Selective harmonic elimination pulse width modulation (SHE-PWM) offers tight control of the harmonic spectrum of a given voltage and/or current waveform generated by a power electronics converter. ...Owing to its formulation and focus on elimination of low-order harmonics, it is highly beneficial for high-power converters operating with low switching frequencies. Over the last decade, the application of SHE-PWM has been extended to include multilevel converters. This paper provides a comprehensive review of the SHE-PWM modulation technique, aimed at its application to multilevel converters. This review focuses on various aspects of multilevel SHE-PWM, including different problem formulations, solving algorithms, and implementation in various multilevel converter topologies. An overview of current and future applications of multilevel SHE-PWM is also provided.
Control of circulating current can help modular multilevel converters (MMCs) improve the internal performance, but only the conventional circulating current suppression control has been considered in ...impedance models to investigate MMC-grid interactions. In this paper, the sequence impedance responses of circulating current injection methods based on instantaneous information are modeled for stability analysis. Compared with circulating current control schemes based on predefined references, the circulating current injection methods based on instantaneous information introduce couplings among circulating current, output current and capacitor voltage control loops, which influence the MMC impedance in the low frequency range. This paper reveals that injection of second harmonics in the circulating current reduces the capacitor voltage ripples, but the system stability is compromised due to the couplings. The MMC stability boundaries for different circulating current controllers are defined with the help of sequence impedance model and Bode-plot-based analysis. The impedance model and stability assessment are validated by frequency scan results and case studies.
In this study, a model predictive control (MPC) strategy that combines finite-control-set MPC with selective harmonic elimination (SHE) modulation pattern in its formulation is proposed to govern ...multilevel power converters. Based on a desired operating point for the system state (converter current reference), an associated predefined SHE voltage pattern is obtained as a required steady-state control input reference. Then, the cost function is formulated with the inclusion of both system state and control input references. According with the proposed reference and cost function formulation, the predictive controller prefers to track the converter output current reference in transients, while preserving the SHE voltage pattern in steady state. Hence, as evidenced by experimental results, a fast dynamic response is obtained throughout transients while a predefined voltage and current spectrum with low switching frequency is achieved in steady state.
Hypoxia induces an inflammatory response in the lung manifested by alternative activation of macrophages with elevation of proinflammatory mediators that are critical for the later development of ...hypoxic pulmonary hypertension. Mesenchymal stromal cell transplantation inhibits lung inflammation, vascular remodeling, and right heart failure and reverses hypoxic pulmonary hypertension in experimental models of disease. In this study, we aimed to investigate the paracrine mechanisms by which mesenchymal stromal cells are protective in hypoxic pulmonary hypertension.
We fractionated mouse mesenchymal stromal cell-conditioned media to identify the biologically active component affecting in vivo hypoxic signaling and determined that exosomes, secreted membrane microvesicles, suppressed the hypoxic pulmonary influx of macrophages and the induction of proinflammatory and proproliferative mediators, including monocyte chemoattractant protein-1 and hypoxia-inducible mitogenic factor, in the murine model of hypoxic pulmonary hypertension. Intravenous delivery of mesenchymal stromal cell-derived exosomes (MEX) inhibited vascular remodeling and hypoxic pulmonary hypertension, whereas MEX-depleted media or fibroblast-derived exosomes had no effect. MEX suppressed the hypoxic activation of signal transducer and activator of transcription 3 (STAT3) and the upregulation of the miR-17 superfamily of microRNA clusters, whereas it increased lung levels of miR-204, a key microRNA, the expression of which is decreased in human pulmonary hypertension. MEX produced by human umbilical cord mesenchymal stromal cells inhibited STAT3 signaling in isolated human pulmonary artery endothelial cells, demonstrating a direct effect of MEX on hypoxic vascular cells.
This study indicates that MEX exert a pleiotropic protective effect on the lung and inhibit pulmonary hypertension through suppression of hyperproliferative pathways, including STAT3-mediated signaling induced by hypoxia.
This paper introduces a low complexity implementation of the voltage balancing algorithm aiming to reduce the switching frequency of the power devices in modular multilevel converters (MMCs). The ...proposed algorithm features a relatively simple implementation without any conditional execution requirements and is easily expandable regardless of the number of submodules (SMs). Two modulation techniques are evaluated, namely the staircase modulation and the phase-disposition pulse width modulation (PD-PWM) under the conventional and the proposed algorithm. Using a circulating current controller in an MMC with 12 SMs per arm, PD-PWM yields better results compared to the staircase modulation technique. The test condition for this comparison is such that the power devices operate at a similar switching frequency and produce similar amplitudes to the capacitor voltage ripples in both modulation techniques. The results are verified through extensive simulations and experiments on a low power phase-leg MMC laboratory prototype.
