Grid-connected/Islanding Characteristics And Control Strategies Of Microgrid

As an important development direction of distributed generation, microgrid technique has been a hot area. This dissertation focuses on AC microgrid with only battery inverters and renewable resources, especially the microgrid grid-connected/islanding characteristics and control strategies.Firstly, principles and control methods of microsource inverter are investigated. Base on the analysis of working principles, this dissertation points out that the R/X and module value of equivalent impedance has big influence on the performance of microsource P-f and Q-V droop control. Base on the disscusstion of four microsource droop control methods’equivalent impedance and virtual inertia, the reason of voltage droop in virtual impendance droop control method, and the influence of control parameters in virtral inertia droop control method are pointed out, which constitute the theory basis of the microsource droop control method in this dissertation.Secondly, base on the above analytic results, a modified microsource droop control method is proposed:considering the static droop characteristics, an automatic power limitation function is added by modifying the P-f and Q-V droop characteristic modules; considering the frequency response, the virtual inertia module is modified, which can avoid the influence to P-f droop characteristic caused by the microsource control parameters, and overcome the shortage of virtual synchronous generator control method; considering the voltage behaviour, the voltage control module is redesigned, which can improve the voltage drop problem in virtual impedance control method; different simulation and experimental tests validate the performance of this microsource control method.Thirdly, considering the adverse effects to the microgrid quasi-synchronize paralleling control caused by the microsource virtual inertia, a modified microgrid quasi-synchronize paralleling control method is proposed. Base on the discussion of two current methods’working principle, this dissertation proves that their simplified equivalent models are the same, and their stability margin reduces when the microsource virtual inertia increases. From this point of view, a modified microgrid quasi-synchronize paralleling control method is proposed to extend the select range of microsource virtual inertia time constant, which is beneficial for the improvements of microgrid quasi-synchronize paralleling control stability. Fourthly, considering the requirement of microgrid black start function in islanding mode, an optimized islanding microgrid black start control method is proposed:ⅰ) base on the comparison of conventional power system top-down and bottom-up restorations, the advantages of bottom-up restorations in microgrid is discussed, and a microgrid black start scheme is proposed; ⅱ) base on the facts that the distribution transformer needs pre-magnetization process during the black start, a microsource black start method with raising voltage from zero is proposed, and the rate of raising voltage is designed, which can make sure that the black start microsource has both the voltage source and droop characteristics performances during the black process.