Research On Advanced Power Sharing Control Strategy Of Distributed Generation In Islanding Microgrid

Nowadays,due to the development of society,the non-regenerative energy consumption is increasing continuously,energy demand and environmental protection are becoming more and more prominent.Therefore,the distributed generator based on renewable energy is particularly important.With the penetration of renewable energy in the grid becoming higher and higher,the distributed energy provides clean energy for the power grid,meanwhile more and more adverse impacts emerge.Therefore,in order to maintain the stability of the power grid,the distributed generator not only needs to provide power to the power grid,but also offers the capability of grid voltage support.In order to suppress the problems caused by the distributed generator,the concept of microgrid is proposed.The microgrid can operate in both grid-connected mode and islanding mode.In grid-connected mode,the grid-connected inverter is characterized as the controlled current source type interface,the output voltage and frequency are supported by the power grid.In islanding mode,the output inverter of microgrid is characterized as the controlled voltage source interface,at this time,due to the difference between the line impedance and the output impedance of the distributed generation source between the inverters and the common bus,the voltage drop difference appears,and it results in poor balance of power sharing.In this paper,power sharing control strategy of distributed generator in islanding microgrid is studied and one improved method is proposed.This paper is mainly divided into the following parts.First,this dissertation introduces the research background and significance,and introduces the research status of micro power grid,the inverter control mode,power equilibrium strategies of micro power supply,the overall control strategy and the system structure of microgrid,then points out the importance of power balance in parallel operation of multi micro power supplies in microgrid.In addition,according to the power balance control strategy based on virtual synchronous generator,the virtual synchronous generator control strategy is analyzed,which makes the microgrid with the corresponding supporting ability of grid voltage amplitude and frequency,and provides the necessary damping effect for the micro grid with a relatively poor stability.Then the mathematical model of the virtual synchronous generator is analyzed and the control algorithm of inverter is introduced.The characteristics of voltage amplitude and frequency variation caused by load change are common phenomena in power system,therefore,the operation control of islanding mode of microgrid needs to start with these basic problems.Virtual load model was presented in this dissertation to simulate the load,and the power electronic devices flexible programming was proposed to realize the flexible control of ac impedance,current and power,and achieve simulation of three-phase ac load.Then this dissertation introduces the ZIP(constant impedance,constant current,and constant power)virtual load model to simulate the control method of local load,and the strategy based on virtual synchronous generator(VSG)and ZIP virtual load are proposed to realize the control and simulation of microgrid running in the island mode.Then the simulation model is established in Matlab/Simulink,and the simulation results show the effectiveness of the control method.Furthermore,this dissertation introduces the power balance control strategy based on the traditional droop control,and analyzes the selection of connection inductance,the design of voltage loop,current loop and,the filter parameters,the inverter output voltage external characteristic.Then an improved droop control strategy is proposed,namely droop control strategy with virtual impedance,and the characteristics of single inverter output is analyzed in detail,as well as the effect to the output impedance of three types of virtual impedance,which are pure resistance virtual impedance,pure virtual impedance and complex virtual impedance.Three types of virtual impedance experiments were performed on the experimental platform,and the results obtained from the experimental results showed that the complex virtual impedance was the best in the system.Then the conformance between power equalization control strategy based on virtual synchronous generator and droop control method is derived,which can bring reference value to system design.After that,a kind of control mode is proposed.When the power fluctuations cause the output voltage and the frequency offset larger,the voltage and frequency compensation are added to the droop control,so that the voltage and frequency can be restored to the rating operation.On the basis of above,an improved control strategy is proposed,which improves the part of droop control to the communication droop control.It can control the voltage and frequency,and the active power and reactive power of the circulation can be adjusted faster at the same time.Finally,a simulation model was established in Matlab/Simulink to compare the simulation waveform of the proposed control mode and the traditional droop control mode,and verify the improvement of the proposed control mode.The experimental platform of two inverters in parallel is built,and the experimental results show that the proposed control strategy can adjust the voltage and frequency back to their rating value after the voltage and frequency offset.Moreover,it has a good balance of current,indicating the correctness of the whole control system and the feasibility of the experimental platform.This dissertation also summarizes the topic and prospects the unfinished work.