Partial Discharge Process Research Articles

Contribution of humidity to the evolution of slot partial discharges

The impact of humidity on the slot partial discharge (PD) process was evaluated in a laboratory experiment conducted in a climatic chamber under constant humidity and temperature. Two stator bars, subjected to slot PDs at two temperatures (28°C and 85°C), were aged at twice their nominal phase-to-ground voltage (16 kV) under three absolute humidity levels (5 g/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , 9 g/ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and 13 g/ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ). The phase resolved partial discharge (PRPD) patterns were monitored during short aging sequences and visual observations of the surfaces were made at the beginning and end of each aging sequence. Observation confirms that humidity has a significant impact on the discharge phenomenon but other parameters, such as surface condition and temperature, affected the discharge process just as much. This paper presents the way slot PD activity evolves and affects the related PRPD pattern under different conditions of humidity, temperature and surface degradation.

Severity Diagnosis and Assessment of the Partial Discharge Provoked by High-Voltage Electrode Protrusion on GIS Insulator Surface

High-voltage electrode protrusion is one of the major causes of insulation failure in gas-insulated substation (GIS) equipment. The present research is designed to diagnose and assess the severity levels of the partial discharge (PD )provoked by high-voltage electrode protrusion on the GIS insulator surface. With the application of boosting voltage approach, long-term tests were conducted on a well-established 252-kV GIS experiment platform to observe the entire developing process of PD from its very initiation until the eventual flashover. Real-time measurement was also undertaken during the tests to capture the trend curves and the spectra of the PD. The study results indicate that 1) PD initiated by high-voltage electrode protrusion on GIS insulator surface undergoes through three severity levels, namely, petty discharge, medium discharge, and threatening discharge, and 2) as PD exacerbates, the phase distribution keeps widening and corresponding shape changes are shown in the scatter plot, the histogram, the gray-scale map, and the time-frequency map. It is suggested that the features of phase distribution and the corresponding spectra be used as important referential criterion in diagnosing and assessing the severity levels of the PD triggered by high-voltage electrode protrusion on the GIS insulator surface.

Partial discharge behavior within a spherical cavity in a solid dielectric material as a function of frequency and amplitude of the applied voltage

Modeling of the partial discharge (PD) process allows a better understanding of the phenomena. In this paper, a simulation model for spherical cavities within a homogeneous dielectric material has been developed. The model is implemented using Finite Element Analysis (FEA) software in parallel with a mathematical package. This method provides many advantages over previous PD models because discharge events can be simulated dynamically and the electric field in the cavity can be calculated numerically. The model has been used to study the effect of different amplitudes and frequencies of the applied voltage and simulation results have been compared with experimental measurement results. It is found that certain model parameters are dependent on the applied stress and parameters that clearly affect PD activity can be readily identified, these parameters include; the electron detrapping time constant, the cavity surface conductivity, the initial electron generation rate and the extinction voltage. The influence of surface charge decay through conduction along the cavity wall on PD activity has also been studied.

An advanced model of partial discharge in electrical insulation

The paper presents an advanced concept for a computer model of partial discharge (PD) in insulation. The advanced model concept is based on a well-known model, using condensers for modeling of electrical network and the cavity where discharge occurs. The fundamental advancement is the modeling of initial conditions of each discharge event by introducing a controlled voltage source and modeling its control, the modeling of parameters of PD current pulse, and advanced modeling of all circuit elements. The paper presents basic analysis of PD processes and the analysis of basic shortcomings of the well-known model. The elements of the advanced model are established, and the way to implement it. It is shown that commercial software packages for electric circuits analysis are indeed suitable for PD modeling. Physical and electrical parameters of PD pulse are established, as well as the response of observed object's electric network to the ensuing excitation. The results of the model show the possibility of modeling of PD current pulse in a wide range of parameters.

