Dielectric prototype test of a full-scale 420 kV power transformer HV lead exit insulated with natural ester liquid

A high voltage bushing electrode/lead assembly for a 420 kV power transformer was built considering the same test program as applied previously with mineral oil. The lead exit combined high-quality solid insulation components impregnated with the natural ester Envirotemp™ FR3™ fluid together in a compact geometric turret, keeping the same outside diameter of the original solution applied for mineral oil. The electrical tests were done in accordance with IEC and IEEE test standards at the High Voltage Test Institute of Graz, Austria. Lightning impulse voltage applied to the test specimen reached a BIL level of 1950 kV, switching impulse of 1560 kV, AC tests up to 850 kV maintained a partial discharge level of less than 5 pC. The paper presents testing results of the lead assembly in FR3 fluid. Preparation of the solid and liquid insulation is reviewed in regards to drying, degassing and particles management. Testing limits imposed by the test bushing, dimensions and clearances of the test lab, rather than the specimen in the natural ester FR3 fluid, are discussed. It is concluded that using advanced design concepts and high-quality insulation components in the same turret arrangement as previously tested, FR3 natural ester insulating liquid proved dielectrically as valuable as mineral oil at transformer testing limits according to IEC and IEEE standards. The insulation assembly has been fully approved using FR3 fluid.