Fig. 14 shows the inverter waveforms and u-phase
switching patterns Sup, Sum, Sun of the T-type three-level inverter
when the grid is stable. From the experimental results, due to
the switching operation of the inverter at 300Hz, it is confirmed
that the output voltage becomes the same as the input voltage
of 200V 50Hz. In other words, the proposed converter achieves
the sinusoidal load voltage without using PWM modulation. As
a result, the proposed circuit can achieve downsizing and loss
reduction of an EMI filter. However, the output voltage
waveform has a surge voltage and the switching pulses have
pulse noise periodically. The cause of the surge voltage will be
found out and suppressed in the future.