Research On Control Strategy Of Micro-grid Inverter Based On Virtual Synchronous Generator Technology

With the continuous increase of energy demand,fossil energy is gradually consumed,and the distributed generation technology utilizing wind energy and solar energy is widely applied.Grid inverter is the key of the distributed power supply and power grid connection device,a flexible control,the advantages of fast response,but the power electronic devices have inertia and damping is small,with the increase of the distributed power grid permeability as well as the decline in proportion of synchronous generator,inertia and damping of large power grid will decrease,the risk brings to the safe and stable operation of power grid.By simulating the features of traditional synchronous generator,the virtual synchronous generator technology endowing the grid-connected inverter with certain inertia and damping characteristics,which has good frequency support capability and grid compatibility.In this paper,the operation principle and control strategy of micro-grid inverter based on virtual synchronous generator technology are studied.The main research contents are as follows:(1)Virtual synchronous generator technology is introduced into the inverter for the lack of inertia in the traditional inverter control strategy.Firstly,by analyzing the frequency regulation principle of synchronous generator,the frequency modulation link and rotor motion equation are introduced into the active frequency regulator of virtual synchronous generator.Based on the analysis of excitation voltage regulation principle of synchronous generator,the reactive voltage controller structure of virtual synchronous generator is given.Then,in order to optimize the output of the virtual synchronous generator,the virtual impedance link and voltage current double closed-loop control structure are introduced,and the overall control structure of the virtual synchronous generator is derived.Finally,the simulation results show that the virtual synchronous generator technology has the characteristics of primary frequency modulation and inertia.(2)In order to take full advantage of the flexible design of power electronic equipment parameters,further strengthen the inertia and damping characteristics of the virtual synchronous generator,improve the active frequency controller,and propose an adaptive control strategy of rotational inertia and damping.Power ring small signal model is established firstly,and analyze the moment of inertia and damping coefficient of the effect the dynamic performance of the system,then analysis Angle oscillation curve and frequency transient process to get the moment of inertia adaptive rule,design the moment of inertia and damping coefficient of the adaptive function expression,according to the frequency change dynamically adjust the moment of inertia and damping coefficient,to damp powerfluctuations,reduce frequency concussion.Finally,the simulation results show that this control strategy is superior to the traditional control strategy.(3)An improved control strategy is proposed for the current distortion and power oscillation of the virtual synchronous generator caused by the negative sequence components generated under the condition of unbalanced power network.First according to the symmetrical component method and instantaneous power theory analysis under the condition of unbalanced power grid,the negative sequence component in the influence of the inverter output,and designed to extract the positive and negative sequence component second-order generalized integrator,and without changing the virtual synchronous generator voltage source and the inertial characteristics under the premise of a given value respectively by controlling the negative sequence voltage balance of constant current,constant active and reactive power control target,according to the condition of different demand to choose the appropriate control target.Finally,the effectiveness of the improved control strategy is verified by simulation experiments.