Phase-resolved Partial Discharge Patterns Research Articles

Investigation of Free-Moving Particles under AC Electric Field in Different Insulating Gas Mixtures

Free-moving conductive particles may affect the dielectric reliability of gas-insulated systems, e.g., gas-insulated lines (GIL) or gas-insulated switchgear (GIS). This paper reports on investigations on the behavior of free-moving particles under AC electric field, using different insulating gas mixtures. Multiple particles were positioned inside a GIL test setup with each particle placed within the same distance to a particle trap. To ensure reproducible and realistic particle geometry, cuboid particles were used. In this paper, a width correction factor was empirically determined for a well-known formula from the literature, which is used to calculate the lifting field strength for cuboid particles. While increasing the applied voltage, the movement of the particles was observed with a camera and further evaluated using ultra-high frequency (UHF) partial discharge (PD) measurements. The UHF phase-resolved partial discharge (PRPD) patterns of steel and aluminum particles in SF6, as well as in different alternative insulating gas mixtures, are compared and discussed. To determine the effectiveness of particle traps for different gas mixtures, further parameters are evaluated, such as the duration of each particle's movement and the number of particles reaching the inner conductor.

A Convolutional Neural Network-Based Model for Multi-Source and Single-Source Partial Discharge Pattern Classification Using Only Single-Source Training Set

Classification of the sources of partial discharges has been a standard procedure to assess the status of insulation in high voltage systems. One of the challenges while classifying these sources is the decision on the distinct properties of each one, often requiring the skills of trained human experts. Machine learning offers a solution to this problem by allowing to train models based on extracted features. The performance of such algorithms heavily depends on the choice of features. This can be overcome by using deep learning where feature extraction is done automatically by the algorithm, and the input to such an algorithm is the raw input data. In this work, an enhanced convolutional neural network is proposed that is capable of classifying single sources as well as multiple sources of partial discharges without introducing multiple sources in the training phase. The training is done by using only single-source phase-resolved partial discharge (PRPD) patterns, while testing is performed on both single and multi-source PRPD patterns. The proposed model is compared with single-branch CNN architecture. The average percentage improvements of the proposed architecture for single-source PDs and multi-source PDs are 99.6% and 96.7% respectively, compared to 96.2% and 77.3% for that of the traditional single-branch CNN architecture.

Partial Discharges Characteristics Analysis in Power Transmission Lines Artificial Defects

The paper describes the research on partial discharges (PD) characteristics in various types of power transmission lines insulation defects. Thirteen types of insulation defects were considered. PD measurements were performed using the AC high voltage source and commercial PD registration device. Phase-resolved partial discharge (PRPD) patterns, average apparent charge, and PD intensity were obtained for each defect type. The research results showed that most overhead lines insulators defects had relatively high PD intensity with small average apparent charge. Several insulators’ defects showed a low average charge and intensity, but insulators’ bodies in these cases were cut and could be destructed by mechanical loads. Power cable samples showed relatively high intensity and average charge in case of the cable termination defect. However, the needle grounding electrode defect showed considerable average charge and intensity. The obtained results will be useful for the defect type recognition tools development and PRPD patterns base extension.

New model of explanation for phase angle pattern of online partial discharge measurement in winding insulation of turbine generator

Abstract On-line partial discharge (PD) measurement in electrical insulation of the turbine generator (TG) is an essential approach to control the quality of insulation and to avoid any undesired shutdowns of TG. Although in the last few decades the number of research in PD on-line monitoring methods in the stator winding of TG has increased significantly, it is still not clear yet why PD only appears at certain phase angles of the AC cycle in the phase resolved partial discharge (PRPD) pattern. Moreover, there is not yet any clarification on how the winding configuration of stator may affect PD phase angle pattern. For this reason, this work examines detailed study of the impact of the winding diagram on PD occurrence in the stator winding of real turbine generator class TVF-60-2 (60 MW, 10.5 KV). In addition, a computer simulation of various sizes of ellipsoidal cavities from 0.1 to 2.5 mm between conductor bar and stator core were carried out by using Laplaces equation in Finite Element Analysis (FEA) software ComSol to investigate the relation between the cavity size and coefficient of electric field. As a result of that, the phase angle of PD occurrence in the stator winding will highly depend on the configuration of bars connection, and the computer simulation has helped identifying the PD inception voltage and applied voltage of various ellipsoidal defects size. These two results have helped proposing an initial new theoretical model of explanation the relationship between voltage distributions and phase angle of PD occurrence to determine the degradation level of insulation caused by ellipsoidal defects in the insulation bars of the stator winding which can be used for stator windings of TG rated 6 kV and higher. This model is the initial step to develop a further comprehensive model of explanation for PRPD patterns which will consider all other types of defects as well as the space charge effects from the previous PD.

