Arm Inductance and Sub-module Capacitance Selection in Modular Multilevel Converter

Arm inductor and sub-module (SM) capacitor are two key components in the modular multilevel converter (MMC). Optimizing the selection of arm inductance and sub-module capacitance is thus critical for the converter design. This report aims at developing a selection principle for arm inductance and sub-module capacitance in MMC. Arm inductors in MMC are used to limit the circulating current which flows within the converter. The switching frequency harmonic is found to be the dominant component in the circulating current when an active circulating current suppressing controller is implemented. The analytical relationship between the arm inductance and switching frequency circulating current is derived, based on which the arm inductance requirement is obtained by limiting the circulating current to meet the defined specifications. In some applications, the arm inductors can also be used to limit the overcurrent during a dc side short circuit fault. The relationship between the arm inductance and fault current is investigated, as well as its impact on arm inductance selection. The sub-module capacitance in MMC is selected mainly based on the capacitor voltage fluctuation constrain. The voltage unbalance among sub-module capacitors is revealed to have a significant impact on the sub-module capacitance selection, as the unbalanced voltage would increase the total capacitor voltage fluctuation. The impact of sub-module capacitors’ unbalanced voltage on the total voltage fluctuation is evaluated. An analytical expression of the unbalanced voltage is derived; it can be used to calculate the maximum capacitor voltage fluctuation, and thus used for the sub-module capacitance selection. A simulation has been carried out in the MATLAB, and the simulation results verify the theoretical analysis. A scaled-down MMC prototype has been built, and the experimental results validate part of the analysis

Control and grid integration of MW-range wind and solar energy conversion systems

Solar-based energy generation has increased by more than ten times over the same period. In total, worldwide electrical energy consumption increased by approximately 6340 TWh from 2003 to 2013. To meet the challenges created by intermittent energy generation sources, grid operators have increasingly demanded more stringent technical requirements for the connection and operation of grid-connected intermittent energy systems, for instance concerning fault ride through capability, voltage and frequency support, and inertia emulation. Ongoing developments include new or improved high-voltage converters, power converters with higher power density, control systems to provide ride-through capability, implementation of redundancy schemes to provide more reliable generation systems, and the use of high-voltage direct current (HVdc) links for the connection of large off-shore intermittent energy systems

Experimental validation of a reduced-scale rail power conditioner based on modular multilevel converter for AC railway power grids

Rail power conditioner (RPC) has the ability to improve the power quality in AC railway power grids. This power conditioner can increase the loading capacity of traction substations, balance the active power between the feeder load sections, and compensate for reactive power and current harmonics. At present, there is increasing use of multilevel converter topologies, which provide scalability and robust performance under different conditions. In this framework, modular multilevel converter (MMC) is emerging as a prominent solution for medium-voltage applications. Serving that purpose, this paper focuses on the implementation, testing, and validation of a reduced-scale laboratory prototype of a proposed RPC based on an MMC. The developed laboratory prototype, designed to be compact, reliable, and adaptable to multipurpose applications, is presented, highlighting the main control and power circuit boards of the MMC. In addition, MMC parameter design of the filter inductor and submodule capacitor is also explained. Experimental analysis and validation of a reduced-scale prototype RPC based on MMC topology, are provided to verify the power quality improvement in electrified railway power grids. Thus, two experimental case studies are presented: (1) when both of the load sections are unequally loaded; (2) when only one load section is loaded. Experimental results confirm the RPC based on MMC is effective in reducing the harmonic contents, solving the problem of three-phase current imbalance and compensating reactive power.This work has been supported by the Portuguese Foundation of Science and Technology (FCT), in Portuguese, Fundação para a Ciência e Tecnologia within the R&D Units Project Scope: UIDB/00319/2020. Mohamed Tanta was supported by FCT grant with a reference PD/BD/127815/2016. Jose Cunha is supported by FCT grant with a reference PB/BD/143005/2018. Luis A. M. Barros is supported by FCT grant with a reference PD/BD/143006/2018

