Normally the double ratio transformer are manufactured with a single voltage ratio ( like 11/0.433KV, ,33/0.433KV,22/0.433KV,11/0.550KV,33/11KV etc.) but in some cases we need transformer which have double voltage ratio like (11-6.6/0.433,11-3.3/0.433KVetc.)These type of transformer can work on any incoming voltage i.e 11KV or 6.6KV and will give the same output i.e 433V.