Research On Control Strategy Of Grid-Forming Of Power Conversion System

Centralized power supply cannot be carried out in some areas with difficulty in power supply,and distributed independent microgrid is generally used to supply power to loads.However,a large number of power electronic equipment and distributed energy are connected to the microgrid,which makes the inertia support ability of the microgrid decrease,and it is unable to provide voltage and frequency support for loads.Therefore,energy storage converters with network capacity are needed to provide support for micro-grid and improve the characteristics of low inertia and weak damping of power grid.This kind of energy storage converters are called network type energy storage converters.In addition,with the increasing requirements on the capacity of micro-grid,the energy storage converter needs multimachine parallel operation to realize the large-capacity network of micro-grid.However,the multi-machine parallel system is prone to power oscillation and other problems,resulting in uneven distribution of output power and affecting the parallel effect.Virtual synchronous generator(VSG)can improve the low inertia and weak support capacity of the microgrid.It provides voltage and frequency support for microgrid,and realizes power equalization of multiple converters by using its droop characteristics.In this paper,the topology of Type I NPC energy storage converter is taken as the research object,and the control strategy of grid-type energy storage converter is studied as follows:(1)Summarize the research status of energy storage converter topology,analyze the advantages and disadvantages of topology from different perspectives,and summarize the current control strategy of energy storage converter.(2)The frequency and voltage regulation characteristics of synchronous generator were analyzed,and the mathematical model was established,and then the VSG control block diagram of single network type PCS was obtained.In order to select the appropriate inertia J and damping D,a small signal model was established,and the inertia J was designed according to the national standard constraints.The rationality of the parameters is verified by the semi-physical simulation.(3)According to the requirements of large-capacity network construction of micro-grid,multi-machine parallel mode is adopted to achieve uniform power distribution by using VSG droop characteristics.There are some problems in grid control,such as coupling of active and reactive power and power oscillation caused by line impedance.Virtual stator impedance is introduced into the outer voltage loop to realize power decoupling and suppress power oscillation by adjusting output impedance.In order to solve the problem that low SOC level can limit the converter’s active power output,the design process of power self-regulation at SOC level is given,and the power self-regulation VSG control strategy based on SOC level is proposed.The control model is built in Matlab/Simulink,and based on this,the effectiveness of the control strategy is verified by the semi-physical simulation platform.(4)According to the communication requirements of power electronic converter on microgrid,such as display function,setting function,etc.,the universal communication protocol of power electronic converter is designed.Based on this,the software design process of Kunlun on-state touch screen upper computer and the interaction logic with DSP are given to provide reference for engineering debugging personnel.