Spectral Power Analysis of Partial Discharges Waveforms During Electrical Tree Growth under Different Excitation Frequencies
Universidad Tecnica Federico Santa Maria, Chile
Electrical trees are the main mechanism of failure in solid polymeric insulation. Their growth is associated to partial discharge (PD) activity. Measuring and analysing PDs during tree growth is crucial for the insulation condition assessment and thus, the risk of failure of power equipment. Here, PD pulse waveforms are analysed using power ratios maps. The aim of the study is to evaluate the ability of power ratio maps to determine the state of progression of electrical trees. Electrical trees were grown in epoxy resin samples at different frequencies, while PDs were measured using very-high frequency (VHF) and ultra-high frequency (UHF) techniques. The progression of the tree was observed using an optical camera and it was correlated with PD data. The results showed that power ratio maps obtained from VHF and UHF signals yield similar patterns. Especially in VHF, it was observed that the centroids of the clusters associated to each stage of tree-growth moved with tree-progression. PDs at the beginning of tree-growth were located at the top-right of power maps, while in more advanced stages, they were located more to the bottom-centre of the maps. This means that the PD pulse waveform had higher frequency content in the beginning than in the end of tree growth. In base of this initial results, it is concluded that power ratio maps are sensitive to the state of growth of electrical trees, and thus, can be used for assessing the risk of failure of insulation subjected to electrical treeing degradation mechanism.
Comparison of a Thermally Accelerated Aged Oil-Paper Insulation of an Open and Hermetically Sealed System
1Leibniz Universität Hannover, Germany; 2GRIDINSPECT GmbH, Germany
Even nowadays, the oil-paper insulation is still one of the most important insulation systems for high-voltage units, such as power transformers. However, the insulation is subjected to continuous ageing due to thermal, electrical, mechanical and chemical influences. In fact, the rate of ageing depends on many factors, e.g. the location and load of the transformer. As paper insulation ages, it becomes brittle and fragile over time, reducing its mechanical strength. This increases the probability of failures of the transformer in the occurrence of a fault. Therefore, the condition of the paper insulation is a limiting factor of the lifetime of the transformer. Currently, the measurement of the DP value is the most reliable method for evaluating the current paper condition. For this, a paper sample is required, which can only be obtained from a transformer in operation at a very high cost and effort. However, samples of the insulation liquid are relatively easy to obtain, hence they are often used to evaluate the condition of the paper insulation. For this purpose, various ageing markers of the insulation liquid are measured to achieve an indirect assessment of the paper condition.
This study compares the ageing behaviour of an oil-paper insulation in an open breathing system and a hermetically sealed system. In this regard, paper samples were impregnated with an uninhibited insulation oil and two inhibited insulation oils, which differ in their manufacturing process. The hermetically sealed system was reproduced in order to avoid contact with the ambient air while an open system had a continuous contact with the ambient air. The study was conducted for a long-term thermal stress to provide paper insulations with different ageing conditions. Therefore, the entire study prolonged over a period of 15 weeks at a temperature of 130 °C. Oil sampling was performed regularly and parameters like moisture content, the amount of acid as well as the interfacial tension (IFT) and etc. were determined. Furthermore, the DP-value of the paper samples, impregnated with the different oils, was also measured. The investigation showed that the oil types influence the ageing process of the insulation system and consequently production of its corresponding parameters. Moreover, the correlation between the aforementioned oil parameters and ageing status of paper was evaluated. This study presents the main differences in ageing behaviour, as well as the effects on the formation of ageing markers, of the different systems.
