The common insulating oil used in oil-filled transformer applications is mineral oil. In recent years, significant attention has been paid to alternative insulating fluids such as synthetic and natural ester liquids. The natural ester liquid is considered to be the best substitute of mineral oil in future. In order to apply natural ester liquid in transformers safely, it is important to have detailed information on its dielectric properties and its performance should be proven to be comparatively at par with mineral oil. This paper mainly focuses on a comparative study of ac breakdown voltage, dielectric constant, dissipation factor (tan δ) and resistivity of five different types of natural ester liquid under a variation of temperature. The natural ester liquids used in this experiment are rice bran oil, palm oil, corn oil, sunflower oil and canola oil. Based on the results, the dielectric constant and resistivity of all types of natural ester oil decreased with the increase in temperature. However, the dissipation factor properties in this experiment increased with the increase in temperature. In general, among all types of natural ester oil, the canola-based ester oil has the best potential to replace the current transformer oil due to its good dielectric properties as shown in this paper.

This paper presents lightning (1.2/50 µs) breakdown voltages of palm oil (PO), coconut oil (CO) and mineral oil (MO) in a quasi-uniform sphere to sphere electric field at two gap distances. The type of PO used in this study is Refined Bleached Deodorized Palm Oil (RBDPO) Olein type. The effect of voltage polarities (positive and negative) and testing methods (rising voltage, up and down and multiple level) on the lightning breakdown performance were investigated. The results indicated that lightning breakdown voltages of CO and RBDPO are comparable to those of MO under various test conditions. The results indicated that there is no polarity effect for lightning impulse breakdown tests in a quasi-uniform field. The testing methods, including rising voltage method, up and down method and multiple level method have a notable influence on the breakdown voltages. The effect of the 50% breakdown voltage on rising voltage method, up and down method and multiple level method for RBDPO and CO is comparable to MO. The withstand voltage at 1% and 50% breakdown probabilities were obtained using the Normal distribution fitting on the cumulative probability plot of impulse shots. Based on a normal distribution fitting, withstand voltages 1% breakdown probability of POA were close to the MO. Finally, based on statistical studies and simulation using ANSYS software, the prediction formulas for breakdown voltage for larger gap distances for all samples were derived.

Vegetable oils are being considered as potential alternatives to mineral oil, due to their better environmental performance and for their high fire point. Although these liquids have been used in distribution transformer, it is still a significant step to adopt vegetable oils in power transformer due to high cost and high level of safety and reliability required in service for these units. Vegetable oils such as Palm oil (PO), Corn oil (CO), and Rice Bran oils (RBO) offer suitable alternative for mineral oil. It is anticipated that most of the un-aged oil could satisfy the minimum requirement for dielectric insulation liquids in transformer. However, since transformers in service could be subjected to heat and multiple environmental parameters, the oil could be subjected to ageing. The chemical properties of the oil may change and its performance could be affected by the presence of ageing-by-products such as moisture and acids. Therefore, considering the application of new dielectric insulation liquids such as PO, CO and RBO in transformer, it is crucial to first examine its ageing performances at laboratory level. This paper focused on the effect of ageing on the electrical and physicochemical properties of PO, CO, and RBO. Sealed ageing experiments were set at 90°C for 30 days, 90 days and 180 days. Before the ageing process, the samples were dried in a vacuum oven at pressure less than 0.8kP at 85°C for 48 hours in order to remove the moisture content in the oils. Then the oils were impregnated with the Kraft paper and continue to age for selected duration time. The electrical properties (relative permittivity, dielectric losses, resistivity and breakdown strength), mechanical properties (viscosity and tensile strength) and chemical properties (moisture and acidity) of the oils were measured throughout the ageing periods. It can be concluded that the laboratory accelerated thermal ageing experiment reveals that all vegetable oils in this study are resistant toward oxidation based on the stable viscosity and low acidity measurements of vegetable oils throughout the ageing duration even with the presence of oxygen. The AC breakdown voltages of vegetable oils can still comply with the recommended limit of new vegetable oil set by ASTM 6781 even after subjected to ageing. In general most of properties of vegetable oils are comparable with mineral oil.

Dissolved gas analysis (DGA) is often use by the utility companies to monitor the health and to detect any faults of their transformers. The concentration and proportion of various gases dissolved in the oil are used to indicate whether there is an electrical or thermal fault within the transformer. A lot of data have been published in the literature on DGA levels for a mineral oil. However, the data for vegetable oils are relatively sparse even though the vegetables oils have been used for more than 10 years in distribution transformers. In this paper, the concentration of dissolved gasses under electrical performance in rice bran oil (RBO) and palm oil(PO) were investigated. These two oils were selected since they show promising potential as alternative insulating dielectric liquid for transformer and easily obtained in Malaysia. Three different gaps (2.5mm, 5mm and 10mm) with sphere to sphere configuration were used in this study. The samples were filtered for three times and dried at 85°C for 48 hours before testing. The AC breakdown experiments were performed in the high voltage laboratory for 100 times. The dissolved gasses from the electrical breakdown voltages were obtained in 2 liter sealed test cell. The gasses concentrations were measured using Gas Chromatography equipment. There were six concentrations of gases contained in the samples which are Hydrogen (H2), Methane (CH4), Ethane (C2H6), Ethylene (C2H4), Acetylene (C2H2) and Carbon monoxide (CO). These gasses were interpreted using IEC ratio, Roger ratio, Dornenburg ratio method and Duval Triangle method in order to investigate whether the existing techniques are suitable for RBO and PO. From the experiments, C2H2 and H2 were the highest amount of gases detected for all samples. The Duval triangle method is the best method in detecting the fault for these oils under electrical breakdown. However, the ratio methods have some limitation in predicting the faults.

Among the alternative fluids considered for application in transformers are Palm Oil (PO) and Coconut Oil (CO). Among the important studies that need to be carried out before it can be applied in-service is the lightning impulse performance. This paper presents the experimental work on the lightning impulse breakdown voltage of different PO, CO and Mineral Oil (MO). The PO used in this is the Refined Bleached Deodorized Palm Oil (RBDPO) type. The test was carried out under quasi-uniform field using sphere-sphere electrode configuration with different testing methods (rising-voltage and up-and-down). It was found that the lightning impulse performance for some of RBDPO and CO were comparable to MO.

Insulation oils are a vital part in power transformers. Insulation oil is not only work as electrical insulation but also as a coolant inside the transformer. Due to the increasing tight regulations on the environment and safety in recent years, vegetable oils are being considered for insulation oils in power transformer. This paper presents two conditions of Rapeseed Oil (RO), which are as received (new) and dried (dry) under difference uniform field electrodes configuration (mushroom-to-mushroom and sphere-to-sphere) with gap distance at 2.5 mm as recommended by the international standards. A comparative study of AC breakdown voltage, dissipation factor (tan δ), and resistivity under variation of temperature were investigated. The experimental works were done according to the IEC 60156 and IEC 60247 standards. The results indicated that the breakdown voltages of both condition are comparable to mineral oil. The dielectric constant and resistivity of two conditions are decreased along with the increasing temperature. However, the dissipation factor properties rose up along with the temperature. The Weibull distribution was used to determine the withstand voltages at 1% and 50% for RO in two probabilities conditions.