Research On Virtual Synchronous Generator Control Technology And Its Application Under Unbalanced Conditions

Improving the utilization efficiency of renewable energy,reducing the dependence on fossil fuels,and building a clean,efficient,green and low-carbon power grid are the development direction of power system in the future.The distributed generation in AC microgrid mostly uses inverter interfaces for energy exchange.With the penetration of large-scale renewable energy,the inherent fast response characteristics of inverter reduce the inertia and damping of the system.Virtual synchronous generator(VSG)technology is of great significance to the friendly access of distributed energy and the improvement of system stability.In this paper,a detailed study of the virtual synchronous generator control technology and its application under unbalanced conditions is carried out.First,the active frequency control loop and the reactive voltage control loop of the VSG are designed based on the mathematical model of the synchronous generator and the characteristics of frequency and voltage regulation.The voltage and current double closed-loop controller is designed,and the parameters are adjusted by the pole configuration method.Then the VSG small-signal model for grid-connected conditions was established,and the fourth-order small-signal characteristic equations were used to analyze and discuss the influence of key parameters on system stability in detail.In the simulation,the frequency and voltage regulation characteristics of the designed VSG model under grid-connected conditions are verified.Aiming at the power command disturbance in grid connected condition,the active response characteristics and transient process of VSG are analyzed in detail.In order to suppress power overshoot and frequency deviation,an adaptive parameter control method for VSG under grid connected condition is proposed.Then the operating characteristics of VSG off-grid condition are analyzed,and the frequency recovery control is designed to solve the problem of frequency deviation during off-grid when load changes.The simulation models of off-grid and grid-connected conditions were built to verify the improved control method.Finally,for the off grid condition,the traditional VSG does not have the ability to operate under unbalanced conditions,so the four-leg topology is selected as the main circuit topology,and the reference voltage is modulated by space vector modulation algorithm,and the resonant controller is used to improve the voltage and current double closed-loop controller,and the zero sequence controller is designed.Considering that the Microgrid is mostly in the medium and low voltage distribution network,and the line impedance has an impact on the power quality and power coupling of the bus,based on the sequence network model of VSG,the virtual negative impedance is constructed by DCCF and SOGI to compensate the line impedance,which reduces the unbalance factor of bus voltage The positive sequence power and zero negative sequence current of fourleg VSG parallel operation under unbalanced condition are distributed in proportion by adjusting the total impedance of the system.