Interleaving and Harmonic Cancellation Effects in Modular Three-Phase Voltage-Sourced Converters

This paper presents a systematic study of harmonic cancellation effects in three-phase voltage-sourced converters (VSCs), both among parallel, interleaved VSC modules and within individual phase of each VSC module. For inter-module harmonic cancellation, we will prove that the effects of interleaving on output current ripple are similar to those in interleaved buck DC-DC converters, and that inter-phase coupling and continuous variation of the duty ratio don't affect the basic harmonic cancellation mechanism. For intra-module harmonic cancellation, we will show that, over each carrier cycle, there is no fundamental difference in the PWM output between trailing-edge modulation and double-edge modulation; the later modulation method eliminates certain sideband harmonic components as reported in the literature because of the periodicity and unique structure of its PWM output spectrum over different carrier cycles. The mathematical tool we used to analyze the harmonic cancellation effects is different from conventional double-integral Fourier analysis method, and is based on first calculating the spectrum of the PWM pulse pattern over each carrier cycle, and then piecing together different cycles using frequency-domain convolution to determine the overall spectrum over a fundamental cycle. This approach provides the necessary insights into the structure of the harmonics and leads to the findings described above