It is well known that multilevel converters can offer significant benefits in terms of harmonic performance and reduced switching losses compared to their two-level counterparts. However, for lower voltage applications the Neutral-Point-Clamped (NPC) inverter suffers from relatively large semiconductor conduction losses because the output current always flows through two switching devices. In contrast, the T-Type multilevel inverter has less conduction losses because only a single outer loop switching device is required to connect the converter output to the upper and lower DC buses, albeit at the expense of increased switching losses since these outer switches must now block the full DC link voltage. Silicon Carbide (SiC) MOSFET devices potentially offer substantial advantage in this context with their lower switching losses, but the benefit of replacing all switching devices in a T-Type inverter with SiC MOSFETs is not so clear-cut. This paper now explores this issue by presenting a detailed comparison of the use of Si and SiC devices for a three-level T-Type inverter operating in grid-tie applications. The study uses datasheet values, switching loss measurements and calibrated heat sink thermal measurements to precisely compare semiconductor losses for these two alternatives for a T-Type inverter operating at or near unity power factor. The results show that replacing only the DC bus connection switches with SiC devices significantly reduces the semiconductor losses, allowing either the converter power level or the switching frequency to be significantly increased for the same device losses. Hence the use of SiC MOSFETS for T-Type inverters can be seen to be an attractive and potentially cost effective alternative, since only two switching devices per phase leg need to be upgraded.
This paper presents a theoretical harmonic analysis of phase disposition (PD) and phase shifted carrier (PSC) pulse width modulation (PWM) strategies for MMC converters. It is shown that when these strategies are implemented on a per MMC arm basis, their spectral performances converge because of cancellation of odd carrier sideband groups between each phase leg's arms. An improved PD modulation strategy is then presented that uses a single PD modulator for the entire phase leg, followed by a state machine decoder that evenly distributes switching pulses to all sub-modules across the phase leg upper and lower arms to balance the distribution of sub-module commutation events. The resulting strategy achieves optimum phase leg PD spectral performance, and also achieves natural voltage balancing of the MMC sub-modules. All theoretical findings are supported by simulation and experimental results obtained using a five level MMC prototype.
Model predictive control (MPC) has been widely advocated as a design strategy for many aspects of industrial electronics. The methodology has been strongly promoted by some researchers but has also attracted criticism from others. In this context, the purpose of this paper is twofold. First, we show that many existing and popular control strategies, including finite set MPC and linear controllers [proportional integral, proportional resonant (PR)], can be viewed as special cases of MPC. Second, we show that the predictive control framework allows one to embellish these classical control architectures with novel features and to design new and advanced control architectures to address various challenges posed by power electronics applications. The findings of the paper are supported by a practical example of designing of a novel form of PR controller with superior tracking performance and delay compensation, confirmed via simulation and experiments. Index Terms-Control design, current control, dcac power converters, model predictive control (MPC), pulsewidth modulation inverters.
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