Shell Type Transformer

Not a lot of info but look towards the end of the document http://mysite.du.edu/~jcalvert/tech/transfor.htm

In a shell type transformer, the primeary and secondary windings are laid over the central core.

Since these windings are sorrounded by the ferromagnetic shell. the leakages and the magnetic interferences much more limited.

Additionally in case of high current low voltage cases (where the stress on conductors - the bursting force is high) the outer shell provides mechanical support to the windings.

You must try to visualise the magnetic circuit and flow lines and then the matter will be clear and you will see that there is a higher possibility of the leakage fluxes emanating from the core type transformers.

However the shell transformers need more insulation since the primary and secondaries are wound neadrby and insulation grading is not as efficient as possible in core type.

The flip side is (in case of 3 phase transformers) the zero sequence currents/fluxes are better handled by the shell type transformers than the core type.

There are advantages and disadvantages of both. A transformer for any specific rating and application can be designed to be of either type. The end decision will be by the cost and the complexicity of manufacture (which also will translate into cost)