Query: Control strategies for losses optimization in modular multilevel converter

Candidate papers:

abstract: The Modular Multilevel Converter represents a really promising solution for several high power applications. Nevertheless, to furtherly enhance the penetration of such conversion structure and to fully exploit its advantages, some issues related to its operation need to be addressed. In particular, this converter features some circulating currents that deeply affect both the conduction losses occurring in the semiconductor devices and the voltage ripple across the capacitors of this conversion structure, which in turn influences the converter efficiency. In order to minimize the converter losses, this work proposes innovative control strategies of the circulating currents by calculating the optimal value of their reference. Alongside the increase of the converter efficiency, the proposed control strategies allow additional advantages in terms of enhancing the converter reliability and extending the overall system lifetime.
author_names: Luis Camurca

Giovanni Monopoli

Leonardo Falco

Frederik Hahn

Vito Giuseppe Monopoli

Marco Liserre
title: Control Strategies for Losses Optimization in Modular Multilevel Converter
venue: IECON 2019 45th Annual Conference of the IEEE Industrial Electronics Society
year: 2019

abstract: The modular multilevel converter (MMC) is a new kind of voltage source converter (VSC) with a high number of voltage levels and considered to be suitable for HVDC transmission. Based on circuit configuration and operation mechanism, this paper presents an mathematical model of MMC. Then the vector control strategy widely used in two level VSC is applied to the control of MMC. Due to modular structure and cascaded connections, a very high number of voltage levels is obtained by MMC in HVDC applications, for which the nearest level modulation (NLM) become feasible. A combination of NLM and optimized capacitor voltage balancing control is presented to reduce the switching frequency as well as the switching losses of power semiconductors. Time domain simulation performed in PSCAD/EMTDC shows that the control and modulation methods provide good steady state and dynamic system performance and significantly reduce the switching frequency of power semiconductors.
author_names: Minyuan Guan

Zheng Xu

Hairong Chen
title: Control and modulation strategies for modular multilevel converter based HVDC system
venue: IECON 2011 37th Annual Conference of the IEEE Industrial Electronics Society
year: 2011

abstract: A particle swarm optimization (PSO) algorithm based staircase modulation strategy for modular multilevel converters (MMC) is proposed. To reduce switching losses and device stress, the staircase modulation method has been adopted in high voltage and high power energy conversion applications. In particular, the selection of the appropriate iterative initial values of switching angles is a significant step to realize a staircase modulated MMC. The proposed method is able to find the optimum initial values of switching angles, while it has the advantages of global optimization and quadratic convergence, which benefit from the PSO algorithm and Newton method, respectively. The paper presents analytical discussion of the voltage balancing control approach with rotation of switching angles. The main benefit of the efficient switching patterns is that the MMC has lower switching losses and minimum dV/dt stress. Simulation and experimental results are presented to verify the practical feasibility of the proposed scheme for the MMC.
author_names: Ke Shen

Dan Zhao

Jun Mei

Leon M Tolbert

Jianze Wang

Mingfei Ban

Yanchao Ji

Xingguo Cai
title: Elimination of Harmonics in a Modular Multilevel Converter Using Particle Swarm Optimization Based Staircase Modulation Strategy
venue: IEEE Transactions on Industrial Electronics
year: 2014

abstract: Unlike conventional converters, modular multilevel converter (MMC) has a higher switching frequency which has direct implication on important parameters like converter loss and reliability mainly due to increased number of switching components. However, conventional switching techniques, where submodule sorting is just based on capacitor voltage balancing, are not able to achieve switching frequency reduction objective. A novel modulation algorithm for modular multilevel converters (MMCs) is proposed in this paper to reduce the switching frequency of MMC operation by defining a constrained multi objective optimization model. The optimized switching algorithm incorporates all control objectives required for the proper operation of MMC and adds new constraints to limit the number of submodule switching events at each time step. Variation of severity of the constraints leads to a desired level of controllability in MMC switching algorithm to trade off between capacitor voltage regulation and switching frequency reduction. Finally, performance of the proposed algorithm is validated against a seven level back to back MMC HVDC system under various operating conditions.
author_names: Saroj Khanal

