A Comparative Study of Physicochemical, Dielectric and Thermal Properties of Pressboard Insulation Impregnated with Natural Ester and Mineral Oil

Natural ester is considered to be a substitute of mineral oil in the future. To apply natural ester in large transformers safely, natural ester impregnated solid insulation should be proved to have comparable dielectric strength and thermal stability to mineral oil impregnated solid insulation. This paper mainly focuses on a comparative study of physicochemical, ac breakdown strength and thermal stability behavior of BIOTEMP natural ester/pressboard insulation and Karamay 25# naphthenic mineral oil/pressboard insulation after long term thermal ageing. The physicochemical and dielectric parameters including moisture, acids and the ac breakdown strength of these two oil/pressboard insulation systems at different ageing status were compared. The permittivity and ac breakdown strength of these two oil/pressboard insulation systems at different temperatures were also investigated. And a comparative result of the thermal stability behavior of these two oil/pressboard insulation systems with different ageing status was provided at last. Results show that though natural ester has higher absolute humidity and acidity during the long ageing period, the lower relative humidity of natural ester helps to keep its ac breakdown strength higher than mineral oil. The pressboard aged in natural ester also has higher ac breakdown strength than that aged in mineral oil. The lower relative permittivity ratio of natural ester impregnated paper to natural ester is beneficial to its dielectric strength. Using natural ester in transformer, the resistance to thermal decomposition of the oil/pressboard insulation system could be also effectively improved

Effects of deposition time and counter-electrode size on the fabrication of lscf-sdc carbonate composite cathode for sofc

The electrophoretic deposition (EPD) process has shown great potential in the development of cathodes for solid oxide fuel cell (SOFC). This study thus aimed to determine the feasibility of the electrophoretic deposition technique in producing composite cathode films. Two parameters were investigated, namely, the effects of counter electrode size and those of deposition time on the thickness and quality of an LSCF-SDC carbonate cathode composite deposited onto an SDC carbonate substrate. The effects of the changed parameters were observed by applying constant suspension pH and voltage. Five different deposition times ranging from 10 to 30 min were selected. The counter electrode sizes used were 25 × 25 mm2 and 50 × 50 mm2 . Then, the cathode composite films were sintered at 600°C for 90 min. Microstructural characterization and film thickness measurement were performed using a scanning electron microscope (SEM). The 50 × 50 mm2 counter electrode was found to produce a cathode composite film with higher thickness. The effects of the selected parameters (deposition time and counter electrode size) were also determined by analyzing the weight and thickness of the obtained LSCF-SDC carbonate films. The results showed that for the selected time interval, a film thickness of 4.6 to 30.8 μm is generated. Further studies on fabricating LSCF-SDC carbonate cathode composites by electrophoretic deposition present promising potential given that the film thickness obtained agree well with those derived in previous studies on various types of cathode materials

Effect of side chains on the dielectric properties of alkyl esters derived from palm kernel oil

Alkyl ester derivatives were synthesized from laboratory purified palm kernel oil. The steps in the synthesis involved transesterification of palm kernel oil to produce a methyl ester, followed by epoxidation and then the grafting of side chains by esterification with propionic and butyric anhydride. The dielectric and thermal properties of the ester derivatives were analyzed and compared with the methyl ester. The melting point of the ester derivatives were found to reduce with side chain attachment and antioxidant improved its thermal stability. The dielectric loss was dominated by mobile charged particles and the chemical modification appeared to increase the rate at which electric double layer was formed at the electrode-liquid interface. The esters possessed excellent breakdown strengths suggesting that the processing to optimize their physical properties did not have a negative influence on their electrical breakdown strength. This product may prove useful as an insulation fluid in Electrical Power Transformers

A mixed-method analysis on students’ critical thinking and problem solving skill development in Malaysian public universities

