This article deals with the problem of electrical discharge initiation in mineral oil in the setup of insulated electrodes. The results of laboratory experimental studies were collated with the results of the numerical calculations of electrical field stress. Both kinds of the research methods were applied to analyze the two model electrode setups immersed in mineral oil: setup with insulated HV electrode and setup with bare HV electrode having the same outer dimensions as the insulated one. The obtained similarities in the values of maximum inception electrical field stress and the equality in the experimentally evaluated initiation delay indicated that the most stressed oil volume and weak points included in it may be responsible for discharge initiation in paper-oil insulation setups with oil of technical purity. The same number of weak points included in oil may be, in both the cases considered, an equally productive source of initiation sites.

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