Research On The Key Technologies Of Three-level Shunt Active Power Filter

In modern industrial and civilian occasions, power electronic equipment and other non-linear devices apply more and more widely, leading to the public grid harmonic pollution becoming worse and worse. On the other hand, more and more new equipments require for a higher power quality. Thus, the governance of harmonics is already an urgent matter. As an ideal means to improve the power quality, Active Power Filter has a very wide range of applications.Firstly, this paper introduces the grid harmonic pollution which is prevalent in the current, and leads to the object of study in this paper-Active Power Filter. Then specifically analyzed the harmonic generation, hazards, and governance, and give a brief overview of the development status of APF.Active power filter is divided into four categories. In this paper, various types of active power filter are simply introduced, in which Three-level parallel Active Power Filter is highlighted by analyzing its system structure and working principle. Through theoretical analysis, a mathematical model of APF in a-b-c coordinates and d-q coordinates is set up.There are two key technologies in active power filter, which are the harmonic and reactive current detection technology and the directive instruction tracking technology. The harmonic and reactive detection technology need to implement both all times harmonic detection and specified number of harmonic detection. This paper focus on the most commonly harmonic detection method of fourier decomposition method and id-iq method based on instantaneous reactive power. According to the advantages and disadvantages of different algorithms, use a sliding window iterative discrete Fourier transform RDFT harmonic detection method, to achieve the specified number of harmonic detection; use a id-iq harmonic detection method which based on synchronous rotating coordinate system, to achieve the all times harmonic detection.Directive current tracking technology is divided into two modules of control and modulation. In this paper, the control module is designed based on the predicted cycle deadbeat control algorithm, using a state observer for the next shot APF output current forecast; using a cycle predictive observer for the next two beats command current forecast. In addition, the design of the charge control algorithm of three-phase three-wire and three-phase four-wire APF is done through three steps.In the diode clamp three-level inverter, the most commonly used modulation algorithms are carrier layered PWM modulation and space vector PWM modulation. Both methods have their advantages and disadvantages, each on different occasions has a lot of applications. In this paper, both the two modulation methods are conducted in-depth research. Because of the common carrier PWM modulation algorithm’s shortcomings of low voltage utilization and midpoint potential imbalance, we studied a dual carrier PWM wave modulation method, and described its midpoint potential balancing algorithm. The traditional three-level SVPWM algorithm is just an extension of the two-level, with a large amount of calculation, and poor real-time. This paper adopts a new three-level SVPWM modulation techniques based on60°Coordinate System, and designs the midpoint potential balancing algorithm.At last, all of the algorithms are validated through simulation. Simulation results show that the harmonic and reactive detection algorithm quickly detect the harmonic current with a high accuracy. Deadbeat control algorithm can accurately obtain the command voltage value. Both of the two modulation algorithms have very good compensation effect. APF charge control algorithm can smooth charge the capacitor to standard value.