Power Transformer Insulation Research Articles

Estimation of paper moisture content based on dielectric dissipation factor of oil-paper insulation under non-sinusoidal excitations

Frequency Domain Spectroscopy is a popular non-invasive method that is widely used for condition assessment of oil-paper insulation in real-life power transformers. Conventional Frequency Domain Spectroscopy is done using sinusoidal excitation over a wide frequency range. In this work, a different approach has been taken to investigate some more possibilities of Frequency Domain Spectroscopy using non-sinusoidal excitation for a better condition assessment of oil-paper insulation. For that, triangular waveshape was chosen as non-sinusoidal excitations whose slope can be changed to vary the harmonic content and amplitudes of constituent components. Samples with preset moisture contents that emulate the oil-paper insulation of real-life transformer have been prepared in the laboratory for the assessment of insulation condition. For a comparative study, both sinusoidal and triangular excitations having same peak as well as time period are applied to the prepared samples under controlled environmental condition and corresponding dielectric response currents are measured. From the applied voltages and corresponding dielectric response currents, dielectric dissipation factors are calculated and compared. It has been observed that use of nonsinusoidal excitation provides better results than sinusoidal excitation in the estimation of paper moisture content. It has also been observed that among the applied non-sinusoidal excitations, a particular waveform provides more accurate information about the paper moisture content.

Investigation of characteristic parameters for condition evaluation of transformer oil-paper insulation using frequency domain spectroscopy

SUMMARY Frequency domain spectroscopy (FDS) is employed to evaluate the moisture content and aging state of oil-paper insulation in power transformers. Oil-paper insulation samples with different moisture content and aging states were primarily prepared in the laboratory. A method was proposed based on a study of the influence of the aging state and moisture content on the frequency domain dielectric property of oil-paper insulation. This method distinguishes the influence of moisture content and aging state on FDS through the imaginary part of complex relative permittivity (er″). Relevant characteristic parameters that characterize the moisture content and aging state of oil-impregnated pressboards were also extracted. Results reveal that the extracted characteristic parameters are sensitive to moisture content in the range of 10−1 Hz to 101 Hz and in the range of 10−3 Hz to 10−1 Hz for the aging state. An obvious exponential relationship exists between the maximum values of the characteristic parameters in the given frequency ranges and degree of polymerization and moisture content. The new method can be utilized for a non-destructive condition assessment of oil-paper insulation of transformers. Copyright © 2014 John Wiley & Sons, Ltd.

Aging rate of grade 3 presspaper insulation used in power transformers

Paper materials are used as the insulation of power transformers. Over time these materials slowly degrade, until they reach a point when they no longer function effectively as transformer insulation, and the transformer has then reached its end of life. The aging rate of paper is affected by temperature, water, oxygen and acids. Investigations have been performed previously on Kraft and Kraft thermally upgraded types of paper. However, a different type of paper used in transformer insulation, Grade 3 presspaper (which contains cotton), has not been extensively tested and compared to Kraft paper. In these experiments we studied the aging rate of Grade 3 presspaper and compared it to our previous studies of Kraft paper. Traditionally, the aging rate of paper has been studied in sealed vessels. The problem with this approach is that the chemical environment within the vessel will change during aging, and so the aging rate will be affected. In our experiment setup we controlled the water and oxygen content to more accurately determine the aging rate. Similarly to Kraft paper, the aging rate of Grade 3 presspaper with the same water content increased with oxygen content in the oil. Life curves were developed based on the water content of the paper and the oxygen content of the oil.

