Research On Three-Level Shunt Active Power Filter Based On DSP

With the rapid development of our society, the problem of power quality has become a burning issue as the non-linear loads in Power Grid have been widely used. Active Power Filter (APF) is an efficient way to solve this problem. Compared with the Two-Level Active Power Filter, Three-Level Active Power Filter can achieve better wave performance under high voltage with lower total harmonic distortion (THD) and less power loss. Three-level Active Power Filter is more suitable for high power application. This paper focuses on Active Power Filter using NPC converter. The converter construction, modulation, LCL-filter and current track algorithm of APF are researched.The neutral point clamped converter design is a feature of this paper. The compact structure of NPC Three-Level converter and reasonable layout make the system stable with low peak voltage, good thermal management and compatibility.LCL-filters have a broader prospect in high power application to reduce the size and cost. An improved design of LCL-filter based three-level Active Power Filters is proposed. The first step is to set the resonant frequency of LCL-filter according to the highest order harmonic current needed to be compensated. Then it optimizes the parameters based on the design principles and constraints. This kind of LCL-filters can achieve high compensation bandwidth and the switching frequency current simultaneously, which can reduce the inductor and cost efficiently.A simplified SVPWM modulation is proposed that it divides the three-level vector sector into six small two-level sectors, which can be implemented based on DSP more efficiently, thus simplifying computation process. It uses the appropriate redundant vector to achieve voltage balance based on the polarity of capacitor difference and the direction of power flow. The modulation is simple to implement and can be applied to multi-level Active Power Filters.The current tracking algorithm is the core part of Active Power Filters and it decides the effect of compensation. This paper presents a tracking current control method based on LCL-filter. PI and parallel resonant controllers under dq rotating coordinate are adopted to track the harmonic current. Each resonant controller represents two harmonic components under stationary coordinate, which can improve the efficiency of the system, achieving good dynamic performance and high tracking accuracy of the system.