| In recent years, as the high speed development of the information technology, lots of information equipments and the correnlative electronic equipments are installed and employed in modern buildings. Most of these equipments are single-phase nonlinear loads which would produce a lot of harmonic current when working. The harmonic problem in power grid is becoming more and more serious. Using active power filters (APF) to compensate the harmonic current is a good solution to tackle the 3-phase 4-wire system harmonic problems. In recent years, because of the advantages, such as lower du/dt, smaller current ripple and lower switching frequency, the 3-phase 4-wire tri-level three-leg neutral-point split APF is paid more and more attentions to. In the past two years, some foreign and domestic companies have begun to launch 400V/200V class tri-level products. However, due to the quantity of switches and the DC bus neutral-point unbalance problem, compared with the two-level 3-phase 4-wire APF, the control of the tri-level one is much more complex. So far, some of its key issues have not yet been solved perfectly.In this paper, the following key issues of the three-phase four-wire tri-level three-leg APF is researched and some correlative research results are achieved:(1) The mathematic models of different structures of 3-phase 4-wire tri-level APF are analyzed and compared in detail. The unified mathematic model of the 3-phase 4-wire tri-level APF is deduced and established in dqO coordinates. Based on the unified mathematic model to research the 3-phase 4-wire tri-level three-leg APF, the control scheme would be compatible among different structure of 3-phase 4-wire tri-level APFs.(2) The relationship between the outputted currents of APF and the DC voltages across the DC capacitors is detailedly analyzed. Base on the analysis results, an evaluation index is proposed to evaluate the influence exerted by the outputted current of APF on the neutral-point potential. And meanwhile, another relevant evaluation index is proposed to evaluate the modulation scheme's control capability of neutral-point balancing.(3) The modulation scheme for 3-phase 4-wire APF inα-β-0 coordinates is analyzed detailedly. The influence exerted by the 27 space vectors on the neutral-point potential under different conditions is researched. Based on the research results, a concept named neutral-point control grade is proposed to evaluate the influence exercised by the vectors on the neutral-point potential. Based on this concept, a novel space vector modulation scheme with neutral-point potential balancing capability is proposed for the first time. In this scheme, vectors with higher neutral-point control grade are selected to approximate the reference vector. By this scheme, the neutral-point potential unbalance problem in the small sectors is effectively solved. In a-b-c coordinates, a novel neutral-point unbalance controllable 3-D space vector modulation scheme base on the neutral-point balancing factor is proposed in which the vectors selection scope can be adjusted automatically according to the relationship between the outputted current and DC voltage. As many complex modulation conditions are unified to be three typical ones, this modulation scheme is very easy to realize by a DSP chip. The phenomenon named'self-balancing'is discoveryed and explained.(4) The design method and stability condition of the current loop controlled by a PI controller is detailedly analyzed. Further, the error transfer function of the close loop system compensated by a cascaded lead compensator is established and analyzed. From the bode diagram, it's obviously that the amplitude-frequency characteristic of the system compensated by a cascaded 2-order lead compensator is effectively improved. The state feedback control is utilized to place the poles of the APF system to the original point of the z-plane. In this control system, a compound repetitive harmonic predictor is designed to predict the harmonic current and meanwhile a state observer is utilized together to make the system realize deadbeat response. Improving PI current controller by the proposed prediction tools, the close loop frequency domain character of the PI control system is effectively improved.(5) The mathematical model of the DC side of the tri-level APF is deduced and analyzed in dqO coordinates. The design method of the DC bus voltage controller is detailedly discussed. To tackle the neutral point potential unbalance problem in the DC bus voltage establishing process, a reactive power injection scheme is proposed to solve this problem.(6) A control hardware platform with a TMS320C28335 float point digital processor (DSP) is designed and built to satisfy the high speed calculation requirement of the 3-phase 4-wire tri-level three-leg APFs.
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