Shell-type transformer has double magnetic circuit and three limbs. Both windings are placed on the central limb. The coils occupy the entire space of windows. The coils are usually multi-layer disc type or sandwich coils. The low-voltage coils are placed nearest to the iron core to reduce the amount of high-voltage insulation. Core is laminated. Special care is taken to arrange the laminations of the core. All the points at alternate layers are staggered properly to avoid narrow air gap at the joint, right through the cross section of the core. The joints are known as overlapped or imbricated joints. The shell-type construction is preferred for a few high-voltage transformers. Since the windings are surrounded by core, natural cooling does not exist. To remove any winding during maintenance, removal of a large number of laminations is required. Figure 1.8 shows a shell-type transformer.
Figure 1.8 Shell-type Transformer
Due to better provision for mechanical support and bracing of coils in the shell-type transformer, better resistance to combat high mechanical force is obtained. High mechanical forces are developed for a high current during short circuit.