e & i Elektrotechnik und Informationstechnik

, Volume 135, Issue 8, pp 536–542 | Cite as

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

  • Florian BachingerEmail author
  • Peter Hamberger
CIGRE 2018


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.


power transformer low temperature cold start climatic test ester 


  1. 1.
    Bachinger, F., Hamberger, P. (2017): Thermal measurement of an ester-filled power transformer at ultra-low temperatures: steady state. In Proceedings of the 4th international colloquium on transformer research and asset management, Pula, Croatia. Google Scholar
  2. 2.
    Jordaens, P. J., Cloet, B., Nuri, J., Van Schevensteen, R. (2015): Cold start of a 5.5MVA offshore transformer. Transformers Mag., 2(2), 28–35. Google Scholar
  3. 3.
    M&I Materials Ltd., MIDEL | MIDEL 7131 Kinematic viscosity,, last visited 2017-03-14
  4. 4.
    Knuts, N. (2014): Cold startup behavior of natural ester based transformer dielectric liquids. Bachelor’s thesis, NOVIA, University of Applied Sciences, Vaasa, Finland. Google Scholar
  5. 5.
    Moore, S. P., Wangard, W., Rapp, K. J., Woods, D. L., Del Vecchio, R. M. (2015): Cold start of a 240-MVA generator step-up transformer filled with natural ester fluid. IEEE Trans. Power Deliv., 30(1), 256–263. CrossRefGoogle Scholar
  6. 6.
    Rapp, K. J., Gauger, G. A., Lukisch, J. (1999): Behavior of ester dielectric fluids near the pour point. In IEEE conference on electrical insulation and dielectric phenomena, CEIDP, Austin, TX, USA, Oct. 17–20, 1999. Google Scholar

Copyright information

© CIGRE - Reprint from with kind permission 2018

Authors and Affiliations

  1. 1.Siemens AG Österreich—Transformers LinzLinzAustria

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