Research Of D-STATCOM In Three-phase Unbalanced System

In order to solve the power quality problem, with better harmonic suppression performance, faster dynamic response speed and more significant economic, D-STATCOM is very important in research field. By the use of PWM(Pulse Width Modulation) control, Traditional voltage source four-quadrant converter has the advantages such as controllable line current, bidirectional power flow, which makes the converter widely used. In high power applications, Traditional voltage source four-quadrant converter generally has the low switching frequency to ensure efficiency. In order to satisfy harmonic standard, line inductor needs to be larger which will decrease system dynamic response speed and increase size and cost of the device. To satisfy the same harmonic standard, due to the better attenuation characteristics in high frequency, the total inductor value of LCL filter is much smaller than the inductor value of L filter, which makes the device smaller and cheaper. But LCL filter is a third-order system, which has obvious resonance peak to make system unstable. Therefore the resonance suppression method need to take into consideration. D-STATCOM can be used to compensate in the closed-loop control system, and each sequence current needs to be detected.This paper firstly introduces the working principle of voltage source four-quadrant converter. Under the assumption that the circuit is ideal, mathematical models of voltage source four-quadrant converter based on LCL filter is established in the three-phase stationary coordinate system, two-phase stationary coordinate system and the two-phase rotating coordinate system. After establishing models, the paper analyzes the frequency characteristics of LCL filter comparing to L filter. According to the frequency characteristics of LCL filter, the paper introduces the controller design of conventional dual-loop control system. Based on the design, the method for LCL filter resonance suppression has been studied, including passive damping method and active damping method. Given the advantages of active damping method on loss, efficiency and other factors, the paper uses the current dual-loop control system to replace conventional current single loop, and studies the current loop controller design for the current dual-loop control system, and validates the design by simulations and experiments. The experiments proves that the design is reasonable and effective and the system works properly.For the study on D-STATCOM, the paper uses the voltage source four-quadrant converter based on LCL filter as the circuit structure and the current dual-loop control system based on the feedback of machine side inductance current as the basic control strategy. After detecting the sequence of the unbalanced current, the paper establishes mathematical models of sequence, studies a control strategy of D-STATCOM and validates the design through simulations and experiments. The experiments proves that the system compensates well.