The application of cascaded multilevel converters in large-scale photovoltaic plants connected to the utility grid has many advantages, such as the absence of sinusoidal filters and line-frequency transformers. Improved overall performance under partial shading conditions is possible due to the distribution of arrays among a large number of maximum power point trackers. In this situation, although the cells operate at different power levels, the three-phase currents injected into the point of common coupling must be balanced. A suitable control technique is required to achieve this goal. This paper presents a complete algorithm to handle three-phase power balance and single-phase power harvesting simultaneously. Simulation results assuming a medium-voltage system connected to a typical utility grid illustrate the dynamic and steady-state performance capability of the proposed strategy. Experimental results of a low-voltage, solid-state transformer-based, three-cell prototype are also included.

Central inverters based on conventional topologies are the current preferred solution in solar farms because of their low cost and simplicity. However, such topologies have some disadvantages such as poor maximum power tracking, use of bulky filters, and low-frequency transformers. An interesting alternative in this case is the SiC-based cascaded multilevel converter (CMC), which provides a distributed maximum power point tracking control with reduced footprint and high flexibility. Each cell of a CMC usually has, as an intermediate stage, a solid-state transformer based on a dual-active-bridge dc-dc converter. Due to the unidirectional power flow characteristic of the photovoltaic application and aiming at further reduction in the converter volume, this work proposes a forward dual-active-bridge (F-DAB) topology, which reduces the number of active switches. This paper shows through analytical, simulation, and experimental results that the cell using an F-DAB is superior to other unidirectional topologies in three aspects: higher power density with commercially available power modules, reduced part count, and simplicity of control.

In metals industry, rolling is the most widely used steel forming process to provide high production and control of final product. Rolling mills must be able to change the speed of the strip at the same time that the speed is controlled within precise limits. Furthermore, this application has a severe load profile, with high torque variations during the lamination process. These characteristics include rolling mills among the classical Mission Critical Industry Applications (MCIA). In addition to the high cost/failure rate, rolling mills have a critical dynamic loading, making the design of a reliable system doubly challenging. Design For Reliability (DFR) is the process conducted during the design of a component or system that ensures them to perform at the required reliability level. In the context of the power converters for rolling mills and other MCIA, the DFR should be known and adopted in the design of the converter proper (component level) well as in the specification of power converters (system level). This paper contributes to the knowledge in the field by proposing a methodology covering the necessary steps for decision-making during the design (component level) and selection (systems level) of power converters for MCIA. A rolling mill system from a large steel plant in southeastern Brazil is adopted as case study. The standard high power converter solution is compared with two high reliability converter topologies: the FT-ANPC and the TSBC-MMC. The importance of DFR in mission critical applications is demonstrated.

Multiphase machines have several advantages, when compared to three-phase machines. Reliability is one of the most important of those advantages, since the multiphase machine can operate with one or more phases open-circuited. However, the traditional models and control of a balanced induction machine can no longer be used under such open phase conditions. This paper addresses important issues in the modeling and control of nine-phase induction machines operating under open-phase faults. The development of the proposed model is presented in detail, including an algorithm to guarantee the proper representation of isolated neutral motors. Simulation results illustrate the post-fault behavior of the nine-phase motor due to an open phase, including the elimination of torque ripple by applying the proper control methods. Experimental results from a lab test setup illustrating the operation of a nine-phase induction motor under one open-phase fault are presented and used to validate the proposed model.

Multiphase machines have several benefits compared to three-phase machines. Their inherently higher reliability makes them attractive in several applications. For instance, the nine-phase motor can operate with up to six open phases in its stator, but with torque ripple production. However, the modeling and control schemes for a balanced motor cannot be used since now the stator is unbalanced. In order to allow the multiphase machine to work magnetically balanced even in open phase conditions, an adequate dq current control should be employed. To achieve this control scheme, it is necessary a synchronous-frame modeling of a nine-phase induction motor (9PIM) under open phase faults, which is developed here. Using it, synchronous-frame control is derived, still using proportional controllers in the dq current loops. This is possible due to the use of an appropriate voltage decoupling considering the fault condition. Hence, the unbalanced multiphase induction motor can be controlled very similarly as a balanced one, additionally resulting in torque ripple minimization. Experimental results characterize the operation with open phases. Simulation and finite element results show the success of the proposed control scheme.