THD Analysis for a High Frequency Link SCR-Based PWM Inverter

Two commutation methods for an AC link inverter are compared in this paper. In a high frequency link inverter, a smaller size transformer can be used to provide galvanic isolation and voltage step-up instead of a bulky 60Hz transformer. Furthermore, an AC-link inverter removes one power conversion step that exists in a DC-link inverter. However, in the high frequency AC-link inverter, safe control of the ACAC output stage, which converts the higher frequency link voltage into the lower frequency output voltage, is challenging. Conventionally, dead time is inserted when the output current changes direction. Previous results presented two different commutation methods which did not require dead time to guarantee safe commutation. In this paper, the total harmonic distortion (THD) values of the output current from the two commutation methods are compared at different switching frequencies, load inductance values and load resistance values. This paper finds that both methods are generally good. However, for some switching frequencies and some output load values, one method produces better quality output current than the other method, and in other switching frequencies and load conditions the other method produces better output current. Therefore, this paper concludes that if one of the commutation methods should be chosen, the switching frequency and the load values should be considered carefully in the design.