Research On Several Problems Of Modular Three-Level Active Power Filter

With the rapid development of power electronics,the performance of a large number of non-linear devices such as unregulated rectifiers and switching power supplies is getting better and better.Nonlinear devices are widely used in the field of converter technology.The extensive use of these nonlinear devices has become the main source of grid harmonic currents,seriously affecting the power quality of the grid.The active power filter(APF)can effectively reduce the harmonic content of the power grid and improve the power quality.The theoretical study of the active power filter has attracted wide attention from domestic and foreign scholars.In this paper,neutral point clamp(NPC)three-level converter as the active power filter topology introduces several important issues of modularized three-level active power filter.They are the extraction of harmonic currents,the operation principle of three-level converter,the phase-locking method and the current loop controller.The method of synchronous rotation coordinate transformation was used to extract harmonic currents.Two low-pass filtering methods,cycle average method and moving average method,were proposed.The methods for extracting specific subharmonics and extracting zero-sequence currents were given.The basic principles of the operation principle,modulation strategy and neutral point potential balance of NPC three-level converter are introduced.The mechanism of deadband compensation and neutral point potential balance is analyzed.The deadband compensation method and the neutral point potential balance algorithm are given respectively,A method of grid positive and negative sequence separation based on grid iterative filtering and a method of grid positive and negative sequence separation method based on grid rotation transformation are proposed.The real-time phase of grid voltage is calculated by using the positive and negative sequence components.Aiming at the current tracking control method of APF.this paper adopts the deadbeat control strategy.In order to solve the possible sampling random noise,a current correction algorithm is proposed to suppress the influence of random noise on current tracking performance during sampling and digital processing.When the harmonic current changes rapidly,the traditional deadbeat controller is difficult to track the given current in time due to the limitation of its response speed.In this paper,a current tracking error compensation algorithm is proposed to compensate the output voltage increment.Finally,an experimental platform of active power filter was set up in the laboratory and relevant experiments were carried out to verify the feasibility of the proposed control method.