Phase-resolved Partial Discharge Patterns Research Articles

Towards Optical Partial Discharge Detection with Micro Silicon Photomultipliers.

Optical detection is reliable in intrinsically characterizing partial discharges (PDs). Because of the great volume and high-level power supply of the optical devices that can satisfy the requirements in photosensitivity, optical PD detection can merely be used in laboratory studies. To promote the practical application of the optical approach in an actual power apparatus, a silicon photomultiplier (SiPM)-based PD sensor is introduced in this paper, and its basic properties, which include the sensitivity, pulse resolution, correlation with PD severity, and electromagnetic (EM) interference immunity, are experimentally evaluated. The stochastic phase-resolved PD pattern (PRPD) for three typical insulation defects are obtained by SiPM PD detector and are compared with those obtained using a high-frequency current transformer (HFCT) and a vacuum photomultiplier tube (PMT). Because of its good performances in the above aspects and its additional advantages, such as the small size, low power supply, and low cost, SiPM offers great potential in practical optical PD monitoring.

A simple approach of partial discharge source location in high voltage cables

This article presents a new approach of partial discharges (PD) source location in power cables. The main advantage of this approach is the detection of the propagation direction of the PD. The method provides a pattern named by the authors PD+D, which, in contrast to the well–known phase-resolved partial discharge (PRPD) pattern, graphically shows the direction of PD propagation and uses the polarity of a sensor array on top of the magnitudes of the positive and negative parts of each PD pulse to detect the direction of propagation, is described in detail. The proposed technique is useful for the evaluation and diagnosis of power cables, since it can identify incipient faults. The attained results that demonstrate the value of this methodology and their scope and limitations are discussed.

Effect of High Voltage Impulses on Surface Discharge Characteristics of Polyethylene

This paper investigates the influence of high voltage impulses on the surface discharge characteristics of polyethylene (PE) during subsequent application of 50 Hz AC voltage. The investigation is done on two types of PE to examine the generality of the phenomena. The effect of impulses on the surface discharge features of PE are studied for both cases of having either a streamer or flashover. For each case, the consecutive surface discharges behave differently when the insulation has been exposed to the transient voltages. The effect of impulses on surface discharge of PE is shown by the change of PD numbers and phase-resolved PD patterns. The results show a prompt change in the total number of discharges which decays with the time. After a surface flashover has occurred due to the impulse, the PD behaves quite differently compared to the similar case without flashover.

Polar coordinate pattern approach to partial discharge on floating electrode defect in GIS

This paper proposes a polar coordinate pattern approach to characterize the floating electrode defect in gas‐insulated metal‐enclosed switchgear (GIS). A 220‐kV GIS bus bar cavity is taken as an object to design the physical model of the floating electrode defect. The partial discharge (PD) development process is studied when the applied voltage is gradually increasing. The phase‐resolved partial discharge (PRPD) patterns and the polar coordinate patterns at different voltage levels are accessed through phase‐resolved pulse sequence (PRPS) data in the ultrahigh‐frequency (UHF) measurement. The proposed polar coordinate pattern approach does not use the traditional positive half‐cycle and negative half‐cycle analysis method in the PRPD pattern. It uses the clustering results to analyze the characteristics of the PD. It can be concluded that there are several advantages to the polar coordinate pattern. First, in the polar coordinate pattern, the phase is in a head‐to‐tail arrangement, so the observation of the PD phase characteristics can be more intuitive. Second, the polar coordinate pattern makes the calculated phase median, amplitude median, and centroid meaningful. With the voltage increasing, the variation law of the phase median and centroid can be observed. Third, the discharge parameters, which are based on the quadrant and statistical theory and the cosine similarity proposed in this paper, are from a new point of view. They can make a clear description of the characteristics of the floating electrode defect quantitatively, which can be distinguished from other insulation faults. Additionally, the continuous mode and phase mode at different voltage levels measured by acoustic detection are obtained to further demonstrate the PD development process. The acoustic results agree with the UHF measurement results. The efficiency and rationale of the polar coordinate pattern are further verified. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Phase resolved PD patterns in treeing in the presence of voids

One of the important aspects of insulation condition monitoring through partial discharge (pd) detection is the need to establish a trend in the discharge pattern for a given insulating system. Treeing is one of the principal routes to failure in insulating materials, and its growth is accompanied by pd activity. In this work, a study of the evolution of the phase resolved pd patterns during the various stages of tree growth in the presence of void is undertaken. Air-filled artificial voids are injected into the polymer, and their effect on the growth of a pre-existing tree observed. The pd patterns were seen to change drastically on tree channels reaching the void. The tree channel reaching the void was seen to undergo intense pd activity followed by widening of the channel. Electrical pd detection is performed simultaneously with online visual observation of tree growth with a high resolution high speed camera.

