Analysis And Suppression Of Resonance In Grid-Connected System With Long Transmission Line

In recent years,renewable energy such as solar energy has developed rapidly.Among them,the development of photovoltaic power generation has attracted much attention.Photovoltaic power generation is recognized as one of the most promising technologies with its unique characteristics.But there is a resonance problem in the photovoltaic power system,which seriously affects the power supply quality of the system and even threatens the safe operation of the system.According to the resonance frequency,the resonance that causes the photovoltaic grid-connected inverter system to be unstable is divided into harmonic resonance and fundamental resonance.Therefore,the resonance analysis of the photovoltaic grid-connected inverter system is divided into harmonic resonance analysis and fundamental wave resonance analysis.Based on the grid-connected photovoltaic inverter system with long transmission lines,the transmission line model and the Norton equivalent model of the inverter are established.The harmonic resonance mechanism of the grid-connected system is analyzed.It is pointed out that there are multiple resonance points in the grid-connected inverter system at this time.When there are background harmonic voltages in the power grid,the background harmonics propagation law and resonance amplification along the background harmonics are analyzed.A complex-impedance active damper(CI-AD)suppression scheme is proposed.This scheme is equivalent to introducing a complex impedance at the point of common coupling,selecting an appropriate coefficient to adjust the impedance value and then solving the system multi-resonance problem,at the same time,when there are background harmonic voltages in the power grid,this solution can also suppress the resonance amplification along the background harmonics,effectively improve the power quality.The specific transmission line length will cause the fundamental resonance of the grid-connected inverter system,resulting in overvoltage at the grid-connection point.By analyzing the relationship between the transmission line length and impedance characteristics,it is concluded that the problem of the fundamental resonant overvoltage ofthe fundamental wave in the photovoltaic system is related to the characteristic of the fundamental wave impedance of the transmission line,the greater the fundamental impedance module,the more serious the grid-connection point overvoltage.This paper proposes a thyristor controllable reactor scheme at the grid connection point from the perspective of impedance reconstruction,which can not only solve the problem of fundamental harmonic resonant overvoltage,but also stabilize the PCC point voltage and make the system run stably.Finally,an experiment platform of single-phase inverters is build and the hardware and software of the system is designed.The correctness of the theory analysis of the complex impedance active damper proposed in this paper is verified by experiments.