Search

Application of low pour point esters in transformers insulation serving in cold climatic regions is still a challenge. To address this concern, the pre-breakdown and breakdown analyses of thermally aged low pour point ester fluids are investigated. The breakdown measurements are performed at different aging conditions under AC stress. Needle-plane electrode geometry (with varying radii of the tip) is employed since this configuration is necessary for any practical diagnostic based on partial discharge (PD). This allows studying the effect of the chemical impurities (aging by-products) as assessed by some physicochemical properties (acidity, particles count, turbidity, and dissolved decay products) on the partial discharge and breakdown characteristics. The complete analysis is carried out for liquids having the pour points less than -50 °C, including a mineral oil (MO) and two synthetic ester liquids (SE1 and SE2). Changes in the streamer properties, such as partial discharge inception voltage (PDIV), breakdown-streamer inception voltage, and breakdown voltage are reported as a function of the needle tip radius and aging condition. It is observed that the breakdown properties of SE1 are better than that of SE2 and are comparable to MO. To understand the influence of aging on the PD, correlations between decay products, acidity, BSIV, and PDIV are reported. It is observed that the acidity and concentration of decay products directly impact the PD characteristics.

Power transformers represent one of the most abundant and expensive components in the electric power industry. Dissolved gas analysis (DGA) of transformer is the most widely accepted diagnostic tool across the globe to understand insulation incipient failures. Nevertheless, DGA fault gas interpretation is a remarkable challenge for transformer owners and utility engineers. Several computational techniques have been adopted for DGA fault classification along with offline methods. However, limited data availability, high ambiguity in DGA interpretation, suitability, and model accuracy are critical challenges in the DGA fault classification using computational techniques. In this work, highly diverse and large DGA data samples of in-service transformer fleets from five different utilities have been used to develop an efficient fault classification methodology. A total of 4580 DGA samples and IEC TC 10 database are used for training and testing, respectively, for various machine learning algorithms. Discussions on performance indicators and evaluation of several algorithms to verify the most suitable class algorithms are also the focus of this work. Furthermore, a best-performing model is identified based on various performance indicators. The hyperparameters of the best model are further tuned to achieve a most precise fault classification. It is inferred that non-parametric methods and non-linear SVM are best suitable for transformer DGA fault classification. Importantly, the rankings in the present study suggest that transformer DGA fault prediction is better with ensemble learning methods.

In the last decades, a large focus is being placed on the sustainability and safety of the power transformer spectrum. Ester liquids, which have interesting properties such as high fire point and biodegradability, are gaining needed attraction. Since in-service condition, thermal aging deteriorates the physicochemical and electrical properties of liquid dielectrics, it is important to study their long-term behavior. In this contribution, the pre-breakdown and breakdown behavior of ester fluids (synthetic and natural) under AC stress are investigated. Important characteristics, such as partial discharge pre-inception voltage, partial discharge inception voltage, breakdown voltage, average streamer velocity, and inception electric field, were assessed. The influence of the radius of curvature (of high voltage needle electrode) as well as the thermal degradation of typical ester liquids are also discussed. Mineral oil was also included in the tests loop as a benchmark for comparative purposes. It is found that the pre-inception voltage of ester liquids was, in most cases, higher than that of mineral oil. For a given radius of curvature, the streamer inception and breakdown voltages decreased with thermal aging. During the streamer initiation, the electric field at the electrode tip decreased with the increase in the radius of curvature. The velocity of the streamers seems to increase with the decrease in the radius of curvature. The period of vulnerability, the so-called “delay time”, seems to be independent of the aging or the radius of curvature for a given condition of the liquid.

