Validation of long term performance of natural ester and Kraft paper as high temperature insulation system

In early 2003, CPFL Energia, a Brazilian utility, was interested on evaluating the performance of natural ester filled transformers, especially regarding the highly increased overloading capability. As a field test, in association with the transformer manufacturer “Itaipu Transformadores”, they built two prototype transformers. Based on original overhead distribution line three phases transformers of 15kV/45kVA, after small adjustments in the windings and cooling system, keeping same core and tank, associated with the use of natural ester based dielectric liquid, and increasing the average winding temperature rise limit from 55°C to 85°C and a hotspot temperature rise limit of 100°C, the transformer nominal rating was shifted to 88kVA.Trying to ensure very high loading conditions, this transformer has been installed in the network replacing a 15kV/112.5kVA transformer. Submitted to an average loading of 82kVA and a peak loading of 123kVA (273% of nominal 45kVA rating), one could expect it to have a premature failure on such extremely harshening conditions. However, both transformer presented no problem along the years of operation. In 2015, after 12 years of interrupt high loading condition, one of them has been removed from the network for a complete teardown to be performed, including full screening of insulation materials. This paper will present full details of the performed test and the surprisingly positive achieved results, both regarding the negligible degradation of the dielectric liquid and the excellent performance of the solid insulating materials. The reduction of the degradation rate of the Kraft paper has been completely confirmed, as lowest value of degree of polymerization from several samples of paper that have been taken was at the hotspot region of the LV winding, but still above 830, while the expectation would be to have consumed above 50% of the total life expectation.