Degradation Of Insulating Oil Research Articles

Cross Capacitance Sensor for Insulation Oil Testing

On-site reliable transformer oil testing to assess the health of an oil-immersed transformer is a great challenge. Different sensors used for the purpose are having several limitations because of the oil and temperature. In this paper, a cross capacitance sensor cell is proposed for field testing of transformer oil which circumvents most of the limitations of thin-film chemical sensors used for the purpose. The cross-capacitance sensor cell based on the Thompson-Lampard theorem has been utilized for the first time to simulate a reliable and accurate sensor to determine degradation in insulating oil. The dielectric constant of the oil, which is determined for different oil samples experimentally, is an essential input for the finite element analysis of the sensor. Finite element analysis of the sensor cell is performed using ANSYS Maxwell software to determine its feasibility as a sensor to estimate the concentration of insulation degradation by-products dissolved in oil. The sensor is simulated to estimate the concentration of moisture and 2-FAL (2- Furfuraldehyde) in oil and to characterize aged oil. Finally, an experimental validation of the simulated sensor response for different fresh insulating oils is done. The experimental results closely match the simulation results. The sensor response is found to be approximately linear and very sensitive. Moreover, unlike thin-film chemical sensors, the same cross capacitance sensor can be used for the detection of different insulation degradation by-products dissolved in oil with proper calibration. Therefore, the proposed cross capacitance sensor cell can be used as a highly reliable on-site insulating oil degradation by-products detector.

Application of digital image processing to detect transformer bushing faults and oil degradation using FRA polar plot signature

Frequency Response Analysis (FRA) technique is commonly used to assess the mechanical integrity of power transformer core and windings. A few papers investigating the ability of FRA technique to detect bushing faults and transformer oil degradation can be found in the literatures. However, in all of these papers, only magnitude of the FRA signature along with visual analysis was used for fault identification and quantification which may lead to inconsistent conclusions for the same FRA signature when interpreted by various personnel. As such, there is an essential need to standardize and automate the FRA interpretation process to improve its reliability and accuracy. This paper is taking a step forward toward this goal through presenting a new FRA interpretation approach by incorporating the magnitude and phase angle plots of the measured FRA signature into one polar plot that comprises more features than the conventional magnitude plot currently used for fault diagnosis. Moreover, the proposed polar plot facilitates the application of digital image processing techniques (DIP) to automate and standardize the whole process. The new proposed technique is assessed through its application to detect minor transformer bushing faults and insulating oil degradations. In this regard, two transformers of different ratings, winding structure and physical dimensions are simulated using 3D finite element analysis to implement various levels of transformer bushing faults and oil degradation. The obtained FRA polar plots of the two investigated transformers under various bushings and oil health conditions are manipulated using the developed DIP codes to investigate the impact of each fault type / level on the proposed polar plot signature. Also, Practical measurements are performed to validate simulation results and evaluate the feasibility of the proposed technique. Results show the ability of the proposed technique to automate the detection of minor levels of bushing faults and insulating oil degradation with a high degree of accuracy.

Analysis of Partial Discharge Signal Characteristics of Nano-Mineral Oil for Transformer Condition Monitoring Applications

Transformer is an expensive and key component in an electric power transmission network. Most of the failure of transformer occurs due to degradation of insulation oil, which further leads to major economical losses and power shutdown. Advancements in nanotechnology has given horizons for the development of nanofluids for transformer insulation applications. Understanding the enhancements in dielectric properties of nanofiller added mineral oil is a hot research area. In recent times, partial discharge (PD) detection and analysis is a well recognized insulation condition monitoring technique for transformers. However, reports on PD signal characteristics of nano filler added mineral oil is scanty. This paper attempts to investigate the PD characteristics of SiO2 nanofiller added mineral oil at different %wt concentrations of nanofiller material. Major PD source normally experienced in transformers i.e. corona discharge due to non-uniform electric field was simulated in the laboratory using needle-plane electrode configuration and the corresponding PD signals were measured using wide band PD detection system. Important PD characteristics such as inception voltage, stable PD formation voltage, PD amplitude with respect to increase in voltage stress and PD signal frequency spectrum were evaluated. Results clearly show that addition of SiO2 nanofiller material in the base mineral oil enhances PD performance of mineral oil and these results will be useful for electrical utilities for developing effective condition monitoring system for transformers.

