Publications
[HTML] from mdpi.com Nearest Vector Control Method Applied to an MMC for PV Generation
Apr 9, 2024Journal Energies
Publisher MDPI
DOI https://doi.org/10.3390/en17081795
Issue 8
Volume 17
This paper proposes a new and simplified Nearest Vector Control (NVC) modulation technique for a grid-connected photovoltaic (PV) system using a Modular Multilevel Converter (MMC). Compared to the Nearest Level Control (NLC) technique, which defines three independent states for the three phases of medium to large four-wire multilevel converters, NVC offers a more coordinated behavior for three-wire converters. The proposed scheme is easy to implement, and it simplifies the understanding of using vectors when detecting the vector of the converter nearest to a given reference. Because it uses natural coordinates, namely, ab, bc and ca, the proposed method is easier to understand and more useful for further developments. Compared with earlier NVC methods, this approach offers full independence of the number of levels at the converter and it can readily accommodate changes in the number of levels, with no need for lookup tables or artificial coordinate transformations. The proposed NVC method was implemented on a 16-cell MMC used for PV generation and then it was compared to NLC, leading to a smaller and more consistent low-order harmonic distortion, requiring about the same complexity of implementation. Furthermore, in comparison to NLC, when applying the proposed NVC modulation, a behavior more insensitive to changes in the grid voltage was found, the most hazardous odd harmonics from the 5th to the 19th were reduced, and a consistent reduction of about 25 dB was achieved on the 5th and 7th harmonics. The newly proposed method is supported by simulations and experimental results with constant and sharply changing solar irradiance, leaving or removing the 100 Hz component of the MMC circulating currents.
MMC-based topology for grid connection of wind generators with phase and arm power balancing
Feb 10, 2023Journal Electric Power Systems Research
Publisher Elsevier
DOI https://doi.org/10.1016/j.epsr.2023.109218
Volume 218
A new topology of a wind farm wherein the wind generators are integrated into the structure of a modular multilevel converter (MMC) is presented. It allows connecting a group of small/medium size wind turbines to the grid, replacing all the grid side converters of the wind generators by the MMC. The different power generated by each wind generator causes power imbalances that can lead to circulating currents; this paper addresses this problem by analyzing it in a simple way. The analysis shows that power differences between phases are naturally balanced by the DC component of circulating current, without the need to use a specific regulator. However, the power differences between the arms result in a high 50 Hz circulating current. This paper presents the design of a new regulator that corrects these power imbalances by generating a phase difference between the upper and lower arm voltages resulting in an optimal 50 Hz circulating current. The operation of the MMC-based wind farm topology has been tested when all the turbines generate the same power and when they generate significantly different powers. Numerical and real-time simulation results show that the proposed regulator keeps the powers of the MMC phases and arms balanced.
Local Carrier PWM for Modular Multilevel Converters with Distributed PV Cells and Circulating Current Reduction
Oct 22, 2022Journal Energies
Publisher MDPI
DOI https://doi.org/10.3390/en13215585
Issue 21
Volume 13
A new topology has been recently proposed for grid-connected photovoltaic (PV) systems, using modular multilevel converters (MMCs) and distributing PV panels throughout the MMC cells. This topology has two main advantages: it reduces the power losses related to moving the energy into the MMC capacitors from an external source, and it removes the losses and costs related to the DC to DC converters used to track the maximum power point on string converters or central converters, because that task is delegated to MMC cells. However, traditional pulse width modulation (PWM) techniques have many problems when dealing with this application: the distortion at the output increases to unacceptable values when MMC cells target different voltages. This paper proposes a new modulation technique for MMCs with different cell voltages, taking into account the measured cell voltages to generate switching sequences with more accurate timing. It also adapts the modulator sampling period to improve the transitions from level to level, an important issue to reduce the internal circulating currents. The proposed modulation has been validated using simulations that show a consistent behavior in the output distortion throughout a wide operation range, and it also reduces the circulating currents and cuts the conduction losses by half. The behavior of this new topology and this new modulation has been compared to the mainstream topology with external PV panels and also to a fixed carrier modulation.
POSSIBLE LENSING SCHEMES FOR FIBER-OPTIC COUPLING IMPROVEMENT
May 15, 2018Journal Journal of Theoretical and Applied Information Technology
Publisher Journal of Theoretical and Applied Information Technology
Issue 9
Volume 96
Integrated coupling between the laser source (such as laser diodes, LD's) and single-mode fibers (SMFs) are one of the most important and fundamental techniques of optical communications. Integrating lenses in source-fiber and fiber-detector receivers was found a great efficiency enhancement way. Meanwhile, on the other hand Lensed fibers have desirable features of the coupling scheme over ordinary Micro-Lenses, such as compactness, simplicity, stability, and freedom from bulky sizes. Three ways of lens coupling schemes are mentioned. Using a micro-lens, then other ways using lensed fiber-ends to achieve efficient coupling of an optical fiber with a light source from a side and detector on the other side. These scheme findings could guide the evolution of a mass-produced at a low price in comparison to the other complex technical ways that'll achieve not more than 5% higher coupling efficiency.