Research And Design Of Parallel Operation In Three-level Active Power Filter

Because the three-phase four-wire active power filter(APF)is generally limited by switching voltage stress,large output current ripple and high switching frequency,APF is gradually developed toward the multi-level topology.But the continuous improvement of the number of levels leads to the existence of cumbersome and poor real-time control of APF,complex structure and high cost and other defects,so the modular development of three-level three-arm mid-point clamp topology(Neutral Point Clamped,NPC)is an inevitable trend.Modular development is conducive to the realization of three-dimensional layout,improve the device power density,the device cooling in the appropriate range,and real-time compensation,to achieve a good compensation effect.This paper studies the three-level NPC-APF two modules running in parallel,and the work in this paper is as follows.Firstly,the main circuit topology of three-level NPC-APF is analyzed and a unified mathematical model of APF is established based on synchronous rotating coordinate system.Under the condition of non-ideal power supply voltage,the shortcomings of the Double Second Order Generalized Integrator Software Phase Locked Loop algorithm are analyzed.On this basis,a novel SPLL is proposed,that is,constructing the trigonometric function matrix to quickly and accurately extract the fundamental positive sequence voltage in the asymmetric distortion voltage component and tracking the fundamental voltage phase.In contrast,the algorithm has a certain superiority,and the harmonic current and reactive current component are got quickly and accurately within the non-ideal power supply voltage.Secondly,the double closed-loop control strategy system of NPC-APF is analyzed.The PI control and repetitive control of current loop is established,and the PI controller of voltage loop and the DC-side voltage potential balance algorithm based on zero sequence current injection are also established.Then,the three-phase four-wire three-level APF Space Vector Modulation method and Carrier-based Pulse Width Modulation are analyzed in this paper.The results show that the switching time and DC voltage utilization of the two methods are same.However,the calculation process of CBPWM is simple,and there is no sector judgment and calculation,which greatly reduces the operation process and reduces the operation cost.Thirdly,the common parallel control method of PWM inverter is analyzed,and the mathematical model of multi-module parallel system structure is established.Taking into account the same capacity and structure of the APF which are produced by the large-scale industrial enterprises have more excellent reliability and lower cost.Therefore,the current sharing control compensation strategy can be used to detect quickly the load harmonics and reactive current of the system in real time,and solve the problem of large capacity compensation.Finally,this paper builds the rated 25 A and 400 V APF device based on the theoretical analysis and simulation to verify the correctness of the above theory system and parameter design.The results show that the designed APF can effectively compensate the harmonic and reactive power and other components.