An experiment for evaluating the scale effect of moisture on high voltage transformer insulation

Many experiments have been undertaken and results published on ageing of high voltage transformer insulation. The objective of these experiments has been to quantify how insulation is affected by moisture and temperature. However, the experiments have invariably been performed on small scale bench top equipment. The validity of scaled up results is always questionable and is exacerbated by the lack of published field data for comparison. A new apparatus has been developed to explore the effect of scale. The goal is to perform experiments representative of conditions inside a high voltage transformer. Electric stress of up to 10 kV can be applied to a pressboard insulated electrode immersed in the bulk oil. A series of experiments are performed to quantify the bulk insulation behaviour of pressboard in virgin oil. The apparatus has a test cell holding 370 litres of oil. The temperature of the test cell can be controlled up to 80 degC and the condition of the oil/paper system evaluated using the usual techniques of moisture analysts, voltage breakdown, tandelta etc. The paper contains a technical description of the apparatus and initial results taken from pressboard samples using the recovery voltage measurement (RVM) technique