In the future, new aspects from decentralized generation using different dc voltage levels are expected to influence the general concept of power exchange. Converter are needed to adapt the voltage between low voltage (LV), medium voltage (MV), and high voltage (HV). In this paper, a galvanic isolated bidirectional dc/dc converter based on the modular multilevel converter is studied, with a large potential for secure and flexible dc power flow control. The use of medium frequency transformation allows savings in copper and iron. A fundamental frequency modulation method is introduced for the presented converter, that enables variable stepup or step-down between primary and secondary dc voltage in discrete steps. The balancing mechanism of the internal power storage components is explained and verified by simulation and experiment.

This paper presents a new non-insulated quadratic three-level DC-DC boost converter. The quadratic feature is interesting for application where a wide voltage range is necessary. Since it is a current-source converter, its application in a fuel cell energy conversion system is interesting. Theoretical analysis for CCM of operation and experimental results are presented. A comparison between the two cascaded boost converter, the single switch quadratic boost converter and the proposed converter is made regarding commutated power. The efficiency of the proposed converter is higher than the others.

This paper presents a voltage-controlled distribution static synchronous compensator (DSTATCOM)-based voltage regulator for low-voltage distribution grids. The voltage regulator is designed to temporarily meet the grid code, postponing unplanned investments while a definitive solution could be planned to solve regulation issues. The power stage is composed of a three-phase four-wire voltage-source inverter and a second-order low-pass filter. The control strategy has three output voltage loops with active damping and two dc bus voltage loops. In addition, two loops are included to the proposed control strategy: the concept of minimum power point tracking (mPPT) and the frequency loop. The mPPT allows the voltage regulator to operate at the minimum power point, avoiding the circulation of unnecessary reactive compensation. The frequency loop allows the voltage regulator to be independent of the grid voltage information, especially the grid angle, using only the information available at the point-of-common coupling. Experimental results show the regulation capacity, the features of the mPPT algorithm for linear and nonlinear loads, and the frequency stability.

The modular multilevel converter is one of the most interesting transformerless multilevel converter topology intended for medium and high-voltage power conversion. Thus, as a new topology, a fewer design guidelines are presented in the literature, on the other hand there are many restrictions. This paper presents aims to be a design guide lines to project a MMC system. The minimum submodule capacitor requirement is presented as the capacitance related to the load displacement angle, operating frequency and output power. The arm inductor design include three parameters: current ripple, fault condition and circulating current. Semiconductor current stress are calculated using average quasi-instantaneous values and rms quasi-instantaneous values will be used. From the performed analysis some normalized abacus are derived and can be used as start point for a new MMC design. The results were validated using MATLAB simulation. Furthermore, experimental results based on 2.2 kW, 5 kHz switching frequency prototype are also include and analyzed.

This paper presents a bidirectional modular DC/DC converter. It consists of two Modular Multilevel Converters (MMC) linked by a transformer to achieve galvanic isolation. This topology serves as “DC transformer” interfacing low and medium voltage DC grids. By the means of the “front-to-front” connection, the converter is able to handle short circuits without additional protection. A higher operating frequency makes it possible to decrease the size of the transformer, the capacitors and the branch inductances. Two fundamental frequency modulation methods are presented with the aim to reduce the switching frequencies without penalizing the performance of the transformer. The multilevel modulation yields a conventional staircase voltage waveform, whereas the two-level modulation, which is inspired by the conventional double active bridge converter, results in a two-level voltage waveform. A case study of a wind power converter will serve as point of comparison between the two methods. The semiconductor losses are evaluated and the efficiencies for different transformer designs are discussed.