Interests on ester liquids as replacement of mineral oil in large power transformers are growing in recent years. This paper presents experimental studies on streamer and breakdown in synthetic and natural esters. Lightning impulse voltage was chosen when considering the practicality of standard tests of transformer industry. First, streamer length, velocity, shape and mode were analyzed based on optical images obtained by a multi-channel ultra high speed camera. Under the same voltage level, streamer in both natural and synthetic esters propagates faster and further, with more branches, than in mineral oil. Breakdown tests at various gaps from 15 mm to 100 mm were conducted, and it was found that fast streamer led breakdown becomes statistically dominant in a large gap under lightning impulse and ester liquids have lower breakdown voltage than mineral oil due to their low tolerance to fast streamer. Therefore a relationship between the results under lightning impulse and previously published results under step voltage was built up and an empirical formula was obtained to predict the breakdown voltage of esters at very large gaps.

Vegetable oil is one of the alternatives to replace mineral oils in transformers. This paper focuses on oxidation stability assessment of a vegetable transformer oil under accelerated thermal aging at 110°C with various durations up to 2500 hours. Oil samples were aged under open condition to stimulate the oxidation process, as well as under sealed condition to provide a comparison. A number of parameters including acidity, DC conductivity, dielectric dissipation factor (DDF) and viscosity were measured to see their responses to oxidation process of the vegetable oil. The results showed that viscosity responds towards oxidation more than hydrolysis whereas acidity responds strongly to both oxidation and hydrolysis, since besides oxidation, hydrolysis of the vegetable oil also generates acid byproducts. Of more importance, oxidation stability, in the term of oxidation induction time (OIT), of thermally aged vegetable oils was investigated by using Pressure Differential Scanning Calorimetry (PDSC) method. As there was no standard on PDSC method available for vegetable oils, various measuring temperatures were tried and 130°C was found to be suitable for the investigated vegetable oil. The results showed that OIT of open samples decreases obviously with aging time until reaching the bottom plateau at 1000 hours, while OIT of sealed samples stays at a high level until the end of aging at 2500 hours. Therefore, OIT measured by using PDSC is appropriate to reflect oxidation stability of the vegetable oil. This was further confirmed by the corresponding consumption of antioxidant and generation of peroxides, dissolved decay products.

This paper presents breakdown and withstand strengths of a synthetic ester, a natural ester and a mineral oil in a quasi-uniform sphere-to-sphere electric field under impulse voltages. The influences of impulse waveforms (lightning and switching), voltage polarities (positive and negative) and testing methods (rising-voltage, up-and-down and multiple-level) on the breakdown behaviour were investigated. The results indicated that breakdown voltages of both the ester liquids are comparable with those of the mineral oil under various test conditions. The withstand voltage at 1% breakdown probability was also obtained based on Weibull distribution fitting on the cumulative probability plot, which was built using the ~1000 impulse shots. It was found that the withstand voltages at 1% breakdown probability of the ester liquids are almost the same as that of the mineral oil. The results in this paper were further compared with other available test data in the literature. By comparing the performances between the ester liquids and the mineral oil, recommendations for insulation design were given to aid the application of ester liquids in high voltage large power transformers.

This paper presents experimental studies on the effect of cellulose particles on breakdown voltages and breakdown mechanisms of a synthetic ester and a natural ester under standard lightning impulse voltages. A mineral oil was also tested and used as the benchmark. In total, liquids with four particle contamination levels, i.e. low, normal, marginal and high; two moisture levels, i.e. dry and wet conditions, and three types of electrodes, i.e. plane-plane, rod-rod, and VDE electrodes were investigated. Results showed that the presence of cellulose particles reduces the 50% breakdown voltages of the ester liquids and the mineral oil in uniform fields, which is further enhanced when the particle is combined with the moisture. The particle effect on breakdown voltages also becomes more prominent when the effective stressed area/volume of the electrode configurations is increased. Nevertheless, at the same conditions, 50% lightning impulse breakdown voltages of the ester liquids are always comparable to that of the mineral oil. Linking with previously published results under AC voltages, it is summarised that the particle effect on liquid breakdown is associated with the effective stressed area, the duration of electric stress and the viscosity of the liquid itself. In addition, the streamer imaging tests revealed that the detrimental effect of cellulose particles on breakdowns in liquids is due to the promotion on streamer initiations rather than on streamer propagations.

Mineral oil is traditionally used in liquid immersed transformers to act as a coolant, an information carrier, and as an electrical insulator. Emerging alternative transformer liquids provide advantages, such as improved fire safety and better biodegradability, of which transformer operators would like to utilize. In this paper, an experimental study is conducted to compare the thermal performance of a mineral hydrocarbon transformer oil, a gas-to-liquid hydrocarbon transformer oil, and a synthetic ester transformer liquid as coolants in a zig-zag disc type winding model. Comparisons are made under liquid directed cooling modes and under liquid natural cooling modes. It was found that under both cooling modes, the mineral based transformer oil and the gas-to-liquid based transformer oil behaved almost with comparable liquid flow and temperature distributions. Under liquid directed cooling modes, the synthetic ester gave more uniform flow distribution and delayed the occurrence of liquid reverse flow compared to the other oils. Under liquid natural cooling modes and using the zig-zag disc type winding model, synthetic ester, due to its higher viscosity which caused lower inlet flow rate to develop under the specific tested retrofilling conditions, caused less uniform oil flow distribution within the pass and higher hot spot temperature.

In this paper, a method for predicting pressure drop and flow distribution in disc-type transformer windings in an oil-forced and directed cooling mode is proposed. First, dimensional analyses are conducted to identify the independent dimensionless variables that affect pressure drop over and flow distribution in the winding. Next, parametric sweeps are performed with computational fluid dynamics simulations under isothermal flow conditions. Finally, pressure drop and flow distribution results obtained from the simulations are correlated with the previously identified dimensionless variables to derive correlation equations. These equations have been verified over a range of different isothermal and non-isothermal flow cases and applied to identify the criteria for the occurrence of reverse flow resulting from a combination of pass inlet flow rate and winding geometry. In addition, the method provides an insight into the controlling parameters to determine the minimum oil velocity in horizontal cooling ducts.