Dynamic Characteristics Analysis of Distributed PV Plants with Panel-level DC Optimizers Under Severe Partial Shading Conditions

DC optimizers (DCOs) based distributed MPPT (DMPPT) technologies are recently developed to mitigate the waste of solar energy due to the partial shading in photovoltaic (PV) arrays. However, the effect of DMPPT may be limited by the topology of cascaded DCOs and the voltage of the DC link when the mismatch problems become severe. In this case, some PV panels cannot work in the MPPT mode, even resulting in the reverse power consumption of PV strings, which is also named the "hot spot effect" of PV panels and causes security issues. Therefore, for DCO-based PV systems involving a large number of power electronic devices, it is necessary to study the dynamic characteristics of series-connected PV-DCO units and their DMPPT control systems under severe partial shading conditions so that the control strategies can be improved. To solve this problem, the electromagnetic transient model of the DCO-based distributed PV grid-connected system is established in this paper, in which the Boost topology is considered. The dynamic response of the PV system under serious mismatch is then analyzed. The main conclusion is that when suffering severe partial shading conditions, the DCO-based PV system will stabilize at a new forced equilibrium point due to the shoot-through state of DCOs, which is also the theoretical maximum power output point in this case. Besides, the improvement designs for the DMPPT control are proposed. Finally, case studies are performed to validate the correctness of the analysis.