Four-leg inverters have been selected as one of the preferred power converter topologies for applications that require a precise control of neutral current, like active filters. The main advantage of this topology lies in an extended range for the zero sequence voltages and currents. However, the addition of a fourth leg extends the space vectors from two to three dimensions, making the selection of the modulation vectors more complex. Most of the algorithms that deal with this problem require an αβγ transformation. This paper presents a new space vector modulation algorithm using abc coordinates (the phase voltages) avoiding the αβγ transformation. Thanks to the use of abc coordinates, the algorithm is much simpler and more intuitive than in αβγ representation, drastically reducing the complexity of modulation algorithm and the computational load associated to it. Index Terms-Four-leg voltage source converter, three-phase four-wire, 3D-space vector modulation, active filter. 51 V3 V11 V15 v b-ref-v a-ref v a-ref-v c-ref 52 V3 V7 V15-v a-ref + v b-ref v a-ref-v c-ref 56 V3 V7 V8-v a-ref + v b-ref v a-ref-v c-ref v c-ref 57 V9 V13 V15-v a-ref v a-ref-v b-ref v b-ref-v c-ref 58 V5 V13 V15 v a-ref-v b-ref v b-ref-v c-ref 60 V5 V7 V15 v a-ref-v b-ref v b-ref-v c-ref 64 V5 V7 V8 v a-ref-v b-ref v b-ref-v c-ref v c-ref

In this paper, a novel three-dimensional (3-D) space-vector algorithm for four-leg multilevel converters is presented. It can be applied to active power filters or neutral-current compensator applications for mitigating harmonics and zerosequence components using abc coordinates (referred from now on this paper as natural coordinates). This technique greatly simplifies the selection of the 3-D region where a given voltage vector is supposed to be found. Compared to a three-level modulation algorithm for three-leg multilevel converters, this algorithm does not increase its complexity and the calculations of the active vectors with the corresponding switching time that generate the reference voltage vector. In addition, the low-computational cost of the proposed algorithm is always the same and it is independent of the number of levels of the converter.

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