently, Method II, which was developed by the authors, is now being studied as the international round robin test proposed by a domestic task force. This paper describes the new test method concretely and shows some typical test results. The test electrode system is characterized by adhesion of test specimen to the embedded sphere electrode, and it may be applicable to all sorts of insulating material for the internal PD resistance test under uniform partial discharges. The most important point of this method is to realize the complete adhesion by skillful techniques. It was ascertained that the spread of test data may depend only on the uniformity of materials by this method, and excellent life data can be obtained for some specific materials.     

Charging and discharging of insulating particles on the surface of a grounded electrode

The aim of the present paper is to analyze the corona charging of millimeter-size insulating disks, as well as their discharging when they are no longer exposed to the action of an external electric field. The experiments were carried out on a roll-type electrostatic laboratory separator, equipped with a wire-type corona electrode, simulating the actual charging/discharging conditions in an industrial unit. Disks of various sizes were charged on the surface of the roll electrode, then the high voltage supplied to the corona electrode was turned off and the particles were collected in a Faraday pail, connected to an electrometer. The charge measurements were performed at various time intervals from high-voltage turn-off. In this way, the charge decay could be recorded and the discharge process fully characterized. The measured data show that the discharge process depends on the nature, size, and shape of the particles, as well as on the contact conditions between the particles and the grounded roll electrode. These data could guide the design of the electrostatic separation experiments that precede any new industrial application of this technology.     

Qualification of fiber-reinforced insulating materials based on PDanalysis

For fiber-reinforced insulating materials used e.g. in gas-insulated system (GIS), a test procedure has been created which stresses tube shaped specimens simultaneously dielectrically and mechanically. Mechanical stress is applied by four-point bending at electric ac fields up to 12 kV/mm. To prevent surface discharges the specimens are tested in a 600 kPa SF₆ atmosphere. For diagnosis a partial discharge (PD) system with a sensitivity of 100 fC has been employed. Even for high-quality vacuum-impregnated materials internal PD was detected and a clear influence of mechanical stress was found. The study of PD patterns resulted in the proposal of a prebreakdown model, which reveals the physical failure process taking place in the fiber/matrix-interface of the specimens. The model provides suitable criteria for a clear ranking of the tested materials. Also general suggestions for the improvement of fiber-reinforced insulating materials are given. For a quantitative estimation of life expectancy accelerated multifactor aging-tests are recommended     

Influence of insulating barrier on the creepage discharge intransformer oil

The propagation aspects of a creepage discharge in transformer oil are observed simultaneously and in detail under a positive impulse voltage condition, by means of a high-speed Schlieren optical system, an LED current measurement system and a charge measurement system using a capacitor. In the present paper, the influence of the material type and the thickness of insulating barriers on the propagation of the impulse creepage discharge are discussed for a point to plane electrode geometry. In addition, the influence of a barrier surface to the electric field direction on the propagation of the impulse creepage discharge is discussed for a parallel plane electrode gap with a protruding point     

Charge relaxation due to surface conduction on an insulating sheet near a grounded conducting plane

Electrostatic charges are responsible for a variety of problems in industrial processes and customer equipment that use webs or sheets. Problems include particle contamination from attracting dust, sheets that stick to each other, and electrical discharges resulting in logic resets or damage to electrical components. These problems can be mitigated by increasing the surface conductivity of the insulating sheets by coating the surface with a conductive layer or by increasing the relative humidity. To mitigate problems, electrostatic charge must dissipate quickly compared with the mechanical transport time of the process. Reported here are the results of a model calculation of the charge relaxation time showing explicitly that the charge relaxation time depends on both surface conductivity and geometry. The charge relaxation time is found to increase as the distance to a nearby, grounded conducting plane decreases. Charge relaxation is slowed because the tangential electric field needed to drive surface current becomes smaller as the distance to the grounded plane decreases. Inferred from this analysis is the dependence of charging on the electric Reynolds number (ratio of the electrical charge relaxation time to the mechanical transport time). Web charging can be divided into three regimes: dissipation (R/sub e/<0.1), transition (0.1相似文献   

Degradation of insulating materials under electrical stress

An avalanche of physical and physicochemical phenomena, often self-sustained, leads to the end of life of an insulating material. The collapse of the insulating role of a material is mostly due to the electrical field stress. The path leading from the catastrophic situation, the breakdown, back to its origin is followed in the present paper. A defect, pre-existing or created by the in-service stresses, gives rise to an ionization process in a volume bounded by insulating walls. With time the process does not remain identical due to both changes in the material and gas evolution, which makes it difficult to interpret the detected electrical signal. The end of life will occur through microcracks in which new discharges are initiated, activating the propagation of these structural failures. This change of structure of the insulating material when due to the existence of water molecules will lead to the same kind of final degradation. The polymeric insulating materials could contain additives but they lead only to a partial healing. From polymers of low complexity such as the PE used in cables, assuming that there is no pre-existing defect, one endeavors to understand how a microstructure reacts with a local field or accumulated charges. The present paper summarizes the thoughts of the author pertaining to the problems of aging and dielectric breakdown in organic insulating materials, processes often given a concrete form through experiments.     