This article proposes a robust Lyapunov’s demodulator (LD) based on an orthogonal signal generation (OSG) approach for single-/three-phase applications. The LD is not normally capable of rejecting the dc-offset in the grid signal, and an additional estimation loop is, therefore, required, which may affect the dynamic performance. Nevertheless, an application of a harmonically polluted grid voltage signal to the LD may severely affect the steady-state performance of the parameters estimated. These issues are addressed by proposing an enhanced LD-based-OSG in which a moving average filter is incorporated into the LD-OSG structure. The rapid rejection of dc-offset and harmonics is, therefore, easily achieved without any additional loop. The proposed structure can accurately estimate the fundamental in-phase and quadrature components. However, these orthogonal components may suffer from amplitude imbalance and errors in the phase information under off-nominal frequency conditions. Nevertheless, the errors in the amplitude and the phase information are eliminated using an open-loop frequency deviation detector and a feed-forward curve-fitting approach. The dynamic performance of the proposed scheme has been validated by means of numerical and hardware studies whose results show that, with the less sensitivity toward the phase angle jump and good immunity to the fundamental negative sequence, the proposed scheme is a potential technique with which to synchronize single-/three-phase grid-connected power electronic equipment.

Natural ester dielectric fluids are being used as alternatives for mineral insulating oils in oil-filled power apparatuses. Although partial discharges (PDs) are recognised as one of the main causes of degradation of dielectrics used in power equipment, their characteristic voltages appear to be temperature-dependent. In this contribution, the pre-breakdown and breakdown behaviour of two natural ester fluids, extracted from canola oil (NE1) and soybean oil (NE2) is reported. To assess the effectiveness of the temperature dependence, −5, 0, and + 20°C have been identified as test temperatures. It was found that the different pre-breakdown and breakdown characteristic voltages increase with increasing temperature for both types of esters. The delay time is always longer for canola oil when compared to soybean oil regardless of the test temperature. The analysis of the average propagation velocity of NE1 and NE2 indicates that PDs propagate faster in NE2 than in NE1. The field strength at the tip of the needle electrode during the inception of the streamers leading to breakdown indicates that streamers in both oils are propagating in the slow mode. It is found that the temperature has a significant influence on the PDs’ parameters for both fluids.

In this article, two synthetic esters along with a natural ester are investigated to monitor the generation of colloidal and dissolved decay particles under thermal aging. The thermal aging of the oils is carried out as per a modified ASTM D1934 in the presence of cellulose and copper at elevated temperatures. Later, the colloidal and dissolved decay particles are monitored as per ASTM D1698 (centrifuge treatment) and ASTM D7150 (Fuller's earth filtration), respectively. The aged and treated dielectric liquids are characterized for the number of decay particles, oil absorbance curves, the concentration of dissolved decay content, and turbidity as per ASTM standards. The evolution of colloidal and dissolved decay contents is individually reported for ester fluids and mineral oils. The degradation in mineral oils has been witnessed by both dissolved and colloidal particles. The generation of colloidal particles following the degradation of ester fluids is almost negligible, whereas dissolved decay content is noticed with thermal degradation time. Importantly, the obtained results depict the ability of esters to dissolve more sludge. This is due to the high (oxidative) stability of ester leading to reduced radical decomposition products. Also, the temperature solvent nature of the ester group tends to dissolve sludge particles back to the liquid phase into the bulk liquid.

In this Letter, a demodulation based pre-filtering technique is proposed for fast detection of the single-phase grid variables. The estimation of grid voltage parameters is achieved through a Type-I synchronous reference frame phase-locked loop (PLL). The modified bandpass filter in the pre-filtering stage ensure a better rejection of the DC-offset and harmonics. Thus, the grid variables are estimated rapidly with the help of a simple proportional controller. Consequently, in-loop filtering burden and the effort required to tune the PLL is minimal. However, steady errors are witnessed in the fundamental amplitude and the phase information which are mitigated by a curve fitting technique. Importantly, the proposed PLL scheme has improved the dynamic performance under off-nominal frequency conditions with immunity against grid disturbances. The robustness of the proposed algorithm is experimentally verified vis-a-vis existing literature.

