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Characterising the behaviour of suspended particles in liquid insulation using partial discharge testing: the case of mineral oil and ester fluids

Particles are commonly found within insulation oil such as in power transformers and for various reasons. This report discusses the effect of these particles on the insulation strength of both mineral and ester oil. An experimental analysis is conducted to determine the behavior of a suspended particle in mineral and ester oil by using partial discharge testing. A test cell is designed such that a suspended particle can be placed between two electrodes. The suspended particle is modelled as a floating needle. The cell is tested without a needle, with a cut-out needle tip and with a suspended full needle. The cut-out needle tip is found to reduce the insulation strength the most, followed by the suspended needle and then oil without a suspended particle. It is found that the electrode gap size influences how much the insulation is affected by the particle with the breakdown voltage reducing from 18.3 kV to 14.5 kV for mineral oil and 18.3 kV to 11.6 kV for ester oil in corresponding electrode gaps of 3 mm, 5 mm, 7.5 mm and 10 mm respectively. From the investigation it was seen that the PD inception voltage is lowest when the suspended particle is a cut-out needle tip. For the mineral oil the inception decreases by 16.5% and the ester oil by 3.05%. It can be tentatively concluded in that regard that Ester oil performs better than mineral oil as electrical insulation fluid in the presence of particles in addition to being less damaging to the environment.