Analysis of Cavity PD Characteristics’ Sensitivity to Changes in the Supply Voltage Frequency

The supply voltage frequency effect on partial discharge (PD) phenomena has continued to draw research interest. Although most high voltage equipment operates at power frequency (50/60 Hz), testing is often done at different frequencies for various reasons. Despite some agreements and inconsistencies for the research findings of PD activity’s frequency dependence, there has been consensus on the recognition of the discharge mechanism parameters that influence how the supply voltage frequency affects PD activity. These parameters include statistical time lag, discharge area surface conductivity, and the residual charge decay. In this paper, a 3-capacitor model (ABC) is used to simulate how the changes in the discharge mechanism parameters influence PD characteristics as a function of the supply voltage frequency. The findings are that the phase-resolved partial discharge pattern (PRPDP) and PD repetition rate (PDRR) characteristics are more sensitive to variations in the probability of the seed electron availability at higher frequencies of the supply voltage. The opposite trend is observed for the cavity surface resistance. At lower resistance of cavity surface, the PRPDP and PDRR characteristics are more sensitive to changes in the supply voltage frequency than at higher resistances. The paper also confirms that incorporating equivalent resistances in the ABC model makes it more authentic than the model comprising of capacitors only.

Исследование характеристик частичных разрядов в СПЭ-изоляции кабеля с множественными дефектами

The paper simulates the electric field intensity picture in the cable with one and several defects. Modeling has shown that the presence of several closely spaced defects does not lead to an increase in the electric field intensity in comparison with one defect. On the basis of experimental studies, an analysis of defects' number influence on various parameters of partial discharges (PD) is performed. Artificial defects are created in the area of the XLPE-insulated cable termination. The following parameters of PD are measured: the magnitude of the apparent charge, the power (intensity) of partial discharges, the phase-resolved partial discharge (PRPD) patterns, the shape of PD signals. The analysis shows an increase in the number of defects does not affect the characteristics of partial discharges, despite the fact the defects are located in different sections of the cable with distributed parameters. Thus, it can be difficult to quantify defects during cable partial discharge measurements.

A Review of Knowledge-Based Defect Identification via PRPD Patterns in High Voltage Apparatus

Partial discharge (PD) as one of well-known signatures of defects in the insulation system of high voltage equipment has been studied over eight decades. It is of a great interest to power grid operators and owners to identify and distinguish local weak spots in insulation system of the key components of power system by using the obtained PD information to make timely decision thus to avoid any unexpected failures in the system. This article reviews the research progress of PD in high voltage apparatus, including motors and generators, cables, transformers and gas insulated equipment, with attempts to assess the severity of different types of PD initiating defects as well as key PD features that can be selected for classifying defects. Experimental and on-site test results for each type of discharge and defect associated are summarized and discussed. Also, development of these defects during operation of equipment caused by changes in physical and chemical parameters of defect and/or insulation, and its impact on phase-resolved PD pattern variation are thoroughly explained. It is expected that this paper can provide a comprehensive guideline for PD data analysis of available PD information extracted by conventional PD acquisition instruments for both engineers and asset managers of high voltage apparatus.