Transformers are generally considered to be the costliest assets in a power network. The lifetime of a transformer is mainly attributable to the condition of its solid insulation, which in turn is measured and described according to the degree of polymerization (DP) of the cellulose. Since the determination of the DP index is complex and time-consuming and requires the transformer to be taken out of service, utilities prefer indirect and non-invasive methods of determining the DP based on the byproduct of cellulose aging. This paper analyzes solid insulation degradation by measuring the furan concentration, recently introduced methanol, and dissolved gases like carbon oxides and hydrogen, in the insulating oil. A group of service-aged distribution transformers were selected for practical investigation based on oil samples and different kinds of tests. Based on the maintenance and planning strategy of the power utility and a weighted combination of measured chemical indicators, a neural network was also developed to categorize the state of the transformer in certain classes. The method proved to be able to improve the diagnostic capability of chemical indicators, thus providing power utilities with more reliable maintenance tools and avoiding catastrophic failure of transformers.

Ester-based dielectric fluids have gained widespread popularity for applications in high voltage apparatus. Synthetic and natural esters have been subjected to research for decades vis-à-vis mineral insulating oils around the world. Although many researchers favor the application of ester fluids, utilities are still uncertain and application of these alternatives remains a challenge. The intent of this survey is to present recent research progress and highlight the state of the art of key aspects that should be emphasized in future research. The contemporary research scenarios pertaining to the performance of ester fluids versus mineral oils, miscibility, and retrofilling of insulating fluids are discussed. In addition, pre-breakdown phenomena, usage of esters in on-load tap changers, environmental and fire resistance properties, and use of esters in cold climates are also discussed. Importantly, challenges and future aspects that should be investigated to improve the existing knowledge of ester dielectric fluids for applications in transformer technology are highlighted.

Study of alternate fluids for transformer and their combination with traditional ones is an interesting area of research. The intent of this work is to investigate the thermal ageing performance of ester oils (synthetic and natural), and mixed oil vis-a-vis mineral oil. All the samples were thermally stressed at 110, 140, 160, and 185°C for 2 weeks each. Dissolved decay contents in oil and linear thermal expansion coefficients of solid insulation have been studied using ultraviolet visible infrared spectroscopy and dilatometry analysis, respectively, as per ASTM standards. Change in functional groups of the insulation oils with ageing have been understood by Fourier transform infrared spectroscopy analysis. Additionally, diffusion of oil-moisture mixture into paper with ageing and the corresponding effect on dielectric strength of paper has been also examined using ASTM standards. It is observed that the use of synthetic ester (SE) and addition of SE to mineral insulating oil lead to improved performance of oil-paper insulation system with retarding the rate of degradation of the insulation system.

In this work, thermal aging aspects of mineral oil, ester oils (synthetic and natural), mixed oil have been reported. All the samples were thermally stressed at 110°C, 140°C, 160°C, and 185°C for 2 weeks each. Concentration of dissolved decay contents in oil, flash point of insulation oil, and linear thermal expansion coefficients of solid insulation with aging have been studied using ultraviolet visible spectroscopy, flash point and dilatometry analysis respectively. Change in functional groups of the insulation oils with aging have been understood by Fourier Transform Infrared Spectroscopy analysis. Additionally, diffusion of oil-moisture mixture into paper with aging and corresponding effect on dielectric strength of paper has been also examined. Further, design and monitoring aspects of ester and mixed oil filled transformers have been also emphasized. It is observed that ester dielectric fluids and ester content as an additive in mineral oil improves the performance of solid insulation. Importantly, blended oil offer an advantage of retarding the degradation of oil as well as solid insulation accompanied by reduced cost of oil when compared to pure synthetic esters.

International audience; In view of the contemporary scenario and importance of alternative liquid dielectrics, IEEE DEIS Technical Committee on Liquid Dielectrics laid emphasis on pre-breakdown phenomena of ester dielectric fluids. The present article reports the background knowledge on the breakdown theories and behavior of streamers in insulating oils. The major influencing properties of the streamers aroused in liquid insulating systems are discussed keeping in view of the ester based dielectric fluids. Importantly, modelling of streamers, testing approaches, and review of pre-breakdown phenomena in ester dielectric fluids has been presented.

