Non-linear control of a grid-connected multi-cell photovoltaic inverter that operates under variable temperature and irradiance

This work evaluates the performance of a nonlinear control strategy of a modular photovoltaic (PV) plant that is used to support grid frequency control. The PV plant is designed based on a real study case and it has a modular structure consisting in several generating units. Each generating unit consists in PV arrays interfaced to the grid with a three-phase three-level cascaded H-bridge converter based on single-phase current-source inverters. It is shown that, under changing temperature and irradiance conditions, the output dc currents of the PV arrays become unbalanced. A non-linear strategy is proposed to control the output voltages of the H-bridges and to overcome the problem of unbalanced dc currents. Additionally a control technique called virtual synchronous generator (VSG) is applied to the PV plant in order to support the control of the grid frequency emulating an inertial response and providing damping and droop power. For the simulated cases, the nonlinear strategy was always capable of keeping the output voltages balanced. However, when the PV plant performed the VSG functions, the nonlinear strategy demonstrated a poor tracking capability of the output power reference. These results suggest that higher order controllers (instead of only gains) have to be tested in the nonlinear strategy in order to improve the tracking of the VSG power reference.