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In this paper, we have investigated the dielectric properties (losses, conductivity) of a modified rape-seed oil (RS50). Dielectric spectroscopy has been done over a wide frequency range (10-2-106Hz) and conductivity measurements according to CEI 61620 standard. The parameters investigated are temperature (-20degC to 120degC) and water content. Frequency spectra of the dissipation factor tgzeta at different temperatures are typical of losses by conduction (in the low frequency range) and of losses by dipole relaxation (in the high frequency range). The influence of temperature is well explained by conduction and relaxation theories taking into account viscosity change. The increase of conductivity with temperature is rather large: a factor of 103 from -20degC to 120degC. Up to about 200% of relative humidity, no significant influence of moisture on the dielectric properties (permittivity, losses, conductivity) has been observed. These results are then discussed in correlation with potential applications of this oil in electrical systems.

Rape-seed oil is considered for electrical insulation at high voltage. In this work, breakdown and streamer propagation in rape seed oil are studied in point-plane gaps up to 20cm, under impulse voltage of both polarities. In both polarities, two main streamer types are observed: "slow" streamers with a velocity of about 2km/s, and much faster streamers up to 150km/s. Compared to mineral oil in the same conditions, fast streamers in rape-seed oil appear at much lower voltage. They are able to propagate over large distances within a short time at moderate voltage. From these results, it is concluded that the properties of streamers in rape-seed oil are less favorable for high voltage insulation than in mineral oil.