Research And Engineering Realization Of Three-Phase Four-Wire APF

In the medium and low voltage three-phase four-wire distribution system,large scale of power electronic equipment accessing has caused serious reactive power and harmonic pollution.To solve the harmonic and reactive power problems,the three-phase four-wire active power filter(APF)is studied in this paper,including harmonic detection,compensation,and converter configuration and its control strategy.In the three-phase four-wire system,the three-phase unbalanced load causes positive sequence,negative sequence and zero sequence components in each harmonic current.The traditional detection methods cannot accurately detect the content of each harmonic current.A random harmonic current detecting method is proposed in this paper,which is capable of simultaneously detecting different orde of positive,negative and zero sequence components.Amplitude and phase correction algorithm is introduced to reduce the high order harmonics detecting error.In the process of harmonics detection,the effect of detection will be directly influenced by the accuracy of the phase.To solve the problem that the traditional phase-locked loop(PLL)cannot accurately lock the phase when the network voltage is distorted,this paper deeply analyzes the working mechanism of the phase-locked loop and compares different phase-locked loop schemes.Finally,a software phase-locked loop based on second-order generalized integrator with strong anti-interference ability is adopted.The APF with three-phase four-wire system can realize compensation functions for reactive power,the three-phase imbalance and harmonics,etc.However,due to the limitation of capacity,only part of them may be compensated.Therefore,how to flexibly set and control the compensation target under a certain compensation capacity of the APF is an important research content.The existing full compensation mode can only compensate all current components except the fundamental active current.This paper introduces the mode of specified component compensation,and gives the detection method and control strategies of harmonic of the APF with three-phase four-wire system in this mode.In order to overcome the disadvantages of high-level output voltage THD and large switching losses in two-level topology,this paper chooses the T-type neutral point clamped(TNPC)structure as the main circuit topology,and gives the control method of adjusting by adding the DC current component under the SPWM modulation strategy with regard to the inherent midpoint imbalance problem in the three-level topology.The APF control strategy adopts the basic control method of the outer voltage loop and the inner current loop.Since the command current contains harmonic currents of multiple frequencies at the same time,this paper deeply analyzes the basic principle of the proportional resonance(PR)controller and designs a current inner loop control method in which a single proportional controller and multiple resonant controllers running in parallel to make the current inner loop realize no static error following to the command current.Based on the research results discribed above,an three-phase four-wire APF prototype is developed,and the methods of the parameter calculation,main circuit device selecting,and control system design are all present in detail.The control strategy and system software is developed on a dual-core DSP platform,with elaborate planning and full utilizing the onchip hardware resource and parallel computing.Experiment result is present and can validate the effectiveness of the proposed harmonic detection method and control strategy.