In-rush current mitigation on toroidal transformers with composite cores

Abstract

Toroidal transformers play an important role in the transformer industry specially in high end applications due to their superior performance, over the conventional laminated transformers. But toroidal transformers lag in performance when comes to high power requirements, specially due to their extremely high inrush currents compared to the laminated transformers. There are many options that can be used externally to the toroidal transformer to avoid this issue, but due to the reliability concerns, transformer based inrush current mitigation methods are always preferred in the industry. Conventional transformer based inrush current mitigation methods fall short on toroidal transformers, because those methods tend to mitigate their superior performance also, together with the inrush current. The proposed transformer based inrush current mitigating method with composite cores will reduce the inrush current extensively, while protecting the typical superior performance characteristics of toroidal transformers. Also the proposed method will have better control over the inrush current than the conventional methods, while being competitive in the market. The proposed method involves two cores; one is lower grade NGOSS (Non Grain Oriented Silicon Steel) core in the centre for the normal operation, and the other is higher grade GOSS (Grain Oriented Silicon Steel) core positioned around the NGOSS core with a controlled air-gap, for inrush current controlling purpose. Due to the uncut NGOSS core in the centre, the composite core retains high performance in the normal operation without compromising. This dissertation includes practical development of the composite core together with silicon steel types CK37-35H300 and M0H-M103‐27P, and then experimental testing on inrush current and finally converge the research findings for developing a new design guideline for the optimized solution, while discussing the cost and the manufacturing aspects.