Carrier-Modified Pulsewidth Modulation for a Single DC Power source driven Cascaded Three-level Inverter Drive

Three level voltage space vector can be generated using the cascaded three-level inverter comprising of two, two-level inverters. Traditionally, this circuit demands two isolated DC power sources to power the individual two-level inverters. This paper proposes a new carrier-modified pulsewidth modulation (PWM) method for the cascaded three-level inverter that involves modifying the carrier signals using suitable offset-voltage in place of the commonly used approach that involves modifying the reference signals. The neutral-point current, the capacitor voltages and the deviation in the capacitor voltages are carefully studied and based on the studies, suitable offset-voltage to be injected to the carriers is determined that is dependent on the DC-link capacitor voltage deviations. It also presents equivalence of the proposed modified-carrier PWM with the traditional CBPWM that involves voltage-injection to the modulating signals. The envisaged carrier-modified PWM method obviates the need of two isolated power sources and thus enables the cascaded three-level inverter to be operate from a single DC power source. The efficacy of the proposed carrier-modified PWM method for the single DC source driven cascaded three-level inverter drive is demonstrated through simulations and experiments by running the induction motor load over a wide speed range. Besides, superior load current performance is also achieved with the proposed PWM method that is supported from vWTHD profiles.