Partial Discharge Model Research Articles

Brittle fracture in nonceramic insulators: electrical aspects of microscopic flaws in glass reinforced plastic (GRP) rods

Glass Reinforced Plastic (GRP) rods used in Non-Ceramic Insulators (NCI) were examined for microscopic defects in order to evaluate their role in causing brittle fracture. The rods evaluated utilized different types of glass fibers and resin systems. Niemeyer's generalized approach to Partial Discharge (PD) modeling was used to investigate the development of electrical discharges inside the rods. The results indicated that the PD inception voltage was well above the highest system voltage used presently. Thus, it was concluded that the reactive species required for the formation of acids could not be developed inside the rod. The results also suggest that chemical reactions other than acid production can promote stress corrosion cracking of the rod and these may play a significant role in the failure process.

Fluctuation analysis of partial discharge pulse occurrence with an integral equation

AbstractThis paper proposes an integral equation to describe the stochastic fluctuation of partial discharge (PD) occurrence under sinusoidal voltage stress based on a simple PD model. In the model, the stochastic behavior of PD fluctuation is assumed to arise from the fluctuation of the PD delay time after the inception voltage is built up across a discharge gap. For simplicity of calculation, the delay time is assumed to have an exponential distribution. Based on these assumptions, it is found that the proposed integral equation provides the basic characteristics such as the PD pulse distribution in the applied voltage phase angle domain. The authors have solved the equation numerically and demonstrate several ϕ–n distribution patterns with average delay times of 0.05 to 5 ms at low applied voltage. The solution is compared with PD characteristics obtained by Monte Carlo simulation based on the same PD model, and good agreement is found. © 2001 Scripta Technica, Electr Eng Jpn, 136(1): 16–28, 2001

Simulation of partial discharges in conducting and non-conductingelectrical tree structures

Electrical treeing is of interest to the electrical generation,transmission and distribution industries as it is one of the causes ofinsulation failure in electrical machines, switchgear and transformerbushings. Previous experimental investigations of electrical treeing in epoxyresins have found evidence that the tree structures formed were eitherelectrically conducting or non-conducting, depending on whether the epoxyresin was in a flexible state (above its glass transition temperature) or inthe glassy state (below its glass transition temperature). In this paper weextend an existing model, of partial discharges within an arbitrarily definednon-conducting electrical tree structure, to the case of electrical conductingtrees. With the inclusion of tree channel conductivity, the partial dischargemodel could simulate successfully the experimentally observed partialdischarge activity occurring in trees grown in both the flexible and glassyepoxy resins. This modelling highlights a fundamental difference in themechanism of electrical tree growth in flexible and glassy epoxy resins. Themuch lower resistivities of the tree channels grown in the glassy epoxy resinsmay be due to conducting decomposition (carbonized) products condensing on theside walls of the existing channels, whereas, in the case of non-conductingtree channels, subsequent discharges within the main branches lead toside-wall erosion and a consequent widening of the tubules. The differingelectrical characteristics of the tree tubules also have consequences for thedevelopment of diagnostic tools for the early detection of pre-breakdownphenomena.

Characteristics of partial discharges in artificial simulated tree channels during tree propagation

In order to clarify partial discharge (PD) mechanism in an electrical tree, we investigated PD pulse sequence and light emission from a single PD in an artificially-simulated tree channel during the tree propagation in low-density polyethylene (LDPE). For positive PD, the light emission was observed near the needle-tip and there was a positive correlation between the PD magnitude and the length of luminous region. The pulse-sequence analysis showed that PD took place at constant voltage increments. These results suggest that PD starts at the needle-tip and the PD magnitude is determined by the PD length. On the other hand, for negative PD, there was no correlation between the PD magnitude and the length of luminous region. The PD pulse sequence can be explained by a simple PD model, since the voltage increment is related to the magnitude of preceding PD. These results suggest that there is a difference between positive and negative PD characteristics and their mechanisms in an artificial channel during tree propagation.

