Research On The Stability Analysis And Design Of Islanded Microgrid With Constant Power Loads

Microgrids,as smart peripherals of future smart grids,are becoming popular these days.Because a microgrid can operate both in grid-connected mode and islanded mode,it can provide electric energy to the loads in a more reliable way.Typically,on the supply side,to address the growing environmental concerns,large amount of renewable resources are connected to microgrids.At the same time,on the demand side,more and more conventional linear loads like incandescent bulbs,induction motors,etc are continuously replaced by power electronics feeding loads,like LED lighting and inverter appliances,aiming to obtain much higher efficiency to improve energy utilization.However,power electronics feeding loads and moto-drive systems,when tightly regulated,behave as constant power loads(CPLs)which have negative incremental input impedances.This eventually decreases the system's damping and makes the system more prone to oscillation.Therefore,stability study on microgrids is of great importance and has become the research focus worldwide.This paper focuses on the stability issue of islanded microgrid,which is caused by interactions among subsystems.Small signal and large signal stability analyses and designs are successively studied.The main efforts done in this paper include:(1)The typical architecture of a microgrid has been selected as the research object based on the state-of-the-art analysis of microgrid structures.Dynamic model(or large signal model)and small signal model are then being deduced for stability analyses.(2)To conduct small signal stability analysis to the microgrid,CPLs are firstly divided into ideal CPL and dynamic CPL,which refer to power electronics feeding load with small time constant and motor drive system with large time constant respectively.Then,small signal stability analyses are made on conditions that ideal CPL,dynamic CPL and both of them are connected to the microgrid.Parameter designing criterions are finally obtained based on analyzing results.(3)To address the issue related to the system stability under large disturbances,a sliding mode controller(SMC)is designed and applied to the microgrid.Because the system's parameter uncertainties and disturbances are considered during the controller design,the SMC features high robustness and reliability.(4)A simulation model of a 30 kVA typical islanded microgrid is established in Matlab/Simulink,all theoretical analyses are verified by simulations.