Oil Insulation System Research Articles

A Hybrid Estimation-Based Technique for Partial Discharge Localization

This article demonstrates the use of five different methods to estimate partial discharge (PD) location in an oil insulation system from noisy measurements. The measurements are obtained from three ultrasonic sensors located in three different places. The sensors map the PD location utilizing a nonlinear model. The estimation techniques used in this article are the extended Kalman filter (EKF), the unscented Kalman filter (UKF), the smooth variable structure filter (SVSF), the EK-SVSF, and the UK-SVSF. The last two filters use the combination of EKF or UKF with SVSF, respectively, to consider possible PD model uncertainty. The proposed integrated UK-SVSF algorithm achieves the following objectives. First, the use of the Kaman-based filter enhances the optimality of the filter to system dynamics and measurements noise. Second, the use of the UKF reduces the calculation complexity and errors by replacing the Jacobian calculation with statistical linearization. Finally, the use of the SVSF enhances the estimate’s robustness to model uncertainty. The experimental results verify the claim that the PD location estimate with minimum error is achieved by the UK-SVSF.

Multilayers oil and oil-impregnated pressboard electric field simulation based on space charge

Space charge build-up has been considered as the major issue in the development of HVDC apparatus such as the converter transformer. The existence of space charge can distort the local electric field, which could lead to the degradation and even breakdown of insulation materials. Therefore, it is vital to investigate factors that can affect space charge formation and dissipation characteristics such as the temperature, moisture, ageing, thickness, multi-layered structure and electric fields. This paper mainly focuses on the effect of multilayers and thickness on space charge behaviour of oil and pressboard insulation system. Space charge was measured using the pulsed electroacoustic technique (PEA) method. The space charge results are quantitatively analysed to establish the relationship between interfacial charge density and different pressboard and oil thickness ratios. A new space charge interpolation methodology is utilized to input space charge into the multilayers oil and pressboard model using COMSOL software. The local electric field of multilayers oil-impregnated pressboard and oil could be simulated, with the emphasis on the electric field after the polarity reversal operation. From space charge results, they indicate that the increased thickness of pressboard could prohibit the interfacial charge increase while the increased oil thickness could facilitate interfacial charge increase. Moreover, from the electric field simulation results, they indicate that there is the electric field gradient caused by the space charge for multilayers oil and oil-impregnated pressboard structure. After the polarity reversal, the maximum electric field of the oil caused by the space charge is higher than electric field calculated based on the Maxwell-Wagner theory.

Accurate Estimation of Partial Discharge Location using Maximum Likelihood

In this article, a novel algorithm for high-accuracy partial discharge (PD) localization in an oil insulation system is proposed. This study aims to identify the statistics of the dynamics and measurement noise sequences in a PD localization system and employ that information in realizing better estimates. An extended Kalman filter (EKF) is used to estimate the PD location. The performance of the filter is enhanced by identifying the true statistics of the noise sequences using a maximum likelihood estimation approach. The accuracy of the proposed optimal algorithm is verified experimentally by estimating the PD location in an oil insulation system under two experimental settings.

Ultrastable Natural Ester-Based Nanofluids for High Voltage Insulation Applications.

Nanofluids for high voltage insulation systems have emerged as a potential substitute for liquid dielectrics in industrial applications. Nevertheless, the sedimentation of nanoparticles has been so far a serious barrier for their wide and effective exploitation. The present work reports on the development and in-depth characterization of colloidally ultrastable natural ester oil insulation systems containing iron oxide nanocrystals which lift the problem of sedimentation and phase separation. Compared to state-of-the-art systems, the final product is endowed with increased dielectric strength, faster thermal response, lower dielectric losses (decreased dissipation factor: tan δ), and very high endurance during discharge stressing. The developed nanofluid was studied and compared with a similar system containing commercial iron oxide nanoparticles, the latter demonstrating extensive sedimentation. Herein, the dielectric properties of the nanofluids are analyzed at various concentrations by means of breakdown voltage and dissipation factor measurements. The characterization techniques unequivocally demonstrate the high performance reliability of the reported nanofluid, which constitutes a significant breakthrough in the field of high voltage insulation technologies.

