| With the massive access of new energy sources and the gradual increase of non-linear loads,the problem of power quality in microgrids has become increasingly prominent.Among them,frequent voltage sag,interruption and other voltage quality problems threaten the normal operation of critical loads.Meanwhile,the intermittent power generation of new energy sources and the random access of loads are likely to cause power fluctuations,which may cause fluctuations in the AC bus voltage in the microgrid and affect the normal operation of the load.Therefore,in order to ensure the good operation of the microgrid,it is necessary to manage voltage problems and adjust power fluctuations.Aiming at the voltage quality problem,dynamic voltage restorer(DVR)has been studied and applied as an effective control device.However,it often only works when the grid voltage fluctuates,and can only control the power quality of the downstream load of the feeder,which has shortcomings in equipment utilization and governance scope.If the traditional DVR can also play a role when the grid voltage is normal,and the governance scope is extended to the upstream of the feeder,it will be of great significance to improve the equipment utilization and the economic and reliable operation of the microgrid.Based on this,this paper extends its power regulation capability on the basis of traditional DVR,and optimizes the operation constraints and the influence of harmonic current.The main work and innovations are as follows:Firstly,the expansion method of DVR power regulation capability based on structure decoupling and its control strategy are proposed.The topology of MF-DVR is introduced in detail,and the realization mechanism of voltage quality regulation and power regulation is described in detail.According to the operation state of power grid,two working modes are divided,and the circuit conduction under different modes is analyzed.The control strategy is designed under decoupling method,and the control parameters,capacitance voltage fluctuation,device electrical stress and output performance are analyzed.In addition,the performance of the proposed scheme is compared with the other three functional expansion schemes.Secondly,the expansion method of DVR power regulation capability based on vector control and its optimal operation strategy are proposed.The realization mechanism of power regulation and voltage quality regulation under vector control method is analyzed in detail,and power regulation states of MF-DVR are analyzed according to the variation range of load voltage phase.The constraints of the output voltage and output power of the converter are analyzed,and a topology improvement scheme of series coupling capacitor at the outlet of the filter is proposed.The working principle and the working performance of the improved topology are analyzed in detail.The design rules of improved topology are analyzed,and the cost changes of device before and after topology improvement are compared,and the control strategy of MF-DVR based on vector control and topology improvement is designed.Again,the output performance of MF-DVR under nonlinear load and the optimal operation strategy to eliminate the influence of harmonic current are analyzed.The influence of harmonic current on the output of compensation voltage is analyzed under the two methods of structure decoupling and vector control,and the influence relationship and influence degree are described by using transfer function and bode diagram.The elimination effect of the independent control strategy on the harmonic current under the structure decoupling method,as well as the elimination effect of the closed-loop control based on the filter capacitor voltage and the compensation voltage on the harmonic current under the vector control method,and optimized operation strategies are obtained.Finally,the correctness of the theoretical analysis and the effectiveness of the control strategy are verified by simulation and experiment of each chapter. |
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