Analysis of a three-phase to seven-phase transformer under unbalanced input

Abstract

In the recent past, multiphase power generation, power transmission, and multiphase drive system are the main focus of research due to their several advantages over existing three phase system. Multiphase drives have better performance over three-phase drives. A multiphase transformer is required at the input of a fixed frequency multiphase drive, rectifier circuit for HVDC applications and multiphase generation and transmission systems. This paper investigates an static three-to seven-phase conversion technique and presents a design of transformer and control strategy to analyze the effect of unbalanced supply on a three-phase to seven-phase transformer. The transformer presented in this paper takes three phase input and a seven-phase output is obtained at the secondary terminals. The paper also discusses input-output unbalancing. A complete design, analysis and simulation of the proposed transformer is presented and experimental validation is also reported in this paper. Experimental and simulation results show that the presented transformer can produce a seven-phase power from three-phase ac power. Input unbalance is reflected at the output of the transformer but is less than the input unbalance. It is also shown that seven-phase output is not produced if one phase of input is open and the transformer has more than three limbs for flux to flow. But a balanced seven-phase output in steady state is produced even in one phase open condition for a three limb core type transformer. A new definition of sequence components of an unbalanced seven-phase system is required for unbalancing study so it is also defined in this paper.