Highly Efficient GaN-Based Single-Phase Transformer-less PV Grid-Tied Inverter

Khaled Alatawi, University of Denver


Growing energy demand and environmental concerns have led to an increased interest in renewable energy resources to provide a sustainable and low carbon emission energy supply. Among these renewable energy resources, photovoltaic (PV) systems have been the focus of many scientific researchers. The most vital component of a PV system that needs to be improved is the power converter. Grid-tied transformer-less inverters have gained a lot of interest in recent years because of their higher efficiency, reduced volume and lower cost compared to traditional line transformer inverters.

This dissertation discusses single-phase transformer-less inverter challenges and provides solutions that could lead to a next generation, high performance, grid-connected, single-phase transformer-less inverter. A new topology with new current paths is proposed to increase efficiency and reduce the leakage current. A comparison study of the proposed topology and multiple transformer-less inverters is carried out in terms of leakage current, power losses and efficiency.

This dissertation also investigates the impact of emerging Gallium Nitride (GaN)-based power devices on a single-phase transformer-less inverter in terms of efficiency, high switching frequency capability, volume and cooling efforts. GaN device structure, as well as static and dynamic characterization, are discussed.

Furthermore, this dissertation studies GaN power devices’ reverse conduction capability to provide the proposed inverter with reactive power control. Existing PWM techniques cannot provide a freewheeling path in the negative power region to generate reactive power in a single-phase transformer-less inverter. Thus, a new PWM technique is proposed to provide new modes of operation to achieve reactive power generation capability in the proposed inverter.

Due to the increased penetration of PV systems into the grid and the updated grid codes concerning PV systems, next-generation PV systems will be required to have several features like high efficiency, high power quality, voltage regulation and fault ride through capability. This dissertation also explores these future requirements for PV system integration into the grid. To comply with the new grid codes and to enhance the PV inverter capability, a simple and flexible multifunctional control strategy is developed to provide PV inverters with advanced functions that will support the grid.

The simulation results validate the theory that the proposed topology reduces the conduction losses of the system. The conduction losses, switching losses, and thermal analysis at different output powers and switching frequencies verify the benefits of replacing Silicon (Si) MOSFET with Gallium Nitride (GaN) HEMTs. Moreover, the use of GaN HEMTs provides superior performance at higher frequencies when compared to their Si counterparts. Consequently, the filter volume is reduced, heatsink requirements are also reduced, and the cost is lowered. Furthermore, the simulation results validate the improvement of the proposed high efficiency transformer-less inverter with the new pulse width modulation (PWM) techniques to generate reactive power. The results also prove the effectiveness of the multifunctional control strategy to provide maximum active power injection, ride through faults, and support the grid by providing reactive power during grid faults. The high efficiency PV inverter equipped with advanced functions is the key to providing a reliable and cost-effective future grid tied to a PV system that can improve power quality.