An ever-increasing interest on integrating solar power to utility grid exists due to wide use of renewable energy sources and distributed generation. The grid-connected solar inverters that are the key devices interfacing solar power plant with utility play crucial role in this situation. Although three-phase inverters were industry standard in large photovoltaic (PV) power plant applications, the microgrid regulations increased the use of single-phase inverters in residential power plants and grid interconnection. This paper presents a detailed review on singlephase grid-connected solar inverters in terms of their improvements in circuit topologies and control methods. Even though there are many reviews have been proposed in the current literature, this study provides a differentiating approach by focusing on novel circuit topologies and control methods of string and micro inverters. The single and multi-stage solar inverters are reviewed in terms of emerging DC-DC converter and unfolding inverter topologies while the novel control methods of both stages have been surveyed in a comprehensive manner. The isolated and transformerless circuit topologies have been investigated by reviewing experimental and commercial devices. The soft computing, evolutionary and swarm intelligence based algorithms have been summarized in MPPT methods section while current injection and grid-connection control methods of unfolding inverters stage have been presented with and without PLL architecture. There are many papers have been compared and listed in each section to provide further outcomes which is followed by a summarizing discussion section and conclusion.

Introduction

The use of renewable energy sources (RESs) is increasing day by day in electricity generation due to their variety and support to utility grids. Besides the opportunities, the wide integrations of RESs to utility grid have brought several challenges in terms of planning, operating, maintenance and management issues. The distributed generation (DG) policies set by governments and demand side management (DSM) requirements of distribution system operators (DSOs) have leveraged the interoperability plans for generation plants including solar photovoltaics (PVs), wind turbines, fuel cells, and micro sources. A DG system comprised by various type of energy sources requires appropriate power electronic devices for power conversion for coupling at a single bus bar. The grid-connected inverters which are required for RES and DG integration to utility play crucial role in resource management. Moreover, the DC-DC converters are also required for regulating the DC power generated by PV or fuel cell sources while inverters are interfacing the entire power plant with grid (Kabalci, 2015; Kabalci and Kabalci, 2018, 2017). The microgrid applications have promoted efficient power conversion requirements especially in low-voltage range (Schweizer and Kolar, 2013) where the wide variety of power converter studies on DC-DC converters and cascaded multilevel inverters have been proposed in the literature. The PV power plants hold the biggest share by increasing their installed capacity up to 400 GWp by the end of 2017 which is estimated to tackle 1000 GWp capacity by 2022 (JaegerWaldau, 2017; Masson et al., 2018; Schmela, 2018). The total installed capacity of global PV power plants has been increased more than 100 GWp in 2017 as seen in Fig. 1. The main contribution is supported by subsidies provided by US and European governments.