Classification of partial discharge sources in mica-epoxy-glass insulation sample using statistical analysis

Partial Discharge (PD) in rotating machine insulation system is deleterious. Even though the main insulation of rotating machine is Mica, which is highly PD resistant, the discharges taking place in slot region and end windings are highly deteriorating to insulation system. In this paper, PD measurements are taken in Mica-Epoxy-Glass insulation sample using cylindrical-plane electrode system. The sample contains a defined void on the surface. Distinguished Phase Resolved Partial Discharge (PRPD) patterns are obtained for different location of void. The different PRPD patterns obtained corresponds to internal void in the bulk of insulation, void between high voltage electrode and insulation surface, void between ground electrode and insulation surface; and surface discharge. The PRPD patterns obtained from void between high voltage electrode and insulation surface, in the actual scenario, indicates PD originating near the conductor surface inside the insulation system of rotating machines. The PRPD patterns obtained from void between ground electrode and insulation surface indicates PD originating on the surface of insulation such as slot discharge, end winding discharge in rotating machine insulation system. The variation of PD parameters like Charge magnitude (q) with respect to applied voltage is shown for different location of void. Further statistical parameters like skewness and kurtosis are calculated for charge distribution (n-q) and phase distribution (n-Φ) for different PRPD patterns. It is inferred that statistical analysis of PD parameters is a good tool to classify of partial discharge sources.

Partial discharges and noise separation using spectral power ratios and genetic algorithms

Accurate measurements of partial discharge (PD) activity is essential for the application of this technique to condition-based monitoring. Noise and PD source characterization is necessary to fulfil that goal, since the interpretation of classical phase-resolved partial discharge (PRPD) patterns is usually complex for the measurements done in field. A successful pulse source separation prior to the identification seems to be the best option. The authors proposed in a previous work a method based on spectral power ratios (PR) to separate pulse sources with quite good experimental results. This technique calculates the spectral power in two frequency bands to obtain two parameters which, represented in a 2-dimensional map (PR map), characterize each pulse source by a cluster of points. The main difficulty of this technique is the choice of the appropriate frequency intervals that give a good separation of clusters, which sometimes can be cumbersome by manual means. Thus, this paper presents an unsupervised technique to select the two frequency intervals that gives the best separation among several clusters. This will give a great support for the system user to separate PD and noise sources in real measurements. The authors used genetic algorithms (GAs) to select these frequencies, with good results in several real experiments.

Comparison of capacitive and inductive sensors designed for partial discharges measurements in electrical power apparatus

In the paper results of simultaneously conducted measurements achieved using capacitive and inductive sensors are presented according to different PD model sources immersed in a mineral transformer insulation oil. All measurements are preceded under laboratory conditions using typical measurement set up commonly applied for on-site PD detection: measuring impedance and capacitor and high frequency current transformer (HFCT) are used respectively. Measuring frequency and voltage level influence as well as phase resolved PD patterns analysis are investigated in the research. Various fundamental PD signal descriptors assigned for selected frequencies are also proposed and compared for chosen sensors. The main purpose of the presented research is to compare PD measurement results achieved using selected type of sensors during laboratory measurements and to point the best application areas in fields of PD detection in high voltage apparatus under normal operating conditions. Furthermore a proper measurement results interpretation coming from different sensors as well as measurement conducting problems and achieved patterns disparities are also discussed in the paper.