Employers nowadays are looking for potential fresh graduates who are able to think outside the box, so that they can produce with quality works. However, they are failed to do so and this situation result in high unemployment rate among graduates, mainly in Malaysia. With this concern, this paper aims to examine students’ critical thinking and problem solving skill development after undergoing an industrial training. Further investigation also examines relationship of the skill development with respect to supervisors’ leadership styles. Both quantitative and qualitative methods were employed, with three hypotheses were tested. A total of 2,000 undergraduate students from six public universities in Malaysia involved in pre- and post-training surveys. In assessing students’ critical thinking and problem solving skill, eleven items in Belbin inventory on Thinking role was used. Meanwhile, nine employers and twelve students were participated in the interviews and they were asked regarding their perception of industrial training in assisting the development of critical thinking and problem solving skill. Descriptive analysis (such as mean), paired samples t-test, correlation and content analysis were utilized for the data analysis. Overall, results reveal that students developed in their critical thinking and problem solving skill upon completion of the industrial training. The results also demonstrate that there is a relationship between transformational leadership style the skill development. The findings of this study provide the necessity for industrial training stakeholders to improve industrial training program, mainly by implementing appropriate leadership style

Streamer evolution arrest governed amplified AC breakdown strength of graphene and CNT colloids

The present article experimentally explores the concept of large improving the AC dielectric breakdown strength of insulating mineral oils by the addition of trace amounts of graphene or CNTs to form stable dispersions. The nano-oils infused with these nanostructures of high electronic conductance indicate superior AC dielectric behaviour in terms of augmented breakdown strength compared to the base oils. Experimental observations of two grades of synthesized graphene and CNT nano-oils show that the nanomaterials not only improve the average breakdown voltage but also significantly improve the reliability and survival probabilities of the oils under AC high voltage stressing. Improvement of the tune of ~ 70-80 % in the AC breakdown voltage of the oils has been obtained via the present concept. The present study examines the reliability of such nano-colloids with the help of two parameter Weibull distribution and the oils show greatly augmented electric field bearing capacity at both standard survival probability values of 5 % and 63.3 %. The fundamental mechanism responsible for such observed outcomes is reasoned to be delayed streamer development and reduced streamer growth rates due to effective electron scavenging by the nanostructures from the ionized liquid insulator. A mathematical model based on the principles of electron scavenging is proposed to quantify the amount of electrons scavenged by the nanostructures. The same is then employed to predict the enhanced AC breakdown voltage and the experimental values are found to match well with the model predictions. The present study can have strong implications in efficient, reliable and safer operation of real life AC power systems

Impact of Power Transformer Insulating Mineral Oil Degradation on FRA Polar Plot

Frequency response analysis (FRA) has been employed as an effective tool for the detection of various mechanical winding and core deformations within electrical power transformer. Traditional FRA signature interpretation relies on the magnitude of the FRA plot although all practical frequency response analyzers are able to provide both magnitude and angle of the FRA signal in wide frequency range. Moreover, no attention has been given to the impact of power transformer insulating oil degradation on the FRA signature. This paper presents is aimed at introducing a new interpretation approach of the FRA signature using polar plot which is obtained by incorporating FRA signature magnitude and phase into one plot. The paper also investigates the impact of insulating mineral oil degradation on the proposed polar plot signature. Digital image processing (DIP) technique is developed to automate the interpretation process. In this regard, the physical geometrical dimension of a single-phase transformer filled with insulating mineral oil is simulated using three-dimensional finite element analysis to emulate real transformer operation. FRA polar plot signatures are measured and analyzed for various health conditions of the mineral oil. Results show that insulating mineral oil degradation has an impact on the transformer FRA polar plot signature. The proposed FRA polar plot technique is easy to implement within any frequency response analyzer

Dielectric Strength Improvement of Natural Ester Insulation Oil via Mixed Antioxidants: Taguchi Approach