Исследование отличительных особенностей характеристик частичных разрядов в проходных изоляторах

The paper is devoted to partial discharge measurement and analysis in switchgear bushings. PD bushing structure analysis is described to assess possible defect sources in bushings. An experimental 10 kV bushing sample with the natural defect is obtained from the bushings’ manufacturer. It is tested using the PD measurement technique. Test results showed significant PD intensity at voltages from 12 kV and higher. We have an assumption that a part of registered discharges occurred in the air close to the high voltage electrode sharp edges. To check this assumption we grind them off and repeated the test. The second test does not show considerable PD characteristics change. Therefore, we assume that the bushing sample has an inner defect because the bushing’s surface is not contaminated to generate surficial discharges. The bushing is researched by a destroying method for defect localization. However, inside the bushing, possible defect locations are not found. It might be connected with the fact that the defect could not be found visually at the test time or the defect is located in the gasket between the high voltage electrode and insulator’s body. Besides, there are determined features of phase-resolved partial discharge patterns in switchgear bushings.

Construction and Experimental Verification of Novel Online Partial Discharge Monitoring System Using Power Line Communication

In this research, we aim to construct a completely new partial discharge (PD) monitoring system that applies inexpensive power line communication (PLC). We have proposed a PD detection method and monitoring system (PDMS-PLC) on the basis of PLC signal transfer rare is affected by an error due to disturbance. An attempt is made to identify PD source from the quadrant dependence of information transmission rate (hereinafter referred to PHY rate [Mbps]) by selecting a pair of quadrants of applied voltage phase; i.e. 1st and 3rd quadrants, or 2nd and 4th ones. It was found that the PHY (Abbreviation for the lowest physical layer in the OSI (open systems interconnection) hierarchical model.) rate number distribution reflects the PD characteristics obtained by the phase resolved PD (PRPD) pattern. Environmental noise level is also measured at an actual substation using PDMS-PLC. Consequently, the applicability of PDMS-PLC to PD signal detection and defect type identification is suggested.

Partial Discharge Analysis of SF6/N2 Gas Mixture Filled MV Circuit Breaker Using Time–Frequency Map Clustering Technique

Breakdown of gas insulated switchgears due to partial discharges (PD) is a major threat for the safe and reliable operation of power system network. In recent times, PD based diagnostics has been widely adopted by electrical utilities in order to evaluate the health of high voltage insulation system. However, very few reports are only available on the PD characteristics of gas insulated medium voltage (MV) circuit breaker prototypes, in particular with mixture of gases, over a wide range of pressure and voltage stress. Deep analysis of nature of PD signals in mixture of gas insulation and its time–frequency characteristics is important in order to develop efficient online condition monitoring systems for circuit breakers. This paper is aimed at the analysis of PD characteristics of a 22 kV MV circuit breaker prototype filled with SF6/N2 gas mixtures using time–frequency (T-F) mapping and clustering technique. PD tests are carried out at different gas pressures (1–5 bar) and at different gas mixture ratios. Important PD parameters such as PD inception voltage, phase resolved PD (PRPD) pattern and PD repetition rate are evaluated. Clustering analysis of PD signals from the time–frequency mapping is carried out and formation of three different clusters consisting of short, long and repetitive PD pulses is identified. Online real time monitoring of individual cluster density will be helpful to understand the degradation of gas insulation of circuit breakers. Keeping in mind the minimal usage of SF6 gas, from the results, it is noted that 10% SF6 gas can be mixed with 90% N2 at 3 bar pressure to get satisfactory PD performance of a 22 kV medium voltage circuit breaker.

Investigation of Electrical Tree Growth of XLPE Nano Composites using Time–Frequency Map and Clustering Analysis of PD Signals

