Seamless Control for a Photovoltaic System

Abstract

This thesis proposes a multifunction interactive PV system that has the ability to feed linear and nonlinear local loads in either grid-tied topology (GTT) or off-grid topology (OGT) and transfers between these two topologies seamlessly. The interactive PV system is built using a two-level three-phase voltage source inverter (VSI) and utilizes a sliding Fourier transform based phase-locked loop (SFT-PLL) as an adaptive and robust technique for extracting the fundamental load current and estimating the fundamental grid current. Further, the SFT-PLL is also used to synchronize the inverter output voltage with the grid during the GTT. The proposed synchronization technique has a superior immunity to a multitude of weak grid problems. The SFT-PLL has a very good ability to mitigate the nonlinear load harmonics at the point of common coupling (PCC) and to deliver active current to the grid with a unity power factor in case of sufficient solar power. Moreover, the proposed technique is able to balance the three-phase grid current when the connected load is unbalanced whether the PV source is available or not. The DC-link voltage is controlled during GTT operation mode by the VSI controller such that the available maximum power from the PV source is extracted. During the OGT operation mode, the AC load voltage and frequency are regulated by the VSI controller while the DC-link voltage is regulated by the boost converter controller to satisfy the feeding requirements of the connected local load. The proposed system is simulated using MTALAB/SIMULINK (v2018) and experimentally verified using a combination of rapid control prototyping platform and a Hardware-in-the-loop platform. A dSPACE MicroLabBox DS1202 runs the real-time co