Reliable Diagnostics on Rotating Machines Using FRA
1Munich University of Applied Science, Germany; 2OMICRON electronics GmbH, Klaus, Austria; 3voestalpine Stahl GmbH, Linz, Austria
The frequency response analysis (FRA) is a known highly sensitive method to detect mechanical and electrical defects in power transformers. Investigations already showed, that this technique is applicable to detect broken windings or shorted turns in rotor and stator of rotating machines. This contribution focuses on the applicability of FRA to other defects, like broken rods of damping windings in synchronous machines or in a squirrel cage of induction motors, which often can be detected only by disassembling the machine. The reason lies in the design: damping windings as well as the squirrel cages have no accessible outer terminal, which can be used for electrical measurements. As the FRA method is capable to measure also capacitive or inductive couplings, it might be a possible low effort alternative. The main focus of this investigation was on whether it is possible to reliably detect defects named above. For a clear statement on reliability and reproducibility, numerous tests were performed on rotating machines of different types. It was found, that the frequency response can be measured on assembled rotating machines highly reproducible up to 1 MHz in different setups. Main factors to be considered are a good connection technique and a clear description of the measurement setup for later tests. A comparative analysis is only possible for the same phase configuration (star / delta / single phase connection) as well as the same rotation angle of the rotor. Similar to power transformers a time-based comparison is preferred. Different phases have similar, but not exactly the same frequency responses. For the investigation described in this contribution, broken rods in a squirrel cage of a 55 kVA induction motor and in the damping winding of a 37 kVA synchronous machine were reproduced. It was found, that the FRA method is sensitive enough to identify even single defects.
Research on Live Detection Method of Transformer Winding Deformation Based on Noise and Vibration
Guangzhou Power Supply Company Limited，China Southern Power Grid
Winding fault is one of the components that cause more problems in the transformer.The main faults of transformer winding are irreversible changes in the mechanical structure of winding under the action of electromagnetic force or mechanical force. Common faults include loosening, warping, bulging and dislocation of winding. The live detection technology of transformer winding deformation based on noise and vibration is of great significance. The project has developed a transformer vibration detection device with vibration and noise detection functions. The vibration and noise characteristics of the transformer and its influencing factors are studied, including voltage, current, temperature, aging degree of insulating materials and power factor.
In this paper, the mechanism and characteristics of transformer vibration are studied firstly. The vibration of transformer is caused by the vibration of its main body (core, winding, etc.) and the vibration of cooling device. The core vibration mainly comes from magnetostriction of silicon steel sheet, which is expressed by magnetostriction ratio. Through mathematical deduction, it is found that the vibration acceleration is based on two times of the power supply frequency, and the amplitude is proportional to the square of the voltage. Winding vibration is closely related to structural stiffness and dynamic characteristics. When the transformer winding coil flows through the load current, due to the existence of leakage magnetic field, dynamic electromagnetic force is generated between windings, wire cakes and turns, which causes winding vibration.
In the study of the mechanism and characteristics of transformer vibration, the axial structure of winding coil is simplified to MASS-SPRING-DAMP model. The upper and lower pressure plates are considered as rigid bodies and fixed constraints. Each layer of wire cake is equivalent to a mass module, and the insulation pad is equivalent to an elastic element. The distributions of radial and axial electromagnetic force and acceleration are obtained by dynamic simulation analysis.
Under typical mechanical faults such as transformer winding and core loosening, or short-circuit shock, the causes and mechanism of abnormal vibration of transformer are studied. The relationship between vibration and noise characteristics and the type of mechanical faults is analyzed. The characteristic parameters of vibration and noise reflecting the mechanical state of transformer and its extraction methods are obtained. The measures of restraining the interference of non-fault factors such as electric quantity and non-electric quantity are analyzed. A method for diagnosing the mechanical state of transformer by using vibration and noise signals synthetically is proposed.
GIS Insulation State Evaluation Based on Multi-source Information Fusion
1Electric Power Research Institute, Chongqing Power Company, Chongqing, China, People's Republic of; 2Wuhan University, China, People's Republic of
With the wide application of Gas Insulated Switchgear (GIS) in the field of ultra-high voltage, the identification and evaluation of insulation state in GIS equip-ment has been a hot topic in this field, Therefore, how to effectively integrate the multi-source information representing GIS insulation state and establish a scien-tific state evaluation model is a major problem to be solved. In this paper, the in-sulation degradation process of typical insulation defect inside GIS were simulat-ed, and ultra-high frequency (UHF) and ultrasonic information of defect in dif-ferent degradation stages were obtained. At the same time, the genetic algorithm is used to further characterize the PD severity assessment feature, and the PD as-sessment index set based on UHF and ultrasonic information was constructed. Based on this, the fuzzy hierarchy analysis was used as the evaluation method framework, the factor analysis weighting method and fuzzy evaluation function based on mutual information was used to obtain the preliminary evaluation result. Finally, the preliminary evaluation result was used to form a complete compre-hensive evaluation model. The evaluation scheme constructed in this paper can ef-fectively complement the multi-source information, thereby improving the relia-bility of the GIS insulation state evaluation result.
