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Modular multilevel converters with non-sinusoidal ac voltage output can reduce cost and volume in medium-voltage-connected electric vehicle battery charging applications. The use of full-bridge submodules in such converters enables single-stage ac/ac voltage conversion, allowing a medium-voltage grid to be directly connected to a medium-frequency isolation transformer. The application of a square wave voltage at the medium-frequency transformer's single-phase port enhances the converter's efficiency and power density in comparison to a sinusoidal voltage. This paper presents the analysis and modelling of a modular multilevel converter, comparing its operation with sinusoidal and square wave output voltages. A single control scheme for both output voltage waveforms is proposed for the three-phase and single-phase ac currents, circulating currents, and the energy stored in the submodule capacitors. The control strategy of the three-phase and single-phase port currents is verified through simulation and experiments using a scaled-down prototype, thereby validating its suitability for high-power bidirectional battery chargers. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is a research hotspot in medium-voltage and high-voltage applications. The measurement offset error would cause an increase in the monitoring error of the submodule (SM) capacitance of the MMC, affecting the estimation accuracy of the SM capacitance monitoring. This paper proposes a capacitor monitoring strategy based on the offset error compensation, where two reasonable capacitor monitoring periods are selected in one fundamental period under the proposed voltage-balancing control (VBC) based on the virtual capacitor voltage (VCV) to compensate for the offset error impact on the capacitance monitoring. The proposed strategy can effectively eliminate the offset error impact on the capacitance monitoring, which ensures the accuracy of the SM capacitance monitoring in the MMCs. The effectiveness of the proposed monitoring strategy is confirmed by the simulations and experiments. [ABSTRACT FROM AUTHOR]

This article proposes a novel submodule capacitor voltage balancing control for multiport modular multilevel converter (MMC)-based solid-state transformers. To balance each submodule capacitor voltage, the balancing control is usually applied to the MMC. However, the low switching frequency operation and bulky arm inductance of the MMC limit the controller bandwidth and stability margin. These issues are addressed by moving the balancing control to the back-end dc/dc converter, which is operated at high switching frequency. The controller gains are designed based on the small-signal model. The superiority of the proposed controller over the conventional one has been verified through Bode plot, pole-zero map, and Nyquist path analyses. Experimental results for a 2.4-kW prototype system have also verified the accuracy of the model and effectiveness of the proposed balancing control for step load changes.

Modular multilevel converters (MMCs) represent the forefront of power converter technology, with wide-ranging applications across diverse industries. Over recent decades, substantial research and development efforts have been dedicated to enhancing MMCs' performance. A significant challenge in power conversion processes is the presence of total harmonic distortion (THD) in output waveforms, which can have adverse effects on electrical equipment. In response, extensive studies have been conducted to address THD-related challenges by refining the control and operation of MMCs. This study investigates the effect of the amplitude modulation index (Ma) on the total harmonic distortion (THD) in nine-level MMC output waves. For this, a standard three-phase and nine-level MMC model was built and simulated in MATLAB/ Simulink environment, and the Ma value was shifted between 0.1 and 1.5. The output current and voltage waves were analyzed, and the optimal limits for the Ma values yielding the lowest THD values were determined. The simulation outcomes reveal a crucial Ma range between 0.6 and 1.2, where THD is significantly minimized. Ma values below 0.6 introduce significant harmonic distortion in the voltage waves, while values surpassing 1.2 lead to appreciable harmonic distortion in the current wave. This study contributes valuable insights for engineers and researchers and aids in the refinement of MMC control strategies and the mitigation of THD-related challenges in power systems. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is a research hotspot in medium-voltage and high-voltage applications. The measurement offset error would cause an increase in the monitoring error of the submodule (SM) capacitance of the MMC, affecting the estimation accuracy of the SM capacitance monitoring. This paper proposes a capacitor monitoring strategy based on the offset error compensation, where two reasonable capacitor monitoring periods are selected in one fundamental period under the proposed voltage-balancing control (VBC) based on the virtual capacitor voltage (VCV) to compensate for the offset error impact on the capacitance monitoring. The proposed strategy can effectively eliminate the offset error impact on the capacitance monitoring, which ensures the accuracy of the SM capacitance monitoring in the MMCs. The effectiveness of the proposed monitoring strategy is confirmed by the simulations and experiments.

Summary: The intrinsic characteristics such as modular structure, filterless operation, and absence of DC‐link capacitor have made the modular multilevel converter (MMC) superior to the conventional multilevel converters. However, the submodule (SM) capacitor voltage balancing is crucial. This paper presents a voltage balance control method for MMC. The proposed method generates 2N + 1 levels (where N is the number of SMs per arm) which halves the step size of output voltage and improves the harmonic performance as compared to N + 1 level generation. It requires only one carrier per phase and no voltage and current measurements are needed for the implementation. Therefore, the proposed method makes the system cost‐effective and reduces the implementation issues related with carrier‐based methods substantially. It also alleviates the non‐uniform charge distribution problem with the phase disposition pulse width modulation (PDPWM) technique. Further, a detailed mathematical analysis is presented which enhances the understanding of balancing algorithm. In addition to this, the circulating current with 2N + 1 modulation is analyzed and a control technique is presented. The proposed voltage balancing method is simulated comprehensively in MATLAB/SIMULINK software and validated by the experimental results obtained from a laboratory prototype. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) have attracted a great deal of attention in medium-voltage motor drive applications. However, the excessive fundamental frequency submodule (SM) capacitor voltage ripple at low frequencies is a major technological challenge. In this paper, an SM capacitor voltage fluctuation suppression strategy based on model predictive control (MPC) is proposed. A cost function is employed to replace multiple PI controllers, which makes the control strategy simpler, and avoids the process of the high-frequency injection. To avoid the common-mode voltage (CMV) generated by the proposed suppression strategy, a flying capacitor is added between the upper and lower arms of a traditional MMC. The MPC strategy is divided into two stages. In the first stage, the SM input number of each arm is obtained through an output current prediction model. In the second stage, the optimal switching states are output through a model for SM capacitor voltage fluctuation suppression. When compared with traditional MPC, the design of the stepwise MPC strategy and finite control set omits the design of partial weight factors, which reduces the number of calculations of the SM capacitor voltage fluctuation suppression strategy. Simulation and experiments are carried out at 5 Hz to verify the effectiveness of the proposed strategy. The obtained results show that the proposed strategy can suppress low-frequency SM capacitor voltage fluctuation without increasing the CMV. [ABSTRACT FROM AUTHOR]