Operational factors are known to affect the health of an in-service power transformer and to reduce the capabilities and readiness for energy transmission and distribution. Hence, it is important to understand the degradation rate and corresponding behavioral aspects of different insulating fluids under various fault conditions. In this article, the behavior of mineral oil and two environmentally friendly fluids (a synthetic and a natural ester) are reported under arcing, partial discharges, and thermal fault conditions. Arcing, partial discharges and thermal faults are simulated by 100 repeated breakdowns, top oil electrical discharge of 9 kV for five hours, and local hotspots respectively by using different laboratory-based setups. Some physicochemical properties along with the gassing tendency of fresh and aged insulating liquids are investigated after the different fault conditions. UV spectroscopy and turbidity measurements are used to report the degradation behavior and dissolved gas analysis is used to understand the gassing tendency. The changes in the degradation rate of oil under the influence of various faults and the corresponding dissolved gasses generated are analyzed. The fault gas generations are diagnosed by Duval’s triangle and pentagon methods for mineral and non-mineral oils. It is inferred that; the gassing tendency of the dielectric fluids evolve with respect to the degradation rate and is dependent on the intensity and type of fault.

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 ℃. 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 to that of the mineral oil.

This study reports experimental study of cellulose performance in mixture of mineral and synthetic ester (SE) oils. Solid insulation impregnated with mineral oil (MO) and mixture oil has been subjected to accelerated thermal stresses to understand the deterioration rate of cellulose. Diffusion of oil-moisture mixture into paper and corresponding variation in dielectric strength of paper has been examined. Compatibility of solid insulation with MO and mixture oil is analysed by thermal expansion coefficients, X-ray diffraction analysis and dissolved decay products. Furthermore, chemical functional groups present in aged oils are identified by Fourier transform infrared spectroscopy analysis. It is observed that the performance of cellulose insulation is improved with addition of SE to MO. This is achieved by higher water solubility of esters, which enables higher moisture consumption during hydrolysis and keeps cellulose dry.

A critical survey has been conducted on high energy-efficient bidirectional converters, various topologies that effectively meet the automated vehicle requirements, and 24 GHz/77 GHz low-profile antennas (for automotive radar applications). The present survey has been identified into two parts on the current topic of study as perspectives and challenges. Part 1 of this survey covers energy-efficient power electronic convertor topologies and condition monitoring aspects of convertors to enhance the lifespan and improve performance. Condition-monitoring issues concerning the abnormalities of electrical components, high switching frequencies, electromagnetic interference, leakage currents, and unwanted joint ruptures have also been emphasized. It is observed that composite converters are proficient for automated hybrid electric vehicles due to fast dynamic response and reduced component count. Importantly, electrical component failures in power electronic converters are most common and need attention for the effective operation of the bidirectional converters. Hence, condition monitoring implementation schemes have also been summarized. Part 2 of this survey focuses on 24 and 77 GHz low-profile (microstrip-based) antennas for automotive radar applications, types of antenna structures, feed mechanisms, dielectric material requirements, design techniques, and performance parameters. The discussion in Part 2 also covers feed methodologies, beam scanning concepts, and side-lobe levels on the autonomous vehicle communication activities.

Automated vehicles are becoming popular across the communities of e-transportation across the globe. Hybrid electric vehicles and autonomous vehicles have been subjected to critical research for decades. The research outcomes pertinent to this topic in the literature have been motivated by the industry and researchers to emphasize automated vehicles. Part 1 of this survey addressed the critical aspects that concern the bidirectional converter topologies and condition monitoring activities. In the present part, 24- and 77-GHz low-profile printed antennas are studied for automotive radar applications. These antennas are mounted on automated vehicles to avoid collision and are used for radio tracking applications. The present paper states the types of antenna structures, feed mechanisms, dielectric material requirements, design techniques, performance parameters, and challenges at 24- and 77-GHz resonating frequency applications. The recent developments in feed methodologies, beam scanning concepts, and the effect of sidelobe levels are addressed. Furthermore, the reasons behind the transition from 24 to 77 GHz are reported in detail. The recent advances in the application of various sensor schemes in an automated vehicle have also been discussed.