INVESTIGATION ON THE DEGRADATION BEHAVIOR OF CREEPAGE DISCHARGE ON PRESSBOARD IMMERSED IN PALM FATTY ACID ESTER (PFAE) OIL

Creepage discharge at the oil-pressboard interface is known as the development of a conducting path which is characterized by white and carbonized marks. This phenomenon tends to cause damage on the cellulose pressboard insulation which is subsequently promote to the catastrophic failure of a transformer. One of major defects that may cause the creepage discharge to occur along the pressboard surface is an excessive moisture content in pressboard insulation. Previously, there is an extensive research concerning the effect of moisture content in pressboard on the degradation of creepage discharge at mineral oil-pressboard interface. However, none has been found on the study of the effect of moisture content in pressboard on creepage discharge using ester oil. Therefore, this paper attempts to present the investigation on the degradation behavior of creepage discharge at Palm Fatty Acid Ester (PFAE) oil-pressboard interface by using a needle-bar electrode configuration under a constant AC voltage. Dry and wet pressboard samples (moisture content of 3 %) are used in this experiment to differentiate the degradation behavior. The development of creepage discharge is analyzed by correlating the visual records of creepage discharge and phase-resolved partial discharge (PRPD) pattern. An unexpected result has been found that the number of partial discharges (PDs) for wet pressboard is lower than the dry one as the formation of white mark with bush-like pattern is observed. Eventually, as the white mark is seen about halfway of the gap distance, the PDs number for wet pressboard sample significantly dropped, i.e. by 99 % compared to the dry board which is only 38 %. Variations in the PRPD data is due to constructive and destructive superpositions of electric field. The results suggest that the destructive effect is higher on the wet pressboard compared to the dry one.

Statistical Analysis of Partial Discharges in SF6 Gas via Optical Detection in Various Spectral Ranges

Partial discharge (PD) detection is essential to the operation of high-voltage systems. In this context, we investigate the basic characteristics of light emission during PDs in SF6 gas from the perspective of insulation diagnosis. A synchronous system is constructed using three optical photoelectric instruments with separate wavelength responses in the ultraviolet (UV, 189–352 nm), visible (VIS, 381–675 nm), and near-infrared (NIR, 737–920 nm) spectral ranges and a wide-band PD current pulse detector with a response of 1 pC. The results indicate that light emission depends upon the type of insulation defect and discharge energy. An increase in PD charge gives rise to more components in the spectral range from UV to VIS, and the presence of an insulator surface in discharges yields a more complex VIS-to-NIR spectrum. The phase-resolved partial discharge pattern (PRPD) of UV light pulses can reasonably reflect the electroluminescence process in the presence of the insulator surface and weak corona at negative voltage points. The PRPD of VIS light describes the features of the actual PD pattern in most cases. In comparison with the other two spectral ranges, light intensity in the VIS range is more sensitive to changes in gas-pressure-normalized voltage (Vrms/p). The linear fitting analysis of the relationships between the light intensity and PD charge shows that UV light detection has a greater sensitivity to the PD charge and that UV detection exhibits a greater degree of linearity. NIR detection is applicable only to severe PDs. We believe that our findings can significantly aid in application of optical PD diagnosis in SF6 gas insulated systems.

Classification of Single PD Sources of HV Transformer Insulation Faults using PRPD Pattern Features and ANN Approach

Transformer is an expensive and vital component in electric power transmission and distribution system. Most of the failures of transformers occurs due to the failure of insulation. Hence electrical utilities are spending a large amount of money in the early prediction of insulation problems inside the transformers. In recent times, partial discharge (PD) detection and analysis is a well recognized insulation condition monitoring technique for power equipments. However, accurate classification of PD signals originating from different PD sources is always a vital and hot research issue. This paper attempts to use PRPD pattern features and artificial neural network approach, which produces accurate results on large data bases, to deal with partial discharge classification. In this work, corona discharges, internal discharges and surface discharges, which are the major sources of PD activities inside the transformer, were simulated in the laboratory using different electrode configurations and the corresponding PD signals were acquired using wide band detection system. Phase resolved PD pattern and time-frequency characteristics of PD pulses were analyzed and important statistical features were extracted. Artificial neural network model was trained and tested using the extracted statistical features of PRPD patterns of PD signals and reported results show that the proposed ANN based PD source classifier is efficient and useful for understanding single PD sources of transformers.

Study on the Partial Discharge Characteristics and Development Process in Use of the Multiple Discharge Patterns for the Typical Defects in Gas-Insulated Switchgear

This paper presents an experiment to study the partial discharge (PD) characteristics and development process of the physical models of general insulation defects in gas-insulated switchgear (GIS). Four types of typical artificial insulation defects are designed to imitate the insulation faults in an actual GIS. The experiment PD data are obtained from a 220-kV GIS equipment using ultrahigh-frequency (UHF) PD measurement. The PD data obtained by UHF detection are stored as phase-resolved pulse sequence (PRPS) data. Then, the multiple PD patterns, such as PRPS analysis pattern, phase-resolved PD pattern, and $\Delta u$ pattern, can be calculated from the PRPS data sets. On the basis of the present experimental research, the multiple PD patterns can be used to classify different PD defects and evaluate the stage of PD development process comprehensively of each typical defect.

