Research On Power Sharing Strategy Of Virtual Synchronous Generator Parallel System In Micro-grid

As an intelligent distribution network,microgrid can not only avoid the impact of large-scale and decentralized distributed power on the distribution network,but also highlight the advantages of distributed power.In order to diversify the functions of distributed power supply and meet the requirements of the development of microgrid,the control strategy of virtual synchronous generator is deeply studied in this paper,and the control strategy is applied to the inverter in microgrid.Aiming at the power distribution problem in the islanding operation,a reactive power sharing control strategy is proposed to ensure the stable operation of the microgrid.Firstly,the realization mechanism of Virtual Synchronous Generator(VSG)is studied.The mathematical model of VSG control strategy is established by referring to the characteristic equation of synchronous generator.According to the mathematical model and considering the influence of digital delay,the design method of VSG key parameters and voltage-current dual loop controller parameters is obtained.At the same time,the steady-state and dynamic characteristics of VSG are studied,and compared with droop control,the relation and difference between them are analyzed.Secondly,because the line impedance of the microgrid is mostly resistive,the active power and reactive power of VSG are coupled.Moreover,the line impedance of each VSG in the microgrid does not match,leading to the difficulty of the reactive power sharing between VSGs,which affects the stability of the system.To solve the above problems,this paper analyzes the power coupling and power distribution mechanism,and introduces virtual impedance to achieve power decoupling and power sharing.At the same time,the equivalent circuit of the microgrid is given,and the stability of the system and the effect of virtual impedance on the system stability are analyzed.Finally,in view of the disadvantages of the traditional virtual impedance reactive power sharing strategy,such as large voltage sag,this paper proposes a reactive power sharing strategy based on the adaptive virtual impedance.In this strategy,the maximum value of the equivalent line impedance in the system is set as the "global impedance",and VSG adjusts its virtual impedance to match the equivalent line impedance with the "global impedance".Since the line impedance is difficult to measure,it can reflect the line impedance through the excitation voltage,so the decentralized network and consensus mechanism of block chain technology are adopted in this paper to respectively realize the transmission of excitation voltage information and the selection of "global voltage" in the system.The difference value between local excitation voltage and "global voltage" is used as the input of fuzzy controller,based on the fuzzy relationship between the line impedance and the voltage difference value,then an adaptive virtual impedance is obtained to match the "global impedance",thus realizing the reactive power sharing and circulation suppression,improving the public bus voltage drop extent at the same time.