Will there will be any transformer de-rating by changing from mineral oil to an ester fluid?
Distribution transformers: unlikely
Power transformers: an assessment may be required
Transformer rating after retrofilling with ester fluids
For distribution transformers there is unlikely to be any need to de-rate the unit after retrofilling. Due to the higher viscosity of MIDEL fluids the transformer may run a few degrees hotter, but this is offset by the enhanced paper lifetime provided by an ester fluid.
For transformers in excess of 66kV a more in-depth assessment may be needed to determine the temperature rise change.
If the transformer specification reads ’55 AWR’ and the mineral oil filled unit was factory tested at 54 AWR, it may be the position of the manufacturer that the transformer would exceed the temperature rating.
However, this is more than offset by the slower paper aging and MIDEL’s enhanced thermal capabilities. As indicated in IEC 60076-14 and IEEE C57.154 ester-based liquids can operate at a higher temperature than mineral oil for a given paper lifetime. Taking into account this slower paper aging, there is no need to de-rate a retrofilled distribution transformer, even if the operating temperature increases.
More questions on ester fluids?
This post is one of the 15 top questions we get asked on retrofilling transformers with ester fluids. Here is the full list of the frequently asked questions on ester fluid retrofilling: