A Novel Delta-Type Transformerless Unified Power Flow Controller With Multiple Power Regulation Ports

As the most comprehensive flexible AC transmission system (FACTS) device, the unified power-flow controller (UPFC) plays a vital role in the transmission network. The conventional UPFC with a structure of two back-to-back voltage source converters achieves isolation, and the voltage level increases through complex zig-zag transformers. However, expensive and bulky transformers add to the overall size and expense of the device. To address this issue, a multiport transformer-less unified power flow controller (MTUPFC) is proposed. Although the MTUPFC offers benefits in terms of cost and size reduction, the limited power regulation range impedes its broader application in complex transmission networks. To address this issue, this paper proposes a novel delta-type transformer-less UPFC (DTL-UPFC) with multiple power regulation ports. Benefiting from the delta-type structure, a circulating current injection scheme could be employed in the DTL-UPFC to expand its power regulation range. First, the topology feature and working principle are introduced. Second, the corresponding control strategy is designed. A comparison between MTUPFC and DTL-UPFC is subsequently performed. Last, the simulation model and experimental prototype are built to verify the feasibility of DTL-UPFC.