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High cost and large volume are always associated with the conventional unified power flow controller (UPFC) formed by two back-to-back connected voltage source converters and two transformers. Both additional series-connected converters and their adjacent transformers are necessitated when controlling the power flow among multiple transmission lines. Such implementation will further increase the cost and volume of the equipment owing to the port expansion. In this regard, this article proposes a multiport transformer-less UPFC (MTUPFC) with easy terminal expandability. The decoupled active and reactive power flow are also controlled independently. This device presents several advantages, including low cost, lightweight, fast dynamic response characteristics, and easy port expansion. Therefore, the MTUPFC has significant advantages in power grids with complex configurations and large-scale renewable energy integration. The operation principle and control strategies of the proposed MTUPFC are elaborated. The feasibility and effectiveness of the MTUPFC are verified by a 60-MVA simulation model and a 2.5-kVA scaled-down experimental platform, respectively.
Zhang et al. (Thu,) studied this question.