Coordination Control of V/V-Connected Hybrid Distribution Transformer for Comprehensive Power Quality Improvement Under Grid Faults

In rural or remote areas, single-phase power distribution systems have been commonly adopted for electrical power supplying, due to historical and economic considerations. Residential distributed energy resources (DERs) and single-phase loads typically connect to the medium-voltage grid directly via separate single-phase transformers and converters. These separate connections result in power imbalances, equipment redundancy, increased costs, and issues with power quality and converter coordination for both low-voltage users and the medium-voltage grid. This paper proposes a novel V/V-connected hybrid distribution transformer (VV-HDT) designed to integrate loads and DERs efficiently, while actively controlling voltage and current to enhance power quality and isolate faults. It utilizes a modified V/V transformer for voltage conversion and primary power transmission, with partial power managed by smaller capacity converters. The V/V transformers offer mutual power circuits, providing hardware basis for resolving power imbalances and facilitating the expansion of needed transformer capacity. The paper elucidates the VV-HDT’s voltage and current control mechanism, the equivalent model of the modified V/V transformer, port power flow analyses, and the coordination control strategy under normal and grid fault conditions. Simulations and prototype experiments confirm the VV-HDT’s effectiveness and control strategy.