Abstract
The main function of a dielectric liquid for transformers is its ability to withstand voltages that the transformer may be exposed to. If the electric withstand performance of the liquid is too low, breakdown will occur after a conducting streamer channel has propagated through the liquid, short-circuiting the insulation. There is now a variety of liquids of different chemical nature arriving in the market. Testing these liquids in point-to-plane gaps shows that the breakdown voltage and the streamer velocities varies depending on liquid chemistry and voltage polarity. For positive polarity in particular, the ratio between so-called acceleration voltage, when the velocity jumps from some millimetre per microsecond to above a decimetre per microsecond, and breakdown voltage varies a lot. A low acceleration voltage, associated with a high streamer velocity, will pose an increased risk for breakdown during short overvoltages, compared to a slow streamer that would not be able to cross the insulation during e.g. a short lightning impulse. Hence, this ratio is suggested as an important functional property of a dielectric liquid.
The processes that drive the formation and propagation of a streamer is governed by the high field in front of the streamer. Recent studies have shown that – contrary to what has earlier been assumed – charge injection and space charge limitation of the electric field in front of a streamer may occur at very short timescales. The charge injection behaves as space-charge-limited currents (SCLC) with different voltage thresholds for activation. There are indications that the threshold for SCLC can be related to the onset limit for acceleration of streamer.
Streamer velocities and ratio between acceleration voltage and breakdown voltage are important functional properties for dielectric liquids. A better understanding of the physiochemical processes governing streamer propagation could provide a path towards predicting thew se macroscopic properties.