This paper proposes a new topology for ac-to-ac power conversion, which is a three-stage conversion. It comprises of a diode rectifier (ac-to-dc), a buck- boost converter (dc-to-dc) and anH-bridge inverter (dc-to-ac) working as an ac chopper. The topology works as V/f drive wherein the frequency is varied by the buck-boost converter and the voltage is varied by the inverter, which is used as a chopper. Thus, it provides variable output voltage and frequency for all three- phases, which can be used for V/f control of induction motor. As compared to the conventional two-stage conversion i.e. ac-dc-ac (ac-to-ac conversion with intermediate stiff dc-link), proposed topology has advantage of improved THD in output voltage, as the input to the inverter is not a stiffed dc but it is a pulsating dc, provided from output of buck-boost converter. Moreover the blocking voltage of each switch of inverter is not constant voltage but varies according to the pulsating input of inverter, thus the stress across switch, as well as machine winding will reduce as compared to two stage conversion system. The proposed scheme offers linear variation of output voltage from zero to rated, avoiding nonlinear overmodulation range used in conventional inverters. The simulation studies are carried out in Matlab/Simulink 2014 and various results are presented.
Neutral point potential balancing method for three-level power converters in two-stage three-phase four-wire power conversion system
