Low Carrier–Fundamental Frequency Ratio PWM for Multilevel Active Shunt Power Filters for Aerospace Applications

Active power filters create sideband harmonics over a wide frequency range around the multiple carrier-frequency harmonics, and these can encroach into the low frequency range. This issue is particularly critical when low carrier-fundamental frequency ratios are used such as in aerospace applications, where high fundamental frequencies exist. A three-phase multilevel active shunt filter with a low switching frequency is proposed to mitigate the lowest order carrier-frequency terms. However, low carrier frequencies lead to reference voltage phase delay and attenuation and can introduce significant baseband harmonics. These effects cannot be hidden by employing multiple modulator converters. In addressing these problems, an improved modulation approach is proposed in this work that allows duty cycle updating (N - 1) times per switching period for each H-bridge of one phase of the N -level converter [rather than only once or twice as in the regularly sampled pulse width modulation (PWM)]. The proposed modulation approach is then combined with predictive current control in order to enhance the system performance. The control loop performance compared to regularly sampled PWM is verified through simulations and experimentally by employing a three-phase five-level active shunt power filters in a 400-Hz power network.