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

Committee:

Dr. Deepak Divan, ECE, Chair, Advisor

Dr. Maryam Saeedifard, ECE

Dr. Santiago Grijalva, ECE

Dr. Mahesh Morjaria, Terabase Energy

Dr. Rhett Mayor, ME

Abstract: For utility-scale photovoltaic (PV) projects, solar-plus-storage (SPS) has become an increasingly favored configuration owing to significantly reduced PV and battery storage costs, improved energy dispatchability, and grid-support services with added storage. However, the state-of-the-art power conversion system (PCS) architectures based on central and string inverters feature a low-voltage direct-current (DC)/alternating-current (AC) distribution with underground cables inside solar farms, inducing significant copper losses and costs. Furthermore, these two approaches require additional converters to integrate the paired battery storage, resulting in extra investment and maintenance effort. These factors result in an increased Levelized Cost of Electricity (LCOE) of utility-scale SPS farms and thus dampen the continued proliferation of solar energy. The objective of this research is to propose three new medium-voltage AC (MVAC) PCS architectures to reduce the LCOE of utility-scale SPS farms and thus accelerate the deployment of dispatchable and low-cost solar energy. These three proposed approaches, namely tri-port medium-voltage string inverter (TMVSI), multi-port DC transformer (MDCT), and massively distributed micro-multiport converter (µMC), enable localized DC-coupled battery storage, an MVAC distribution network using standard and low-cost overhead lines, and distributed layout of power conditioning units across the plant with scalable SPS farm building block design. Throught this dissertation, a 300 kVA/4 kVac TMVSI has been designed, built, and tested to validate its effectiveness and viability, with a focus on the medium-frequency transformer design and control optimization. In addition, enhanced energy dispatchability and grid-support services of a 20 MW/80 MWh TMVSI-based SPS farm have been demonstrated. Finally, a framework for system-level LCOE analysis has been established to validate the advantages of the proposed MVAC architectures in reducing system LCOE of utility-scale SPS farms over a wide range of inverter-loading-ratios.