Removing the Oscillations of Injected Active Power of Grid Connected PV System under Grid Voltage Sag

The development of high power grid-connected photovoltaic (PV) systems and their adaptability with grid conditions is a crucial issue. In fault conditions, it can be considered with two sides, first being connected with the grid due to economical problems, second minimizing damages into the grid due to technical problems. The improvement of grid-connected PV systems performance under grid voltage sag (which is the most probable faults) is proposed in this paper. In point of common coupling in case of voltage sag, the active power will oscillating and the grid synchronization system would not work properly, which both are detrimental for unbalanced grid. In order to improve the performance of the exchanged power between the converter and the unbalanced grid, method of Positive-Negative-Sequence Control (PNSC: able to remove oscillation of active power) is suggested. This method is also effective to reduce the oscillations in DC voltage that can be detrimental for DC-link capacitor. In order to track the desired unbalanced reference current generated by PNSC, a proportional–resonant (PR) current controller has been designed. Furthermore, instead of Synchronous Reference Frame Phase Locked Loop (SRF-PLL) which suffers the distorted response in polluted grid, a Double Second-Order Generalized Integrator-Frequency Locked Loop (DSOGI-FLL) is used to synchronize inverter with the grid. Finally, to validate the proposed approach, the whole system is simulated via MatLab/Simulink® and simulation results have been compared with conventional method. Results verify the grid condition has improved in comparison with implementing the conventional method.