Title:  Optimal Power Conversion System Architectures for Utility-Scale Solar-Plus-Storage Farms

Committee: 

Dr. Divan, Advisor        

Dr. Saeedifard, Chair

Dr. Grijalva

Abstract: The objective of the proposed research is to investigate and validate optimal power conversion system architectures for utility-scale solar-plus-storage farms to reduce their Levelized Cost of Electricity (LCOE) from a system perspective and increase their value stacking. Solar-plus-storage has become a prevalent configuration for newly commissioned large-scale photovoltaic (PV) plants, thanks to declining PV and battery storage cost, and improved energy dispatchability and grid services with added storage. However, the state-of-the-art technologies, including central and traditional string inverters, feature either a low-voltage DC or a low voltage AC distribution with underground cables inside solar farms, inducing significant power losses and costs. Furthermore, these approaches require additional isolated converters to integrate the added battery storage, resulting in extra investment and maintenance effort. To address these challenges, a tri-port current-source soft-switching medium-voltage string inverter (MVSI) is proposed and developed to interface the PV and battery storage directly to 4/13 kV medium-voltage (MV) AC grid, using standard, low-cost overhead MV distribution networks inside PV farms. Compared to the state-of-the-art approaches, the proposed approach integrates battery storage without added complexity, lessens the losses and costs of distribution cables, thus enabling an extended LCOE reduction. It also presents attractive features of single-stage conversion, bidirectional power flow, galvanic isolation, soft-switching capability across the entire load range, controlled low dv/dt and EMI, and benign fault tolerance, facilitating the MVSI to leverage wide-bandgap devices. In addition, the proposed approach empowers PV farm operation as a virtual power plant to provide grid-support services such as black-start, frequency response, reactive power support, etc., enhancing value stacking of PV farms. The successful demonstration of this proposed research will yield an economically and functionally favored candidate for utility-scale solar-plus-storage farms and therefore proliferate deployment of low-cost and increasingly dispatchable solar energy.