Carrier-Based Continuous PWM for Compensating the Effect of Unbalanced and Oscillating Neutral-Point Voltage in T-Type Inverter

The three-level T-type inverter incorporates two cascaded DC-link capacitors that share a common coupling node (CCN) known as neutral-point. During the operation of T-type inverter, the CCN voltage oscillates at triple the line frequency. Furthermore, in some applications like dual-input photovoltaic systems, there may be a DC voltage imbalance across DC-link capacitors, resulting in an unbalanced CCN voltage. The presence of an unbalanced or oscillating CCN voltage in T-type inverter affects the quality of output current and voltage waveform by introducing low-frequency harmonics. To address this issue, this paper proposes a novel modulation method to compensate the effects of unbalanced and oscillating CCN voltage, enabling the generation of non-distorted phase voltage and current. By enhancing the output waveforms quality, the proposed method can result in reduction of the DC-link capacitor size and improvement of power density in the T-type inverter. In this method, the modulation signal is adjusted using simple mathematical calculations based on the sensed CCN and DC-link voltage, which imposes negligible processing time on the micro-controller. This method is applicable across the entire range of operation, regardless of load power factor or modulation index. Furthermore, the proposed method preserves the high-quality and sinusoidal shape of output current waveform even under dynamic disturbances in the CCN and DC-link voltage. To provide further insight, the normalized increase in total semiconductor loss due to the implementation of the proposed method is also evaluated. Finally, the analytical and experimental results are conducted to validate the effectiveness of the proposed compensating method.