Effect of surface degradation on slot partial discharge activity

To improve our understanding of slot partial discharge (PD) mechanisms, an accelerated aging test was initiated two years ago. This long-term experiment is being performed on six stator bars subjected to slot PDs under electrical, thermal and mechanical stresses. It is well known that slot PD activity in air-cooled generators is harmful to the stator winding ground insulation. The degradation induced by slot PDs will modify the physical properties of the surfaces of the cavity where PDs take place and this in turn will influence the slot PD process. To understand the evolution of slot PD activity and the changes in its PRPD (Phased Resolved Partial Discharge) pattern, it is therefore essential to recognize and understand the interdependence between surface modifications and PD signals. This paper presents results of visual observations of stator bar surface degradation, changes in surface conductivity and the effect on the evolution of PD signals in the presence of slot PD activity under conditions of electrical, thermal and mechanical stresses.

A generic knowledge-based approach to the analysis of partial discharge data

Partial Discharge (PD) diagnosis is a recognized technique to detect defects within high voltage insulation in power system equipment. A variety of methods exist to capture the signals that are emitted during PD, and this paper focuses on the ultra high frequency (UHF) and IEC 60270 techniques. Phase-resolved patterns can be constructed from the PD data captured using either of these techniques and due to the individual signatures that different defects generate, experts can examine the phase-resolved pattern to classify the defect that created it. In recent years, knowledge regarding PD phenomena and phase-resolved patterns has increased, providing an opportunity to employ a knowledge-based system (KBS) to automate defect classification. Due to the consistent physical nature of PD across different high voltage apparatus and the ability to construct phase-resolved patterns from various sensors, the KBS offers a generic approach to the analysis of PD by taking the phase-resolved pattern as its input and identifying the physical PD processes associate with the pattern. This paper describes the advances of this KBS, highlighting its generic application through the use of several case studies, which present the diagnosis of defects captured through both the IEC 60270 and UHF techniques. This paper also demonstrates, in one of the case studies, how a limitation of previous pattern recognition techniques can be overcome by mimicking the approach of a PD expert when the pulses occur over the zero crossings of the voltage waveform of the phase-resolved pattern.

Partial Discharge Activity of Inter-turn Insulation under Different Parameters of Square Wave Voltage

Partial discharge (PD) activity of inter-turn insulation of electric motor winding is investigated under different parameters of square wave voltage, and the memory effect of deposited space charge in a cavity on PD activity is also studied. Under different frequency and rise time of square wave, discharge number, maximum discharge magnitude and phase resolved PD pattern is investigated. Increase in frequency and decrease in rise time both results in increase in maximum discharge magnitude and discharge number at square wave. Comparing the results of phase resolved PD patterns at different frequency of square wave, most of PD moves to zero point of square wave voltage with the increase in frequency. Meanwhile, with the reduction in rise time of square wave, a similar phenomenon can be observed. Through the analysis of a PD process at square wave voltage, the memory effect of deposited space charge after a discharge in cavity leads to superposition of electric field when the polarity of square wave voltage rapidly reverses. With the increase in frequency and reduction in rise time at square wave, superposition of electric field enhances. As a result, available discharge time prolongs during the rise time of square wave and overvoltage across the cavity increases. They result in the increase in discharge number and maximum discharge magnitude. Furthermore, for this reason, the position of PD shifts to zero point of square wave voltage.

방사전자파를 이용한 고분자애자의 오손량 분류기법

Recently, diagnosis techniques have been investigated to detect a Partial Discharge associated with a dielectric material defect in a high voltage electrical apparatus, However, the properties of detection technique of Partial Discharge aren't completely understood because the physical process of Partial Discharge. Therefore, this paper analyzes the process on surface discharge of polymer insulator using wavelet transform. Wavelet transform provides a direct quantitative measure of spectral content in the time~frequency domain. As it is important to develop a non-contact method for detecting the kaolin contamination degree, this research analyzes the electromagnetic waves emitted from Partial Discharge using wavelet transform. This result experimentally shows the process of Partial Discharge as a two-dimensional distribution in the time-frequency domain. Feature extraction parameter namely, maximum and average of wavelet coefficients values, wavelet coefficients value at the point of <TEX>$95\%$</TEX> in a histogram and number of maximum wavelet coefficient have used electromagnetic wave signals as input signals in the preprocessing process of neural networks in order to identify kaolin contamination rates. As result, root sum square error was produced by the test with a learning of neural networks obtained 0.00828.

