Cascaded Loop Control of a Three-Phase Grid-Connected PV Inverter System

Abstract

Owing to the advantages Photovoltaic (PV) systems possess over other renewable energy sources, more resources have been diverted into their advancement in recent years. These advantages combined with the increasing energy demand of the world have led to the commercialisation of PV power generation. One way of achieving large-scale utilisation of PV sources is through grid-connection. This paper focuses on the control of a three-phase grid connected PV inverter system that comprises a regulated boost DC-DC converter and a Heterojunction with Intrinsic Thin Layer (HIT) PV array. Perturb and Observe (P&O) Maximum Power Point Tracking (MPPT) Algorithm was used in the control of the boost converter and the Sanyo HIP-215NKHE5 solar module was chosen and simulated for system verification. For grid integration, a three-phase voltage source inverter was modelled together with an LCL filter. The inverter control used was a voltage-current cascade loop control scheme that employed Proportional Integral (PI) controllers in conjunction with a Phase Lock Loop (PLL) and the synchronous d-q frame control. The optimal PI gains were obtained using the Ziegler-Nichols method and the system was simulated in MATLAB/Simulink. With this control, the input to the inverter was maintained constant irrespective of the changing PV output voltage. The amount of current injected into the grid was regulated. The inverter system output was successfully synchronised with the grid in terms of phase and frequency, thereby improving power factor control. Thoughrelatively not robust, this control strategy was relatively simple, easy to implement and efficient