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Future distribution grids will require to integrate renewable energies in higher penetration levels than we are used to today in conventional power systems. To make this possible, key aspects of their operation such as reliability and robustness must been in safe levels. To accomplish the latter, two different technologies have been proposed: (a) Virtual Synchronous Machines and (b) Smart Transformers. Despite all the benefits that each one of these technologies provide, solid-state transformers have not been very successful in operating as smart transformers. Then, hybrid transformers were proposed as a more feasible solution and with the capability of providing ancillary services to the distribution grid. In this paper, a new type of hybrid transformer that incorporate features of a virtual synchronous machine is proposed. The proposed transformer is capable of emulating virtual inertia as an ancillary service, and, at the same time, providing other basic services such as reactive power injection and voltage compensation. Simulation results provide evidence of the advantages of using this type of hybrid transformers in distribution grids.

Multi-cell topologies based on three-phase input / single-phase output power converters have the drawback of dealing with an important oscillatory second harmonic current in the DC link. This requires the use of a large DC link capacitor in order to minimize the DC link voltage ripple. Different control schemes have been presented to reduce the second harmonic generated for the H bridge inverter aiming the design of the DC link capacitor without oversizing it. However, this is achieved at expense of much distorted AC supply currents and increased modulating signals amplitudes. In this paper, these problems are accurately characterized with the objective of mathematically identify the limitations of those schemes.

Design criteria are addressed to select the parameters of an asymmetric multilevel shunt active power filter based on a Cascaded H-Bridge topology. The proposed approach is meant to simultaneously compensate both fundamental reactive power and current harmonics. The presented methodology allows to systematically design the main parameters of the power converter, particularly the inductive filter, the DC capacitors and the DC voltage level of the power modules. The procedure allows to consider not only a single compensation condition but the entire operating region. The obtained results are supported with mathematical derivations and proved through simulated waveforms.

Multilevel converters are widely used in medium voltage applications. One of the most popular topologies in this field is the 18 pulses diode rectifier, which uses a mu Iti pu I so transformer that realizes a current harmonics cancelation that are generated by the three phase diode rectifiers. In this work an efficiency comparison of the aforementioned topology and a Multi-Cell Active-Front-End (AFE) Rectifier operating with Finite Control Set — Model Predictive Control is performed. This efficiency comparison is focused on the rectifier losses divided into two types. The first one is the semiconductors switching and conduction losses. The second one evaluates the transformer losses, which are studied under two criteria: copper losses (Standard IEEE C57.10-1998) and the K — Factor (Standard IEEE / ANSI C57.110). The results show that the diode based rectifier topology has lower semiconductors losses, while the topology based on AFE rectifiers has lower transformers losses and a less complex design. Indeed, a reduction of a 7.2% in copper losses is achieved in the transformer secondary with respect to a multi-pulse transformer secondary.

In the last decades, finite control set-model predictive control (FCS-MPC) has been extensively investigated. This control strategy uses the system model to determine the optimal input that minimizes a predefined cost function. This procedure is performed by evaluating the admissible inputs of the converter model. In order to achieve a proper performance of the control strategy, a small sampling time must be used. In practice, the computational effort and processing time are critical factors that depend directly on the number of admissible states of the topology. With the objective of reducing the execution time needed by the algorithm, it is possible to take advantage of the high degree of parallelism that can be obtained from a field-programmable gate array (FPGA). This paper deals with the implementation of FCS-MPC in an FPGA XC3S3500E to control a power cell of a cascaded half bridge (CHB) converter. The design is divided into subparts, called modules, which can be projected to control multiple cells using only one chip. In addition, with the aim to restrict the use of hardware to the available resources, the document presents a series of considerations related to the algorithm and its implementation. This paper presents a detailed guide with emphasis on the design and outlining of each module in order to reduce the developing times for researchers in the area of power converters.

Multi-cell converters allow the use of AC motor drives for medium and high voltage level applications using low voltage semiconductors. This type of converter has a multi-pulse transformer which performs the cancellation of the low frequency harmonics generated by the three-phase diode rectifiers. Despite this advantage, the multi-pulse transformer is bulky, heavy, expensive, and must be designed according to the number of power cells, making the transformer topology dependent. This work proposes to reduce the complexity of the transformer in multi-cell converters using Active Front End rectifiers in combination with a Finite Control Set - Model Predictive Control strategy. This is achieved by the strategy as it emulates the multi-pulse transformer harmonic cancellation thanks to the predictive algorithm and the generation of input current references with a phase shift angle for each Active Front End rectifier.

Multilevel inverters are the alternative for medium voltage applications. Within the inverters types there are symmetric and asymmetric topologies. The asymmetric inverters have different DC voltage values. The most common topology is when the different cells are implemented in cascade arrangement, where the DC voltage are in multiples of 3, obtaining an AC voltage with 3n = 27 levels (n = 3 cascaded inverters). This topology provides a load voltage with low harmonic content, THD

The asymmetric multi-cell inverters are an alternative to provide an AC voltage with low THD. Considering a voltage DC ratio 1∶ 3∶ 9 for three inverters connected in series an AC voltage featuring 27 levels is obtained. The most common modulation used in asymmetric inverters is the Nearest Level Modulation (NLM) which has the drawback of bringing some cells into regeneration for some values of the modulation index, regardless of the load conditions. This paper presents a new modulation strategy for this asymmetric inverter that forces all the cell to have identical power flow direction. This strategy is based on the fact that the fundamental voltage amplitude of each inverter is required to be always greater than or equal to zero. To confirm the proposed technique, an active power flow analysis is performed as well as simulated and selected experimental results are included.

The main drawback of a Multi-Cell topology based on Current Source Converters is the oscillating power drained by the single-phase inverters from the DC link which requires a large DC reactor in order to avoid a significant variation of the DC current. In this work, a control scheme is proposed in order to compensate the oscillating power, avoiding to oversize the DC reactor at the expense of increasing the distortion of the input currents in each power cell and the instantaneous peak of the modulation index. However, the overall input currents present very low distortion and the instantaneous peak of the modulation index can be reduced by properly designing the multi-pulse transformer in terms of voltage ratio and phase shift. Both, the input currents distortion and the effect in the modulation index are mathematically characterized. Simulation result illustrates the effectiveness of the proposed control approach.

Multi-cell topologies based on three-phase input / singlephase output power modules have the drawback of dealing with an important oscillatory power in the DC link. This requires the use of a large DC link capacitor in order to minimize the DC link voltage ripple. Instead, this paper proposes a control scheme based on predictive control aimed to compensate the oscillatory power that allows the design of the DC link capacitor without oversizing it. However, this is achieved at expense of much distorted AC supply currents and amplified modulating signals. Such effects of the proposed compensating technique based on predictive control are mathematically characterized. The results show that the multi-pulse transformer guarantees distortion free overall AC currents. Simulated results show the feasibility of the proposed scheme.