Transformerless Utility Interface for Solar PV and Battery Hybrid Systems

Abstract

This research will provide new knowledge essential for the establishment of a transformerless utility interface for hybrid solar PV and battery systems. The dissertation is divided into four main parts. The first responds to the growing interest in deploying medium voltage DC collection grid technology in renewable energy applications to improve energy efficiency and power density. Thus, a new medium voltage DC collection grid method for large-scale PV plants with a DC-DC interleaved modular multilevel (boost) converter (IMMC) is proposed. This proposed IMMC is synthesized with lower voltage half-bridge SiC inverter blocks connected in a series to support medium voltage DC-DC conversion. A power sharing stage is an integral part of this proposed converter, enabling two series of connected PV plants to supply unequal power under partial shading conditions. The second part of this dissertation explores a proposed single-stage transformerless (SSTL) microinverter that resolves recent microinverter-related challenges. Current microinverters require a high-frequency transformer and DC link buffers. This adds additional power loss and size constraints to the design. Most currently available microinverters do not incorporate a battery storage system (BSS). Thus, an auxiliary DC-DC converter is needed for battery state of charge control. In response to these challenges, an SSTL microinverter with an integrated BSS is proposed to eliminate transformer and DC link buffers. These buffers are replaced with low voltage half-bridge modules along with film capacitors. The third part of this dissertation expands the SSTL microinverter concept for three-phase hybrid PV/battery inverters for commercial grid applications. The proposed converter uses a minimum number of sensors to integrate both solar PV and battery sources, offering the ability to control for maximum power transfer, as well as charge/discharge functions in the BSS. It was found that the resulting zero voltage switching operation yields higher conversion efficiency. The fourth part of this dissertation involves implementation of a defense control mechanism in a PV inverter system, offering protection against cyber-attacks via a hardware-in-the-loop platform. A harmonic injection type of cyber-attack was used to test the inverter’s behavior. Additionally, a watermarking technique was added to the DSP controller to allow for fast intrusion indication.