Failure of underground cables due to electrical treeing phenomena, i.e. formation of electrical discharges in the imperfections of cable insulation, is a major problem faced by electrical utilities. Many research works on nanocomposites are being carried out to improve the electrical treeing resistance of the XLPE cable insulation material. Technological advancements in communication and data analytic systems have shown the way for implementing online continuous partial discharge (PD) monitoring systems for high voltage apparatus. Hence collection of PD database of XLPE nanocomposites in the laboratory during entire electrical tree growth process is important for implementing efficient condition monitoring systems and relatively little work has been published in this area. In this work, PD characteristics of XLPE nanocomposites with 1, 3, 5 and 10 wt% silica were investigated. Electrical tree growth and corresponding phase resolved PD (PRPD) pattern were analysed with respect to time. Cluster analysis of equivalent time–frequency mapping of PD signals was carried out with respect to tree growth time period. Statistical analysis was performed for the entire set of PD data. Results show that cluster analysis of T–F map of PD data is useful in estimating early failure of insulating material due to treeing. Addition of silica nano fillers in the range of 3–5 wt% concentration significantly improves the PD resistance and breakdown time of XLPE material.

Applying HF and VHF/UHF Partial Discharge Detection for Distribution Transformer

This paper represents application of high frequency (HF) and very high frequency/ultrahigh frequency (VHF/UHF) partial discharge (PD) detection for a distribution transformer. A capacitive sensor is used to detect the HF electric field caused by charge transfer inside oil–paper insulation due to PD at the defect site, and an electromagnetic sensor or antenna is used for detecting electromagnetic PD transients in the air outside the investigated transformer in the near-field region. Three types of artificial PD sources in air and insulating liquid, which are corona discharge, surface discharge and air void discharge in pressboard, were investigated. Three identical distribution transformers were rated at 22 kV, 400 V and 50 kVA, and were designed and constructed. The first transformer was filled with mineral oil, the second was filled with natural ester and the third was filled with palm oil. The PD generated by the air-filled voids in the insulating papers and pressboards of these transformers with five different conditions were investigated, i.e., non-impregnated paper, impregnated paper for 3 hours, 6 hours, 9 hours and 12 hours. The impregnation process was done with 65°C liquid temperature, and the pressure in the oven was around 5 mbar. From the experimental results, it can be concluded that the electromagnetic PD transients radiated from the corona discharge of both high-voltage (HV) and low-voltage sides in the air are in the VHF range, and surface discharge frequency is extended up to the UHF range. For the PD in the insulating liquid, the phase resolved PD (PRPD) pattern in the HF range is a valuable tool to characterize the PD sources. The PD in an air-filled void inside the insulating paper of the mineral oil transformer is obviously different compared with those of the natural ester transformer and the palm oil transformer. For the manufacturing of distribution transformers in this research, it is found that after the paper insulation is dried out, the impregnation process for a period of 9 hours is suitable for improving the oil–paper insulation with an acceptable PD level. This paper is the cross-field application by applying the antenna and communication theory for detecting the discharge problems in HV equipment.

Effects of temperature on electrical treeing and partial discharges in epoxy/silica nanocomposites

This article reports the effect of temperature on tree propagation and phase resolved partial discharge (PRPD) pattern in epoxy/silica nanocomposite. The results show that tree shapes in 0-wt% and 5-wt% samples change from branch-like to bush-like as temperature rises. The tensile stress tends to decrease when the temperature is high, and a tree can select numerous paths to propagate; however, tree shapes in 10-wt% samples keep bush-like regardless of the temperature. It is suggested that nano-particle effect of inhibiting trees is more likely to occur at high temperatures rather than at room temperature. When a tree starts to propagate, the PRPD pattern tends to form wing-like one regardless of the amount of nano-particle and the temperature.

Separation of Partial Discharge Sources Measured in the High-Frequency Range with HFCT Sensors Using PRPD-teff Patterns.

