Study Of Harmonic Suppression Method Based On Improved Droop Control In Micro-grid

With the development of society and the progress of mankind,the demand for electricity is higher and higher.The advantages of micro-grid application in new energy sources are very obvious.Many experts and scholars pay more and more attention to it.Because of the use of power electronic converter and nonlinear load in the micro-grid,there are a lot of harmonics in the power grid.The harm of harmonics is well known.The emergence of this problem seriously affects the operation and transmission of the power system,and causes the hidden danger to the complete operation of the power grid.The harmonic suppression method based on SOGI virtual impedance is studied in this thesis.Firstly,from the point of view of DGs’power,the characteristics of power transmission are analyzed.According to the direction of droop control,the working principle of each controller is analyzed,and the simulation model of voltage control,current control,power control and droop control is set up.Secondly,according to the shortage of traditional droop control in low voltage micro-grid and the influence of nonlinear loads on the voltage of PCC point,the virtual impedance control loop is introduced on the basis of the traditional droop control strategy,and a droop control strategy based on SOGI virtual impedance is established.By comparing the traditional droop control strategy,the droop control strategy based on virtual impedance and the virtual impedance droop control strategy based on SOGI,the effectiveness of droop control method based on SOGI virtual impedance is compared and analyzed.Finally,the droop control of the micro-grid control adopts multiple proportional resonance controllers of voltage and current loop controls in combination with the virtual impedance droop control strategy.This can eliminate the 5^th and 7^th voltage and current harmonic distortions at the output of the micro-grid inverter,satisfying the grid-connected standard of distributed generations.In addition,the nonlinear loads will cause harmonic currents,making the voltage at the PCC point to be distorted.In order to compensate this voltage distortion,the micro-grid control strategy was proposed based on the harmonic voltage compensator.This control method can offer harmonic suppression effect in the micro-grid by means of a harmonic voltage compensator.A simulation example of harmonic suppression in micro-grid is set up in Matlab/Simulink,and the harmonic suppression effect of the improved droop control is studied.The droop control strategy based on SOGI virtual impedance proposed in this thesis can maintain the voltage stability of the common connection point of the system,and the power can be automatically allocated and equal when the load changes.The harmonic suppression strategy based on SOGI virtual impedance and harmonic voltage compensation can suppress the 5^th and 7^th harmonics in the micro-grid and improve the power quality.