Dielectric behavior of insulating materials under liquid nitrogen

Measurements were made to assess the AC breakdown voltages in liquid nitrogen (LN/sub 2/) with different electrode configurations such as sphere-sphere, needle-needle, hemisphere-hemisphere, plane-plane, sphere-needle, etc. Experimental results reveal that the breakdown voltage is a function of electrode geometry and gap length. This study also addresses the effect on the breakdown strength of solid insulating materials under LN/sub 2/ environment with a sphere-sphere electrode configuration. In this paper, special emphasis has been attributed to the effect on loss index of a variety of dielectrics dipped in LN/sub 2/. The measured values of breakdown strength and loss index have been compared with those obtained under atmospheric condition. The study reveals that the breakdown strength of cellulosic materials like paper or pressboard increases manifold while the loss index decreases significantly when dipped in LN/sub 2/ with variations of the order of 50% to 90%. However, for impervious noncellulosic materials like Perspex (acrylic glass) or presspahn, the increase in breakdown strength is not that pronounced and the decrease in loss index is of the order of 2 to 30%.     

Aging of polymeric insulating materials in power cables

More than 160 years of history go with the use of polymers in the cable industry. Polyethylene of low and high density and crosslinked polyethylene gradually succeeded paper and polyvinylchloride. This review discusses some aspects of the aging processes of polyethylene under working conditions. Electrical and water treeing are considered, dependent upon the morphology and the induced characteristics of the insulating material     

Analysis of electrical activity associated with inclined-planetracking and erosion of insulating materials

Surface electrical activity and physical changes were measured and compared for several types of outdoor insulation materials, using a standard test method. The leakage current activities during the inclined-plane test (ASTM D2303) were measured by using four representative materials: a silicone with high (>70% by weight) loading of alumina trihydrate (ATH), a silicone with no ATH, a poly ethylene vinyl acetate (EVA), and a glazed porcelain. Quantitatively, the severity of the inclined-plane test was defined by detailed leakage current measurements. Qualitatively, it is observed that the test was severe enough to damage the glaze on porcelain. The study enabled us to compare polymers with porcelain, silicones containing no ATH vs. high loading of ATH, and silicones with EVA. The analysis showed that electrical activity, particularly the average leakage current and the distribution of peak current, depended on the surface wettability of the materials by the contaminant solution. The compound formulation is more important than the generic polymer types. It was demonstrated that silicone with no ATH, and EVA with only a medium level of ATH, exhibited excellent tracking and erosion resistance, comparable to the silicone highly filled with ATH. The presence of ATH is not absolutely necessary to achieve the superior tracking and erosion resistance of a silicone elastomer     

Simulation of electrical tree growth in solid insulating materials

Contents In this paper the electrical tree growth in solid insulating materials is modeled using von Neumann's Cellular Automata (CA). The model is based upon the assumption that the electric stress at the end of a conducting tip quite often approaches the dielectric strength of the material and that progressive breakdown can occur by electrical tree formation. Because of tree advancement, the potential distribution into the insulating material changes with time and is calculated at each time step. An algorithm for the simulation of electrical tree growth in solid dielectrics based on this model has been developed. The algorithm is also used to simulate breakdown in solid dielectrics containing square or spherical voids.

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Simulation des Wachstums elektrischer Bäume in Festisolierstoffen

Übersicht In diesem Artikel wird das Wachstum der elektrischen Bäume in Festisolierstoffen mittels von Neumannscher zellularer Automaten modelliert. Dieses Modell geht davon aus, daß das elektrische Feld an der Spitze der Elektrode oft in der Nähe der dielektrischen Festigkeit vom Material liegt und das ein fortschreitender Durch-schlag durch elektrische Bäume möglich ist. Durch das Wachsen der elektrischen Bäume ändert sich die Spannungsverteilung innerhalb des Festisolierstoffes mit der Zeit und wird in jedem Zeitschritt berechnet. Ein Algorithmus für die Simulation des Wachstums eines elektrischen Baums in Festisolierstoffen ist entwickelt worden. Der Algorithmus kann auch den Durchschlag in Festisolierstoffen mit zylindrischen oder kugelförmigen Hohlräumen simulieren.

