Evaluation of flitch plate losses in power transformers

Although eddy current losses in flitch plates of large power transformers may form a small part of total transformer losses, they are important because they can appear concentrated in a small area and cause hazardous hot-spots. An analytical method to calculate flitch plate eddy loss is a very useful practical guide to a transformer designer, but for high power transformers, a more accurate analysis by techniques such as the finite element method (FEM) is desirable. A series of 2-D FEM simulations, using a statistical technique-orthogonal array design of experiments, have been carried out to find the effect of various factors on the losses in a mild steel flitch plate. The procedure for carrying out the experiments and the results obtained thereof are presented. The more involved analysis of slotted flitch plates, has been done using 3-D FEM. Loss and eddy current patterns in mild steel and stainless steel flitch plates have been studied. In both cases, the effect of the number of slots and dot length on the losses is discussed. Effect of slots on eddy current pattern is explained. Results of simulation of laminated flitch plate are presented. The eddy loss distribution obtained by 3-D FEM electromagnetic analysis is used in 3-D FEM thermal analysis to estimate temperature rise of the pitch plate Verification of 3-D FEM analysis has been done by measurement of temperatures on a slotted mild steel flitch plate of a 33 MVA, single phase, 220/132/11 kV autotransformer. The estimated temperatures have been found to be in good agreement with that obtained by measurements.