National Grid uses transformer thermal ratings for planning purposes and day-today operation of the transmission system. The thermal ratings use transformer models based on IEC methods that are known to have shortcomings, particularly with oil temperature behaviour and where changes of cooling state occur. National Grid is now acquiring transformers cooled only by natural circulation or by three-stage cooling and these require modifications to the existing ratings process to be modelled properly. Transformer thermal capability is calculated from known test certificate data. However, some older transformers in key locations do not have test certificate data, resulting in the use of conservative ratings that will be restrictive. The relevant thermal parameters could be determined by the application of appropriate models to the measured data for these units.

Objectives

This project map and data will deliver an improved transformer thermal model that enables accurate ratings to be calculated. A method for determining thermal parameters for those transformers without test certificates will be developed. The project will specifically address the effects of ambient conditions, changes in cooling state and the influences of the transformer surroundings, of particular interest in built-up locations. Met Office data from a previous scheme will be used to assess the effects of heat wave conditions, of especial importance in the South-East.