Reaearch On The Effects Of Inverter's Digital Time Delay On The Stable Grid-connected Capacity Of Large-capacity Photovoltaic Plant And Its Control Method

With the rapid development of renewable energy,the role of PV energy is gradually changed from supplemental energy to substitute energy in recent years.The digital time delay that exist in the grid-connected PV inverter not only reduces the stability of PV inverter,but hinder the capacity expansion of PV plant.Therefore,this thesis mainly studies the effects of digital time delay on the grid-connected large-capacity PV(LCPV)system and the control method of grid-connected inverter with digital time delay,which aims to improve the steady operation of grid-connected LCPV plant,based on National Natural Science Foundation of China(“Resonance Mechanism and Suppression Strategy Research of the Large-Scale Grid-Connected Photovoltaic System”,51477021).The researches have a theoretical and practical significance for the design and stable operation of the grid-connected LCPV plant.Based on the comparison of the topology of grid-connected LCPV system,considering the wide usage of digitally controlled PV inverter,the Norton model of grid-connected LCPV system with considering the digital time delay of PV inverter is built in this thesis for the first time.According to the system model,this thesis studies the effects of digital time delay on the system stability.The results indicates that,the digital time delay coupling between the inverters that caused by the grid impedance will lead to the decreases of the stability of grid-connected LCPV system,and the higher the digital time delay value,the lower the stability of grid-connected LCPV system.The research of the effects of digital time delay on grid-connected system stability is expanded from the single inverter system to the multi-inverter system in this thesis,and thus realize the comprehensive analysis of the effects of digital time delay on the stability of grid-connected inverter system,which set a foundation for further research.In consideration of the stability of LCPV plant is affected by the digital time delay of PV inverter and the allowable grid-connected capacity of LCPV plant is directly determined by the system stability,the effects of digital time delay on the stable grid-connected capacity of LCPV plant is researched based on the obtained LCPV system model.The research results show that stable grid-connected capacity of LCPV plant decreases with an increasing of digital time delay value of PV inverter;The adding of the inverter with a smaller digital time delay value will increase the system stability and thus raise the upper limit of the stable grid-connected capacity,and the LCPV plant that have more PV inverters with a smaller digital time delay value will have a higher stability and thereby obtaining a bigger stable grid-connected capacity;Moreover,a higher proportion of PV inverter with a smaller digital time delay value will lead to a higher system stability which make the system have a larger extensible capacity when the inverters in LCPV plant have different digital time delay values.The discussions about the allowable grid-connected capacity of LCPV plant provide a reference for the determination of the capacity in the design and expansion of LCPV plant,and the selection of the inverter with different digital time delay value,which is beneficial to the promotion of the economic efficiency of LCPV plant.In order to reduce the digital time delay impact on the stability of grid-connected LCPV system and increase the stable grid-connected capacity of LCPV plant,a new capacitor-current feedback based on the Infinite Impulse Response principles is proposed in this thesis.The new capacitor-current feedback uses the exact linear phase characteristic of digital filter to approach the required performance and extend the system damping region,which enable the system stability while maintaining a high robustness to the variable grid impedance when the digital time delay exists and system sampling mode changes.In addition,compared with the LCPV plant that contains the PV inverters with conventional capacitor-current feedback,the stable grid-connected capacity has risen by more than half in the LCPV plant with the PV inverters using the new capacitor-current feedback.It indicates that new capacitor-current feedback will reduce the effects of digital time delay on the grid-connected LCPV system and thus raise the stability of the grid-connected LCPV system,which is conducive to the development of LCPV plant.