Assessing insulating oil degradation by means of turbidity and UV/VIS spectrophotometry measurements

Oil is a vital part of the transformer body and (similarly to blood in a human being body) keeps responsibility for the condition of the entire organism. Oil is particularly responsible for functional serviceability of the entire insulation system. The insulating oil must be kept in pristine condition, since its condition can be a decisive factor, which determines the life span of the transformer. Fields and laboratory experiences have shown that transformer oil contains a vast amount of information. Oil analyses can be extremely useful in monitoring the condition of power transformers. To meet pressing needs of power industries, fast, inexpensive and reliable laboratory testing procedures are necessary. To ensure long-term reliability of oil filled power transformers, it is important to identify early sign of degradation of the insulating oil. In this paper, oil degradation was monitored with various ASTM test methods. Investigations were performed on service-aged oil samples as well as on oil samples aged in laboratory conditions. Many key parameters actually used to monitor the condition of transformer oil relative to oxidation/degradation were investigated. From the obtained results, correlations were found between some of them. The results indicate that Dissolved Decay Products (DDP) and turbidity, which change with a higher rate than interfacial tension (IFT) and Acid Number (AN) values, can be possibly used as an effective index for insulating oil degradation assessment. Limits are suggested which provide a “picture” of the fluid condition.

변압기용 ONAF 방식 방열기의 팬 배치에 따른 냉각특성 연구

Owing to the trend of an increase in capacity and high-efficiency requirement, the life and reliability of power transformer depend significantly on the amount of heat generation per unit volume and the degradation of insulation oil. These problems can be solved by enhancing the cooling performance of the radiator. The purpose of this study was to find a suitable position of fans for a better cooling effect given by the forced-convection of air in an ONAF (Oil Natural Air Forced) type transformer. In the simulation, commercial software was used for flow analysis, and the cooling passage of the oil was simplified to shorten the time taken for computation. With the diameter of the fan fixed at a constant value, the analysis was performed only by changing the position of the fans. As a result, a vertical position change of the fans does not affect the cooling performance significantly. However, the temperature drop given by the fans positioned on the front region of the transformer is larger than that on the rear region.

Techniques to inhibit transformer insulating oil degradation - Effectiveness evaluation of the removal of degradation products using adsorbents

For long-term operation of power transformers, as well as ensuring their insulating reliability, an adequate study must be conducted on the age-related decline in various insulating oil characteristics and the method to inhibit the decline. In the present paper, to inhibit the degradation of insulating oil, a study was conducted on a method of removing the degradation products generated in insulating oil due to aging and the effect of removing these products. Consequently, for simulated degraded oil subjected to heat stress of 10 years old, degraded through the accelerated degradation of new oil, the performance of insulating oil subject to such accelerated degradation was improved through adsorption treatment and short-term improvement was able to be observed. Following reheating after the adsorbent treatment, a long-term improvement effect to prevent the decline in insulating oil characteristics due to degradation was able to also be observed. On the other hand, for the insulating oil aged about 30 years in the field, the effect was less than that for simulated degraded oil equivalent to about 10 years old. The effect of the adsorbent was able to vary depending on the amount used and the degree of insulating oil degradation. Accordingly, volume resistivity, representing the degree of insulating oil degradation, was varied to study the effect of the adsorbent with respect to its amount. As a result, it emerged that, as the amount of adsorbent was increased, the inhibitory effect on degradation increased but the effect was less significant for insulating oil with volume resistivity significantly decreased. Based on these results, it was clarified that, for aged oil with a low degree of degradation, an inhibitory effect on the short- and long-term decline in insulating oil characteristics was able to be obtained through adsorbent treatment.

Inhibition technique of transformer insulating oil degradation - evaluation of the effectiveness of oxidation degradation inhibitors

To operate power transformers long-term, as well as ensuring their insulating reliability, an adequate study must be conducted on the age-related decline of various insulating oil characteristics and the method to inhibit the same. Accordingly, in the present study, 1,2,3-benzotriazole (BTA) and 2,6-dibutyl-4-methylphenol (DBPC), which are expected to be effective in preventing oxidation degradation, were added to the test oil and an accelerated degradation test was conducted to examine their effect. Consequently, for new oil, an deactivating effect on copper by the addition of BTA was confirmed and the ability to prevent the decline in characteristics caused by the degradation of the dissipation factor and volume resistivity could be observed. For aged field oil, however, the effect was less than that for new oil. Since phenol compounds structurally similar to DBPC may have been present in aged field oil, a study was conducted on the increasing tendency of concentration and saponification value of the former. As a result, it emerged that the presence of phenol compounds inhibited the increase in the saponification value and boosted oxidation degradation inhibition, even if BTA or DBPC had not been added. Based on the above results, it was clarified that, where an oxidation degradation inhibitor was added or structurally similar compounds were contained, an inhibitory effect on the aging of insulating oil could be obtained.

