Research On Control Strategy Of Bidirectional AC/DC Converter In AC/DC Hybrid Microgrid

The AC/DC hybrid microgrid combines the advantages of AC microgrid and DC microgrid,and becomes the general trend of microgrid construction in the future.As an important hub of energy exchange,the bidirectional AC/DC converter in AC/DC hybrid microgrid has the core function of completing the energy flow between two sub microgrids and ensuring the voltage and frequency stability of AC/DC bus.However,when there is a nonlinear load in the AC sub-microgrid,harmonics will be generated.When the harmonic current passes through the power supply line,the current in the same power grid will fluctuate at the same frequency,which further leads to the distortion of voltage and current in the power grid and reduces the power supply quality in the system.In addition,the frequency,phase and voltage amplitude of the microgrid system and the public grid are different when the AC/DC hybrid microgrid completes the grid-connected/isolated island mode switching,and the sudden switching will produce instantaneous over-current impact,which threatens the stable operation of the microgrid system and the public grid.The reasonable control strategy of bidirectional AC/DC converter can reduce the influence of nonlinear load in AC sub-microgrid and grid-connected/isolated island mode switching on the microgrid system.In this thesis,theoretical analysis,control strategy improvement and comparative experimental simulation are studied,and the specific work is as follows:Firstly,the topology network of AC/DC hybrid microgrid system is designed,and establishes the common distributed generation unit(DG)model in AC/DC hybrid microgrid,including photovoltaic power generation unit,wind power generation unit and energy storage unit.Combined with the operation mode of AC/DC hybrid microgrid,the control of DG under different modes is designed,and the control of DG is verified by simulation experiment.In the grid-connected mode,the AC sub-microgrid contains nonlinear load,which causes the current distortion of AC bus and reduces the power transmission quality.In this thesis,the control strategy of droop control combined with Fryze-Buchholz-Depenbrock(FBD)harmonic detection method is improved,and an improved grid connected harmonic current suppression control strategy is designed.The improved FBD harmonic current detection method can eliminate the influence of zero and negative sequence fundamental frequency component by decoupled double synchronous reference frame phase locked loop(DDSRF-PLL)with self decoupling network,and can realize accurate phase detection when it is disturbed by nonlinear load.At the same time,LMS adaptive filter is used instead of traditional FBD harmonic detection method low pass filter(LPF),which can enhance dynamic response ability when load current suddenly changes,and solve the problem that the cutoff frequency of low-pass filter can not take into account the detection accuracy and response speed of FBD harmonic current detection method.Through the simulation experiment,compared with the original control strategy,the total harmonic distortion(THD)of the improved control strategy is reduced by 0.45%under non-ideal conditions,which proves the effectiveness and feasibility of the improved control strategy.In order to solve the problem of transient over-current and voltage in grid-connected/isolate island mode switching of AC/DC hybrid microgrid,which leads to sudden change of transmission power and affects the stability and security of microgrid system,an improved pre-synchronous droop control seamless switching control strategy with disturbance observer(DOB)is designed.According to the different power transmission calculation methods of droop control in grid-connected and islanded mode,the adaptive switching is realized by detecting the current and frequency at the point of common coupling(PCC)the public power grid.When island is switched to grid-connected mode,the impact is caused by the frequency,phase and amplitude difference between microgrid and public grid,which will cause system collapse in serious cases.The pre-synchronization control is designed.Based on the pre-synchronous droop control,the current mutation can be quickly tracked by combining the current inner loop and the disturbance observer.The purpose of suppressing the difference is achieved by tracking the difference,and the seamless switching from island to grid connection can be realized.The simulation results show that the improved seamless switching control strategy can reduce the impact of over-current impact in the process of mode switching,and the power mutation is effectively improved,which proves the effectiveness of the designed control strategy.