| The microgrid energy storage system introduces energy storage link for distributed generation,which transforms the rigid system into a flexible system,and improves the flexibility and stability of distributed generation operation.As the key interface equipment between energy storage device and microgrid system and even large power grid,the control strategy of energy storage converter is of great significance in parallel off-grid and power grid fault scenarios,which is related to the operation performance of the whole system.This paper investigates the research status of microgrid and energy storage converter,summarizes the application scenarios of energy storage converter,and then studies the smooth switching,islanding detection and fault ride-through.The main contents are as follows:(1)The mathematical models of energy storage converter is established,and a current droop control method of energy storage converter is presented.By establishing small signal model,the influence of key parameters on system performance is analyzed,and the design basis of relevant parameters is given.(2)Research on pre-synchronization control and support vector machine based passive islanding detection method.Firstly,the limitations of traditional off-grid switching method based on control mode switching are analyzed,and an improved pre-synchronization control strategy is proposed on the basis of current droop.By tracking the two rotating coordinates of grid voltage,the smooth switching of energy storage converter from island mode to grid-connected mode is realized.Then,the characteristics of unplanned island are analyzed.In order to avoid the influence of active detection methods on power quality,a passive island detection method based on support vector machine(SVM)is proposed.A classification model is established with the voltage of common connection points,output current of converter harmonics and their ratio as characteristic variables to realize the judgement of grid-connected and island.The parameters of SVM can be optimized by genetic algorithm.The proposed method can identify island effectively.(3)Fault ride-through control of energy storage converter based on power angle hysteresis loop.Aiming at the problem of power network fault in grid-connected operation,the fault characteristics of power droop and current droop are analyzed in detail.A fault ride-through control of power angle hysteresis loop is proposed based on current droop.The power angle is kept within a certain range of the steady-state value before fault,so that it can pass the fault transient stage smoothly and quickly restore to the state before fault clearance.Because of the superior fault characteristic of current sag,the over-current phenomenon of output active current is avoided.And a reactive power support is provided according to the requirement of fault ride-through.(4)An experimental platform for energy storage system is established,and the topology and functions of the experimental platform are described.On this platform,the parallel operation characteristics of current droop,mode switching and fault ride-through control proposed in this paper are verified experimentally. |
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