Hardware Design of a 13.8-kV/3-MVA PV Plus Storage Solid-State Transformer (PVS-SST)

Abstract

Photovoltaic (PV) power generation plant with integrated battery energy storage (BES) is becoming increasingly attractive and necessary as the PV penetration increases. Traditional solutions involve two paralleled inverter systems at the same site. This increases the balance of the system cost and the control complexity. Furthermore, high-power step-up transformers are needed to connect to the distribution grid, further increasing the capital cost, land use, and installation cost. Combining PV and BES into a single inverter system without the 60-Hz step-up transformer is, therefore, very attractive as the next-generation technology for utility-scale solar. This effectively calls for the development of a PV+BES solid-state transformer (PVS-SST). This article proposes a 13.8-kV/3-MVA PVS-SST targeting a 13.8-kV grid connection on the ac side and a 1500-V PV farm on the dc side. The PVS-SST utilizes a modular SST concept with a total of 27 SiC submodules. Each submodule is based on 1700-V SiC power MOSFET devices and single-stage dual-active-bridge (DAB)-based dc/ac power conversion technology. In order to utilize the inevitable parasitic capacitances in the actual hardware, a novel turn-off loss model is proposed and experimentally verified. A comprehensive optimization strategy, including the proposed novel turn-off loss model, ZVS range, dead-time calculation, and minimum circulation current, is proposed. The medium-frequency transformer (MFT) design to achieve very high efficiency and isolation voltage is a grand challenge in the proposed PVS-SST and is discussed in detail. The power density of the SiC power submodule is 1.6 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (26 W/inch <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ). The PVS-SST control architecture is discussed with a focus on voltage balancing and real and reactive power controls.