The ability of a transformer to dissipate its losses without exceeding the specified temperature rises, as laid down in standards such as BS171 and IEC76, determines its rating. An increase in the MVA rating by improving the cooling through a re-design of the existing arrangement or by the addition of ancillary equipment is often possible. Improving the rating in this manner can be a cost effective method of realising the full asset value of transformers. Before recommendations are made a thermal appraisal of the existing cooling arrangement is carried out. New designs are prepared and the necessary equipment specified and procured. Site installation and commissioning complete the service. A re-design of the cooling can require, new cooling radiators, heat exchangers, oil circulating pump or just cooling fans as necessary. All designs are supported by detailed thermal calculations giving estimated top oil rise, average winding rise and hottest spot temperature at the enhanced rating. If desired the emergency rating in a 5°C ambient is also given.   Original Rating New OFAF Rating 7.5 MVA 11.25 MVA 12.5 MVA 18 MVA with emergency rating of 27 MVA 12/24 MVA IST rated transformer 24 MVA to BS 171 22.5 MVA 34 MVA 48 MVA 60 MVA 60 MVA 90 MVA