Research On Topology And Control Method Of Multilevel Active Power Filter

The multilevel technique together with its more output-voltage levels applicable to the active power filter could raise the tracing ability of output current to reference current and improve the compensation effects of active power filter (APF). Based on that the complicated circuit structure and lots of power semiconductor devices of the traditional multilevel inverters have brought much difficulties to the design of control system, this paper proposes a simple inverter topology circuit and its application on APF, the control method is also been studied in this paper.The traditional multilevel inverters are mainly applied in high voltage and large capacity situations with complex circuit structure and large amounts of power electronic devices. A new multilevel inverter topology with simple structure is presented in this article without consideration of high voltage and large capacity. And this topology has the advantages of simple structure, less power electronic devices, no need of clamping diodes and flying capacitors and so on. The new five-level inverter only needs six one-way power switches and two capacitors while the new seven-level inverter only needs eight one-way power switches and three capacitors. The new multilevel inverter is obtained by increasing the auxiliary circuits based on the traditional H bridge inverter, both the H bridge and auxiliary circuits are operated independently so the control system can also be designed independently. This new multilevel inverter is also easily for expanding, the output levels could be expanded to five, seven, nine and so on based on the traditional H bridge inverter.This paper analyzes the principal and control method of the five-level active power filter. The tracing precision of output compensation current to reference current is controlled according to the five different output levels of the five-level inverter by using current hysterics means with the purpose of better compensation effects. In order to reduce switching power loss, there are two power switches working at the system frequency; the other four switches control the output current to trace the reference current effectively. DC voltage control adopts PI closed loop control, thus the reference value of compensation current could be obtained. The circuit structure and operation processes are described in detail, and both the simulation and experimental results verified the accuracy and the validity of the control methods.In order to improve current control methods of APF, this paper investigates an improved deadbeat control method based on the analysis of the traditional deadbeat control method. The improved deadbeat control method is implemented by letting the average value of output compensation current of APF in one on-off cycle track the reference value and computing the duty ratio in next cycle. The method has the advantages of simple computation, small delay, small output current ripple and better tracing performance to current mutation. The methods can be easily applied to the multilevel inverter. The detailed theoretical analysis and calculations together with the tracing ability of output current to reference current of APF are all presented in this article. The compensation results are compared between the traditional deadbeat method and the improved one by simulation, the simulation results confirmed the advantages of improved deadbeat method.This paper also presents the whole design of control system which is quite significant for improving compensation effects of APF. The control system of the novel multilevel APF is composed of two parts including output current control and DC voltage control. The output current control strategy is based on the improved deadbeat rule, it obtains the needed output voltage by different voltage combinations and each operation state time is calculated so as to get the power switch control signals. Control system adopts traditional PI controller to control active power and balance the DC capacitor voltage by different switch combinations. It may realize the balancing of capacitor charge and recharge in the same moment that the output current tracks the reference current. Simulation and experimental models have been built based on the new seven-level inverter, control algorithm is verified by both simulation and experimental results. All above would establish a good foundation for further theoretical research and practical application.