Comparative Research On Harmonic Current Control Strategy Of High Frequency Active Power Filter

As more and more power electronic devices are used in power frequency power systems,aviation power systems and other fields,the grid current and the voltage of the common coupling point of the grid are distorted,and the harmonic problem is serious,which affects the power supply reliability and power of the grid system quality.Active Power Filter(SAPF)is not only insensitive to system impedance changes,but also has controllability and flexibility,so it has been widely used.The research of APF applied to 50 Hz system is relatively mature,and the fundamental frequency of aviation system is 400 Hz,which is higher than the frequency of civil power grid.Therefore,the rapidity and harmonic compensation of SAPF switching frequency and its control strategy,the accuracy requirements are higher.In this paper,high-frequency SAPF with a switching frequency of 100 k Hz is selected as the research object.A comparative study of PI,PI + traditional repetition,PI + 6-times fast repetition,and fractional-order fast repetition control strategies was conducted.The theoretical analysis,simulation and prototype experiment verification of the harmonic compensation effect of 50 Hz and 400 Hz systems are carried out respectively.For the SAPF with a switching frequency of 100 k Hz,if only the PI controller is used,the current loop bandwidth can meet the harmonic compensation requirements of the 50 Hz system.The harmonic current frequency of the 400 Hz system is 8 times that of the power frequency grid.The SAPF current loop bandwidth cannot control harmonics within the 50 th order,and the compensation effect is not ideal.Since the command current of SAPF is a harmonic current component in a non-linear load,the traditional repetitive control can track the command current according to the internal model principle.Therefore,PI + traditional repetitive control can accurately compensate harmonics of 50 Hz system and 400 Hz system.After compensation,the THD of the grid current is within 3%.The traditional repetitive control needs to use the command signal of the last fundamental cycle to build its internal model link,resulting in a delay in the dynamic response.In order to improve the dynamic response speed of SAPF,the control frequency of the traditional repetitive control internal model link is increased to 6 times the original to obtain six times the frequency rapid repetitive control.In the dq coordinate system,the harmonic current of high-frequency SAPF is 6k times the fundamental frequency.The internal model link of this control strategy only controls the 6k order harmonic current,which reduces the delay time of the internal model link,thereby improving the dynamic response speed of SAPF.However,improper selection of the switching frequency will cause the internal model delay point N(which is obtained by dividing the switching frequency and the grid fundamental frequency)from being an integer,resulting in the current loop inaccurately tracking the SAPF command current,and the compensation effect is not ideal.In order to overcome the shortcomings of the 6-times frequency fast repeat control,the number of delay points is not an integer,and the traditional repeat control cannot adapt to the grid frequency offset.PI + fractional fast repeat control strategy can be used.According to the Lagrange interpolation method,the fractional part of the repeated control delay link is converted into a parallel connection with several integer order delay links.By comparing the fitting effects of different interpolation points and interpolation lengths,the controller parameters are optimized when the number of delay points for repeated control of the internal model is not an integer and there is a frequency offset in the power grid.The simulation results show that the high-frequency SAPF under the fractional repetitive control strategy not only has a good harmonic compensation effect,but also has a good dynamic response speed,and also has good adaptability to the frequency shift of the power grid.The research work in this paper is also applicable to other three-phase converters.