     

Electromagnetic wave radiated from an insulating spacer in gas insulated switchgear with partial discharge detection

As a means of diagnosing partial discharge (PD) signals propagate inside a Gas Insulated Switchgear (GIS), a study for the leakage of electromagnetic waves (EM-waves) emitted from the insulating spacer was implemented. The EM-waves leaking out from the solid insulator have the resonance frequencies depend on the spacing between adjacent bolts in the direction of the flange circumference, because the leakage portion is the equivalent of a slot antenna. In this paper, using an electromagnetic analysis model which has a simulated spacer on a concentrically-shaped GIS tank, the output characteristics of the EM-waves that leaked out from the slit were analyzed under various conditions such as the spacing between adjacent bolts, the width of the spacer, the dielectric constant of the spacer and the form of the flange. Also the actual measurement by the experimental equipment used to simulate the model was implemented for comparison with the analytical results. Consequently, the optimal specifications of the sensor and the measurement method used to achieve highly-sensitive detection for practical use were summarized and proposed as well as evaluating the effectiveness of the electromagnetic analysis model adopted in this paper.     

Dielectric heating in insulating materials subjected to voltage waveforms with high harmonic content

Dielectric heating is one potential aging mechanism active below partial discharge inception voltage in materials used as high voltage insulation. When exposed to voltage waveforms containing high amount of harmonics, the heat generation will be larger due to increased power losses as compared with power frequency excitation. This may result in a decreased life or even failure of insulation due to the increased operating temperature or to thermal runaway. An analysis of the power developed due to dielectric heating in two different materials subjected to voltage waveforms with high harmonic content is presented in this paper. By expressing the non-sinusoidal loss as an enhancement factor to the sinusoidal one, a geometry-independent formalism is derived. From dielectric response measurements at low voltage and at several temperatures the dielectric power loss in the material can be calculated for different voltage levels and waveforms. Two important material parameters can be extracted from the calculated dielectric power loss: (i) non-sinusoidal loss compared with sinusoidal loss with the same fundamental frequency (pfact) and (ii) change of loss with changing temperature (dpfact/dT). These two parameters could potentially be used to indicate the suitability of materials for use in applications where voltage waveforms contain high harmonic content.     

Effects of atmospheric pressure on DC resistance to tracking of polymer insulating materials

Polymer insulating materials are required for use in high altitude regions. For the use of polymer insulating materials in such regions, it must be confirmed whether the resistance to tracking at high altitudes is different from that at altitudes near the sea level. In this paper, an artificial atmospheric chamber was set up to investigate the resistance to tracking on a polymer insulating surface at DC voltage application under a reduced pressure. The tests were carried out in accordance with IEC 60112. The test results showed that the dependences of the resistance to tracking for polymer insulating materials upon the decrease in ambient pressure could be classified into three types. One type is for paper-based phenolic laminate and polycarbonate, where the resistance increases with the decreasing ambient pressure; the second is for polybutylene terephthalate where the resistance decreases with the decreasing ambient pressure; the last type is for epoxy resin where the resistance is almost independent of the decrease in the pressure. A Gaussian wavelet analysis was applied to discharge currents to discuss correlations among the resistance to tracking, the discharge energy level and the ambient pressure. It was found that the data of resistance to tracking with some of the materials obtained at altitudes near sea level are inadequate for use in a high-altitude environment. Problems may occur with respect to the safety and reliability of polymer insulating materials when the DC resistance to tracking decreases under low-pressure conditions.     

Discussion on application of the Weibull distribution to electrical breakdown of insulating materials

The Weibull distribution is commonly used for statistical processing of breakdown data of electrical insulation. The statistical theory of breakdown that has been proposed since the 1970s has introduced a two-variables Weibull distribution in order to take into account both the stress applied and the failure times. In this paper this distribution is thoroughly discussed and then rejected. Indeed, the random variable is only one; the other is an independent variable that becomes random only through the first, with the same shape parameter. The case of breakdown after electrical or multiple-stress aging (electric strength test on aged specimens) is also examined and the probability distribution function is written even in the general case, where the probability is a function of several variables. Only one of the latter, i.e. the electric strength ES, is the random variable while the others, such as the applied stresses and the time at which the ES measurement is performed, are independent