Improvement of power transformer insulation diagnosis using oil characteristics data preprocessed by SMOTEBoost technique

This paper proposes a novel method for power transformer insulation assessment using oil characteristics. A hybrid algorithm, named as SMOTEBoost is implemented in the paper to improve the diagnosis accuracy and consistency. The SMOTEBoost can significantly enhance the generalization capability of artificial intelligence (AI) algorithms for transformer insulation diagnosis. This will provide important benefits for applying AI techniques in utility companies, i.e., an AI algorithm with its model built upon on a local dataset can be utilized globally to make transformer insulation diagnosis. The SMOTEBoost adopts Synthetic Minority Over-sampling Technique (SMOTE) to handle the class imbalance problem, in which data points belonging to different fault types or insulation conditions are unevenly distributed in the training dataset. By using this boosting approach for reweighting and grouping data points in the training dataset, the SMOTEBoost facilitates AI algorithms consistently attaining desirable diagnosis accuracy. To verify the advantages of SMOTEBoost algorithm, it is integrated with a number of representative AI algorithms including support vector machine (SVM), C4.5 decision tree, radial basis function (RBF) network and k-nearest neighbor (KNN) to make transformer insulation diagnosis using various oil characteristic datasets collected from different utility companies. A statistical performance comparison amongst these algorithms is presented in the paper.

Condition assessment of cellulosic part in power transformer insulation using transfer function zero of modified debye model

Analysis of Polarization-Depolarization Current, recorded from high voltage oil-paper insulation using insulation model is common among researchers. It is reported that paper insulation of power transformers undergoes non-uniform aging. Unlike Conventional Debye Model (CDM), Modified Debye Model (MDM) is capable of modeling such non-uniform aging. However, factors like insulation geometry affect the values of the MDM branch parameters. Therefore, model parameterized using data obtained from one insulation system finds limited use in assessing the condition of a different transformer, even with similar loading history and power rating. The present paper shows that a performance parameter, which is less sensitive to insulation geometry, can be evaluated from Transfer Function, TFM(s) of MDM. The parameter is the zero Z1 of TFM(s) which is located farthest away from the origin in the Left Half Plane of s-plane. The capability of Z1 as an insulation condition sensitive parameter is first tested on laboratory samples and then on data recorded from several real life power transformers. Results obtained for these transformers show that there is a good correlation between magnitude of Z1 and paper moisture content obtained from Frequency Domain Spectroscopy (FDS) using IDAX 300.

Accelerating Dielectric Response Measurements on Power Transformers—Part II: A Regression Approach

Dielectric frequency response, also known as frequency-domain spectroscopy (FDS), was introduced about a decade ago as the preferred method to estimate the moisture content in the cellulose insulation of power transformers. A conventional FDS measurement is carried out as a frequency sweep down till 100 μHz, thus causing unavoidable large measuring time due to the very low frequency oscillations. Nevertheless, FDS is the favored choice for onsite measurements due to its robustness against noise. This contribution proposes a newly developed modification to the FDS measurement technique. In Part II, of a two-part series, we propose reducing the measuring time by measuring only a fraction of a complete oscillation. Regression techniques are then used to calculate the amplitude and phase shift of the fraction. The capacitance and dissipation factor (tanδ) are then calculated for each individual oscillation. The proposed advancement described in this part could reduce the measuring time by up to 90% compared to a conventional FDS.

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.

Pattern recognition techniques for power transformer insulation diagnosis-a comparative study part 2: implementation, case study, and statistical analysis

Transformer oil tests such as breakdown voltage, resistivity, dielectric dissipation factor, water content, 2-furfuraldehyde, acidity, and different dissolved gasses have been adopted in utility companies for evaluating the conditions of transformer insulation. Over the past 20years, various pattern recognition techniques have been applied for power transformer insulation diagnosis using oil tests results (oil characteristics). This paper investigates a variety of state-of-the-art pattern recognition algorithms for transformer insulation diagnosis. To verify the applicability and generalization capability of different pattern recognition algorithms, this paper implements 15 representative algorithms and conducts extensive case studies on eight oil characteristics datasets collected from different utility companies. A statistical performance (in terms of classification accuracy) comparison among different pattern recognition algorithms for transformer insulation diagnosis using oil characteristics is also conducted in the paper. Copyright (c) 2014 John Wiley & Sons, Ltd.

