A static three-phase to five-phase transformer based on Scott connection

Abstract

Multiphase machines have become challenging candidates for safety-critical applications that require wide fault tolerant capabilities and higher system reliability. However, this adds more complexity to the adopted power converters. In applications such as electrical submersible pump (ESP) systems, motor electric power is commonly fed using a three-phase low voltage variable frequency drive system (VFD) followed by a step-up transformer and a medium voltage long feeder. Application of multiphase machines in such applications was recently shown to provide possible benefits, however, this necessitates a more complex multiphase power converter and a multiphase transformer. Alternatively, a passive transformation is a viable and more economical solution to step up the three-phase output of a standard off-the-shelf three-phase inverter and convert it into n-phase secondary voltages. This paper investigates the existing connections of passive transformation and proposes a transformation, based on the well-known Scott connection, to convert the three-phase grid voltages to an n-phase supply. In comparison with other connections in the literature, the proposed connection uses only two magnetic cores and less number of total coils, hence less transformer volume. The paper also introduces the general per-phase equivalent circuit for three-phase to n-phase transformer, and the required modifications to conventional open-circuit and short-circuit tests to estimate the transformer parameters. A case study is carried out for a three-to-five-phase transformer to validate the proposed connection. Simulations as well as experimental investigations are presented.