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Research On Modified Droop Control Strategy Of Paralleled Inverters With Virtual Complex Impedance In Low Voltage Micro-grid

Posted on:2018-01-03Degree:MasterType:Thesis
Country:ChinaCandidate:Y J JiangFull Text:PDF
GTID:2322330533458763Subject:Electrical engineering
Abstract/Summary:PDF Full Text Request
With the increasing expansion of distributed power generation system,the stability and power quality requirements of micro-grids are becoming higher.The reliability of the distributed power supply operating in parallel is the key factor to determine whether the micro-grid can run stably.Inverters,as the links between distributed generators,need to connect the micro-power to the local load and the utility grid.Therefore the safety and stability of the inverters in parallel operation is of great significance.Most of the distributed power supplies are close to the user side,and have the low voltage level.Thus,the study on the parallel operation of the inverters in the low voltage micro-grid becomes a hot spot.This paper mainly does some research on the parallel control strategy of single-phase inverters in the low voltage micro-grid.The control method without the interconnected communication line has high redundancy and flexibility,and the droop control is one of the most commonly used in this control method.The traditional fP-? VQ-droop control strategy is only applicable to the medium and high voltage grid system where the line impedance is nearly inductive.In contrast,the line impedance of the low voltage micro-grid is resistive.In order to solve the problem of coupling between active power and reactive power in low voltage micro-grid,this paper introduces the virtual complex impedance to design the equivalent system output impedance to be resistive.In this process,the virtual impedance and the equivalent output impedance of the inverter are analyzed by vector diagram analysis to easily improve the accuracy of the introduced virtual impedance,so that the active power and reactive power of the system can be better decoupled.In the pure resistive systems,the active power of the system is approximately proportional to the voltage,and analogously,the reactive power is nearly proportional to the frequency.Therefore,the corresponding droop control method is changed from the original fP-? VQ-droop control to the VP-?fQ-droop control method.As the locations of the micro-powers are different,the distances between the distributed power generation systems and the loads are also unequal,which leads to the different line impedances between them.It will significantly affect the accuracy of power sharing during islanding mode due to the unequal voltage drops.As a result,some power supply systems will get overload,which will finally affect the stable operation of micro-grid.Thus,this paper presents a modified droop control method for low voltage micro-grid.By introducing a PI regulator into the VP-droop control,the traditional fixed droop coefficient is changed to the variable droop coefficient,so that the system can adjust the droop coefficient adaptively under different line impedance to achieve better active power sharing.The proposed method has been simulated by Matlab/Simulink software.The simulation results show that the modified control method can accurately achieve the equalization of the output power among the inverter systems in the case that the load mutate and the line impedances are different.
Keywords/Search Tags:low voltage micro-grid, parallel control, virtual complex impedance, power coupling, vector diagram, droop control, power sharing
PDF Full Text Request
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