Study Of Shunt Active Power Filter Based On Three-level Converter

With the development of power electronics technology and heavy use of various non-linear loads, power quality problems such as harmonic pollution have received great attention. Active Power Filter (APF) is a kind of power electronic device which can dynamically suppress harmonic and compensate reactive power. Compared with the traditional devices such as Passive Filter (PF), APF has the advantages of fast response, good compensation effect and can realize dynamic compensation. However, due to the limitations of voltage level of power devices, the traditional two-level APF is difficult to achieve harmonic compensation for high-voltage nonlinear load. Multilevel converter can produce multi-step and low-voltage distortion waveform. It has many significant advantages such as low pressure and low switching losses of the switching devices, high efficiency and low electromagnetic interference, and is widely used in high voltage and high power occasion in recent years. Among them, APF using the three-level converter has become a hot topic in the field of APF.Research object in this paper is APF based on the NPC three-level converter. Firstly, the research situation and trend of two-level and three-level APF, as well as the key technologies of APF are introduced. Detailed introduction and analysis of the harmonic detection method based on instantaneous power theory and current tracking control strategy are made to lay the theoretical foundation of control method research. Secondly, harmonic extraction method based on d-q transformation is introduced. The impact of phase difference between the reference signals of d-q transformation and source voltage to fundamental active current and reactive current detection is analyzed. The existing method to solve this problem is improved by increasing a PLL of fundamental positive sequence voltage. Simulations are made to verify these two methods.Again, the three-level hysteresis band (HB) control principle is introduced based on the two-level HB control method. To solve the problem of unfixed switching frequency in HB control, a kind of three-level adaptive HB control strategy is discussed based on the two-level adaptive HB control. The hysteresis width calculation method of multiple voltage grades is presented. In addition, a matched method is used to maintain the balance of midpoint potential. Detect the direction of midpoint current and amplitude of midpoint potential to determine whether to adjust the switch combinations, which will make the direction of midpoint current good for the midpoint potential balance.Finally, a shunt three-level APF experiment system based on DSP TMS320F2812controller is designed on the basis of theoretical analysis and simulation research. Device selection and parameter design of the main circuit are also made. Software and hardware design for the experiment system are detailed introduced. An experimental platform is built and the primary experimental results of this shunt three-level APF are consistent with the theoretical analysis.