Vahid Rasouli Disfani
title: A Novel Optimal Modulation Strategy for Modular Multilevel Converter Based HVDC Systems
venue: 2019 IEEE 2nd International Conference on Renewable Energy and Power Engineering (REPE)
year: 2019

abstract: Lesnicar and Marquardt introduced a Modular Multilevel Converter (MMC) topology back in 2003. Although this topology has received a great deal of attention in recent years by both the research community and industry, hitherto no steady state model has been developed which accurately captured all the relevant power losses while being computationally light. Hence, the aim of this paper is to introduce a fast MMC loss model which captures the key sources of power losses in steady state operation. The model only requires information which is known a priori, e.g. datasheet information of the components. The proposed model was compared to a loss model developed by Marquardt. Both models presented similar results under the same assumptions. However, the proposed model captures the switching losses more realistically and considers the temperature of operation of the electronics, as well as the losses of the inductors and cooling system in the overall efficiency of the MMC. To validate these new additions, the proposed steady state model was compared to a dynamic model. Once again the proposed model was able to capture the different sources of power losses. Nonetheless, results demonstrated that the balancing strategy greatly influences the efficiency of the MMC. Therefore, information regarding the envisioned control strategy is necessary to accurately calculate the efficiency curve of the MMC.
author_names: Athanasios I Papadopoulos

Silvio Miguel Fragoso Rodrigues

Epameinondas Kontos

Todor Todorcevic

Pavol Bauer
title: A fast steady state loss model of a modular multilevel converter for optimization purposes
venue: 2015 9th International Conference on Power Electronics and ECCE Asia (ICPE ECCE Asia)
year: 2015

abstract: A new modular multilevel converter (MMC) sub module (SM) topology based on the reverse blocking insulated gate bipolar transistor (RB IGBT) is proposed in this brief for equipping the system with dc fault blocking capability. In addition, the proposed topology has lower conduction loss than all of the existing SM topologies with dc side fault blocking capability. By employing RB IGBTs in the sub module circuit, it is possible to change the fault current path to block the dc side fault without inserting extra semiconductor devices in the normal current path, thus reducing the conduction loss when compared to other topologies. A comparison between the proposed topology and the existing topologies in terms of components number, power loss and dc fault blocking capability is carried out to show the superiority of the proposed topology. To reduce the cost of the topology, a control strategy is introduced to reduce the capacitance of the proposed topology. By optimizing the amplitude of the second order and fourth order harmonic currents when injecting third order harmonic voltage, the capacitance can be reduced to about 37% of that of the conventional method. To verify the dc side fault blocking capability and the effect of the proposed method, the controller hardware in the loop experiment based on RT LAB is conducted. The reported study results confirm the dc side fault riding through capability of the proposed sub module topology and the effectiveness of the proposed method is also validated.
author_names: Cheng Peng

Rui Li
title: A Low Conduction Loss Modular Multilevel Converter Topology With DC Fault Blocking Capability and Reduced Capacitance
venue: IEEE Transactions on Circuits and Systems II: Express Briefs
year: 2020

abstract: Following Europe's 2020 growth program, the Energy Roadmap 2050 launched by the European Commission (EC) has officially set a long term path for a low carbon economy, assuming a reduction of at least 80% of greenhouse gas emissions by the year 2050. Meeting such ambitious requirements will imply a major change in paradigm, including the electricity grid infrastructure as we know it.The breakthroughs in semi conductor technology and the advances in power electronics topologies and control have added momentum to the on going process of turning the SuperGrid into a reality. Perhaps the most recent breakthrough occurred in 2003, when Prof. Marquardt introduced the Modular Multilevel Converter (MMC or M2C) which is now the preferred power electronic topology that is starting to be used in VSC HVDC stations. It does however, introduce a number of rather complex challenges such as "additional" circulating currents within the converter itself, causing extra losses and potentially unstable operation. In addition, the MMC will be required to properly balance the capacitive energy stored within its different arms, while transferring power between the AC and DC grids that it interfaces.The present Thesis project aimed to design adequate "high level" MMC control strategies suited for HVDC applications, under balanced and unbalanced AC grid conditions. The resulting control strategy is derived with a "top to bottom" design approach, inherent to optimization strategies, where the desired performance of the MMC results in the control scheme that will be applied. More precisely, the Lagrange multipliers optimization methodology is used to calculate the minimal MMC circulating current reference signals in phase coordinates, capable of successfully regulating the capacitive arm energies of the converter, while reducing losses and voltage fluctuations, and effectively decoupling any power oscillations that would take place in the AC grid and preventing them from propagating into the DC grid.
author_names: Gilbert Bergna Diaz
title: Modular Multilevel Converter Control for HVDC Operation Optimal Shaping of the Circulating Current Signal for Internal Energy Regulation
year: 2015

