Research On The Control Strategy Of Multi-machine Parallel Connection Of Energy Storage Converter Based On VSG

With the vigorous promotion of clean energy,the microgrid technology with distributed power sources as the core has been greatly improved.Among them,solar energy,wind energy and other clean energy have certain intermittent,random and other defects.Large-scale centralized grid connection is carried out through the interface of power electronic devices.Due to the characteristics of low inertia and low damping of the interface,the microgrid system is unstable.The combination of large-scale energy storage technology and virtual synchronous generator control strategy(Virtual synchronous generator,VSG)can improve the stability of the system and the operating performance of the energy storage converter(Power Conversion System,PCS).For the overall capacity and reliability requirements of the network system,parallel connection of multiple machines is imperative.In this paper,the electrochemical energy storage system represented by the all-vanadium flow battery is selected.On the basis of the energy storage system,the VSG algorithm is used to provide damping and inertial support for the system by simulating the external characteristics of the traditional synchronous generator.In a system with parallel converters,most of the energy storage converters,including the battery on the DC side,and the public load have uncontrollable line impedance characteristics,which will cause the uneven distribution of the output power of each converter.A large circulating current is generated between the energy storage converters(Power Conversion System,PCS)and affects the stable operation of the system.In view of the above problems,the four aspects of modeling for all-vanadium flow battery,virtual synchronous generator modeling,small signal stability analysis of VSG parallel system,and virtual impedance adaptive control strategy are carried out.The main research contents are as follows:(1)Firstly,introduce the composition and construction principle of the vanadium liquid flow,construct the equivalent loss model,electrochemical module,fluid mechanics module,and model the hybrid model,and calculate the stack voltage,stack current,SOC,etc.of the battery The external characteristics of the battery were simulated to verify the feasibility of the model.At the same time,it was verified that the VRB on-load discharge has no significant effect on the input and output voltage and current of the PCS.(2)Based on the characteristics of low inertia and low damping of the power electronic interface,the VSG algorithm is adopted.The mechanical part model and electrical part model of the synchronous motor are used to model in detail,and the virtual synchronous generator is modeled.The principle of the VSG algorithm is explained from the active speed governor control and the reactive excitation control,and the virtual moment of inertia is simulated.And the damping coefficient,etc.on the power and frequency dynamic performance of the system.(3)On the basis of the energy storage system,the VSG grid-connected model and the VSG parallel system are modeled with small signal,constructed into a state space expression,and the roots of the characteristic equation are obtained,and the different moments of inertia,The distribution of the root locus of the damping coefficient,droop coefficient,frequency modulation coefficient,virtual equivalent inductance and other parameters,it can be seen that the trend of the characteristic root changes with the change of the parameter,then the stability of the system also follows the trend of the characteristic root Changes occur,according to the stability judgment conditions of the control system,providing a reasonable range of selection for the selection of relevant parameters.(4)Based on the stability analysis of the parallel system,the parallel system with multiple energy storage converters controlled by virtual synchronous generators suffers from certain limitations in the low-voltage line and uncontrollable factors of line impedance lead to uncontrollable system power distribution.A control strategy of adaptive virtual impedance is proposed to reduce the voltage difference between any two inverter output terminals by introducing an adaptive virtual impedance in the voltage outer loop,so as to suppress the generation of circulating current in the system and eliminate it.The power coupling caused by the line impedance realizes the power distribution according to the rated capacity ratio and the stable operation of the system.Finally,the feasibility of the strategy is verified by simulation.