Contribution of surface conductivity to the current forms of partial discharges in voids

The current form of partial discharge (PD) in a void was divided into fast and slow components, with short and long fall times, respectively. Comparing these with the current shape of PD in a void between two metal surfaces, in which the slow component was not observed, it was suggested that the formation of fast component in a usual void was due to the instantaneous discharge between the discharge areas of the PD, i.e. the region to which the carriers in a PD process can instantaneously propagate, and the slow one might be due to the low conductivity in the regions outside the discharge areas. An equivalent circuit was proposed to describe the effect and numerical simulations were carried out. It was suggested that the fall part of the first PD after polarity reversal might reflect the surface conductivity change in aging process.

Impulse partial discharge characteristics and their mechanisms under non-uniform electric field in N/sub 2//SF/sub 6/ gas mixtures

We have investigated impulse partial discharge (PD) and breakdown (BD) characteristics of a needle-plane gap in N/sub 2//SF/sub 6/ gas mixtures under positive lightning impulse voltage application, and discussed their physical mechanisms. The 50% probability PD inception voltage (PDIV/sub 50/), leader discharge onset voltage (LOV) and BD voltage (BDV/sub 50/) were measured and analyzed as a function of gas pressure and SF/sub 6/ content. Experimental results revealed the stepwise propagation process of the impulse PD and enabled us to classify the impulse PD in N/sub 2//SF/sub 6/ gas mixtures into two types, the streamer discharge and the leader discharge. We also discussed the impulse PD propagation mechanisms in terms of PD parameters such as propagation length, time interval and current pulse magnitude, and suggested a sequential relationship in the PD propagation process under non-uniform electric field.

Trends in partial discharge pattern classification: a survey

Partial discharge (PD) detection, measurement and classification constitute an important tool for quality assessment of insulation systems utilized in HV power apparatus and cables. The patterns obtained with PD detectors contain characteristic features of the source/class of the respective partial discharge process involved. The recognition of the source from the data represents the classification stage. Usually, this stage consists of a two-step procedure, i.e., extraction of feature vector from the data followed by classification/recognition of the corresponding source. The various techniques available for achieving the foregoing task are examined and analyzed; while limited success has been achieved in the identification of simple PD sources, recognition and classification of complex PD patterns associated with practical insulating systems continue to pose appreciable difficulty.

Advances in Online Monitoring and Localization of Partial Discharges in Large Rotating Machines

Some of the problems facing partial discharge (PD) measurements in rotating machines, especially during online measurements are: i) calibration of the measurements; ii) high noise levels interfering with both the measurement system and the winding under the test; as well as iii) attenuation and distortion of PD signals during transmission from the source point to the winding terminals. This paper presents a new technique and a measuring system for online PD monitoring of large rotating machines, developed by the authors. With this technique, the above mentioned problems are improved considerably. PD detection is implemented for every stator coil and the coil that PD comes from can also be identified. Inductive coils are used for decoupling of PD pulses and processing of the detected signals is in accordance with the developed technique, denoted Intech. Remote monitoring and navigation via a telephone link is one of the system features. The system was tested on both hydro and turbo generators with good results. The detection sensitivity of the system for the conventional machines is about 1 nC under operating condition.

