Analysis of five-legged transformer used for parallel operation of rectifiers by coupled circuit-field approach

This paper deals with the modeling and analysis of a five-legged rectifier transformer that is used for parallel operation of two three-pulse rectifiers, specifically, for low-voltage high-current applications. The five-legged system is compared with the conventional rectifier system consisting of three-legged transformer and interphase transformer (IPT). Ensuring equal current sharing between the rectifiers and obtaining output voltage with higher pulse number are the two major requirements which are satisfied in the conventional system. Mathematical explanation for the five-legged system satisfying the mentioned requirements is given. The system is treated as a strongly coupled circuit-field one, wherein, non-linear finite element (FE) model of the five-legged transformer is coupled with non-linear circuit equations. IPT-like behavior of the five-legged rectifier transformer is also investigated. Stated theory and analytical expressions are validated experimentally on 12 V, 5000 A rectifier system.