Research On Active Isolated Islanding Detection Method For Grid-Connected Power Generation System

With the increasing scale of the distributed grid-connected generation,Islanding Phenomenon are also occurring more frequently.When islanding occurs,the service life of electrical power unit might be affected,even the stable operation of large power grids and the safety of maintenance personnel will be threatened.Therefore,the ability to detect island quickly and accurately is indispensable for grid-connected inverters.However,the existing active islanding detection methods have some shortcomings in application,such as harmonic pollution and unmatched load properties,so it is still necessary to study the islanding detection methods more deeply.This paper starts with the definition and harm of island effect,then derived the frequency and amplitude of voltage changes before and after islanding.Based on the above content,the existing methods of islanding detection,detection standards and blind area evaluation are introduced in detail.Several active improved islanding detection methods are modeled and simulated in SIMULINK,and evaluates its advantages and disadvantages.Since the existing single-phase AFD islanding detection method has persistent disturbance and the detection effect is affected by the load nature,a new method of frequency-shift islanding detection based on discontinuous disturbance and frequency change rate is proposed in the paper.This method utilizes the difference of frequency change rate before and after disconnection and monitors its fluctuation in real time,together with the discontinuous disturbance and positive feedback function with fuzzy control,it can accurately judge the occurrence of islanding under low disturbance injection.SIMULINK simulation model is established in the situation when islanding detection conditions are the most stringent.Compared with traditional AFD method,the simulation results verify the effectiveness and superiority of the proposed method.As for more practical three-phase distributed grid-connected generation system,its corresponding islanding detection methods also have some problems,because it's difficult to eliminate the NDZ and it is limited by frequency offset method.Therefore,a new negative sequence current injection islanding detection method based on voltage unbalance is proposed in this paper.The method takes negative sequence current as disturbance and voltage unbalance as criterion of islanding occurrence.Combining with the non-linear positive feedback function expressed by fitting function,it can detect the Islanding Phenomenon without changing the frequency and phase of three-phase current.The simulation model of three-phase islanding detection is built in SIMULINK,and compared with the existing methods.The simulation results show that the proposed method has faster detection speed and less impact on the symmetry of three-phase current,not to mention the power quality.