In this work mineral oil, natural ester, synthetic ester and a mixture of mineral oil and synthetic ester have been subjected to accelerated thermal stressing in presence of copper and cellulose. All these samples were subjected to 110°C, 150 °C, 170 °C and 200 °C thermal stressing for different test durations. Thermal degradation of the samples has been monitored continuously with UV Spectroscopy and visual inspections. Additionally interfacial tension of mineral oil and mixture oil has been monitored under the same test conditions to observe the miscible performance of the fluids. It has been found that the degree of degradation is less with synthetic esters than other samples. Interfacial tension and visual observations revealed less sludge formation in synthetic esters. Furthermore, aged insulation paper has been analyzed using X- ray diffraction technique to find the degradation rate of cellulose in various insulating fluids. It has been found that the rate of degradation is low in synthetic ester and performance of mixed oil is better than mineral oil.

Performance of a fuzzy expert system is related with how good the membership functions are normalized, tuned for a problem statement and correlation of antecedents and consequents. This paper helps in tuning and designing the membership functions that are best suited for the problem statement by integrating subtractive clustering method for fuzzy expert system design. Subtractive clustering algorithm is used to generate the tuned membership functions automatically in accordance to the domain knowledge. The proposed integrated design of clustering based fuzzy expert system acts in improving the accuracy and leads to a precised decision making environment. A practical example for ageing assessment of transformer insulation oil has been included to illustrate the method much effectively; the discussed example has been validated practically in the laboratory and is found that the proposed method is efficient in decision making.

Determining the current prevailing conditions in the transformer is still a challenge and an attempt is made for meeting the same challenge. In this paper a novel system based on fuzzy logic is developed which considers thermal, electrical and mechanical conditions prevailing in the transformer and gives the critical status of the transformer at an aggregate. The developed system also facilitates to give the critical condition of Paper thermal, Oil thermal, Partial discharge, Electrical arching, Dissolved gas analysis(DGA), Break down voltage and sweep frequency response analysis(SFRA) individually so that corresponding preventive measures can be taken by the technologists. This system consists of 10 fuzzy logic controllers whose connections are made by considering technical conditions and reasons, the corresponding rule bases of these fuzzy logic controllers were developed by considering various standards and experience of TIFAC CORE in NIT-Hamirpur. This fuzzy logic based system is tested for various transformers and found that it is highly précised in classifying the critical statues of any transformer as well as all identifying the current prevailing conditions.

Power transformers are to be monitored frequently to avoid catastrophic failures which are more or less related to internal faults for which many techniques and tools are developed, somehow many of these techniques rely on experts analysis and are well effected by environmental conditions which leads to misdiagnosing of the unit. In this paper a new fuzzy logic algorithm (FLA) based technique is developed which gives the vulnerability status of internal faults by considering thermal, electrical and mechanical conditions prevailing in the transformer and integrating them. This system takes a set of test results of dissolved gas analysis, break down voltage, and sweep frequency response analysis so that aliasing effects and misdiagnosing can be reduced at a glance. It also facilitates to give current prevailing condition of Paper thermal, Oil thermal, Partial discharge, Electrical arching, oil break down voltage, and mechanical deformations related with core and windings individually so that corresponding remedies can be taken by the technologists. This system consists of 10 fuzzy logic controllers and is connected by considering technical conditions and reasons, the rule bases of these controllers were developed by considering various standards and experience of TIFAC CORE in NIT-Hamirpur. This fuzzy logic based system is tested and found that it is highly precise in classifying the critical statues of any transformer, reducing misdiagnosing and aliasing effects and identifying the current prevailing conditions.

Diagnostics and proper monitoring of power transformer plays a key role in the life expectancy of proper transformer. Mineral oil in transformer is the inseparable component of the dielectric insulation system. Information about the health of the power transformer that can be use to plan cost, maintenance, renovation and operational criteria can be accurately interpreted using UVSpectrophotometer. As UV scan can only show the pictorial information of the age of the oil hence it is not advantageous in all aspects. In the present paper a decision tree method to determine the age of the transformer oil is introduced. The decision tree uses the UV/VIS spectroscopy absorbance values of the transformer oil which are in service at several locations. The decision tree method is designed so that the results of transformer oil can be examined quickly and automatically. The results obtained are compared with the UV/VIS spectroscopy for testing the accuracy of the decision tree and found that it is 100% accurate.