Study On Islanding Detection Method Based On Closed-loop Frequency Control For Grid-Connected Inverters

Distributed generation system is more and more widely used in today’s society.Its security,reliability and flexibility are more prominent than other power generation modes.In addition,the distributed generation system has a good peak performance for the large power grid,and it is easy to operate and is conducive to full automatic control.However,islanding detection is one of the frontier problems in the field of grid connection.Now widely used on the islanding testing technology can be broadly divided into active testing and passive islanding detection,these two types of testing technology have their own advantages and disadvantages:active islanding detection means high sensitivity,detection blind area is small,but will affect the power grid power quality;The passive islanding detection method is economical and easy to implement,but the detection blind area is large,the algorithm structure is complex,and it is easy to misjudge.Therefore,it is generally used to solve the islanding effect by combining active and passive detection.In this dissertation,a new type of closed-loop frequency-locked control is analyzed.The phase angle of grid tracking generated in this paper is deeply studied.The grid tracking frequency of the inverter output side derived from this phase angle is deduced.The addition of reactive power negative feedback allows the frequency to track the grid frequency without error,which is used for grid-connected inverter to achieve accurate closed-loop frequency control.This dissertation summarized the traditional and new type two kinds big islanding passive detection method,listed the basic principles of various detection methods and quality difference,discussed the disadvantages of existing passive detection method,put forward the method to control and improve the existing island detection technology from the source.Aiming at the problem of large non-detection zone in traditional passive detection method as well as the current non-detection zone passive detection method control algorithm is too complex,in this dissertation,by analyzing the principle of frequency tracking in closed-loop lock frequency control,the high-frequency impedance characteristics of grid-connected inverter controlled by closed-loop frequency lock are studied,discussing the load impedance characteristics of various islanding conditions,here,a new passive islanding method is proposed,this method uses the output frequency of the closed-loop lock frequency control method directly,preventing the occurrence of the island by adding high-frequency impedance detection.Simulation and experimental results show that the passive island detection method proposed in this dissertation is able to detect the inverter disconnection situation quickly and accurately,can also eliminate non-detection zone,and avoid the complex problem of control algorithms.