A Nine Level Inverter Topology with Linear Operation at Over-modulation Region

This paper discusses a nine level inverter scheme formed by cascading a five level T-type and a H-Bridge unit where all the three phase T-type and HB capacitors can be balanced using pole voltage redundancy. The proposed topology can also operate upto eleven level(pole voltage levels)mode. But at extreme level with conventional carrier based Pulse Width Modulation(PWM) HB capacitor balancing issue limits the operation of the inverter at over modulation range. So a unique PWM scheme is proposed which enables to balance the floating HB capacitors of all the three phases at 11-level mode such a way that the modulation range of the inverter can be increased linearly. Hence the output phase voltage is devoid of any lower order harmonics which are critical for drive application as those harmonics may cause low frequency torque ripple. The feasibility of the proposed inverter scheme is verified under different test condition with extensive simulation studies in MATLAB SIMULINK environment. A hardware prototype is build in the LAB and experimental results are also provided for further validation of the proposed scheme. The increase in linear modulation range is achieved till 0.6248V
dc
at unity power factor operation.