Selectivity of damped ac (DAC) and VLF voltages in after-laying tests of extruded MV cable systems

The purpose of HV after-laying tests on cable systems on-site is to check the quality of installation. The test on extruded MV cable systems is usually a voltage test. However, in order to enhance the quality of after installation many researchers have proposed performance of diagnosis tests such as detection, location and identification of partial discharges (PD) and tan /spl delta/ measurements. Damped AC voltage (DAC) also called oscillating voltage waves (OVW) is used for PD measurement in after-laying tests of new cables and in diagnostic test of old cables. Continuous AC voltage of very low frequency (VLF) is used for withstand voltage tests as well as for diagnostic tests with PD and tan /spl delta/ measurements. Review on the DAC and VLF tests to detect defects during on-site after-laying tests of extruded MV cable systems is presented. Selectivity of DAC and VLF voltages in after-laying testing depends on different test parameters. PD process depends on type and frequency of the test voltage and hence, the breakdown voltage is different. The withstand voltage of XLPE cable insulation decreases linearly with increasing frequency in log scale. Experimental studies with artificial XLPE cable model indicate that detection of defects with DAC or VLF voltage can be done at a lower voltage than with DC. DAC voltage is sensitive in detecting defects that cause a breakdown due to void discharge, while VLF is sensitive in detecting defects that cause breakdown directly led by inception of electrical trees.

Pulse Sequence Analysis - a diagnostic tool based on the physics behind partial discharges

The analysis of partial discharge signals is a well-known tool to monitor the degradation of electric equipment. In contrast to the common practice of the analysis of data sets (phase angles of occurrence and discharge magnitudes) accumulated during a predefined time interval, the Pulse Sequence Analysis (PSA) is based on the evaluation of data sets in which also the individual sequence or partial discharge events are registered.The classical analyses are mainly based on the frequency distributions of the pulse heights of discharge events and the phase angles of the applied voltage at which they occur. Detailed analyses of partial discharge sequences reveal that - if liquid or solid dielectrics are involved - the phase angle of the externally applied voltage is not a meaningful parameter in the partial discharge process. The ignition of the partial discharge is determined by the local electric field at the defect site and not by the absolute value or the phase angle of the externally applied voltage.Experiments with different model defects as well as measurements of commercial apparatuses show that there are characteristic sequencesof partial discharge events. In many cases a systematic shift of the phase angles of occurrence occurs, which produces an apparently `statistic scatter' of the phase angles unless the correlation between consecutive discharges is taken into account. Sequence correlated parameters such as the voltage differences of the applied voltage or time differences between consecutive discharges are far more decisive parameters in the process. By means of the pulse sequence analysis, local defects can be characterized more precisely and single and multiple discharge sites can be clearly distinguished.A physically based model to describe discharge phenomena will be discussed and experimental results will be compared with results of numerical simulations of partial discharge processes.

Fundamentals in computer aided PD processing, PD pattern recognition and automated diagnosis in GIS

The partial discharge (PD) measurement is a sensitive method for testing and monitoring the insulation condition of HV apparatus. Digital PD measurements have become state of the art. Computer-based PD diagnostic systems nowadays can be used to increase the reliability of modern gas-insulated system (GIS). Also on-line monitoring of HV equipment is becoming increasingly important to enable prevention of a sudden breakdown of PD affected components. On-line monitoring must be done on-site, but an important problem of on-site PD measurement is the superposition of PD data and interference. Furthermore, reliable recognition of the type or nature of the insulation defect is necessary for any risk assessment. Traditionally, this is achieved with reference to expert knowledge. An alternative approach is presented in this paper, which treats the fundamentals of automated computer-based PD diagnosis, taking into account that successful noise suppression is essential for a successful on-site diagnosis.

Influence of mineral content on the behaviour of electrical papers under high‐frequency partial discharges

AbstractThis work describes the use of the broadband Partial‐Discharge (PD) measurement technique to account for the influence of mineral content of electrical papers, as an alternative technique in quality assessment of cellulose materials. Appreciable differences between the evolution versus frequency of the PD and breakdown processes were emphasized, suggesting that the breakdown analysis, if used as unique method in quality assessment, proves to have limits in accuracy and results interpretation. The PD occurrence is clearly influenced by the mineral purity and an optimum value of 0.42 % ash content was appointed, as leading to the best electrical behaviour of cellulose materials in the whole frequency range. This value is less restrictive and could bring notable potential benefits for paper producers.

