Dual multi-string PV topology fed three level grid connected inverter

Presently, multi-string topology is considered the state of the art for grid integration of high power photovoltaic (PV) systems. In this paper, an improvement of the basic multi-string topology is presented which uses a three-level neutral point clamped (3L-NPC) converter as central grid tied inverter. Each of the two capacitors of the split dc link in 3L-NPC converter is fed from a separate multi-string PV configuration. This modified topology is referred as dual multi-string (DMS) topology. Compared to basic multi-string topology, the dual multi-string topology doubles the number of distributed maximum power point (MPP) trackers, increases modularity and reduces the step-up effort required by the dc-dc stage to half. The control design for grid integration of DMS topology is presented and the limitation of implemented control algorithm is derived. Simulations are done on scaled-down model to validate the control strategy and hardware implementation is done using a 3 kW_p roof top PV installation.