From the editors´ desk

The first article in this issue is "A Comparison of the Voltage Withstand Properties of Ester and Mineral Oils" by Carlos Azcarraga and Andrea Cavallini from the University of Bologna, and Ugo Piovan from Trafoexperts GmbH, Switzerland. Ester oils (EOs) have some attractive properties, e.g., their fire points are 100??C to 200??C higher than those of mineral oils (MOs), they are biodegradable, which favors their use in environments where oil spills cannot be easily confined, and they are much more hygroscopic than MOs, which helps with drying of solid insulation. They also have drawbacks. They can be oxidized easily, and their use is generally limited to non-breathing transformers. Their viscosities are larger than those of MOs, so that replacing MOs with EOs without adjusting the oil gaps between adjacent pressboard barriers requires caution. Also, their pour points are higher, which could limit their use in cold climates. Finally, streamer propagation under positive lightning impulses is faster than in MO, leading to lower lightning impulse breakdown voltages. The authors compared an MO and an EO based on soy seeds, both of commercial grade. In addition to conventional breakdown tests, partial discharge tests were carried out in order to characterize fully the dielectric behavior of both oils. The experimental results were compared with the available body of knowledge and the results of FEM simulations. It was concluded that EO is a viable substitute for MO, at least when new, dried, and highly purified. The smaller permittivity mismatch between pressboard and EO, compared to that between pressboard and MO, leads to a more uniform field distribution at the oil/ pressboard interface.