  A fractional-order (FO) based controller for a grid-connected PV system is
presented in this paper. A single phase two-stage grid-connected photovoltaic
generator (PVG) is used to test the performance of the FO controller. The main
objectives of the proposed controller are: (1) To regulate the output voltage
of PVG at a point where the maximum power is drawn. (2) Constant DC-link
voltage control. (3) Power factor control (PFC) at the inverter output with low
total harmonic distortion (THD). To solve the first problem, a non-linear
control method known as fractional-order back-stepping control (FOBSC) is used
to regulate the output voltage of PVG. A maximum power point tracking (MPPT)
technique known as perturb and observe (P & O) is used to generate a reference
voltage which is suitable for tracking the maximum power generation of PVG. The
generated reference is used to regulate the output voltage of PVG using FOBSC.
The DC-link voltage fluctuation issue is tackled using FO based PI controller.
The last objective is achieved using FOBSC to obtain maximum power factor of
the grid. Lyapunov candidate function is used to verify the stability of the
system. To test the performance of the proposed controller, it is compared to
conventionally known Integer-order (IO) controller. Results have shown a
significant improvement in THD and efficiency of the system. The proposed
controller offers 0.94%, 1.43% and 1.86% lower THD in comparison with IO
controller at 100%, 80% and 70% of the power generation capacity of PVG,
respectively. The overall efficiency of the system for 100%, 80%, and 70% of
the dynamic powers of the system is noticed to be better in case of FO
controller.