The unique advantages of modular multilevel converters (MMCs) have led to wide adoption of the converter topology in high voltage dc (HVDC) transmission systems, with great potential for use in ...medium voltage motor drive and dc grid applications. Inspired by the structure of the MMC, many extended converter topologies have been developed in the literature. These include, in addition to conventional parallel multiphase connection, a variety of series-connected topologies. Extended MMCs integrate the modular nature of cascaded SMs of the MMC but with different topology structure leading to substantially different operating principles. Given the growing number of such topologies, this paper provides a critical review of MMC derived topologies and summarizes individual advantages and disadvantages. The control schemes and application fields of these different topologies are analysed. This review facilitates the classification and understanding of extended MMCs and provides the necessary scaffold for the development of a general framework for new topologies and applications based on the MMC.
The modular multilevel converter (MMC) is currently one of the power converter topologies which has attracted more research and development worldwide. Its features, such as high quality of voltages ...and currents, high modularity and high voltage rating, have made the MMC a very good option for several applications including high-voltage dc (HVdc) transmission, static compensators (STATCOMs), and motor drives. However, its unique features such as the large number of submodules, floating capacitor voltages, and circulating currents require a dedicated control system able to manage the terminal variables, as well as the internal variables with high dynamical performance. In this paper, a review of the research and development achieved during the last years on MMCs is shown, focusing on the challenges and proposed solutions for this power converter still faces in terms of modeling, control, reliability, power topologies, and new applications.
•Availability comparison for MMCs in MVDC systems of all suitable SM topologies.•Evaluation of reliability with different redundancy design in MMCs for MVDC applications.•Analysis of reliability in ...MVDC MMCs under multiple operation modes.•Impact of correlation between lifetime of SMs on converter reliability in MVDC networks.
In modular multilevel converters (MMCs) designed for medium-voltage DC (MVDC) systems, since the required sub-modules (SMs) in one arm are fewer, the converters cannot afford the same level of redundancy as observed in high-voltage DC (HVDC) applications. In order to ensure high availability of converters in MVDC systems, reliability becomes a challenge and requires different design approaches. Considering the redundancy design, operating modes, and correlation across failures of SMs in one arm, this paper analyses and compares reliability in MMCs for MVDC systems with multiple SM topologies. Under different scenarios in MVDC systems, SM topologies with better performance of reliability have been identified in this research. It is demonstrated that, for a given output voltage in converters, three-level SMs generally provide higher reliability than two-level SMs. By implementing passive and load-sharing operation schemes in converters, the MMC reliability can be improved compared with the conventional active mode. In terms of the correlation between lifetime of SMs in the same arm, the smaller disturbance introduced by one faulty SM promotes the improvement of converter reliability. Also, the correlation between failures of SMs in one arm has greater effects on the reliability of an MMC with higher level of redundancy. The outcomes of this work contribute to the optimisation of converter design and operation with high reliability and low investment for an MMC in MVDC systems.
Australia is one of the leading countries in energy transition, and its largest power system is intended to securely operate with up to 75% of variable renewable generation by 2025. High-inertia ...synchronous condensers, battery energy storage systems, and grid-forming converters are some of the technologies supporting this transformation while facilitating the secure operation of the grid. Synchronous condensers have enabled 2500 MW of solar and wind generation in the state of South Australia, reaching minimum operational demands of ≈100 MW. Grid-scale battery energy storage systems have demonstrated not only market benefits by cutting costs to consumers but also essential grid services during contingencies. Fast frequency response, synthetic inertia, and high fault currents are some of the grid-supporting capabilities provided by new developments that strengthen the grid while facilitating the integration of new renewable energy hubs. This manuscript provides a comprehensive overview, based on the Australian experience, of how power systems are overcoming expected challenges while continuing to integrate secure, low cost, and clean energy.
Medium-voltage (MV) multilevel converters are considered a promising solution for large scale photovoltaic (PV) systems to meet the rapid energy demand. This article focuses on reviewing the ...different structures and the technical challenges of modular multilevel topologies and their submodule circuit design for PV applications. The unique structure of the converter's submodule provides modularity, independent control of maximum power point tracking (MPPT), galvanic isolation, etc. Different submodule circuits and MPPT methods to efficiently extract the PV power are reviewed. The integration of the multilevel converters to PV systems suffers unbalanced power generation during partial PV shading conditions. Several balancing strategies to solve this problem are presented and compared to give a better understanding of the balancing ranges and capabilities of each strategy. In addition, the paper discusses recent research advancements, and possible future directions of MV converters-based large-scale PV systems for grid integration.
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