| The smart micro-grid composed of the energy storage system and the distributed power source can be operated both on-grid and off-grid.In this way,new energy can be efficiently used and intelligently controlled.However,the smart microgrid is mostly connected to a low-voltage distribution network dominated by single-phase load.There is a three-phase voltage unbalance caused by an unbalanced load or an asymmetric short-circuit fault.Therefore,it is of great practical significance to study the control of energy storage converters or microgrids under this condition to the safe and reliable operation of the entire system.After comprehensively analyzing the subject background and research status,this paper studies the control of the energy storage converter under unbalanced conditions.The main work of this article is as follows:For the case of multiple energy storage converters with asymmetric loads,the system equivalent models of the positive and negative sequence circuits are established by using the symmetric component method in this paper,then the causes of unbalanced voltage and its compensation mechanism under off-grid condition are analyzed,next,a series of problems and solutions for the energy storage converter in the microgrid under unbalanced conditions are discussed.With the help of Ether-CAT communication technology,the current sharing algorithm is introduced into the micro-grid central controller to achieve the equal sharing of negative sequence current;virtual impedance technique and voltage stratified control technique are used to compensate the negative sequence voltage drop of different lines,the voltage of the AC bus of the microgrid is balanced finally.In grid-connected mode,a droop characteristic based on current control is firstly analyzed.Aiming at the unbalanced AC bus voltage caused by asymmetric short circuit fault,the causes of asymmetric current are analyzed.The algorithm of the circuit simulator is introduced,combined with the positive and negative sequence current sequence control to ensure the balance of the output current of the energy storage converter.The fault characteristics based on droop control are further analyzed,and a fault ride-through control of energy storage converter based on power angle hysteresis control is proposed.This method provide reactive power support for the grid and accelerate the recovery speed after the fault is cleared;and effectively avoid the overcurrent of the output active current during the fault;The dual synchronous rotating coordinate control guarantees the converter's fault ride-through ability in the case of asymmetric short circuit.The simulation model is established on MATLAB / Simulink to verify the proposed control method.An experimental platform for energy storage system is established,and the topology and functions of the experimental platform are described.On this platform,the control algorithms under two various unbalanced conditions mentioned in the article were experimentally verified. |
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