Low Voltage Ride Through Performance And Analysis Of Grid-connected Photovoltaic System

Due to the rapid penetration of photovoltaic(PV)power to the grid,energy regularity authority is compelled to impose much stricter code on this integration of power.The integration of large amount of PV power makes the conventional utility grid more pronounce to fault occurrence.During the grid fault,PV system itself and the interfaced power converter between the grid and PV system encounter different types of abnormalities in their operations.When voltage dips occur,voltage at the point of common coupling(PCC)goes to low value,in response the grid current increases.The DC link voltage increases for the purpose of power balancing.The PV system is forced to be disconnected from the grid according to the grid code requirements.The power system operation is adversely effected by this shutdown of large PV power plant.Low voltage ride through(LVRT)is the most common and simplest technique for current controlling during the grid fault and provides the necessary protection to the power converter from overcurrent as well as the reactive current injection to support the utility grid during voltage sag.Two novel methods of LVRT to deal with voltage dips have been implemented for the PV system.A control strategy of the two-stage three-phase grid-connected PV system to improve the LVRT capability according to the grid connection requirement has been used.With the help of the new strategy,the PV array can generate active current as much as possible according to the depth of the grid voltage dips.It is worthy of special mention that the simulated strategy can ensure the reactive current support when a low voltage fault occurs without additional device.The effectiveness of the proposed control strategy has been demonstrated through various simulation scenarios.Compared with the conventional protection,the proposed control strategy not only enhances the LVRT capability of the PV system,but also provides grid support through the active and reactive current control.The second LVRT technique simulated in this research work is crowbar.The use of crowbar is the most simple and important method to provide protection to the power system which bypasses the overcurrent during the grid fault.Additional DC chopper circuit has also been used in the model at PV side to provide a constant DC link voltage.It decreases the triggering time of crowbar circuit and increase the stability of the system.The aim is to make the grid-connected PV model compatible with new grid code and is able to catch the main dynamics of the PV system in response to voltage dips.Simulation results will investigate the performance of crowbar and chopper circuit for grid connected PV system.The aims of this algorithm are: 1)To study and analyze the LVRT in the grid connected PV system;2)To examine the performance of crowbar for converter current bypassing during the LVRT and to keep the current within the normal converter rating;3)To investigate the performance of DC chopper circuit for constant DC link voltage during grid fault.