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Research On System Modeling And Reactive Power Sharing For Microgrid Inverter

Posted on:2017-02-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:J GuFull Text:PDF
GTID:1312330512968662Subject:Electrical engineering
Abstract/Summary:PDF Full Text Request
As the interface between distributed power supply and microgrid, microgrid inverter should have important roles such as supporting microgrid voltage amplitude and frequency, sharing load power and stabling power fluctuations in AC microgrid. As an important part to ensure the stable operation of microgrid, research on the related problems of microgrid inverter is becoming more and more important.This dissertation,entitled “Study on System Modeling and Reactive Power Sharing for Microgrid Inverter",carries out the small-signal modeling and analysis of microgrid systems with different power network structures, discusses reactive power sharing strategy and studies the optimal design of transfer impedance and the implementation method of virtual reactance. The main work and innovative points include:(1) When modeling microgrid system, the interaction between micro grid inverters and the dynamic characteristics of the power network should be reflected. In the modeling process, if microgrid inverter is considered as ideal controlled voltage source or the dynamic characteristics of power network is ignored, it will lead to the error between microgrid system model and actual microgrid system. In this dissertation,small-signal method is used to model the hybrid-structure microgrid consisted with the basic parallel-structure and series-structure. The hybrid-structure microgrid system is divided into three sub modules, including microgrid inverter module, power network module and load module. The three sub modules are unified by means of coordinate transformation method, and then the small-signal model of the hybrid-structure microgrid is constructed. Based on the small-signal model of the hybrid-structure microgrid, the small-signal model of the most common parallel-structure microgrid is introduced, which includes two cases of resistive load and resistor-inductance load. By using eigenvalue sensitivity and participation factor analysis method, the common parallel-structure mircrogrid system with resistive load is simulated and analyzed in detail, which can provide a reliable theoretical basis for the selection of system electrical components and controller parameters and guide the experimental research of microgrid system. By time domain simulation, the correctness of the small signal modeling and analysis of parallel-structure microgrid system is verified.(2) In the review of existing typical reactive power sharing strategies, PQ decoupled mechanism of the two reactive power sharing strategies such as output impendance method and virtual reactance method is studied by at the same time considering the output impedance of microgrid inverter, line impedance and virtual impedance, and the setting principle of the rating voltage in droop controller is defined.By using the concept of unit transfer impedance and relative power sharing deviation,the problems that the droop control should be paid attention to in engineering realization are studied. Because load effect and Q-E droop can cause the PCC voltage amplitude to descend seriously, the quality of microgrid power supply is affected, an improved reactive power sharing strategy is proposed for low-voltage microgrid. The improved reactive power sharing strategy firstly introduces virtual reactance at the output end of DG unit to ensure that transfer impedance is inductive, which realizes the decoupling between active power and reactive power and achieves the application condition of droop control. Because the virtual reactance introduced in the improved reactive power sharing strategy is only used to decouple and don't need to solve transfer impedance mismatch problem, virtual reactance is greatly reduced. Secondly, the difference between nominal voltage amplitude and PCC voltage amplitude is multiplied by the proportional coefficient to the traditional Q-E droop control structure, which can achieve accurate reactive power sharing and reduce PCC voltage amplitude drop caused by load effect and Q-E droop. The control structure of the improved reactive power sharing strategy is presented and analyzed, and then the quantitative analysis is made on the premise that the PCC voltage amplitude is replaced by the voltage amplitude of the grid side of the access switch to realize the improved reactive power sharing strategy.The power sharing devation caused by the error of the global settings in the improved reactive power sharing strategy is analyzed by using the concept of relative reactive power deviation. In order to study the stability of the improved reactive power sharing strategy, the small-signal stability analysis method is carried out. The comparison between the traditional droop control and the improved droop control is carried out by simulation.(4) Due to the influence of transfer impedance on the stability and dynamic performance of microgrid system, it is very important to optimize the design of the transfer impedance. The transfer impedance designing method based on a simpler microgrid small-signal model is introduced to provide a powerful basis for selecting actual transfer. impedance, especially selecting the virtual impedance, at the same considering DG unit power transfer capability, PQ decoupling, the stability and transient performance of microgrid in grid-connected mode and islanding mode.According to the problem that derivative method and coordinate transformation method make the output voltage of microgrid inverter distorted under the condition of nonlinear load, the three-phase virtual reactance implementation method based on SOGI is presented. Compared with derivative method and coordinate transformation method, the three-phase virtual reactance implementation method based on SOGI is analyzed by the simulation results under the conditions of linear load and nonlinear load. The influences of the three-phase virtual reactances implemented by derivative method, coordinate transformation method and SOGI method on the output characteristic of microgrid inverter are studied.By using microgrid system simulation platform, the reactive power sharing strategy based on virtual reactance introduced by SOGI method is simulated to verify current-sharing effect under the conditions of linear load and nonlinear load. The distortion rates of microgrid inverter output voltages are simulated and compared when virtual reactances are respectively introduced by derivation method, coordinate transformation method and SOGI method.(5) In order to verify the correctness of the design of microgrid system experimental platform, the basic droop control experiment is done on the platform,including the related experiments of a single microgrid inverter and two sets of microgrid inverters in parallel. The related experiments about verifying the three-phase virtual reactance implementation method based on SOGI and the improved reactive power sharing strategy are done on the experimental platform.
Keywords/Search Tags:islanding microgrid, droop control, small-signal analysis, reactive power sharing, virtual reactance
PDF Full Text Request
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