Measurement of thermal behavior of an ester-filled power transformer at ultra-low temperatures

Abstract

At arctic ambient temperatures, the viscosity of cooling fluids for power transformers reaches extreme levels. This leads to a change of fluid flow and cooling performance, which is even more critical in case of ester fluids and cooling by means of radiators. To ensure reliable operation even under such conditions, a power transformer has been tested under different loads at ambient temperatures ranging from −50 °C to −30 °C. In this report, we concentrate on the measurement results for two cold start tests: one smooth start at −50 °C and a second, quicker start with full load at −40 °C. The steady state results have been analyzed in an earlier publication. When power is supplied to a completely cold transformer, we observe that flow inside windings and tank develops relatively fast. However, it may take a long time to start natural flow through the radiators. In the meantime, temperatures in the active part may reach undesired levels. In the cold start at −50 °C, the radiators did not become operational at all, even after several days. The losses were too small to overcome the hydraulic resistance in the radiator panels. When the transformer was started with full load at −40 °C, the cooling equipment stayed cold for a long time, but became suddenly active approximately 14 hours after start. At this point, we noticed an abrupt change of temperatures in the radiator. Afterwards, the transformer gradually approached a steady state situation. Although we observed an overshoot of top oil and hot spot temperatures in this heat run, the measured temperatures remained at an acceptable level. This, however, need not be the case in general. Especially in the situation of separate radiator batteries, critical temperatures must be expected.