Toward High-Accuracy Estimation of Partial Discharge Location

In this paper, high-accuracy estimation of partial discharge (PD) location in an oil insulation system is addressed. The study aims at forming a fault-tolerant PD localization system. Initially, the algorithm periodically and probabilistically checks for possible bias in the acoustic emission sensors’ measurements. Once the detected bias is removed from the sensors’ measurements, multiple-model extended Kalman filters are used to estimate the location of the PD. The proposed multiple-model approach is intended to compensate for increased measurement noise statistics that result from sensor aging or ambient noise. The accuracy of the proposed algorithm is verified using multiple experimental results when the PD is initiated in different locations within the oil-filled transformer tank under different working conditions.

Assessment of paper-oil insulation under copper corrosion using polarization and depolarization current measurements

This paper discusses the effect of temperature on polarization and depolarization current measurements on paper oil insulation system. The influence of copper sulphide formed due to corrosive sulphur in mineral oil, on the changes in polarization and depolarization current at different temperatures are considered. A significant increase in the magnitude of polarization and depolarization currents due to deposits of copper sulphide on paper layers is observed. The conductivity of paper insulation increases significantly with increase in temperature. The increase in current is attributed to thermally active conducting particles of copper sulphide. The Debye model parameters of equivalent circuit under copper corrosion have been computed and a model for failure due to copper sulphide is explained using the data on polarization and depolarization currents.

A Novel Bias Detection Technique for Partial Discharge Localization in Oil Insulation System

This paper presents a sequential fault detection and identification algorithm for detecting a bias fault in the measurements of partial discharge (PD) in a transformer insulation system using acoustic signals. The algorithm identifies a bias fault in any of the ultrasonic sensors by computing the probability of having that bias fault given a carefully constructed measurement residual. The constructed measurement residual is a function of the measurement noise and the possible measurement fault. A set of bias hypotheses is assumed and initially given equal alarm probability. It is assumed that only one sensor will acquire a bias at any given time. Once the probability of a hypothesis approaches 1, that hypothesis is declared as the correct hypothesis and the bias associated with the hypothesis is removed from the sensors’ reading. Therefore, this will enable high-accuracy estimation of the location of PD. First, the algorithm is verified in a simulation environment. Subsequently, the accuracy of the proposed algorithm is verified using the experimental results.

Understanding and analysis on frequency dielectric parameter for quantitative diagnosis of moisture content in paper–oil insulation system

The ageing rate of cellulose insulation materials (paper/pressboard) in oil-impregnated transformers is largely determined by moisture. In this paper, several experiments were conducted under controlled laboratory conditions to quantitatively diagnose the amount of moisture in cellulose insulation using frequency domain spectroscopy (FDS) technique, which has been proven to be an effective tool to predict the insulation condition in oil-impregnated power equipment. The frequency dielectric response in oil-impregnated pressboards, moisture content as well as test temperature, was monitored carefully, and a new dielectric characteristic parameter for the diagnosis of moisture content was proposed. In addition, the master curve technique was introduced to calibrate the temperature effect on FDS measurement results. These experiments revealed that a strong correlation exists between frequency dielectric parameter and moisture content in cellulose insulation. Such correlation can be utilised for better prediction of moisture content in paper–oil insulation. A modified assessment method for the diagnosis of moisture content in paper–oil insulation was investigated and its effectiveness was also demonstrated using a case.

Diagnostic expert system of transformer insulation systems using the acoustic emission method

The subject matter of this paper refers to the improvement of the acoustic emission (AE) method when used for detection, measurement and location of partial discharges (PDs) in oil insulation systems of power appliances. In particular, presents the basic assumptions and describe the individual elements of an expert diagnostic system, which uses an acoustic method to asset the state of the measured power transformer insulation, performed during their normal work in industrial conditions. The system will consist of four basic modules, i.e. measuring system, processing-analyzing system, knowledge base and classifier. These modules are characterized successively in Point 2 of the paper. Special attention is given to the description of the group of multiparametric descriptors characterizing the AE signals in the time, frequency and time-frequency domains, to descriptive statistics indexes and correlative parameters. Accordant descriptors make it possible, at strictly defined metrological conditions, to recognize basic PD forms that may occur in paper-oil insulation. In this way a catalogued knowledge base containing the socalled 'fingerprints' was created for basic types of high-voltage defects of insulation system.

