Research On Control Strategy Of Virtual Synchronous Machine For Photovoltaic Inverter Under Weak Grid

In recent years,with the increase of PV penetration rate,a large number of PV inverters are integrated into the grid in the form of power electronic interfaces,which poses new problems for the safe and stable operation of power systems.The VSG introduces synchronous generators in the control.The rotor mechanical and stator electrical characteristic equations can well simulate the operating characteristics of synchronous generators,provide inertia and damping for the system,and have a good supporting effect on maintaining the stability of the power system.At the same time,with the increase of photovoltaic permeability,long-distance transmission lines will make the terminal grid exhibit weak grid characteristics.For the special operating environment of weak grids,it is of great significance to study the grid-connected control method of inverters based on virtual synchronous machines.This paper firstly models the three-phase photovoltaic inverter under the weak grid,introduces the multi-loop control strategy of the virtual synchronous machine,and the outer loop control is the power loop.Through the introduction of the rotor motion equation and the active and reactive power regulation characteristics of the synchronous generator,The inverter can simulate the synchronous generator running characteristics,and the inner loop is a voltage and current control loop,which can stabilize the output voltage and improve the dynamic response speed of the grid-connected current.The analysis of the output characteristics of the inverter shows that there is coupling between the output active power and the output reactive power in the weak grid environment,and the influence of power coupling is weakened by adding the virtual impedance in the stator electrical equation.Next,the passive control parameters are simple,and the response speed and robustness are weak under the weak grid.The Euler-Lagrangian(EL)equation of the system is established,and the damping is completed to complete the passive controller.Design and improve the current control inner loop of the VSG.Establish the transfer function of inner loop control under weak grid,draw open-loop Bode diagram,compare and analyze the influence of PI-controlled current inner loop and passively controlled current inner loop on system control performance.Finally,the influence of virtual rotor inertia and damping coefficient on system control performance is analyzed.Then,for the case that the system power frequency is prone to disturbance under weak grid,a virtual rotor inertia adaptive method of VSG is proposed.The change,the virtual rotor inertia is adjusted in real time,so as to reduce the system oscillation and reduce the overshoot of the system frequencywhen the active power is given a change.Compared with the VSG with fixed virtual rotor inertia,the superiority of the adaptive algorithm in suppressing system oscillation is verified.Based on the simulation verification of the above research content.Design the hardware circuit and software program of the system,complete a 1.5kVA three-phase PV inverter experimental prototype,and verify the proposed control algorithm.