Impact of Digital PWM Carrier-Related Baseband Distortion on Parallel Converters

Power converters are typically controlled by digital pulse-width modulators (DPWM). In a DPWM, the modulation and carrier signals are discrete. Regular sampling of the modulation signal is known to introduce baseband harmonics due to the effect of time delay on the switching pulses and a significant amount of work has focused on characterising this sampling on modulator baseband distortion. This work, however, focuses on how discretising the carrier signal contributes to modulator baseband distortion and how this can impact the operation of power converters. The time resolution at which the carrier signal is sampled depends on the DPWM clock frequency of a DSP. It is demonstrated, even in the case of oversampling, that sampling of the carrier can introduce significant baseband distortion in the modulated signal. The impact of this carrier-related modulator baseband distortion is demonstrated by a controller hardware-in-the-loop (C-HIL) real-time simulation of two grid-forming controlled parallel inverters. The results show a negative interaction between the two inverters, where a sustained low-frequency power oscillation occurs between them. In addition, it is demonstrated that the increase in carrier sampling resolution effectively dampens these oscillations. The presented work is relevant for control and stability analysis studies, which make use of linear average inverter models where the impact of the carrier may not be considered. It is shown that neglecting the effect of modulator baseband distortion will result in inaccuracies in developed models under certain carrier sample resolutions.