Advanced Technique for Moisture Condition Assessment in Power Transformers

The presence of moisture in a transformer needs to be monitored throughout its service life. Moisture deteriorates transformer insulation by decreasing both electrical and mechanical strength. High moisture content accelerates solid insulation aging, reduces the breakdown strength and makes the transformer vulnerable to the overload conditions due to high temperature spot bubbling. In addition to that, partial discharge can occur in a high voltage region because of the moisture disturbance. Traditional indirect estimation of the moisture concentration in the solid insulation of power transformers includes testing the oil samples as well as measurement of the insulation resistance and loss tangent (50/60 Hz) of the transformer. However, these methods usually give limited information and may lead to wrong conclusions. Direct measurement is not viable and may not be representative to take a paper sample from the surface insulation because moisture distribution is not homogeneous along the insulation geometry. Dielectric Frequency Response, DFR was introduced more than 20 years ago and has been thoroughly evaluated and proven. Several documents have been published summarizing the research work and field tests all over the world. DFR is a practical non-intrusive and non-destructive technique for moisture condition assessment in power transformers, a breakthrough compared with traditional methods. Scientists, researchers and utility operators have shown great interest in the development and application of DFR technique. In this paper, the limitation of traditional methods is presented at first. Later, a comprehensive review of DFR method demonstrates its advantages over traditional methods. Finally, latest research about the mathematical model, temperature correction and test voltage is included to answer common questions regarding the application of DFR method.

Feasibility Study on the Use of Vegetable Oil (Natural Ester) in Malaysia Power System Transformers

Power transformer insulation oil is critical to the useful life of the transformer. Although mineral base insulation oil is the most common type of transformer insulation oil in use, environmental and safety concerns has encouraged the development of biodegradable alternatives. Today, <TEX>$Envirotemp^{(R)}\;FR3^{TM}$</TEX> is in commercial use and this study is aimed at ascertaining the possibility of applying the oil in Malaysia power transformers. A sample of <TEX>$Envirotemp^{(R)}\;FR3^{TM}$</TEX> was tested to measure and compare the technical (including electrical, chemical and physical) properties of the oil according to Malaysian standards. The study found that the oil sample had better qualities, such as higher dielectric strength, lower dissipation factor, higher flash and fire points, higher moisture absorption capability, and less dissolved gases composition amongst others. However, it was also ascertained that further development in this area could be hindered due to Malaysia's lack of standards for biodegradable oil.

Pattern recognition techniques for power transformer insulation diagnosis-a comparative study part 1: framework, literature, and illustration

The condition of the insulation system of a power transformer has a significant impact on its overall reliability and serviceability. Transformer oil tests including breakdown voltage, acidity, dielectric dissipation factor, 2-furfuraldehyde, water content, and dissolved gases analysis have been commonly performed in utility companies to provide information regarding the conditions of transformer insulation. Over the past two decades, various pattern recognition techniques are proposed to interpret the oil tests results and make diagnosis on transformer insulation. However, there are still considerable challenging issues to be investigated before the pattern recognition technique can become a ready-to-use tool at utility companies. This paper provides a comparative study of pattern recognition techniques for power transformer insulation diagnosis using oil tests results. A general pattern recognition application framework will be outlined in the paper. And a comprehensive literature review on various pattern recognition techniques for transformer insulation diagnosis will be provided in the paper. The important issues for improving the applicability of pattern recognition techniques for transformer insulation diagnosis will also be discussed. A case study will be presented to demonstrate the procedure of applying pattern recognition techniques to practical transformer insulation diagnosis using oil test results. Copyright (c) 2014 John Wiley & Sons, Ltd.

