Study On Stability Of Multiple Grid-Connected Inverters With Time-Varying Input Voltage

Most of the electrical energy generated by renewable energy is DC electrical energy.Renewable energy power generation converts DC electricity into a sine wave alternating current with the same frequency and phase as the grid through grid-connected inverter,and then integrates the electrical energy into the grid to achieve grid-connected power generation.When the light intensity or the outside temperature changes greatly,the electrical energy output by the photovoltaic solar cell array also changes continuously.At this time,the DC input voltage of the grid-connected inverter is fluctuating and contains a lot of harmonics,causing the output power quality of the inverter and the stability of the system to be seriously reduced.Therefore,in-depth study of the stability of multi-grid inverters considering the time-varying DC input voltage has important theoretical and practical significance for improving grid-connected power quality.This dissertation first establishes the mathematical model of a single three-phase LCL grid-connected inverter,derives the expressions of DC input voltage and grid-connected voltage and current,and analyzes the grid-connected voltage,grid-connected current and grid-connected when the DC input voltage changes suddenly The impact of power quality.Then,three mathematical models of grid-connected inverters were established,and the influence of the sudden change of DC input voltage on the stability of multi-grid inverters was studied.The research results show that when the DC input voltage changes suddenly,the waveform of the grid-connected current is severely distorted,the harmonic content increases significantly,and the stability of the grid-connected system decreases or cannot continue to maintain stable operation.When multiple grid-connected inverters are connected in parallel,the waveform distortion of the grid-connected current is severe,the harmonic content of the grid-connected current is higher,and the stability of the grid-connected system is lower.Next,a method to improve the stability and power quality of the grid-connected inverter is proposed.By adjusting the parameter value of the filter capacitor C_fin the LCL filter,the harmonic content of the grid-connected current can be reduced,the power quality of the grid-connected system can be improved and the stability of the grid-connected system can be improved.Finally,the effects of different integral and differential orders of the fractional order PI~λD~μcontroller on the system dynamic and steady-state performance are studied,and the fractional order PI~λD~μcontroller of the grid-connected inverter is designed.The research results show that,compared with integer-order PID controllers,the use of fractional-order PI~λD~μcontrollers improves the phase margin of the system,increases the gain at the fundamental frequency,the system’s dynamic performance is better,and enhances the Harmonic attenuation ability greatly improves the stability and dynamic performance of the system.The simulation results verify the correctness of the theoretical analysis.