A Comparison Between Bubble Formation on Kraft and Thermally Upgraded Papers in Synthetic Ester

Bubble formation endangers the integrity of the oil–paper insulation system of power transformers. In this article, a test setup is created to detect bubbling inception temperature (BIT) for Kraft and thermally upgraded papers (TUPs). The paper samples are dried, prepared at six different moisture levels, and immersed in synthetic ester for the experiments. The paper strips are wound around a cartridge heater, and a controller unit raises its temperature. Results of the experiments revealed that BITs for Kraft paper are comparable to those reported in the literature. The usage of synthetic ester is shown to cause insignificant changes in the BITs. The causes of discrepancies among the BITs obtained via different test setups are discussed in detail, and an equation is presented to estimate BIT in Kraft paper based on four experimental datasets. The tested TUP is shown to have significantly lower BITs than Kraft paper, which is in contrast to the previously reported data. Scrutinizing TUP at temperatures beyond BITs revealed that the behavior of bubble formation differs from that in the Kraft paper. In the test setup with Kraft paper, bubbles are formed progressively at temperatures higher than BIT. In contrast, occasional sporadic bubbles emerge from TUP beyond the BIT until a stream of bubbles, similar to the case of Kraft paper, is observed at several degrees higher than BIT in the TUP.