A generalized model for partial discharge processes based on astochastic process approach

A general framework for the physical description of partial discharge (PD) processes is presented that holds for different types of PD causing defects. A PD process is treated as a stochastic process consisting of short duration discharges (point-like in time) and charge carrier drift/recombination intervals between these discharges. It is determined by few basic physical parameters and, in a stochastic process framework, can be described in a closed form by a master equation. Since usually only the fast discharges can be measured as PD signals, a restricted possibility of observing a PD process results. The link between the stochastic process and observable quantities is derived. A specific type of measurements is reported, the so-called phase-resolved partial discharge (PRPD) patterns. Here the total charge transferred during a discharge and the time or alternating current phase at which the discharge occurs are measured. Thus each discharge event is described by the two quantities, charge and phase angle. The modelling of the observation process is explicitly derived for this case. However, the used method can easily be generalized to other types of PD measurements. The proposed approach yields new possibilities for the interpretation and analysis of PD patterns. Features of PD patterns can be derived analytically from the process parameters. Conversely, quantitative information about the discharge physics can be gained from measured patterns. Some limiting cases of model parameter values leading to typical pattern features are discussed explicitly. Examples are presented that demonstrate the applicability of the model for three different discharge types (internal discharge in a gas-filled void, surface discharge in oil, corona in air).

SF&lt;sub&gt;6&lt;/sub&gt;ガス中における部分放電現象のウェーブレット変換を用いた時間周波数解析

Recently, diagnostic techniques have been investigated to detect a partial discharge (PD) associated with a dielectric material defect in a high-voltage electrical apparatus. However, the properties of PD aren't completely understood because the physical process of PD is complicated. For improvement of detection techniques of PD, it is important to make clear the fundamental physical process of PD. Therefore, this paper analyzes the process of PD using Wavelet transform. Wavelet transform provides a direct quantitative measure of spectral content ( Dynamic spectrum ) in the time-frequency domain. As it is important to develop a non-contact method for detecting the insulation fault, this paper analyzes the electromagnetic waves emitted from PD using Wavelet transform. This paper experimentally shows the process of PD as a two-dimensional distribution “Dynamic spectrum” in the time-frequency domain. This method is shown to be useful for detecting prediction of insulation fault.

Practical implementation of ultrawideband partial discharge detectors

It is shown that significant improvements in partial discharge (PD) measurement can be achieved through the use of ultrawideband (UWB) coupling techniques. These enhancements are principally in the areas of increased sensitivity, external noise rejection, and better understanding of the physics of the PD process. Examples of the improvements achievable by the use of UWB techniques are discussed for the cases of rotating machines and GIS. A new PD coupler which has a frequency response extending into the GHz range is demonstrated to enable reliable discrimination of PD signals from noise on operating turbine generators. Preliminary results from a fundamental study on PD in epoxy resins, show that there are two types of PD phenomena, 'fast' and 'slow'. >

Digital acquisition and processing of partial discharges during acceptance test of HV transformers

A six-channel digital recording system has been developed for monitoring partial discharges (PDs) in the HV insulation of power transformers during acceptance tests. The analog PD signals are converted to digital words describing their apparent charge, polarity and the point on the wave (phase) of the test voltage at which the PD pulse appears. The data are recorded during a 1 s acquisition period, processed, displayed and stored during the time (up to 7 s) the system is preparing for the next acquisition. The complete record of PDs acquired during the test is stored on a hard disk, and specialized programs have been developed for post test evaluation of the transformer performance. Different presentation formats allow easy identification of interference signals induced in the test circuit by external sources, correlation of PD patterns recorded on different transformer windings, and evaluation of changes in PD activity during the test.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>