Relative ability of UHF antenna and VHF capacitor methods to detect partial discharge in turbine generator stator windings

On-line partial discharge (PD) monitoring is widely used to detect stator winding insulation deterioration. The most common type of on-line PD sensor for turbine generators is the 80 pF capacitive coupler installed at the machine terminals which detects the PD in the very high frequency (VHF) range. More recently, ultra high frequency (UHF) antennae have also been used to detect stator winding PD in large turbine generators. Both types of sensors have been installed in 16 turbine generators. This paper discusses the results from near simultaneous measurements from both types of sensors on the same machines. It was expected that the UHF sensors would have stronger discrimination of the electrical disturbances internal to the machine compared to the VHF sensors, but the VHF sensors would be more sensitive to PD. As expected, the VHF sensors were generally more corrupted by electrical disturbances. However, in about half of the machines investigated here, the UHF sensors were significantly more sensitive than the VHF sensors. Many examples of phase-resolved PD patterns that illustrate these findings are presented.

Investigation of partial discharge in piezoelectric ceramics

Electrical partial discharges were studied in different piezoelectric ceramics. Epoxy material with micro sized cavities was also tested and compared to the piezoelectric ceramics. It is found that compared to epoxy, partial discharge (PD) occurs at relatively lower electric fields for piezoelectric ceramics. The PD inception voltage was found to be lower for materials with higher relative permittivity. This indicates that the intensification of the electric field within the defects is the main cause for the differences in inception field observed for epoxy compared to piezoelectric ceramics. Furthermore, phase resolved PD pattern analysis was carried out for all materials at elevated electric fields. A broad distribution of the discharge event was observed for both epoxy and hard PZT samples, whereas for soft PZT discharge occurs concentrated at electric fields slightly above the coercive field. This intensification of PDs close to the coercive field suggests that PDs may be enhanced due to an increase of the internal field and electron emission rate induced by the domain switching process.

Modelling of aged cavities for partial discharge in power cable insulation

In this study, the ageing process of cavities in solid insulation of power cables is divided into three stages, and for each stage a circuit model is proposed. Each of these circuit models preserves the basic characteristics of the cavity under the circumstances of the corresponding stage and provides the expected behaviour of generated partial discharge (PD) pulses. This set of models simulate cavity wall effects, the non-uniformity of internal electrical field and charge deployed on the wall surfaces with considering the existence of electronegative gases according to the requirements of the three defined ageing stages. Finally, the simulation results, in the forms of phase resolved PD patterns are presented showing good agreement with the published experimental measurements drawn from accelerated ageing process setups.

Separation of radio-frequency sources and localization of partial discharges in noisy environments.

The detection of partial discharges (PD) can help in early-warning detection systems to protect critical assets in power systems. The radio-frequency emission of these events can be measured with antennas even when the equipment is in service which reduces dramatically the maintenance costs and favours the implementation of condition-based monitoring systems. The drawback of these type of measurements is the difficulty of having a reference signal to study the events in a classical phase-resolved partial discharge pattern (PRPD). Therefore, in open-air substations and overhead lines where interferences from radio and TV broadcasting and mobile communications are important sources of noise and other pulsed interferences from rectifiers or inverters can be present, it is difficult to identify whether there is partial discharges activity or not. This paper proposes a robust method to separate the events captured with the antennas, identify which of them are partial discharges and localize the piece of equipment that is having problems. The separation is done with power ratio (PR) maps based on the spectral characteristics of the signal and the identification of the type of event is done localizing the source with an array of four antennas. Several classical methods to calculate the time differences of arrival (TDOA) of the emission to the antennas have been tested, and the localization is done using particle swarm optimization (PSO) to minimize a distance function.

