Research On T-Type Three-Level Active Power Filter Based On LCL

With the rapid development of technology in recent years,power electronic devices can be seen everywhere in our daily life.While bringing great convenience to industry and people's life,the generated harmonic and reactive current also brings great challenges to the power quality of the grid.Active Power Filters have been widely studied for their ability to compensate harmonic and reactive currents in power grid.Compared with the traditional two-level APF,the three-level APF has a greater application prospect in high voltage and high power occasions.This paper takes three-phase three wire T-type three-level APF as the research object,and makes a deep research on its key technologies.Firstly,this paper introduces the working principle of T-type three-level APF,The mathematical models of three-phase stationary abc coordinate system and two-phase rotating dq coordinate system are established,which lays the foundation for the design of current controller in the following paper.At the same time,the parameter design of LCL filter is analyzed in detail.Firstly,different performance indexes are set.According to the relationship between performance indexes,the constraint graph of filter parameter selection is drawn,and the filter parameters are determined in the constraint range.In view of the characteristics of traditional three-level SVPWM modulation strategy,such as large amount of calculation,large amount of trigonometric function calculation,not easy to be realized in digital processor,a simplified three-level SVPWM modulation strategy based on two-level SVPWM is studied.The calculation amount of this modulation method is small,which can greatly shorten the operation time in the processor.At the same time,the point potential imbalance of three-level topology is analyzed The reason is that a time factor based neutral point potential balance control strategy is proposed,which can effectively suppress the neutral point potential imbalance.The harmonic extraction algorithm based on instantaneous reactive power theory is analyzed in detail,and the low-pass filter in harmonic extraction is improved.The second-order Butterworth low-pass filter and average filter in series can effectively improve the accuracy of harmonic extraction.When the grid is in a non ideal state,the traditional srf-spll phase-locked loop has low accuracy In this paper,a dsogi-spll phase-locked loop based on the second-order generalized integrator is studied.This phase-locked loop can accurately lock the frequency and phase of the positive sequence voltage component of the power grid when the three-phase voltage of the power grid is unbalanced or even contains harmonics.LCL filter can reduce the ripple of the output current,but the system has changed from the first order to the third order,which brings difficulties to the design of the controller.In this paper,the one beat delay brought by digital control is used to advance the 180 ° crossing frequency point,increase the amplitude margin of the system,and make the system closed-loop stable.At the same time,in order to improve the steady-state accuracy of the system,the repetitive control is used as the current outer loop control,and the delayed one beat PI control is used as the current inner loop control.At the same time,the characteristics of harmonic current are analyzed,the traditional repetitive control is improved,and the 1 / 6 cycle repetitive control is proposed.On the premise of little impact on the steady-state accuracy,the dynamic performance of the system is further improved,and the simulation is carried out in MATLAB.The simulation results prove the effectiveness of the control strategy.In order to verify the control strategy proposed in this paper,a T-type three-level APF experimental platform based on LCL filter is built.The hardware design and software design of the platform are introduced in detail.The experimental results further illustrate the effectiveness of the control strategy.