Advanced Control Strategies to Enable a More Wide-Scale Adoption of Single-Phase Photovoltaic Systems

The installations of PhotoVoltaic (PV) systems, including grid-connected PV systems, have experienced a significant increase in the past few decades. More PV capacity is expected for the future power grid to be of intelligence and flexibility. This thriving scenario also raises concerns about the availability, quality and reliability of the whole power grid. Consequently, grid codes/standards are continuously being revised to host more PV energy in the grid. Ancillary services required initially by wind power systems, e.g. fault ride-through and grid support, are becoming more preferable in PV systems today than they were. In addition, achieving high efficiency and high reliability is always of importance for PV systems in order to reduce the cost of energy. Both goals can be enabled by advanced control strategies for PV systems, which are typically based on power electronics technology. In the light of those issues, the Ph.D. project has investigated and evaluated next-generation transformerless inverters for single-phase grid-connected PV systems, and proposed advanced control strategies to enhance the PV penetration with reduced cost of energy. The research work mainly includes two parts: 1) Modelling and Evaluation of Single-Phase PV Systems and 2) Advanced Control Strategies for Single-Phase PV Systems.

The first part, including Chapters 2 and 3, describes the entire models of the most promising transformerless PV candidates (models of PV modules, inverters, and filters) in Chapter 2. Then, a thorough evaluation of those topologies in terms of e.g. efficiency, reliability, leakage current mitigation ability, and reactive power injection capability has been presented in Chapter 3, where a multidisciplinary assessment approach with characterized features of energy production estimation and lifetime prediction based on mission profiles (e.g. solar irradiance level and ambient temperature) has been proposed. Grid detection and synchronization techniques have also been discussed in Chapter 2, since they are of importance in the control of single-phase systems both in normal operation mode with maximum power point tracking and under grid faults.

According to those investigations, advanced control strategies, which are able to ensure a flexible, reliable and efficient power conversion from PV systems in different operation modes, have been proposed and implemented in Chapters 4 to 6. The first discussed advanced control strategy is about the low voltage ride-through control of single-phase PV systems, which can contribute to the grid voltage stability and the avoidance of energy losses during low-voltage grid faults. Then, a constant power generation control (reduced power control) strategy has been introduced in Chapter 5 with the purpose to accept more PV energy without violating the distribution grid capacity. Other benefits of constant power generation control have also been explored in that chapter. Finally, thermal optimized control strategies are discussed in order to improve the reliability of PV inverters, and thus an extended lifetime of the entire system is expected.

It has been found from the research work that the penetration level of PV systems will be much higher with the demand of clean power generation, leading to an evolution of grid standards. The presented verification results reveal that the proposed advanced control strategies can underpin the increase of power electronics based PV systems. It is expected that the outcome of this work documented in this thesis would make a contribution to the development and implementation of new single-phase grid standards regarding PV interconnection and novel control strategies in the future. Finally, a more wide-scale adoption of PV energy can be realized with reduced cost of energy.