PD characteristics as a stochastic process and its integral equation under sinusoidal voltage

This paper proposes an integral equation to describe the stochastic fluctuation of partial discharge (PD) occurrence under sinusoidal waveform voltage conditions based on a simple PD model (an equivalent circuit model). The stochastic behavior of PD fluctuation is assumed to arise from the fluctuation of PD delay time after the discharge inception voltage is built up across a discharge gap. The distribution of delay time is assumed to obey an exponential distribution and the dependence of the delay time on the overvoltage is assumed to be expressed by an exponential function of the overvoltage. The integral equation has been solved numerically under the low voltage condition in which only one PD can occur in a half AC voltage cycle. It is found that the proposed integral equation can provide the basic characteristics such as PD pulse repetition rate vs. applied voltage phase angle.

正弦波電圧条件下の部分放電パルス発生ゆらぎの解析

This paper proposes an integral equation to describe the stochastic fluctuation of partial discharge (PD) occurrence under sinusoidal voltage stress based on a simple PD model. In the model, the stochastic behavior of PD fluctuation is assumed to arise from the fluctuation of the PD delay time after the inception voltage is built up across a discharge gap. For simplicity of calculation, the delay time is assumed to have an exponential distribution. Based on these assumptions, it is found that the proposed integral equation provides the basic characteristics such as the PD pulse distribution in the applied voltage phase angle domain. The authors have solved the equation numerically and demonstrate several ϕ–n distribution patterns with average delay times of 0.05 to 5 ms at low applied voltage. The solution is compared with PD characteristics obtained by Monte Carlo simulation based on the same PD model, and good agreement is found. © 2001 Scripta Technica, Electr Eng Jpn, 136(1): 16–28, 2001

Self-consistent modeling of electrical tree propagation and PD activity

The self-consistent model of electrical tree propagation and partial discharge (PD) activity within growing tree channels is presented. The local electric field and the damage accumulation in the dielectric material surrounding the channels govern the tree growth. The damage increment is proportional to the energy release in the channels due to PD. The electric field distribution is determined by the charge deposition within the tree structure and the electrode geometry. The charge distribution changes within the channels during PD. PD starts when the electric field along the channels exceeds threshold inception value and stops when the field falls below the threshold quenching value. The numerical three-dimensional realization of the model has been used for simulation of electrical treeing with sinusoidal and triangular voltages in a needle-plane geometry. The spatial-temporal dynamics of the tree growth and phase-resolved characteristics of the PD have been studied for various magnitudes of the applied voltage. The simulation results have been compared with experimental data given in the literature.

A novel physical model for partial discharge in narrow channels

In order to reveal the mechanism of partial discharge (PD) in electrical trees, an artificial single channel is studied as model for tree channels. In a long-narrow channel, the behaviour of PD is similar to that in an actual tree. When the length of the channel is decreased, the /spl phi/-q-n pattern of PD in the channel changes from wing to turtle or rabbit like, and the range of light emission in the channel increases when the applied voltage is increased. Considering the influence of a PD on the total field distribution and thus on a later PD, a physical model is put forward to describe the PD behaviour. It is assumed that PD is induced by the high field at the needle tip, then propagates along the channel, and stops propagating when the field in the channel becomes lower than a critical value due to field divergence. Mathematical analysis on an ideal needle-plane electrode system clearly explains the physical meaning of the model. And the computer simulation by using the superposition method agrees with the experimental results very well. This model is also valid to describe the behaviour of PD in electrical trees.

An approach to the modelling of partial discharges in electrical trees

A partial discharge model has been developed to simulate experimental partial discharge activity within electrical tree structures grown in polymeric based insulation. A pin plane electrode arrangement and tree were defined on a grid representing an area 2 mm by 2.1 mm. A sinusoidal 50 Hz voltage was applied to the pin tip and 3600 time steps per cycle were used to give a phase resolution of . Partial discharges were simulated at each time step by adding one or more dipoles of charge onto the tree structure to reduce the potential difference between adjacent points in the tree from to . Each dipole of charge represents a local electron avalanche occurring over a distance equal to the grid spacing. The model parameters, and , represent the discharge properties of the decomposition gas. Partial discharge current pulses in the external circuit and the induced image charge were calculated as a function of time. The spatial distribution of the emitted light in the tree structure over one cycle of the applied voltage was also calculated. The close agreement between the calculated and experimental data suggests that the underlying assumptions used in the construction of the partial discharge model are appropriate for the case of electrical trees grown in the flexible epoxy resin.

