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A comparison of the D.C. breakdown characteristics of synthetic and natural insulating oils

From a statistical viewpoint, the problems associated with “strength of materials” as applied to different states of matter and different phenomena such as mechanical strength or electrical strength are all essentially equivalent. This assumes that we conceive the materials as being permeated by small defects spread randomly throughout the material. These defects cause local weaknesses which may reduce the local strength of a particular specimen considerably below the intrinsic or theoretical strength of the specimen. This concept forms the cornerstone of the weakest link theory which may be described as being based on the following assumptions: 1) A certain number of physical imperfections or other initiating events exist or are created within any material under test; 2) These flaws are randomly distributed throughout the material; 3) The size of these flaws is also randomly distributed; 4) Failure will occur for a certain possible range of experimental values; 5) Each flaw has an equal opportunity for being the cause of failure; 6) Flaws act independently of each other and the effect of their presence is not cumulative; 7) Since no one flaw is dominant the probability of a particular flaw causing failure is very small; 8) Breakdown of any part of the system causes immediate failure of the whole. The flaws affecting electrical breakdown of a liquid dielectric include the presence of colloidal suspensions, impurities, gas bubbles, nonuniform distribution Of the electric field, irregularities on the electrode surfaces and local heating particularly at electrode liquid interfaces. Liquids offer the experimental advantage of a high degree of recovery providing that a suitable testing system is selected. Many measurements may therefore be made on a single sample without having to consider the pronounced variations which are found in supposedly identical samples of solid materials. In this paper the authors have used a number of statistical analyses to consider the behaviour of the breakdown of askarels (synthetic transformer oil) and some comparitive tests with conventional natural transformer oil have also been made.