During the last two decades, on-line partial discharge (PD) measurements have been proven as a very efficient test to evaluate the insulation condition of high-voltage (HV) installations in service. Among the different PD-measuring techniques, the non-conventional electromagnetic methods are the most used due to their effectiveness and versatility. However, there are two main difficulties to overcome in on-line PD measurements when these methods are applied: the ambient electric noise and the simultaneous presence of various types of PD or pulse-shaped signals in the HV facility to be evaluated. A practical and effective method is presented to separate and identify PD sources acting simultaneously in HV systems under test. This method enables testers to carry out a first accurate diagnosis of the installation while performing the measurements in situ with non-invasive high-frequency current transformers (HFCT) used as sensors. The data acquisition in real-time reduces the time of postprocessing by an expert. This method was implemented in a Matlab application named PRPD-time tool, which consists of the analysis of the Phase-Resolved Partial Discharge (PRPD) pattern in combination with two types of interactive graphic representations. These graphical depictions are obtained including a feature parameter, effective time (teff), related to the duration of single measured pulses as a third axis incorporated in a classical PRPD representation, named the PRPD-teff pattern. The resulting interactive diagrams are complementary and allow the pulse source separation of pulses and clustering. The effectiveness of the proposed method and the developed Matlab application for separating PD sources is demonstrated with a practical laboratory experiment where various PD sources and pulse-type noise interferences were simultaneously measured.

Fiber Optic Acoustic Sensor Based on SMS Structure With Thin Polymer Diaphragm for Partial Discharge Detection

This paper proposes a fiber optic acoustic sensor (FOAS) based on a single-mode fiber - multimode fiber - single-mode fiber (SMS) structure attached to a thin polymer film used as a diaphragm. The diaphragm was specially developed to enhance the sensitivity towards the acoustic pressure-wave resulted from the partial discharge (PD) events. The sensitivity and signal-to-noise ratio (SNR) characterizations of the FOAS without and with a thin polymer film were performed. Both time-resolved and phase-resolved partial discharge (PRPD) patterns measurements were carried out in air and oil media. The experiment was conducted with three-electrodes using FOAS in conjunction with the conventional PD measurement as per IEC 60270 standard. The sensor achieved a sensitivity up to -31.21 dBm and -30.8 (0 dBm is defined as 1V/μBar) using broadband and tunable light source, respectively. The discharge characteristics pattern of FOAS was verified with IEC 60270 standard, and the patterns of FOAS resembled IEC 60270 standard. The proposed FOAS was capable for detecting the PD using both broadband and tunable laser lights. The use of the thin polymer film had a significant impact on the acoustic sensitivity. With the simple, low-cost design structure and free from electromagnetic interference, FOAS is found to be suitable as an in-situ sensor for detecting the acoustic signals of partial discharge and can be utilized inside the transformer.

Partial discharge behaviour due to irregular‐shaped copper particles in transformer oil with a different moisture content of pressboard barrier under uniform field

This study presents the investigations on Partial Discharge (PD) behaviour of irregular-shaped copper particle in oil with pressboard barrier placed in two positions under uniform field with different moisture contents. Frequency Domain Spectroscopy (FDS) was used to estimate percentage moisture in pressboard. Recovery Voltage Measurement was used to confirm obtained FDS results. PD characteristics like magnitude, number, discharge power, rise time and duration time of PD pulse were analysed. Electric field due to particle in oil with moisture content pressboard barrier was studied by simulations. Phase Resolved PD (PRPD) pattern for different moisture pressboard observed to have similar patterns with variation in PD inception voltage (PDIV), magnitude and number of discharges. It was observed that PD characteristics as a function of voltage, increases nearly quadratic rate with respect to percentage moisture in pressboard. Simulation results complimented experimentally measured PDIV values for particle in oil with moisture in pressboard. Filtering and conditioning of oil is mandatory practice in a new transformer before going to any high voltage (HV) applications. PRPD pattern and PD characteristics data obtained in this study has great practical significance to assess the condition of transformer insulation systems before proceeding for HV application to avoid any potential failure.

Surface Discharge Characteristics of Different Solid–Liquid Insulation Materials in Power Transformers

The surface discharge along the interface of solid–liquid insulation is one of the main causes of the deterioration of the life of a power transformer. This experimental study investigates the effects of different solid and liquid insulation materials on the surface partial discharge (PD) characteristics. The liquid insulation materials utilized are mineral oil and synthetic ester, whereas the solid insulation materials are cellulose board and Nomex board. The surface PD characteristics in various combinations of solid and liquid insulation materials are investigated using PD inception voltages (PDIV), apparent charges (pC), and phase-resolved PD (PRPD) patterns, and the comparative analysis is presented. Moreover, topography and composition of samples during surface discharges phenomena have been discussed using high-resolution images and chemical composition obtained using the scanning electron microscope (SEM) and energy dispersive spectroscopy (EDAX). The findings of this article will assist in understanding the behavior of surface PDs under various conditions.