Enhanced distinction of surface and cavity discharges by trapezoid-based arbitrary voltage waveforms

Identification of different partial discharge (PD) sources may be improved by the use of non-sinusoidal waveforms of the testing voltages. For instance, surface discharge (SD) and cavity discharge (CD) are not always easy to directly distinguish from the phase resolved PD pattern at traditional 50 or 60 Hz alternating sinusoidal voltage. This study compares PD patterns stimulated by sinusoidal voltage and by several forms of trapezoid-wave voltage, including the limiting cases of triangular and approximately square-wave voltages. Trapezoid-based voltage waveforms are considered as a potential new off-line diagnostic method for PD sources. Partial discharge measurements were performed with each different waveform in two test cells representing canonical cases of SD and CD, with polycarbonate plates as the solid insulating material. The results show that the applied voltage of arbitrary waveform could more clearly distinguish between these PD sources� behavior than normal sinusoidal voltage. The constant-voltage period of peak value in trapezoidal and square voltage waveforms played an important role in the distinction of the two discharge sources. Compared with the cavity discharge�s symmetric features, surface discharge produced in the asymmetric test cell shows strong asymmetric behavior during the constant-voltage period between two polarities under trapezoidal and square voltage waveforms. A faster rise time and increased duration of the constant peak-voltage part of the waveform caused more obvious asymmetry of the surface discharge.

Dielectric and partial discharge investigations on ceramic insulator contaminated with condensable hydrocarbons

This paper investigates on understanding the influence of condensable hydrocarbons (collectively known as tar) over the dielectric condition of ceramic insulators. Multiple dielectric and diagnostic test methods over a wide frequency range are employed for this purpose. Initially, experiments are conducted on chosen clean and contaminated insulator samples. The influence of tar contaminations on the surface of the chosen samples are quantified through surface dielectric properties such as insulation resistance, loss factor or dissipation factor, impedance and so on. In addition, the partial discharge (PD) that arises over the contaminated surface of the insulator sample is measured and the respective phase resolved PD pattern is analyzed. The recorded PD signals are de-convoluted in frequency domain. Following this, similar experiments are conducted on test insulators contaminated under laboratory condition. The findings of the clean and contaminated samples are experimentally cross-verified on test insulators and later validated on an actual electrostatic precipitator unit. In summary, it is believed that such an investigation might provide an opportunity to gain more knowledge about the electrical characteristics of condensable hydrocarbons.

Investigations on PD Characteristics of Thermal aged Palm and Corn Oil for Power Transformer Insulation Applications

Partial discharge (PD) detection plays a major role in the life time assessment of liquid insulation in power transformers. Many research works are being carried out to replace conventional mineral oil insulation in transformers by vegetable oils. It is necessary to understand the PD characteristics of vegetable oils before recommending them as an alternate for mineral oil. In this paper, the breakdown strength and PD characteristics of palm and corn oil were investigated in both unaged and thermally aged conditions. Laboratory experiments were performed as per IEC test procedures. PD signals were measured using wide band detection system. Phase resolved PD pattern of vegetable oils and mineral oil were compared. Effect of increase in voltage stress on the PD pattern of palm and corn oil were studied. Time and frequency domain analysis of PD pulses at needle-plane electrode configuration was carried out. Statistical analysis of PD pattern i.e. skewness and shape parameter variations with respect to applied thermal stress were also carried out. From the results, it is observed that palm and corn oils have better breakdown strength and PD characteristics even under long-term thermal stress and hence they can be used for power transformer applications.

Change in partial discharge activity as related to degradation level in oil-impregnated paper insulation: effect of high voltage impulses

High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular electrical insulation at the instant of interaction with a high voltage transient would determine a level of the degradation of an electrical insulation. This work investigates how high voltage impulses would change Phase Resolved Partial Discharge (PRPD) patterns in test samples consisting of a cavity deliberately introduced between the sheets of oil-impregnated paper used in power transformer bushings. It also investigates how the change in PRPD patterns is related to the degradation level of oilimpregnated paper. In order to accomplish these objectives, the experiments were set in such a way that the effect of HV impulses and an early stage PD activity at an elevated AC stress, HV impulses in combination with a prolonged PD activity at an elevated AC stress, and a prolonged PD activity alone at an elevated AC stress could be investigated separately. The experimental results presented in this paper indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity would not cause a significant change in PRPD patterns, and would not damage oil-impregnated paper to a level which can be noticed by a visual observation. On the other hand, a prolonged AC PD activity alone can cause the change in PRPD patterns, but cannot quickly damage the oil-impregnated paper as it would do when it is combined with HV impulses. However, the combination of both, HV impulses and a prolonged AC PD activity caused a high drop in the PD parameters (total PD charge and a repetition rate). The study found that the decrease of the PD parameters can be considered as a sign of severe degradation of oil-impregnated paper.