Effect of Accelerated Thermal Aging on the Properties of Paper-Ester from Palm Oil Composite Insulation

Mineral oil is widely used for transformer liquid insulation. However, due to environmental consideration and the availability of the liquid in the future, many efforts are being done to find new type of insulations. Recently, new type liquid insulation from natural like ester is being investigated for possible application in high voltage transformers. This paper reports experimental results on the effects of thermal aging on the kraft transformer paper in natural ester made from palm oil. The natural ester from palm oil with paper combined with copper strips were put in closed aging bottles. The sample composition was designed to simulate the condition in an actual transformer. The sample was subjected to an accelerated thermal aging at 120 degrees C in a controllable oven. The aging period was done until up to 1008 hours. The effects of thermal aging on the paper characteristics was investigated by measuring the tensile strength and degree of polymerization of paper. Comparison among measured parameters was done. Chemical change in the paper was examined using energy dispersive X-ray spectroscopy (EDS). Experimental results showed the reduction of tensile strength as well as degree of polymerization (DP) with aging time. The tensile strength and DP are strongly correlated each other. The EDS measurement indicated that the thermal aging increased the C element in the paper and inversely reduced the oxygen in the paper. At the later aging, small amount of K element was observed in the paper which was considered as the esterification by product during manufacturing the natural ester from palm oil. As comparison in mineral oil instead of K, small amount of S was detected.