Circulating Active Power Flow Analysis in a Hybrid Transformer With the Series Converter Connected to the Primary Side

Hybrid distribution transformers (HDTs) have been proposed as an alternative to conventional transformers (low-frequency transformers) and solid-state transformers, combining the reliability of low-frequency transformers (LFTs) and the controllability of power converters. Although there are several HDT configurations, those that connect the series converter on the primary side and the parallel converter on the secondary side can improve the operating conditions of the main LFT. Nevertheless, during voltage sag compensation, there is a circulating active power flow (CAPF) between the main LFT and the power converters. This CAPF increases the total power circulating through the LFT, which finally affects the overall efficiency of the HDT. This article presents an analysis of the different operating conditions of the HDT, their relationship with the CAPF, and their impact on the efficiency of the HDT. Two methods to reduce the CAPF and their advantages and disadvantages are introduced. Finally, a minimum controller for the HDT to operate under balanced and unbalanced conditions and to carry out the experiments is presented.