An Overview of Applications of the Modular Multilevel Matrix Converter

The modular multilevel matrix converter is a relatively new power converter topology suitable for high-power alternating current (AC)-to-AC applications. Several publications in the literature have highlighted the converter capabilities, such as full modularity, fault-redundancy, control flexibility and input/output power quality. However, the topology and control of this converter are relatively complex to realise, considering that the converter has a large number of power-cells and floating capacitors. To the best of the authors’ knowledge, there are no review papers where the applications of the modular multilevel matrix converter are discussed. Hence, this paper aims to provide a comprehensive review of the state-of-the-art of the modular multilevel matrix converter, focusing on implementation issues and applications. Guidelines to dimensioning the key components of this converter are described and compared to other modular multilevel topologies, highlighting the versatility and controllability of the converter in high-power applications. Additionally, the most popular applications for the modular multilevel matrix converter, such as wind turbines, grid connection and motor drives, are discussed based on analyses of simulation and experimental results. Finally, future trends and new opportunities for the use of the modular multilevel matrix converter in high-power AC-to-AC applications are identified.Agencia Nacional de Investigación y Desarrollo/[Fondecyt 11191163]/ANID/ChileAgencia Nacional de Investigación y Desarrollo/[Fondecyt 1180879]/ANID/ChileAgencia Nacional de Investigación y Desarrollo/[Fondecyt 11190852]/ANID/ChileAgencia Nacional de Investigación y Desarrollo/[ANID Basal FB0008]/ANID/ChileAgencia Nacional de Investigación y Desarrollo/[Fondef ID19I10370]/ANID/ChileUniversidad de Santiago/[Dicyt 091813DD]//ChileUCR::Vicerrectoría de Docencia::Ingeniería::Facultad de Ingeniería::Escuela de Ingeniería Eléctric

An optimal full frequency control strategy for the modular multilevel matrix converter based on predictive control

The modular multilevel matrix converter (M3C) is a promising topology for high-voltage high-power applications. Recent researches have proved its significant advantages for adjustable-speed motor drives compared with the back-to-back modular multilevel converter (MMC). However, the branch energy balancing in the M3C presents great challenge especially at critical-frequency points where the output frequency is close to zero or grid-side frequency. Generally, this balancing control depends on the appropriate injection of inner circulating currents and the common-mode voltage (CMV) whereas their values are hard to determine and optimize. In this paper, an optimization based predictive control method is proposed to calculate the required circulating currents and the CMV. The proposed method features a broad-frequency range balancing of capacitor-voltages and no reactive power in the grid side. For operation at critical-frequency points, there is no increase on branch voltage stresses and limited increase on branch current stresses. A downscaled M3C system with 27 cells is designed and experiment results with the R-L load and induction motor load are presented to verify the proposed control method

Dual Channel Control with DC Fault Ride Through for MMC-based, Isolated DC/DC Converter

This study is sponsored by the Engineering and Physical Sciences Research Council (EPSRC) grant no EP/K006428/1, 2013.D. Jovcic and H. Zhang are with the School of Engineering, University of Aberdeen, AB24 3UE, U.K. ([email protected], [email protected]).Peer reviewedPostprin

Analysis and Criterion for Inherent Balance Capability in Modular Multilevel DC–AC–DC Converters

Modular multilevel dc-ac-dc converters (MMDAC) have emergedrecently for high step-ratio connectionsin medium voltage distribution systems.Extended phase-shiftmodulation has been proposed and was found to create the opportunity for inherent balance of SM capacitor voltages. This letter presents fundamentalanalysis leading toclear criterion for the inherent balancecapability in MMDAC. A sufficient and necessary condition,with associated assumptions,to guarantee this capability isestablished. Using the mathematics of circulant matrices, this condition is simplified to a co-prime criterion which gives rise to practical guidance for the design of an MMDAC. Experimentson down-scaled prototypesand simulations on full-scale examples both provide verification of the analysis and criterion for the inherent balance capability of MMDAC

Highly Dynamic AC Current Control for Modular Multilevel Converters

A highly robust and dynamic control of the MMC grid currents is necessary to ensure grid stability. Based on a state space modelling of the MMC, a real time capable indirect modelbased control is presented. Measurements with a laboratory setup show the reliability even under disturbed grid conditions