Prediction of moisture present in cellulosic part of power transformer insulation using transfer function of modified debye model

Polarization-Depolarization Current (PDC) analysis is a well known technique for condition monitoring of oil-paper insulation in power transformers. However, due to variation in insulation geometry, information obtained through analysis of PDC measured for one transformer cannot be used for predicting condition of another transformer even if the loading history of the two units are similar. Furthermore, parts of solid insulation closer to winding are exposed to much higher temperature than the parts away from it. Prolonged exposure to this temperature variation leads to nonuniform aging in cellulosic parts. It is reported that Modified Debye Model (MDM) is capable of modeling this non-uniform aging. In the present work it is shown that a parameter sensitive to insulation condition can be obtained from Transfer Function of MDM. Several laboratory samples having different physical dimensions has been constructed. PDC data recorded from these samples are used to obtain the performance parameter (which is less influenced by insulation geometry) and its relation to paper moisture content. The results obtained from the proposed method have been compared with that obtained using Frequency Domain Spectroscopy (FDS) based commercial instrument (IDAX 300) in the case of real-life power transformer.

Analysis of partial discharge characteristics of natural esters as dielectric fluid for electric power apparatus applications

Petroleum based mineral oils are generally used for electric power apparatus insulation and cooling applications. Since the conventionally used mineral oils are not biodegradable and difficult to decompose, there is a need for the development of alternate vegetable based insulation oils for such applications. Formation of partial discharges plays a major role in determining the life time of liquid insulation. Reports on partial discharge characteristics of thermal aged natural ester fluids for high voltage applications are scanty. The major aim of this research work is to understand the partial discharge characteristics of natural esters as a dielectric fluid for power transformer and other electric power apparatus insulation applications. Laboratory experiments are performed as per IEC (International Electro technical Commission) test procedures in order to understand the breakdown voltage and partial discharge characteristics of vegetable oils such as corn oil and palm oil under both virgin and thermal aged conditions. This paper presents a comparative assessment of the PD characteristics of thermal aged natural ester fluids with respect to mineral oil. Time and frequency domain analysis of PD pulse at both needle-plane and rod-plane electrode configurations are studied. Statistical analysis of PD pattern is also carried out. This analysis collects the typical PD patterns from natural ester fluids in order to find its suitability for power transformer and electric power apparatus applications.

Copper-in-oil dissolution and copper-on-paper deposition behavior of mineral insulating oils

Copper-in-oil dissolution and copper-on-paper deposition may affect the insulation of oil-filled power transformers, shunt reactors and high voltage bushings. Dissolved copper increases the dielectric losses in oil, and copper deposition can dramatically increase insulating papers' conductivity. The available literature does not report researches where the mechanisms of dissolution of copper in oil and copper deposition on the surface of solid insulation (Kraft paper) have been elucidated. This study is aimed at the investigation of the tendency of some commercially available unused mineral oils of different compositions to dissolve copper and to deposit it in the cellulose phase. Different conditions of reaction, presence of oxidation inhibitor (of the hindered-phenols family), addition of triazole-type compound (used as corrosion inhibitor) and general oil properties were studied to investigate their influence on copper concentration in oil and onto paper after artificial aging in controlled conditions.

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.

Applying the Recovery Voltage Method (RVM) to Study the Degradation of High Power Transformer Insulation

Good operation of power transformers is very important because they are among the most expensive equipment in the network generation, transmission and electricity distribution. It became necessary regular monitoring of such systems and determinate the life of a transformer, it preventing breakdown and corresponding loss occurrence.Ageing in the insulation system of power transformers is determinate by influence of air, moisture, temperature, mechanical and electrical stresses and insulation contamination. One of the most important ageing indicators in transformers is the water content in the solid part of the insulation. The water reduces drastically the dielectric strength of solid insulation, accelerate ageing in paper and can cause water vapor bubbles.In RVM method a voltmeter determines the recovery voltage (RV) after charging the insulation with a DC voltage. By subsequent relaxation and repeated charging for varied times the so called polarization spectrum can be created. RV range provides an indication on the condition in which there is the insulation of transformer. The paper analyzes measurements on two high power transformers in operation, determining the moisture content of solid insulation.

