Control Strategies for Grid-Tied Inverters Based on Photovoltaic Solar Systems

Abstract

A two control strategy for a photovoltaic grid-tied system is proposed in this paper. A microgrid (MG) can be operated in a grid-tied mode or be disconnected from the grid (in an islanded mode). To interface renewable energy sources with low-voltage distribution systems, grid-tied inverters are more commonly used, however. When a MG operates in conjunction with the grid, various control techniques have been proposed based on current control (power control) in single or triple loops. In this paper, the proposed controller enables the PV solar system to run based on voltage-control (frequency/voltage) to regulate both active and reactive power injected into the grid. The MG consists of a photovoltaic (PV) array; a DC/DC boost converter to interface the PV array to a common DC-link with double loop strategy; a 3-phase inverter controlled using a triple loop strategy; an LCL output filter; and an impedance feeder used to connect the output of the inverter to the busbar of the grid. A dq-frame for voltage and current control structure is also used. A Phase-Locked Loop (PLL) mechanism is used to provide current control in order to extract (Ɵ), while it is extracted from the frequency control in the voltage control structure. Additionally, the control strategy uses feed-forward techniques to reduce the impacts of the inherent nonlinearity of the control system and inter-couplings. Two control strategies were thus designed, analysed, and validated through simulations under several operating scenarios using detailed switching models of a 23.5 kW a PV unit in PSCAD/EMTDC software.