Recently, natural ester insulation (NEI) oils are found to be the best candidates to replace mineral-based insulation oils for oil-immersed transformer applications. However, NEI oils are prone to oxidation due to their poor oxidative stability which can be improved by adding antioxidants into the oils. Latest studies have also shown that the use of selected antioxidants improves the AC breakdown voltage (BdV) of NEI oils. However, the experiments in previous studies were designed using the conventional one-factor-at-a-time (OFAT) method, which requires a large number of samples to be tested in order to determine the optimum response. Thus, a Taguchi-based designed experiment is introduced in this study in replacement of the OFAT method. It is found that this method is capable of determining the optimum concentrations of propyl gallate (PG) and citric acid (CA) which will maximize the AC BdV and improve the oxidative stability of the NEI oil. An AC breakdown voltage test is conducted in accordance with the ASTM D1816 standard using Megger OTS60PB portable oil tester, in which the electrode gap distance is kept fixed at 1 mm. The results indicate that the addition of PG and CA antioxidants increases the AC BdV of the rapeseed-based NEI oil. It is found that the optimum concentrations of PG and CA antioxidant is 0.05 and 0.25 wt.%, respectively. Lastly, the model developed in this study is analysed using analysis of variance (ANOVA). Validation test is also conducted on the optimized NEI oil to determine its dielectric strength and oxidative stability

Investigation of the degradation of a wood pulp-cotton presspaper in different biodegradable oils

New insulating materials, as biodegradable oils and upgraded paper, need to be investigated. In this paper, the ageing of a wood pulp - cotton presspaper is studied. Solid insulation is impregnated and aged with three different fluids: a mineral oil and two vegetable oils, from sunflower and soybean. Both oils and paper were dried before the ageing process, which was carried out at 150°C for 732 hours in iron vessels. Degradation of oils is analysed through the measurement of their breakdown voltage, dielectric dissipation factor (tan δ), resistivity, moisture content and acidity. Deterioration of presspaper is quantified by its polymerization degree (DP), moisture content and dielectric dissipation factor. Results showed that the dielectric properties of oils are negatively affected by the ageing, since the breakdown voltage and resistivity were reduced, whereas the dissipation factor increased. Also, acidity increased, especially in the vegetable oils. In the case of the solid insulation, its tanδ increased with the ageing, despite the reduction of its moisture content. DP was reduced, reaching the end-of-life criteria (DP<200) in the ageing with mineral oil (134) whereas it remained at higher values for the sunflower (206) and soybean (216) oils.This research is under BIOTRAFO project, which has received funding from the European Union’s Horizon 2020 Research and Innovation Program under the Marie Sklodowska-Curie Action-Research and Innovation Staff Exchange (MSCA-RISE) grant agreement No 823969. The authors of this research wish to thank the Ministry of Economy for its financial support for the National Research Project: Gestión del Ciclo de Vida de Transformadores Aislados con Fluidos Biodegradables (PID 2019-107126RBC22). Cristina Méndez also wants to acknowledge the Spanish Ministry of Science, Innovation and Universities for the financial support for the FPU grant (FPU19/01849)

A new approach to understanding the frequency response of mineral oil

Dielectric spectroscopy is non-invasive diagnostic method and can give information about dipole relaxation, electrical conduction and structure of molecules. Since the creation of charge carriers in mineral oil is not only from dissociation but also injection from electrodes, the injection current cannot be simply ignored. The polarization caused by the charge injection has been studied in this paper. Based on our research, if the mobility of the injected charge carriers is fast enough so that they can reach the opposite electrode, the current caused by the injection will contribute only to the imaginary part of the complex permittivity and this part of the complex permittivity will decrease with the frequency with a slope of -1 which is in a good agreement with the experimental result. The classic ionic drift and diffusion model and this injection model will be combined to make an improved model. In this paper, the frequency responses of three different kinds of mineral oils have been measured, and this modified model has been used to simulate the experiment result. Since there is only one unknown parameter in this improved model, a better understanding of the frequency response in mineral oil can be achieve