Control of Multilevel Cascaded H-Bridge Inverters

THD Study of Seven-level CHB Inverter using Phased-Shifted and Level-Shifted PWM

In phased-shifted modulation, a multilevel inverter with m voltage levels requires (m – 1) triangular carriers. In the phase-shifted multicarrier modulation, all the triangular carriers have the same frequency and the same peak-to-peak amplitude, but there is a phase shift between any two adjacent carrier waves, given by Ɵcr = 360°/(m – 1). In the case of a 7-level CHB (m = 7), as we used in our project, all carrier waves (7-1 = 6) are 60o apart from the neighboring carrier waves. Similar to the phase-shifted modulation, an m-level CHB inverter using level shifted multicarrier modulation scheme requires (m – 1) triangular carriers, all having the same frequency and amplitude. The (m – 1) triangular carriers are vertically displaced such that the bands they occupy are contiguous. The frequency modulation index is given by mf = fcr/fm. In the case of a 7-level CHB (m = 7), as we used in our project, all carrier waves (7-1 = 6) are displaced from the neighbouring carrier waves.