SVPWM-Based Voltage Balancing And Fault-Tolerant Algorithm For Three-Phasre H-Bridge Cascaded Multilevel Converters

The cascaded H-bridge multilevel converter(CHMC)is a promising topology for medium-and high-voltage applications,as it features modularity and scalability.The main issues faced by a CHMC mainly focus on the expansibility of the relevant modulation algorithm,large turn-on and turn-off losses for switching devices,unbalanced DC voltages,and malfunction of switching devices and power cells of the CHMC.In order to optimize actual applications of a CHMC,a(space vector pulse width modulation,SVPWM)-based algorithm is proposed in this thesis to address the aforementioned issues.It can be clarified as follows:(1)A simplified SVPWM incorporated with variety of control factors is proposed.No any complex multiplication and division as well as trigonometrical calculations are required to detect modulation regions and triangles.Subsequently,with respect to the equivalence between SVPWM and(Nearest Level Modulation,NLM),a dwell vector closest to the origin and three-phase duty cycles can be obtained by the proposed SVPWM algorithm.Finally,the duty cycles are compared with the corresponding modulation triangle to generate the required PWM waveforms.(2)Aiming at the unbalanced DC voltages of a CHMC,mathematical modes of its AC side and DC side are established.In addition,a cost function to describe the relationship between the average DC voltages and DC voltages of respective power cells is given.Accordingly,the factor N generated by the proposed SVPWM is utilized to minimize the aforementioned cost function,which is used to balance the mutual-phase DC voltages.Compared with the PI-based generalized algorithms,the corresponding control loops are omitted.Finally,a modulation triangle is introduced to fix the switching frequency of the switching devices for the CHMC.(3)The switching states and duty cycles of the corresponding dwell vectors are used to balance inner-phase DC voltages of a CHMC,in which minimum number of switching transition is guaranteed.Based on law of conservation of energy,model mathematical modes of AC side and DC side of the CHMC in one phase are established to determine the change tendency of output voltages of respective power cells.What is more,in order to reduce switching losses and voltage surge,change range for the output voltages is supplemented.Based on the proposed algorithm,vertical space vectors can be calculated.The proposed algorithm is simplified and applicable to any level number of the CHMC.(4)To resolve the short-switch and open-switch failures of switching devices as well as power cell failure for the CHMC,two fault-tolerant SVPWM-based algorithms are proposed.The proposed algorithms reconfigure the mathematical mode which is used to obtain the active space vectors in Section(1).Subsequently,the remained active vectors are obtained under different failure conditions.The need of switching transition number reduction and harmonic optimization is satisfied simultaneously.Meanwhile,an equal power distribution principle is introduced to reduce the voltage stress among different power cells when short-switch and open-switch failures occur.The proposed algorithms are simplified.Based on them,balanced line-to-line voltages and currents are obtained for the CHMC under different failure conditions.