Electric stress distribution in paper oil insulation due to sulphur corrosion of copper conductors

Abstract Failure of paper oil insulation due to formation of copper sulphide in transformer windings is a serious issue affecting the performance and life of large transformers. Though chemical effects leading to corrosion are well understood, quantification of electric stress and its correlation to formation and migration of copper sulphide has not been reported in literature. Hence electric stress distribution under copper corrosion using FEM simulation has been studied and discussed in this paper. The simulation results are compared and correlated to experimental findings. The study clearly points towards increase in electric stress in unaffected paper and non-uniform electric stress distribution in paper–oil insulation system. Implications of using enamel coating on copper conductor as a mitigation technique to combat copper corrosion is also discussed.

Application of wavelet analysis to acoustic emission pulses generated by partial discharges

The subject matter of this paper refers to the improvement of the acoustic emission (AE) method when used for detection, measurement and location of partial discharges (PDs) in oil insulation systems of power appliances. The detailed subject matter refers to the issues connected with the application of modern methods of digital processing of signals obtained during technical high-power measurements. The paper presents the results of measurements and analyses of the AE pulses generated in setups making the modeling of basic PD forms that can occur in oil insulations possible. The research concentrated mainly on the following types of PDs: point-plane, multipoint-plane, multipoint-plane with a layer of pressboard, surface, generated in gas bubbles and on the indeterminate-potential particles moving in oil. In each case, the acoustic pulses generated by PDs in both positive and negative half times of the voltage supplying the spark gaps under study were analyzed. The AE pulses measured were subject to the wavelet analysis. For each of the PD forms the continuous wavelet transform (CWT) was determined and the corresponding time-frequency distributions were drawn. Moreover, for the AE pulses generated by PDs of the surface and multipoint-plane types, the discrete wavelet transform (DWT) was calculated, the runs of the particular details for six frequency levels and the approximation run were presented. In reference to the DWT analysis for the particular details, the probability density functions (PDF) and the autocorrelation functions (ACF) were determined at the particular levels of decomposition. Also columnar diagrams showing the energy value that is transferred at the particular levels of decomposition were presented. Moreover, the energy density spectrum runs determined through a FFT were presented for comparison. The Morlet wavelet was used to determine the CWT, and the symlet wavelet was used to calculate the DWT. Next, the distributions obtained for the particular PD forms underwent a comparative analysis taking into account the voltage polarization. It was observed that different frequency structures occur for the particular PD forms. Also, a different time of coherent structure duration for the particular PD forms was observed.

Field experience with an on-line gas monitoring device for oil filled instrument transformers

Electrical breakdown in the paper oil insulation systems of high voltage oil filled apparatus can cause severe damage to the apparatus as well as to the surrounding equipment. These failures also cause a loss of revenue from unexpected interruptions. These factors led to requests from electric utilities for additional safety measures to monitor the health of the insulation systems in high voltage oil filled instrument transformers. The direction of these measures was to develop a monitoring device to detect the presence of gassing, an indication of a breakdown in the insulation system and then provide an alarm before the apparatus actually failed. An online gas detector was developed and introduced. The experience gained over the past 35 years has shown that this solution is minimizing the damage and the high costs associated with these types of failures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Study of samples of composite insulating system under electrical and thermal stresses

Factors affecting the performance of small composite samples are investigated, and the capabilities of the uniaxially oriented polyethylene (UOPE) tape/dodecylbenzene (DDB) oil insulation system are evaluated. The apparatus used to study the influence of the combination of electrical and thermal stresses on the performance of the samples is described. Experimental results are reported for both uniform and nonuniform electrode arrangements. The discharge energy/material damage tests showed that a satisfactory correlation between the total surface area damage of the tapes and the energy supplied by the discharge exists. This suggests that the measurement of total discharge energy is a very useful technique in evaluating electrical insulation and may prove useful as a diagnostic tool for predicting the remaining life of insulation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Ultrasonic Spectrum Signatures of Under-Oil Corona Sources

Ultrasonic techniques are useful for the detection and location of partial discharge or corona sources associated with HV power apparatus, particularly oil-filled power transformers. Measurements are commonly made using narrow-band transducers operating in the frequency range from ~20kHz to ~300 kHz, and occasionally at higher frequencies for which there is little published information. Because of the lack of available data on the spectra of ultrasonic emissions from discharges in oil-insulation systems, an investigation was initiated to determine if there was an optimum measuring frequency and whether discharges could be identified by their spectral characteristics. It was discovered that insulation voids and gap-type discharges, for example, have clearly identifiable ultrasonic frequency characteristics, and the actual physical size of voids may be estimated from their frequency spectra.