Controller Design for the Rating-Proportional Current Sharing in Modular Hybrid Critical Load Substation Transformers

Partially rated power electronic modules (PEMs) can be interfaced with conventional line-frequency transformers (LFTs) to realize hybrid transformers (HTs). Hybrid transformers possess higher controllability as compared to conventional LFTs. In this paper, the parallel operation of multiple such hybrid transformer systems to form a modular architecture is proposed. The proposed system utilizes back-to-back power converter units with a lead/follower (L/F) control strategy. It entails reliable and resilient operations for critical load substation deployment during disasters and emergencies. The partially-rated power electronic module is exploited to achieve rating-proportional current sharing and voltage regulation. The controller design for the rating-proportional current sharing is presented. The DC-side stability of the back-to-back power converter unit with the proposed current sharing controller is assessed. Simulation results are included to validate the proposed concept.