Introduction: Optimising modelling techniques to support capital replacement planning for transformers.

Objectives: This project is aimed at optimising capital investment in replacement transformers. For long term replacement planning purposes transformers have been given asset lifetimes of 40 to 80 years based on experience and engineering judgement. Actual replacements are based on condition, using several assessment methods which have been successfully developed and applied. There is however a gap in the knowledge of transformer end-of -life modelling linking the probabilistic and deterministic approaches of the long and short term plans. This project has the objective of building on existing knowledge of ageing mechanisms to provide a model to bridge the gap and provide credible predictions of medium term (4-10 year ahead) requirements for transformer replacement volumes. The basis for the plan would be the existing policy of maintaining system reliability and unplanned transformer replacements at existing levels. The dependence of system reliability on plant reliability would be part of the study. Transformer replacements will cost between £10M and £30M per year for the foreseeable future, failure to plan effectively could have significant implications for regulatory review and system reliability.