| The increase of the proportion of Photovoltaic(PV)grid-connected generation in distributed energy supply has an increasingly significant impact on the power grid.In order to ensure the stability of the power grid,the grid-connected guidelines require PV inverters to have fault ride-through capability when the grid voltage sags.Grid voltage sags,swells,and unbalanced conditions seriously threaten the stable operation of grid-connected inverter.In order to maintain the stable operation of the PV grid-connected system during asymmetrical al power grid faults,it is of practical significance to carry out research on the fault ride-through control method,stability analysis and model parameter identification of PV grid-connected system during asymmetrical fault ride-through.Firstly,the thesis adopts the modular modeling of PV grid-connected system and obtains the simulation model according to the requirements of grid-connected guidelines.The output characteristics of the PV cell under different irradiance and temperature,and the operation characteristics of the PV grid-connected system under normal working condition and irradiance changes are simulated and analyzed.Secondly,the asymmetrical fault ride-through control model of PV grid-connected system is established,and the harmonic generation mechanism of PV grid-connected system during the asymmetrical fault ride-through is deduced.An optimization method for asymmetrical fault ride-through control based on notch filter and proportional resonant controller with negative sequence voltage feedforward control is proposed.Finally,from the perspective of model parameter optimization,the large signal and small signal stability of the controller parameters of the PV grid-connected system model inverter are obtained by using the mixed potential function theory and impedance analysis method,so as to guide the parameter design.Aiming at the parameters acquisition of inverter controller,the appropriate target parameters,observation variables and identification algorithm are selected by sensitivity analysis,probability reliability assessment and comparison of multialgorithm identification results,and the identification strategy is formulated.The identification results of high probability reliability are obtained as the final parameter identification value to improve the accuracy of model parameters.The different fault ride-through control strategies under the condition of asymmetrical grid voltage sags are simulated and compared under the condition of asymmetrical grid voltage sags,and the suppression effect of the proposed asymmetrical fault ride-through control strategy considering harmonic suppression on DC bus voltage fluctuation,output power fluctuation and grid current harmonics is verified.It shows that the proposed optimized control strategy can guarantee the safe and stable operation of the PV grid-connected system during asymmetrical faults. |