Partial discharges in epoxy resin voids and the interpretational possibilities and limitations of Pedersen's model

Partial discharges take place, among others, inside voids enclosed in insulations. The partial discharge behaviour of epoxy resin samples is studied with the aid of a recently developed streamer partial discharge model (Pedersen's model) and its relation to the modem Pulse Height Analysis and Phase Resolved Measurement techniques is investigated. Furthermore, some relations based on Pedersen's model are deduced. The advantages and disadvantages of the aforementioned model are analysed. Scanning electron microscope photographs show the nature of the deterioration of the void walls due to the partial discharges. The question whether initiation of the treeing phenomenon leading to breakdown from an enclosed spherical void is possible, is also investigated.

Partial discharges due to electrical treeing in polymers: phase-resolved and time-sequence observation and analysis

We have investigated the partial discharge (PD) due to electrical treeing degradation in low-density polyethylene (LDPE), ethylene - vinyl acetate copolymer (EVA) and ethylene - acrylic acid copolymer (EAA) by a computer-aided partial discharge measurement system which allowed us to obtain phase-resolved PD pulse data. The experimental results revealed that the PD magnitude was strongly affected by the instantaneous applied voltage and that the occurrence of a PD was determined by the time derivative of the applied voltage (). The PD pulse-sequence analysis revealed the following: (i) a PD occurs in a discharge path which consists of a tree trunk and branches extending from the trunk; (ii) in each discharge path at most one PD occurs per half cycle. Based on these facts, a model of PDs due to electrical treeing was proposed. The influences of applied voltage and frequency were investigated by applying a triangular voltage. The number and average magnitude of PDs increased linearly with applied voltage whereas the PD charge per cycle increased quadratically. These results are in good agreement with the model.

PD recognition with knowledge-based preprocessing and neural networks

Partial discharge (PD) patterns are an important tool for the diagnosis of HV insulation systems. Human experts can discover possible insulation defects in various representations of the PD data. One of the most widely used representations is phase-resolved PD (PRPD) patterns. We present a method for the automated recognition of PRPD patterns using a neural network (NN) for the actual classification task. At the core of our method lies a preprocessing scheme that extracts relevant features from the raw PRPD data in a knowledge-based way, i.e. according to physical properties of PD gained from PD modeling. This allows a very small NN to be used for classification. In addition to the classification of single-type patterns (one defect) we present a method to separate superimposed patterns stemming from multiple defects. High recognition rates are achieved with a large number of single patterns generated by stochastic PD simulations. Our network architecture compares favorably with a more traditional network architecture used previously for PRPD classification. These results are confirmed by classification of patterns measured in laboratory experiments and power stations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A generalized approach to partial discharge modeling

An important tool for improving the reliability of HV insulation systems are partial discharge (PD) measurements. The interpretation of such measurements aims at extracting from the measured data information about insulation defects which then are used for estimating the risk of insulation failure of the equipment. Because the physical understanding of PD has made substantial progress in the last decade, it can now be exploited to support interpretation. In this paper a concept is presented which merges the available physical knowledge about various PD types into a generalized model which can be applied to arbitrary insulation defects. This approach will be restricted to PD of the streamer type in gases and at gas-insulator interfaces which cover a large fraction of the cases encountered in technical insulation systems. The generalized model allows us to derive approximate relations between defect characteristics, insulation design parameters and test conditions on one side, and measurable PD characteristics on the other. The inversion of these relations yields rules for extracting defect information from the PD data. The application of the generalized model is illustrated by two simple examples, namely, spherical voids in an insulator and electrode protrusions in SF/sub 6/. >

On the influence of measuring circuit on a DC partial-discharge repetition rate

A simple model of DC partial discharges running in cable and capacitor dielectrics is described in order to investigate the influence of the measuring circuit on the DC partial-discharge repetition rate. This model shows that the measuring circuit can only influence the discharge repetition rate temporarily. On reaching the stationary state the DC partial discharges are governed by the electric qualities of the dielectric used.