Influence of trace components contained in transformer insulating oil on various characteristics over time

To operate power transformers long-term, as well as ensuring their insulating reliability, adequate consideration must also be paid to the age-related decline in various insulating oil characteristics. Previous studies could roughly clarify the key components causing the insulating oil characteristics to decline by adding the components contained in field transformer insulating oil to oil with few impurities. On the other hand, this degradation causing components are trace components not present in the new oil. To clarify the insulating oil degradation mechanism over time, the components determined as the sources of the degradation causing components (source components) must be investigated. Accordingly, in this paper, a study is conducted on the influence of the trace components present in new oil on the degradation of the characteristics over time. In addition, since the breakdown characteristics of insulating oil are influenced by the water content in oil, the influence of the combined effect of various components and water etc. is also studied. Consequently, it emerged that sulfide (octyl sulfide), or sulfur compounds detected in new oil increased the copper content in oil, showing a tendency toward an increasing dissipation factor and declining volume resistivity characteristics. Conversely, no influence of additives was observed on the increase in water content and the relationship between breakdown voltage and water content remained almost unchanged, regardless of the presence or absence of additives. Following the study of the change based on interaction with water content, no influence of the presence or absence of the additives was observed on the characteristics (acidity, interfacial tension, and breakdown voltage). Based on the above, it was clarified that, among the trace components existing in oil used in the present experiment, octyl sulfide had impact on the insulating oil characteristics over time.

Evaluation of the use of Near Infrared Spectroscopy (NIR) in on-line Monitoring of Power Transformer Insulation Oil

To maximize the lifetime expectance of power transformers, it is important to develop tools to monitor and to give diagnose information, allowing for real-time detection of incoming failures. Information from analysis of gases dissolved or free in insulating oil is one of the most valuable tools in evaluating the health of a transformer and has become an important part of preventive maintenance programs. The condition of the oil greatly affects the performance and the service life of transformers, and on-line monitoring has become a reliable method to ensure the safe operation of transformers. An on-line monitoring of dissolved gases generated by electrical transformers in operation was developed, which can be used as an important parameter for the detection of faults and monitoring the chemical degradation of mineral insulation oil. This paper proposes a method to analyze the generated gases, based on Near Infrared Spectroscopy (NIR) through a developed prototype that can be adapt on the Buchholz transformer relay. The combination used of NIR spectroscopy and chemometric tools has allowed for obtaining information over the samples. Recent developments of chemometrics software and computer technology have made NIR analysis the practical choice for process analysis applications.

Aging effect on electrical characteristics of insulating oil in field transformer

To ensure long-term reliability of transformers, it is important to identify the degradation characteristics of insulating oil in long-term operations, which are the dominant factors of the transformer dielectric strengths. Analysis on aged field insulating oil was conducted to identify how insulating oil in transformers changes and deteriorates with increasing age and what are the impacts on the electrical characteristics such as breakdown voltages. Insulating oil samples were collected from a total of 98 transformers, varying significantly with six manufacturers, manufacture years of 1956 to 1999, and voltage classes of 66 kV to 500 kV. Electrical, physical and chemical characteristics were obtained, and the impact of increasing age and the relationships among characteristics were evaluated. The characteristics found to deteriorate with increasing age were volume resistivities, dielectric loss tangents, interfacial tensions, and total acid values. These characteristics can be used as effective indexes for trend monitoring to identify aging statuses and detect abnormalities at an early stage. The physical characteristics (kinetic viscosities, densities, and flash points) showed different tendencies depending on the oil type and age. Since the physical characteristics depend on the compositions of insulating oil, the possible causes for this result are that the compositions of insulating oil were different between oil types and that the compositions varied in accordance with increasing age. Furthermore, the correlations between characteristics were evaluated. As a result, a correlation was found between volume resistivities and total acid values, and good correlations were found between interfacial tensions and volume resistivities and total acid values. Interfacial tensions, which change with a higher rate than total acid values, can be possibly used as an effective index for insulating oil degradation.

Finding the Degradation of Insulation Oil in a Transformer with a Fiber-Optic Sensor

Finding the Degradation of Insulation Oil in a Transformer with a Fiber-Optic Sensor