Harmonic Suppression Of Back-to-back Converter And Its Application In Micro-grid

Three-phase,two-level ac-dc-ac converters,also known as‘back-to-back’converters,are popular due to advantages such as high power factor and high efficiency and capability of bidirectional power flow.They have found applications in many areas.Based on the analysis of the back-to-back converter model,it was found that switching power converters injected current pulses into the DC link from the rectifier and inverter sides,and these current pulses lead to the loss of the DC link capacitor,and increase the requirement of capacitor ratings and add weight,size and cost,as well as issues such as low power density and reliability.Therefore,this paper presents a new method of suppressing the harmonic current in active DC link capacitors in a back-to-back converter.Based on the analytic solution derived from the harmonic current in the switched-DC link and by the specific control of the phase angle of the pulse width modulated(PWM)carrier,the target harmonics of the rectifier and inverter can be synchronized,so that their cancellation occurs at a direct current link capacitor.The method can significantly reduce the harmonic content in the first and second carrier frequency bands,and the feasibility of the method is verified by MATLAB/SIMULINK simulation software.Based on the bidirectional flow characteristics of back-to-back converters,it is applied to micro-grid to achieve smooth grid connection between micro-grid and public power grid.Due to the traditional grid connection method is connected by circuit breakers or static switches,it has the problems of slow grid connection speed,large inrush current,low power supply reliability and inability to control power flow.In addition,faults on either side of the micro-grid or the large power grid will affect the other side.For example,interaction of harmonics,mutual influence of frequency and voltage cannot control in real time on both sides of the grid.Therefore,this paper analyzes and discusses the key control technologies and the methods of grid connection,and proposes a structure of the micro-grid grid-connected based on the back-to-back converter described above.Back-to-back system proposed in the above based on the control strategy,in which VSC1 by controlling the in-line current and grid voltage in phase,so that the output reactive power is zero to achieve grid.VSC2 achieves the second adjustment of the voltage amplitude and frequency,and achieves bi-directional power flow between the grid and the micro-grid by keeping the DC bus voltage constant.In order to verify the effect of back-to-back VSC on solving the frequency and voltage deviation of the micro-grid during load changes,the MATLAB/SIMULINK simulation software was used to model the grid-connected device based on back-to-back converters.The results show that the grid-connected device can not only improve the grid-connected characteristics of the micro-grid,but also improve the voltage quality of the micro-grid,balance and redistribute the power of the active power and the reactive power.