Toroidal transformers are high performance transformers used in high end applications. The unique toroidal construction drives them for such performance levels, but the gap less core construction itself makes them lag in controlling high inrush currents. Transformer based solutions are always favourable in the industry over the external solutions, but most of the existing transformer based solutions weaken the toroidal transformer's typical superior performance while mitigating the inrush

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... t. This paper investigates an effective inrush current mitigating method with composite cores, while protecting the performance level of typical toroidal transformers and being competitive in the market. The proposed method has two cores made with different steel types which are positioned concentrically, where the inner core is uncut, but the outer core is cut with an air-gap. This paper investigates on the optimum air-gap that should be maintained to minimize the inrush current based on the transformer size and the particular two steel types. Further, this paper describes on the inrush current calculation on composite cores together with the formulae and derived characteristic curves. The results and formulae presented in this paper are verified with laboratory experiments with real transformers, built with composite cores.