Ester fluids as alternative for mineral oil: The difference in streamer velocity and LI breakdown voltage

Mineral oil has predominantly been used as the insulation and cooling liquid in transformer industry for more than 100 years. However, with the concern of fire safety and environmental impact alternative liquids such as ester fluids are of interest. In fact, these liquids are biodegradable, less flammable and have been used in thousands of distribution transformers world-wide. A trend has been seen in recent years that these fluids have been introduced in power transformers. With the aim at qualifying the dielectric performance of ester fluids for use in medium and large power transformers, the authors' group has carried out research work during the last 5 years. Part of results is reported in this paper. In the investigation, needle/plane geometry was used. A streamer was generated at the needle tip and propagated across the oil gap or along an oil/pressboard interface. Breakdown voltage, polarity effect, streamer velocity and acceleration voltages for transition to fast event were studied. Findings show that the streamer fast event feature in ester fluids was quite different from that in mineral oil especially at positive polarity, where significantly lower breakdown voltage values were observed in ester fluids than in mineral oil over long electrode gap distances, and that the transition voltages to fast event in ester fluids were much lower than those in mineral oil and that fast event streamer velocity in ester fluids increased much more steeply at over-voltages than in mineral oil. Electrode gap distances of 25 mm, 100 mm and 200 mm were tested at LI (lightning impulse) voltages of 1.2/50 μs. Streamer velocity up to 120 km/s was recorded. Results are summarized and analyzed. The work contributes to the understanding of the dielectric properties of ester fluids and mineral oil for the insulation system design of medium and large power transformers.