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With the innovative progresses in power electronics in recent years, photovoltaic (PV) systems emerged as one of the promising sources for electricity generation at the distribution network. Nonetheless, connection of PV power plants to the utility grid under abnormal conditions has become a significant issue and novel grid codes should be recommend. The low-voltage ride-through (LVRT) capability is one of the challenges faced by the integration of PV power stations into electrical grid under abnormal conditions. This work firstly provides a discussion on recent control schemes for PV power plants to enhance the LVRT capabilities. Next, a control scheme for a three-phase four-leg grid-connected PV inverter under unbalanced grid fault conditions using synchronous reference frame proportional integral (SRFPI) controller is proposed. Simulation studies are performed to investigate the influence of the control strategy on the PV inverter.

<span lang="EN-US">Power quality is a concern for utility and grid operators due to a large penetration of intermittent and stochastic renewable power generation sources. One of the major concerns, when designing and controlling grid-feeding photovoltaic (PV) inverters is meeting the grid requirements. International grid requirements demand low-voltage ride-through (LVRT) capability and maintaining grid functionality during fault conditions. This paper presents a comprehensive review for several control techniques to assure the LVRT capability of grid-feeding converters as well as discussing their respective advantages and limitations in detail. Areas for further research are identified afterwards. Finally, the conclusion gives a brief summary and critique of the findings.</span>

<p>Recently, researchers have shown an increased interest in renewable energy sources (RESs) to improve power quality, reliability and stability of the power system. However, RESs require proper control strategies to ride through grid disturbances and stay connected to the main grid for the stability contribution. Moreover, the control of them relies mainly on the synchronization algorithms to precisely detect the voltage phase angle, magnitude and frequency. Throughout the years, various synchronization schemes have been introduced and developed. In the renewable-based generation sources, the phase-locked loop (PLL) is a well-known technique for operating grid-tied power converters for the estimation of the synchronization information and grid voltage monitoring. This paper aims to provide a comprehensive review of the recently developed PLL algorithms for grid synchronization applications. At first, various estimation techniques are discussed. Then, a comparison between various PLLs and possible future works are recommended.</p>