Approximately 30% of the 132 kV grid transformers over 40 years old. It has become imperative to determine the integrity of their insulation and estimate their residual life. An investigation was carried out to determine the most useful parameters likely to reflect changes in the integrity of the winding insulation before occurrence of a fault. Another object was to assess if such changes could be related to the residual life of transformer insulation. A set of diagnostic techniques have been used to monitor the magnitude of a number of parameters of oil samples from some 500 transformers. The techniques included dissolved gas analysis (DGA) of hydrogen, hydrocarbons and the carbon oxide concentrations. Moisture, acidity and electric strength were also monitored. A high pressure liquid chromatograph (HPLC) was also used to measure the contents of furfuraldehyde, a degradation residue of molecular scission of the cellulose insulation. From these results a 'norm' has been established for health transformer insulation.
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