Pattern Recognition of Partial Discharges On Power Cable Systems
Central Power Research Institute, India
Partial discharge measurements are considered to be an important diagnostic/condition monitoring tests on Power Cable System. Various off line and online measurement systems have come up with inductive and capacitive sensors. The high frequency partial discharge signals occur in power cable system due to various defects such as voids/cavity in the power cable insulation, defective termination, defective stress control materials and defective joints. However, the partial discharge pattern differs for each type of defects. Hence to identify the type of defect, proper analysis of pattern is required. The statistical parameters such as mean, skewness, Kurtosis etc with respect to the phase angle , highest discharge magnitude etc helps in extracting the feature information of each pattern. PD-fingerprints such as Skewness (Sk) and kurtosis which measures the degree of asymmetry of a distribution & sharpness of a distribution, along with the average value of each half cycle (Mean) are estimated using MATLAB programming for various partial discharge signals of laboratory failed Power Cables and accessories. In this work, an attempt is made to develop some finger prints for various defects on power cable systems using the statistical parameters and PD pattern
Feasibility of Detecting Partial Discharge in Smoothing Reactor Using UHF Method
1State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing, China; 2Beijing Key Laboratory of High Voltage and EMC, North China Electric Power University, Beijing, China
On-line detecting Partial-Discharges (PDs) in smoothing reactors is of great importance for finding out insulation defects before they arouse accidents. The smoothing reactors in service are whole in high voltage area, so Ultra-High-Frequency (UHF) method is the only choice for on-line PD detection. However, the windings are tight, so it is still unknown whether PD UHF signals can propagate through the smoothing reactor. To certify the feasibility of detecting PDs in smoothing reactor, the PD UHF signals from the equipment are collected, and the UHF signals propagation is analysed based on simulation and electromagnetic wave theory. From the test results, we can see that UHF signals can radiate out from smoothing reactor. The propagation path includes two ways, one is diffraction between adjacent winding discs, and another is leakage in cooling channel between two coils. The electric field polarization directions of the first electro-magnetic wave are difference, which are decided by the propagation pathes. These conclusions is benefit for promoting the UHF technology for on-line detecting and locating PDs in smoothing reactors.
Investigation on the Accelerated Ageing Behaviour of an Oil-Paper-Insulation Using Different Weight Ratios
1Leibniz Universität Hannover, Germany; 2GRIDINSPECT GmbH, Germany
The main goal of this research was the comparison of the ageing behaviour of an oil-paper-insulation with different weight ratios and the influence on pertinent ageing markers which are used to estimate the remaining lifetime of an oil-paper insulated transformer. Even today the oil-paper-insulation is still one of the most important insulation systems for high-voltage components like transformers or reactors. This kind of dielectric medium undergoes permanent stresses during operation in the form of thermal, electrical, mechanical and chemical stresses. Accordingly, the insulation is exposed to a slow permanent ageing process, which makes it more vulnerable to different failures. However, the ageing time of every transformer is different and depends on several factors, e.g. the location or the amount of the load. Therefore, the service age of these assets should not simply be seen as a function of the years of operation, but rather influenced by the insulation end-of-life. In general, the pyrolysis has a degrading effect on the cellulose of the paper resulting in a decrease of the mechanical strength and consequently, its insulation quality. Currently, the method of measuring the degree of polymerization (DP-value) is the most accurate technique to determine the condition of the paper insulation. Usually, a DP-value in a range below 200 is considered as end-of-life criteria for a transformer, what can be seen as a practical value. At this value, the paper is fragile and brittle even with reduced mechanical stresses. However, for this measurement a paper sample is needed, for which the transformer has to be de-energized and opened with consecutive very high costs. Therefore, in order to reach an optimal lifetime of the transformer and to give recommendations about their future life expectation, it is necessary to observe the degradation of the cellulose during the operation time. Usually, for the determination of the condition of the paper insulation, oil samples are taken to measure different ageing markers, because oil probes can be easily taken.
Therefore, an investigation has been conducted to measure different ageing markers of a non-inhibited insulating oil during a long-term thermal stress (15 weeks at 130 °C) of an oil-paper-insulation with different weight ratios to examine the influence of the ratio on the formation of ageing markers. Furthermore, the DP-value of the paper samples was also measured. For this, an open system was reproduced, so that a permanent contact with the ambient air was given. The obtained results show that the weight ratio has an influence on the ageing rate and the ageing markers.
