Hydropower Development in Southern and Southeastern Asia

Recently, no-load losses P and noise n of transformer cores have been reduced by replacing the former single-step-lap (SSL) joints by multistep-lap (MSL) joints. On the other hand, the application of MSL in model cores was found not only to decrease P and n, but in some cases they increase. The aim of this paper is the investigation of the difference between SSL and MSL with respect to local magnetic conditions at joint regions and their relevance for global conditions in the core. The results show that the joint is the key region of the transformer core, affecting its dynamical behavior in a distinct way. To minimize the iron losses and the noise of the core, the step lap number N should be chosen carefully to avoid the appearance of "virtual temporary air gaps".