GC Methods for the Determination of Methanol and Ethanol in Insulating Mineral Oils as Markers of Cellulose Degradation in Power Transformers

Methanol and ethanol in transformer oils have been recently proposed as new markers of thermal and mechanical degradation of cellulose (the solid insulation in power transformers). In this work, we optimized and compared the performance of two headspace gas chromatographic methods based on flame ionization (HS–GC–FID) and mass spectrometry detection (HS–GC–MS) to determine methanol and ethanol in insulating mineral oil. For methanol and ethanol, the detection limits were 12 and 27 μg kg−1 (HS–GC–FID) and 1.3 and 3.1 μg kg−1 (HS–GC–MS). Repeatability was evaluated in transformer oils for both the methods at different concentration levels of analytes and RSD values were found to lie between 1.8 and 16 %. The accuracy of the methods was assessed under a proficiency test (Cigré JWG A2/D1.46). The methods were compared by a F-test and a one-sided paired t test performed on 21 transformer oils in service. Correlations of methanol and ethanol content in sampled oils against their actual time of service are provided. For each sample, the content of traditional markers (furan-2-carbaldehyde and CO2) was also measured, finding a correlation between light alcohols and CO2 content. This indicates that methanol and ethanol determination may be helpful in providing further information on the thermal degradation conditions of transformers’ solid insulation. The method developed is currently routinely applied by the laboratories of Sea Marconi Technologies for the assessment of transformers’ conditions.

Moisture aided degradation of oil impregnated paper insulation in power transformers

Moisture aided degradation of oil impregnated paper insulation in power transformers

Mathematical Model and Regression Analysis of Acoustic Emission Signals Generated by Partial Discharges

An improved mathematical model describing acoustic emission (AE) signals generated by different types of partial discharges (PD) that occur in electric power transformer insulation system is presented in the paper. AE signals are analyzed within the AE method as applied for power transformer failure detection due to occurrence of PD. There are several types of basic defects, which are characterized by different types of PD. The mathematical model presented here is crucial for numerical analyses and simulations, where it acts as the function describing the acoustic source in an acoustic model of power transformer insulation system. The regression procedure was performed based on empirical AE signals, registered in a laboratory experiment. The AE signals are described by a mathematical model being a multi-parameter function, which involve both the time domain and the frequency domain. Goodness of the model was evaluated based on analysis of 480 data samples in the time, frequency and time-frequency domains. Also coherence between the registered and modeled signals was calculated. It was stated that the improved model fits very well to the real data, although, due to high level of noise embodied in signals registered in experiments, the coherence values remain low. Moreover, analyses of the estimated data were performed and some example results are presented in this paper. Based on the achieved outcomes a collection of parameter values was prepared for each of the eight considered PD basic types. One can simple use it now in a numerical model for simulation of AE signal source generated by specified type of PD, what corresponds to a particular power transformer insulation system failure. Furthermore, the regression procedure presented in this paper can be easily transferred to any other types of AE sources including processes of compression, tension and cracking.

A new approach for determination of moisture in paper insulation of in-situ power transformers by combining polarization-depolarization current and return voltage measurement results

Moisture content in an oil-paper insulation system of power transformer accelerates the aging process of cellulose that degrades insulation properties. As a result, dielectric response measurement data is also affected by moisture present in insulation. In time domain these measurements include Return Voltage Measurement and Polarization-Depolarization Current measurement. In addition it is a known fact that power transformer insulation consists of complex structures which is difficult to model in laboratory. Therefore, results obtained from samples prepared under controlled environment may not be applicable to real life power transformer. The present paper discusses a new approach to combine the results obtained from these two methods namely, Return Voltage Measurement and Polarization-Depolarization Current measurement for several power transformers, to find the relation between oil and paper moisture content that is relevant to insitu power transformers.