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The capacitor voltage fluctuation suppression is essential for the reliable and safe operation of the modular multilevel converter (MMC). This article proposes a shared capacitor (SC) based MMC with suppressed capacitor voltage fluctuation, where the SC in each submodule is utilized as the power channel for three phases of the MMC. A multivariate sorting-based voltage balancing control (MVS-VBC) strategy for the SC-MMC is also proposed, which can balance the SC voltages based on the sorting of arm currents and SC voltages. The proposed SC-MMC can effectively suppress the capacitor voltage fluctuation, and accordingly improves the reliability of the SC-MMC. Simulation studies with MATLAB/Simulink are conducted and a down-scale SC-MMC prototype is also built in the laboratory. The results both confirm the effectiveness of the proposed SC-MMC, MVS-VBC strategy, and the capacitor voltage fluctuation suppression effects.

For a modular multilevel converter (MMC) operating in the medium-voltage range, carrier-based pulsewidth modulation (PWM) methods are good options for modulating the converter, as harmonic contents of the phase voltage/current can be centered around the carrier frequency and/or its multiples. However, in the relevant literature, arm voltage references are mostly normalized by the submodule (SM) capacitor voltage reference, leading to the so-called direct modulation, while the indirect modulation considering the capacitor voltage ripples receives much less attention. This article investigates the indirect modulation of MMCs, focusing on a specific PWM method where only two SMs per phase operate in PWM mode in each control period. It is found that the direct PWM (DiPWM) generates phase voltages/currents that contain high-amplitude low-order harmonics, while the indirect PWM (IndiPWM) presents well-suppressed low-order harmonics but significant distortions around the carrier frequency. The cause of the above problem of the IndiPWM is revealed theoretically through Fourier analysis, and a solution with simple implementation is developed by rearranging the switching pulses of the PWM-mode SMs. Both simulations and experiments are carried out. The results show that the proposed method combines the advantages of both DiPWM and IndiPWM, achieving low harmonic distortions simultaneously in the low frequency and around the carrier frequency.

Semiconductors in power converters are operated in switched-mode. For a given sampling frequency, a digital controller computes the next actuation according to several objectives. Typically one desires to regulate current or voltage values with minimal error and harmonic distortion. Higher switching frequency of the semiconductor is helpful as the granularity of the problem is improved and the harmonic spectrum is shifted toward high-order harmonics that are easier to filter with passive elements. However, the switching losses increase. Finite control set model predictive control (FCS-MPC) is proven to be an excellent control method with a great ability to maximize harmonic quality in terms of the switching frequency, particularly when the controller is configured to consider long prediction horizons. Nonetheless, in order to avoid computational overruns, the prediction horizon and the sampling frequency cannot be indefinitely increased. In this article, the tradeoff between these two parameters is studied, providing guidelines on FCS-MPC tuning for different configurations.

The power conversion system based on the modular connection has widespread applications in various power electronic systems. To accurately estimate the state of health without recognizing the systematic mathematical model and to extend the lifetime, this article proposes a lifetime extension approach based on the Levenberg–Marquardt back propagation neural network (LM-BPNN) and power routing of interleaved dc–dc boost conversion systems. The LM-BPNN model is constructed based on the voltage, current, and temperature data generated by the system. On the basis of the trained LM-BPNN, the real-time cumulated damage estimation of each power cell in the conversion system can be accomplished. Applying the power routing concept, the dc–dc boost conversion system allocates different power to the cells according to the cumulated damage of each cell, thereby delaying the failure of cells with higher cumulated damage. Numerical simulation results show that the proposed lifetime extension approach can extend the overall system lifetime. Furthermore, an experimental setup of the interleaved dc–dc boost conversion is constructed to verify the proposed approach, which is of great significance for predictive maintenance in the industrial system.