abstract: In this work, a generalized optimization methodology based on Lagrange Multipliers was used to propose circulating current references in order to properly control the Modular Multilevel Converter (MMC) under different conditions. The optimization strategy finds the local minima of the objective function, which is typically the power losses of the MMC system, subject to equality constraints, which are previously defined by the desired operating condition. Furthermore, this control approach is developed in the ABC frame, giving more insight into the physical phenomena related to the MMC internal dynamics. Finally, equivalent controllers to existing ones, which can be found in the literature, are obtained by means of this methodology with the objective to demonstrate the versatility and generality of the present control approach.
author_names: Gilbert Bergna

Erik Berne

Philippe Egrot

Pierre Lefranc

Jean-Claude Vannier

Amir Arzande

Marta Molinas
title: A generalized power control approach in ABC frame for modular Multilevel Converters based on mathematical optimization
venue: 2012 IEEE International Energy Conference and Exhibition (ENERGYCON)
year: 2012

abstract: Multilevel converters are widely employed in the 30 50 kW medium voltage grid connected photovoltaic (PV) generation systems due to their higher power quality and lower switching losses compared with the two level converters. Dual T type three level converters and one three level diode neutral point clamped circuit can be integrated to derive a simplified five level modular composited converter (5L MCC) for the medium voltage PV grid connected systems. However, realizing the capacitor voltage balance is a challenge to enhance its reliability. First, the model of dc capacitor voltage variation of the 5L MCC is discussed to expose a limitation once the nearest three vector synthesis method is employed with conventional space vector modulation (SVM) In order to extend the operational range, a hybrid space vector modulation (HSVM) strategy is proposed, which integrates the optimized SVM switching sequence for the low modulation <inline formula><tex math notation="LaTeX"$M\ ,0.5$/tex math> /inline formula> and a simplified vector synthesis method for the high modulation region <inline formula> <tex math notation="LaTeX"$M\geq 0.5$/tex math>/inline formula> The proposed hybrid modulation maintains the dc capacitor voltage balance over the full modulation range and power factor. Finally, the effectiveness of the proposed HSVM strategy is verified with the simulation and experimental results.
author_names: Wuhua Li

Jiawei Hu

Senjun Hu

Heya Yang

Huan Yang

Xiangning He
title: Capacitor Voltage Balance Control of Five Level Modular Composited Converter With Hybrid Space Vector Modulation
venue: IEEE Transactions on Power Electronics
year: 2018

abstract: Since the introduction of modular multilevel converters (MMC) various additional features have been introduced for MMCs, among them parallel inter module connectivity. However, such a parallel mode requires appropriate modulation and switching strategies to exploit its full potential. Low frequency switching modulation is widely used with half bridge modules and a large body of research focusses on the optimum control and scheduling of these methods. However, so far, a suitable adaptation for MMCs with parallel mode is missing, and existing methods perform suboptimum. This article proposes a generic scheduling algorithm for simple and low cost integration with low frequency switching modulation. The proposed method reduces control complexity, minimizes power loss, and can be combined with most modulation techniques, including nearest level modulation (NLM) and selective harmonic elimination. The text in this article provides a general algorithm for integrating this method with other control levels and analyzes its effect on MMCs with parallel functionality. Moreover, it examines the performance of the system with NLM combined with the proposed scheduler and compares it to other alternatives. Results show up to a 50% reduction in power loss with a similar modulation technique as well as significantly lower THD and power loss compared to optimized phase shifted carrier modulation as another sensorless alternative method.
author_names: Nima Tashakor

Zhongxi Li

Stefan M Goetz
title: A Generic Scheduling Algorithm for Low Frequency Switching in Modular Multilevel Converters With Parallel Functionality
venue: IEEE Transactions on Power Electronics
year: 2021