Sizing of distribution transformers based on "life-cycle-cost"

Abstract

Many energy audits in Sri Lanka have identified the installed presence of oversized transformers and excessive number of transformers. No proper study has been done on the effect of the no load losses added to the system due to oversized transformers. Thus, transformers have not always being properly sized at the design stage. In this research, a sample of bulk consumers from the North Western Province was selected and their installed capacities were compared with the properly sized transformers. Analysis shows that the no load losses can be reduced, on the average by about 40% with the proper sizing. Having identified the importance of proper sizing of transformers, software has been developed to select the optimum transformer capacity comparing the total owning costs of the transformers. The effect of the load pattern, transformer life time, discounting factor and the tariff rate has been considered in the optimization. Also, the software permits the determination of the optimum transformer for a few different load curves as well. The software tool has been validated in comparison with manual calculations. This software is a useful tool for the bulk consumer as well as electrical consultants to determine the optimum transformer capacity for a given load curve based on total owning cost. The consumer can select the optimum size at the initial stage of the project so that unnecessary future costs due to losses can be minimized. Moreover, this would not only be a blessing to the consumer, but would mean that the overall distribution loss in the total electrical network will be reduced.