Research On The Key Technology Of Compound APF For Medium And High Voltage Applications

Installing Active Power Filter(APF)in the power system can not only reduce the interference of load current on various links in the power grid,but also effectively improve the power supply quality of the distribution network and ensure the normal operation of sensitive loads in the power grid.Improve grid operation stability.The compound APF can realize all the functions of the series APF and the parallel APF at the same time,and it can reduce the energy storage requirements on the DC side,reduce the volume and cost of the capacitor,and has a good development prospect.In medium and high voltage distribution networks,the introduction of industrial frequency transformers in traditional APF will increase the size of the APF system,reduce the system power density,and increase the installation site requirements.In order to solve this problem,this paper proposes a new compound type The APF system and the corresponding improved system control strategy further reduce the system volume and effectively improve the DC bus stability of the system.Research on key technologies of APF system in medium and high voltage occasions,the main contents are as follows:First,the basic working principle of APF is analyzed,and the related technologies involved in the operation of series APF and parallel APF are summarized,including two methods for extracting harmonic components,and the generation of three types of compensation signal reference values.Method and three kinds of harmonic tracking controllers,and analysis and comparison of various methods involved in each key technology.Secondly,the structure and control of traditional compound APF in medium and high voltage applications are analyzed.Firstly,the "back-to-back" structure and working principle of the compound APF are analyzed.The "back-to-back" structure includes the series-side APF part,the parallel-side APF part,and the DC bus.The series-side APF is connected in series with the high-voltage grid through a coupling transformer or a capacitor.The control strategy and control parameter design of series-side APF and parallel-side APF for current compensation and voltage compensation are given.At the same time,the significance of the DC bus in the "back-to-back" structure is pointed out,and the relationship between the direction of power flow and various physical quantitiesis analyzed when there is active power transmission in the compound APF system.Thirdly,the shortcomings of the traditional compound APF are analyzed.Aiming at the problem of reducing the power density of the compound APF system caused by the large volume and weight of the power frequency boost transformer,a modular multilevel converter(MMC)new compound APF structure.By introducing high-frequency transformers and omitting the power-frequency step-up transformer,the traditional "back-to-back" APF structure has been changed to a three-stage structure.The three power stages are series-side APF,intermediate stage DC / DC conversion,and parallel-side APF,and the cascades are performed between the stages through a DC bus.At the same time,in order to reduce the fluctuation of the DC bus during the dynamic process,this paper proposes a voltage feedforward strategy to reduce the impact of the response delay of each power stage on the stability of the system and reduce the smooth and stable DC bus voltage to the DC bus capacitor energy storage capacity.Requirements.Finally,the compensation effect of the compound APF in the traditional medium and high voltage situations is simulated.The proposed new compound APF compensation effect and the corresponding feedforward control strategy for the high voltage situations are verified.