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Multilevel Phase-Shifting Carrier PWM Technique in case of Non-Equal dc-Link Voltages

This paper analyzes the phase-shifting carrier PWM techniques in case of multilevel cascaded H-bridges with non-equal DC-link voltages. In the past it has been demonstrated that these techniques succeed in cancelling the sideband harmonics in the produced multilevel voltage only if the DC-link voltages are equal. Two different approaches can be used to modulate the reference signal in multilevel converters when the DC-link voltages are different and in a non-integer ratio: the theory of symmetric polynomials and resultants, and the generalized hybrid modulation (GHM) technique, which extends the hybrid modulation technique to non-integer DC-link ratio. This paper extends the original PSC-PWM technique proposing a method to obtain the proper shifting angles in view of reducing the overall output voltage WTHD under every possible de-link voltages. The proposed new approach is compared, in simulation with the original phase-shifted cascaded multilevel technique