Simulation of Void Discharges and Time Domain Examination of Their Emitted Electromagnetic Signals
Budapest University of Technology and Economics, Hungary
The goal of this paper is to present the time domain behavior of the emitted electromagnetic signals of void discharges and to show the design process of the measured partial discharge (PD) sources. After the introduction of the used physical models and equations, the finite element calculations used to create a modified empirical ignition criterion for the discharge and to determine the optimal dimensions of the samples are presented. Using these sizes, two void discharge sources were created by including different diameter air bubbles in a thin layer of epoxy resin between two electrodes. After this, the emitted electromagnetic signals of the sources were measured using a noise-free GTEM cell and a digital sampling oscilloscope. From the measured signals, PRPD patterns have been created to examine the activity of the emitted void discharge signals as the function of the phase angle of the AC testing voltage. This was followed by the examination of the effect of bubble size on the PRPD characteristics. The results of the emitted measurements were consistent with the conducted measurement results of the literature and the partial discharge physics based model of the air-filled cavity.
Mineral Oil and ester based Oil/Paper Insulation Decaying Assessment by FTIR Measurements
ViAHT - UQAC, Canada
Esters based dielectric fluids have been widely researched since 1990’s for application in high voltage insulation applications. Since then, researchers are affirmative towards usage of ester based insulating fluids as a replicated to mineral insulation oils. The operating properties and aging performance of ester oils proved to be potential candidates for high voltage applications. In view of high temperatures and longevity of insulation systems, there also is a need to understand the chemical perspectives along with aging behaviour of ester oils. Accelerated aging of oil/paper insulation associated with mineral oil and synthetic ester with cellulose insulant has been experimentally simulated as per ASTM D 1934 at 115° C. Fourier Transform Infrared spectroscopy analysis of oils and cellulose papers is carried out at different aging factors. The compositional changes in oils and cellulose kraft paper with aging have been enumerated. The changes in the absorbance area for appropriate functional groups have been also reported. It is found that, the chemical stability of synthetic esters is superior than that of the mineral oil.
RSSI-Based Angle of Arrival Estimation Using Monopole UHF Wireless Sensor Array
Shanghai Jiao Tong University, People's Republic of China
The detection and localization of partial discharge (PD) using ultra-high frequency (UHF) antennas is studied in air-insulated substations. Existing UHF PD localization system mainly based on time-difference of electromagnetic wave signal, which requires very high time synchronization accuracy and sampling rate. This paper proposes a received signal strength indicator (RSSI)-based angle of arrival (AoA) method using the monopole UHF wireless sensor array. Multiple antennas are evenly placed in a circle frame and form an antenna array. Then an AoA estimation scheme is derived through the radiation pattern of designed UHF antenna. Interpolation and clustering are applied to improve the AOA resolution. A field test is performed to verify the effectiveness of the proposed PD localization system. The results show that the estimated error on average is below 10°. Compared to the time-difference-based method, the accuracy of our designed system is more feasible for practical application considering its significant low-cost feature.
Capacitor Voltage Transformer Diagnostic by Partial Discharge on Field
1Cepel, Brazil; 2PUC-Rio
Instrument transformers (IT) have secondary roles in the power electrical system. Moreover, the average age of these transformers is around 30 years of operation. Currently, the only way to evaluate the Instrument Transformers insulation system is by means of power factor tests, in which the shutdown of the system is required. Generally, these tests are performed in 60Hz, making it impossible to detect high frequency signals generated by the partial discharges (PD). Furthermore, due to the electrical system operation complexity, the shutdown are increasingly rare, which requisites a real time methodology evaluation and non-invasive to the equipment. The discharge partial measurement can provide precious information regarding the insulation system conditions of the high voltage equipment. The PDs are symptoms and causes of the dielectric deterioration and it can result in the equipment failure. The present work aims primarily to present some details relevant to measurement and to the IT monitoring, in special case the capacitor voltage transformer (CVT), combining the field measurement and the laboratory validation, in controlled condition, which allows glimpsing the developing of a monitoring technique in real time of these equipment.
