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Constant Switching Frequency State Feedback Control of Cascaded H bridge Multilevel Inverter with Reduced Switches

This paper presents a constant switching frequency state feedback control of a cascaded H-bridge multilevel inverter. The inverter topology consists of a number of full-bridge cells, each equipped with a transformer, connected in parallel to a single dc source. The turns ratio of one transformer is modified to 1: 2 which reduces the number of components required to generate an N-level waveform, thus decreasing the size and cost of the inverter. Level shifted multi-carrier sinusoidal pulse width modulation technique is modified for application to this topology such that each bridge generates a PWM output waveform. For closed loop operation of the inverter, state feedback control is applied to drive the inverter output to the desired operating point. The control law is derived using state space modelling of the inverter and is used as the modulating signal in PWM technique so as to generate a constant switching frequency closed loop operation of the inverter. The inverter operation is verified using PSCAD-EMTDC. Results are presented both for open loop and voltage controlled mode of operation. The working of the controller is verified with step changes in load, reference value and, dc-link voltage.