Topology and Control for Photovoltaic Microinverters

Abstract

Motivated by the continuously increasing world energy demands and greenhousegas (GHG) emissions from conventional fossil fuels, renewable energy, especially solar energy is ready to become a significant part of global energy portfolio. The microinverters, as the interface between the photovoltaic (PV) modules and the ac electrical power grid, have become popular due to the potentials to achieve high reliability, improved system flexibility and enhanced electrical safety compared with the string or central inverters. Thanks to the decreasing costs and disruptive enabling technologies, they have rapidly emerged in the global residential and even commercial PV market.

This dissertation proposes a two stage isolated PV microinverter comprised of dc-dc converter followed by dc-ac inverter. For the front-end dc-dc converter, the with enhancement gallium nitride filed-effect transistors (eGaN FETs) is used to achieve high voltage gain, soft switching and high frequency operation. To further explore the electrical and thermal characteristics of eGaN FETs, the driving signals of eGaN FETs with appropriate overlap are proposed and implemented to not only optimize the reverse conduction performance of eGaN FETs, but also avoid shoot-through current during dead time in single phase leg structure. To evaluate the cooling requirements in advance to ensure effective heat dissipation with minimized heat sink, a simplified thermal resistor model of eGaN FETs is proposed and verified thorugh both finite element analysis (FEA) and LabVIEW/ Multisim co-simulation.

Single phase inverters are inherently subject to the double line frequency ripple power at both ac and dc sides. The general solutions that unify all existing power decoupling techniques is obtained. The component counts, energy utilization and voltage/current ripple of energy storage components, dc voltage utilizations of both main circuit and power decoupling circuit, and the current stresses of power devices in main circuit are derived and investigated. The evaluations on all existing power decoupling techniques for the two stage PV microinverters, are summarized to provide helpful guidance during design. A digitally implemented proportional resonant (PR) and hybrid hysteresis current control with soft switching in the dc-ac inverter of the two stage PV microinverters is proposed to reduced switching losses, optimize zero-crossing distortions and mitigate low frequency harmonics.