Research Of LCL-type Three-level Active Power Filter

With the rapid development of power electronics industry, the injection of high power、nonlinear、shock and asymmetry loads lead to serious harmonic problems and affects the normal operation of the power system a lot. Active Power Filter (APF) is a new kind of power electronic devices which can dynamically compensate harmonics and reactive power. It can realize real-time harmonic compensation with the characteristics of fast response, dynamic continuous and real-time compensation. So it is widely discussed and a large number of circuit topologies and control schemes are developed.In this paper, the inherent imbalance problems of the capacitor voltage in the three-level circuit are analyzed and a control strategy, by which can effectively restrain the imbalance of the capacitor voltage, is proposed; As is known, the resonance peak of LCL filter may cause the system unstable and to address this problem, six different kinds of passive damping technologies are analyzed and the advantages and shortcomings are compared respectively. Based on the evolution of control block, the capacitor current feedback damping strategy is deduced and the virtual resistance is imitated by the controller to realize the same effect produced by the real resistance. To introduce the capacitor current feedback into the digital control system, firstly, the impact of digital control delay of LCL filter capacitor current active damping control is studied in detail. The loop gain in the discrete domain is deduced and current feedback coefficient constraints is achieved to guarantee the system stable by digital controller; Then, by studying frequency domain based Discrete Fourier Transform (FFT)and Recursive Discrete Fourier Transform (RDFT)harmonic detection research, a new sliding window iterative Fourier harmonic detection method, which is based on the Space-Vector Discrete Fourier Transform (SVFT) harmonic detection method, is proposed.In this method, orthogonal vectors is used to eliminate the traditional RDFT multiplication factor to quickly and accurately abstract each harmonic component with a few calculations. Finally, a 400V/100A three-level prototype whose digital controller is a quad-core control chip TMX320F28377D is designed to verify the correctness of the mathematical model and related theory.