A Novel Partial Discharge Detection Method Based on the Photoelectric Fusion Pattern in GIL

Optical detection and ultrahigh frequency (UHF) detection are two significant methods of partial discharge (PD) detection in the gas-insulated transmission lines (GIL), however, there is a phenomenon of signals loss when using two types of detections to monitor PD signals of different defects, such as needle defect and free particle defect. This makes the optical and UHF signals not correspond strictly to the actual PD signals, and therefore the characteristic information of optical PD patterns and UHF PD patterns is incomplete which reduces the accuracy of the pattern recognition. Therefore, an image fusion algorithm based on improved non-subsampled contourlet transform (NSCT) is proposed in this study. The optical pattern is fused with the UHF pattern to achieve the complementarity of the two detection methods, avoiding the PD signals loss of different defects. By constructing the experimental platform of optical-UHF integrated detection for GIL, phase-resolved partial discharge (PRPD) patterns of three defects were obtained. After that, the image fusion algorithm based on the local entropy and the phase congruency was used to produce the photoelectric fusion PD pattern. Before the pattern recognition, 28 characteristic parameters are extracted from the photoelectric fusion pattern, and then the dimension of the feature space is reduced to eight by the principal component analysis. Finally, three kinds of classifiers, including the linear discriminant analysis (LDA), support vector machine (SVM), and k-nearest neighbor (KNN), are used for the pattern recognition. The results show that the recognition rate of all the photoelectric fusion pattern under different classifiers is higher than that of optical and UHF patterns, up to the maximum of 95%. Moreover, the photoelectric fusion pattern not only greatly improves the recognition rate of the needle defect and the free particle defect, but the recognition accuracy of the floating defect is also slightly improved.

Analysis of partial discharge sources in stator insulation system using variable excitation frequency

Identification of partial discharge (PD) sources in stator insulation is an important and challenging task. Distinct phase-resolved PD (PRPD) pattern of different PD sources are well established at power frequency (50/60 Hz) and can help to identify each source. In this study, different sources of PD commonly found in stator insulation were created on a 6.6 kV epoxy-mica stator coil. Each PD source was experimentally investigated at a variable excitation frequency in the range of 50-0.1 Hz. PRPD pattern and PD parameters such as integrated charge per cycle and number of discharge pulses per cycle of each discharge source were compared at different frequencies. The comparison suggests that the parameter of each PD source was dependent on excitation frequency. Some discharge sources have shown a distinct variation in their characteristics with excitation frequency. These distinct characteristics are additional information to the existing ones, which will help to correlate PD characteristics with the source of PD in stator insulation.

Partial discharge of cable termination on electric multiple unit of China high‐speed railway below zero‐degree centigrade

Ethylene propylene rubber (EPR) cable terminations have great potential in improving the operational reliability of electric multiple units, for their high elasticity and low loss. However, the breakdown of EPR cable terminations frequently occurred in northwestern China during winters. In this study, tests were conducted to investigate the effect of temperature (-40 to 20°C) on partial discharges (PDs) characteristics, characteristic transition temperature, and breakdown process of cable termination. In samples with no artificial defects, it was found that the average PD inception voltage (PDIV) decreased obviously by 45% when temperature reduced from 20°C to -40°C. Meanwhile, the PD amplitude increased rapidly with decreasing temperature. The symmetric phase resolved PD pattern also changed from `turtle-like ' to `wing-like ' shape with decreasing temperature. Differential scanning calorimetry results indicated that the decrease of mechanical mismatching between EPR and stress control tube (SCT) at low temperature led to decreasing PDIV. A physical model of electric field (EF) distribution within cracks on EPR/SCT interface inside the termination was designed. Distortion of the EF by simulation was consistent with the discharge channel in the test. These results showed that the decreasing temperature had significant influences on the PD initiation of EPR cable termination.