Hello every one

I am working with an ESP with 4 fields. As we all know every field has it's own transformator/rectifier to execute corona effect and dust collection. this ESP uses a three phase transformator only for the first field and other fields have single phase transformators. We all know three phase transformators have much lower Ac ripple and therefore they provide stronger Corona effect and better efficiency, so I wonder why this ESP does not use 3 phase transformators for all it's fields? Is there any specific reason for that?