Developments in computational methods for transformer cyclic loading evaluations

The paper reports advances in computational procedures for transformer internal temperature and relative ageing calculations which form an essential part of transformer cyclic rating evaluations. The first development relates to the thermal model for transformers in cyclic load operation. Previously, internal transformer temperatures over ambient were found from an iterative procedure. Deriving from the boundary conditions which cyclic loading establishes, the paper develops for the first time a non-iterative form of solution. It gives an exact solution which further advances the all-round dependability of cyclic rating determinations in practice. The second development relates to the evaluation of the time integral by which account is taken in cyclic loading studies of insulation ageing. Computing time requirements can be high when loading studies are to be carried out for substantial periods. The paper shows how a separation of variables can reduce computing overheads to a minimum. The two developments are validated in the paper using examples of rating studies given in IEC 354, the international standard in the subject area of the paper.