Detection of CF4 Gas Using a Nanomaterial-based Gas Sensor Fabricated by Dielectrophoresis
1Graduate School of Information Science and Electrical Engineering, Kyushu University, Fukuoka, Japan; 2Kyushu Electric Power CO., INC., Fukuoka, Japan
Gas Insulated Switchgear (GIS) is one of essential equipment for a power distribution system with high reliability and compact size. Generally, sulfur hexafluoride (SF6) is used as gas insulation medium due to its excellent dielectric strength as well as superior arc extinction property. However, partial or arc discharge, which can happen in the GIS system, will lead to the decomposition of SF6 gas and eventual failure of the entire system. The authors have proposed and demonstrated a diagnosis method of GIS based on detection of PD-generated decomposition products of SF6 gas. The decomposition products such as HF, SO2 and SOF2 were successfully detected using a carbon nanotube (CNT) gas sensor, which was fabricated by dielectrophoresis (DEP). CNTs were trapped onto a microelectrode under the action of positive DEP force, which propelled polarized dielectric material to high electric field region. Recently, carbon tetrafluoride (CF4) has attracted as a SF6 decomposition product for the GIS diagnosis, because it can exist in GIS tank for a longer period than the other decomposition products such as HF and SO2. CF4 is often found when the decomposition reaction involves solid or polymeric, plastic material based on carbon, insulators located in the GIS system. In the present study, we explored the application of the DEP-fabricated gas sensor to CF4 detection, aiming the development of GIS diagnosis based on the gas sampling method. As CF4 sensing transducer, several different kinds of nanomaterial, such as CNT, zinc oxide (ZnO) nanowire, tin oxide (SnO2) nanoparticle, were tested. These nanomaterials were trapped onto a microelectrode by DEP in order to fabricate a nanomaterial-based gas sensor. It was found that SnO2 gas sensor showed the highest response to CF4 gas at 1% concentration in SF6. The conductance of the SnO2 sensor gradually decreased with elapsed time after exposure to the CF4 gas. Effect of the sensor operating temperature was also investigated.
Development of PRPS Conversion Algorithm and Generator using PRPD Pattern of Underground Power Transmission Cable
1Korea Electric Power Corporation, daejeon-si, Korea; 2SMND Co., Ltd., Anyang-si, Korea
Widely used method for PD (partial discharge) measurement and analysis of existing underground power transmission cables is to store the detected signal pulses as PRPD (Phase Resolved Partial Discharge) data and to analyze the PD defect factors by the pattern type. Although the PRPD pattern data of various facility defects are acquired through the underground power transmission cable diagnosis, due to the PRPD data storage structure, the pulse generation information is destroyed over time, and reconversion into 3D PD pattern (PRPS: Phase Resolved Pulse Sequence) is impossible. Also recently, as a method for verifying the reliability of the power utility prevention diagnosis system, a simulated partial discharge is generated for each facility defect, and the diagnostic system verification that is capable of simultaneously measuring pulses and PD patterns using various sensors is performed, and it is necessary to develop a PD pulse generator based on simulated PRPS data in order to generate the pulses based on PRPS pattern using the existing PRPD pattern, and input the pulses into the system to perform the defect judgment performance test. To solve this problem, this study developed and verified a PD pattern generator that can generate pulses based on PRPS data, and an algorithm that converts existing PRPD pattern data currently owned by Korea Electric Power Corporation (KEPCO) to PRPS pattern data
A Novel Approach of Critical Span Analysis
Budapest University of Technology and Economics, Hungary
The maintenance of operational safety and reliability are the major challenges dur-ing the operation of electrical network both in distribution and transmission sys-tem. New technical trends like the spreading of distributed generation or the inte-gration of renewable energy sources are also raising the extend of these challeng-es, while the adequacy to the market trends and consumer satisfaction also should be satisfied.
Dynamic line rating (DLR) technology offers a cost-efficient solution for the optimal utilization of the transmission system through the monitoring of the pow-er lines. Therefore, not only the transmission capacity of the overhead lines can be increased, but system operators receive information about the actual state of the line. Although, there are different algorithms for ampacity calculation and conductor temperature tracking, nevertheless the building conception of the data providing sensors are less discussed in the international literature. The aim of this paper is to demonstrate the problems with critical span analyzation and review the possible solution for the optimal sensor placement to DLR system.