Three-stage eighteen-level hybrid inverter design with novel control method are presented. The inverter consists of main high-voltage, medium-voltage and low-voltage stages connected in series from the output side. The high voltage stage is a three-phase, six-switch conventional sub-inverter. The medium and low voltage stages are made of three-level sub-inverters constructed by H-bridge units. The proposed control strategy assumes a reference input voltage vector and aims to approximate it to the nearest inverter vector. The control concept is based on holding the high voltage state as long as it is feasible to do so. The reference voltage vector has been represented in a 60°-spaced two axis coordinate system to reduce the computational effort. The concept of the stagedcontrol has been presented, the transformed inverter vectors and their relation to the switching variables have been defined, and the implementation process has been described. The test results verify the effectiveness of the proposed strategy in terms of computational efficiency as well as the capability of the inverter to produce very low distorted voltage with low switching losses. IntroductionIn general, multilevel inverters, MLIs, refer to the class of inverters with output points which have more than two voltage levels with respect to a reference point [15]. The capacity to produce output voltage levels higher than those of the power semiconductor switching devices' ratings and the reduced distortion and dv/dt stress are the basic MLIs advantages [5]. While the circuit complexity, cost and control difficulty are the main barriers on the MLI expansion road [19].With the basic MLI topologies, the number of inverter levels is linearly proportional to the number of inverter's switching devices and this limits the practical number of levels to few levels [17]. Inverter with extended number of levels have been built using the asymmetrical MLI structure, where the inverter's cascaded H-bridges have been supplied with different voltage levels [9,10] . This condition is not satisfied with ratio-3 related dc sources, and hence this selection is not appropriate for PWM control. This ratio, however, has been followed by some designs which do not apply PWM control [6,8]. Alternatively, the control method followed is to approximate the reference voltage vector to the nearest inverter vector.Whereas determining the nearest inverter vector to the reference vector is systematic, relating this vector a one inverter switching state is not so. The large number of inverter levels results many switching states sharing the same voltage vector, when a given inverter vector is selected a second stage of processing need to be included to determine which switching state related to this vector is going to be applied. This option has not been utilized in previous research applying voltage approximation of inverters with a large number of stages [6,8].The study presented in [1] have suggested a stage-by-stage vector control structure to mitigate the problem of sw...

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This paper presents a voltage control algorithm for a hybrid multilevel inverter based on a staged-perception of the inverter voltage vector diagram. The algorithm is applied to control a three-stage eighteen-level hybrid inverter, which has been designed with a maximum number of symmetrical levels. The inverter has a two-level main stage built using a conventional six-switch inverter and medium-and low-voltage three-level stages constructed using cascaded H-bridge cells. The distinctive feature of the proposed algorithm is its ability to avoid the undesirable high switching frequency for high-and medium-voltage stages despite the fact that the inverter's dc sources voltages are selected to maximize the number of levels by state redundancy elimination. The high-and medium-voltage stages switching algorithms have been developed to assure fundamental switching frequency operation of the high voltage stage and not more than few times this frequency for the medium voltage stage. The low voltage stage is controlled using a SVPWM to achieve the reference voltage vector exactly and to set the order of the dominant harmonics. The inverter has been constructed and the control algorithm has been implemented. Test results show that the proposed algorithm achieves the desired features and all of the major hypotheses have been verified.

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