Abstract
This paper proposes a generalized method for thegeneration of space vector pulsewidth modulation (SVPWM) signalsfor multilevel inverters. In the proposed method, the actualsector containing the tip of the reference space vector need notbe identified. A method is presented to identify the center of asubhexagon containing the reference space vector. Using the centerof the subhexagon, the reference space vector is mapped to theinnermost subhexagon, and the switching sequence correspondingto a two-level inverter is determined. A new technique is proposedin this paper, by which these two-level vectors are translated to theswitching vectors of the multilevel inverter by adding the center ofthe subhexagon to the two-level vectors. The proposed method canbe extended to any n-level inverter, and a generalized algorithmis proposed. The scheme is explained for a five-level inverter, andexperimental results are presented for a three-level inverter.Index Terms—Multilevel inverter, open-end winding, reversemapping, space vector pulsewidth modulation (SVPWM).
I. INTRODUCTION
IN THE FIELD of medium- and high-power applications,multilevel inverters have emerged as an attractive choice[1]–[3]. The output waveforms of the multilevel inverters aresmoother than those of a two-level inverter as the output voltageis synthesized from multiple levels of dc voltage. The mostwidely used techniques for implementing the pulsewidth modulation(PWM) strategy for multilevel inverters are sine-trianglePWM (SPWM) and space vector PWM (SVPWM) [4]–[24]. Inmultilevel SPWM, the reference sine wave is compared witha number of level-shifted carriers to decide the switches to beturned on [5]. In the SVPWM scheme, the sampled value ofthe reference voltage space vector which is the combined effectof the three-phase voltages is realized by switching the nearestvoltage space vectors among the inverter voltage vectors [6].There are different techniques available for implementingSVPWM for multilevel inverters [7]–[24]. In general,the SVPWM implementation involves the sector identification,switching-time calculation, switching-vector determina tion, and optimum-switching-sequence selection for the invertervoltage vectors [7]–[20], [23]. The sector identificationcan be done by coordinate transformation [8], [9], [16] orby repeated comparison of the three phase reference voltages[7], [15]. The lookup tables can be used for determining theswitching vectors in optimum switching sequence [6]–[20]. Thecalculation of the duration of the switching vectors can besimplified using the mapping technique, in which the identifiedsector of the multilevel inverter is mapped to a correspondingsector of the two-level inverter [13]–[15], [23].The SVPWM methods using the principle of equivalencewith SPWM can generate the SVPWM signals directly fromthe instantaneous reference phase voltages for multilevelinverters without using lookup tables [21], [22]. The fractalbasedapproach for SVPWM generation using a triangularizationscheme to generate the voltage space vectors also does notrequire lookup tables [23].This paper proposes a new approach to generate SVPWMsignals for multilevel inverters. The proposed method uses sectoridentification only at the two-level. In the proposed method,the actual sector (where the tip of the instantaneous referencespace vector lies) in the space vector diagram of a multilevelinverter is not required to be identified. A method using theprinciple of mapping is proposed for generating the switchingvectors corresponding to the actual sector and the optimumswitching sequence of a multilevel inverter from that of the twolevelinverter. An algorithm is proposed for generating SVPWMfor any n-level inverter. The proposed method can be used foran inverter with an even number of levels also. The scheme isexplained with a five-level inverter, and experimental results fora three-level inverter are presented.
II. PRINCIPLE OF THE PROPOSED METHOD
Fig. 1 shows the space vector diagram of a five-level inverter.The redundant vectors are not shown for simplicity. The smalltriangles formed by the adjacent voltage space vectors arecalled sectors. Such six sectors around a voltage space vectorforms a hexagon called subhexagon [14], [15]. The space vectordiagram of a multilevel inverter can be viewed as composed of anumber of such subhexagons. The shaded regions in Fig. 1show two subhexagons. They are represented as “subhexagon I”(referred as inner subhexagon) having the vector 000 as thecenter and “subhexagon II” having the vector 330 as the center.The inner subhexagon can be viewed as a space vectordiagram of a two-level inverter whose inverter voltage vectorsswitch between the lowermost levels.


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