A new Modular Multilevel Converter with Interleaved half-bridge Sub-Modules (ISM-MMC) is proposed in this article. The ISM-MMC exhibits higher modularity and scalability in terms of current ratings with respect to a conventional MMC while preserving the typical voltage-level adaptiveness. The ISM-MMC brings the known advantages of classical MMC to low-voltage, high-current applications making it a novel candidate for the sector of ultrafast chargers for all types of electric vehicles (EVs). This advanced topology makes it possible to easily reach the charging power of the EV charging system up to 4.5 MW and beyond with a low-voltage supply. To operate the new converter, a hybrid modulation scheme that helps to exploit the advantages of the interleaving scheme is implemented and explained in this article. It has been verified that the typical MMC control methods are still applicable for ISM-MMC. A comparative study between classical MMC and ISM-MMC configurations in terms of output characteristics and efficiency is also given. Furthermore, it has been demonstrated that the number of ac voltage levels is synthetically multiplied by the number of interleaved half-bridge legs in submodules. Simulations, hardware in the loop, and experimental tests are carried out to demonstrate the feasibility of the proposed topology and the implemented modulation scheme. [ABSTRACT FROM AUTHOR]

Modularity, scalability, and efficiency of modular multilevel converters (MMCs) have shown clear advantages among the multilevel topologies, being ideal for medium and high voltage applications. However, they require several control loops in order to achieve multivariable control, where the main challenge is the voltage balancing of the submodules capacitors, usually requiring large capacitances. To extend the MMC application to the low-voltage range, the arm link enhanced MMC solves this issues by adding dual active bridges (DABs) between the upper and lower submodules (SMs) in order to transfer the power ripple between each other. This allows the use of small capacitances in the SMs and greatly reduces the control and measurement complexity, while maintaining a low voltage ripple in an extended output frequency range. In this scenario, this work studies how the circulating current suppression control (CCSC) affects the DABs power transfer, as well as the overall effects in the converter and its performance. Considering the variable frequency operation of the converter, the effects at low and high frequencies are analyzed, as well as the consequences in the DABs high-frequency transformers. The main objective is to determine whether it is convenient or not to enable the circulating current suppression depending on the operation conditions of the converter. [ABSTRACT FROM AUTHOR]

Multi-terminal high voltage DC (MTDC) network is an effective technology to integrate large-scale offshore wind energy sources into conventional AC grids and improve the stability and flexibility of the power system. In this paper, firstly, an analytical model of a general applicable MTDC system integrated with several isolated AC grids is established. Then, an improved AC-DC power flow algorithm is used to eliminate the additional DC slack bus or droop bus iteration (SBI/DBI) step of the conventional AC-DC sequential power flow. A multi-objective optimal power flow (MOPF) algorithm is proposed to minimize two optimization targets, i.e., overall active power loss and generation costs of the system. To increase the degree of freedom, adaptive droop control is used in the proposed optimization algorithm in which the voltage references and droop coefficients of the modular multilevel converters (MMCs) are control variables. A multiple objective particle swarm optimization (MOPSO) method is applied to solve the MOPF problem and achieve the Pareto front. A technique for order of preference by similarity to ideal solution (TOPSIS) is incorporated in the decision analysis section and helps the decision maker to identify the best compromise solution.

A bleeding resistor is usually adopted to be parallel with the capacitor in the submodule (SM) to avoid static voltage imbalance caused by auxiliary power supply during start-up process of modular multilevel converters (MMCs). However, the bleeding resistor causes unnecessary power loss during normal operation. In this article, a resistor controllable SM based on a normally-on silicon carbide (SiC) junction field-effect transistor is proposed to reduce the power loss on bleeding resistor at normal operation of MMCs. The proposed control can not only ensure the static voltage balance during start-up of MMCs, but can also completely eliminate the power loss on bleeding resistor during normal operation of MMCs for high efficiency. In addition, the proposed control can shorten the SM capacitor discharging time at shut-down process. Experimental studies are conducted and the results confirm the effectiveness of the proposed control. [ABSTRACT FROM AUTHOR]

Thisarticle presents an efficient analogue sorting algorithm for balancing the submodule (SM) capacitor voltages of modular multilevel converter (MMC). The proposed analogue sorting algorithm offers the advantage of fast convergence rate without any need of recursive loops for the implementation on embedded devices. It can be easily implemented with combinational logic operations on field programmable gate array (FPGA) and provides less hardware and computational overhead. The functionality and performance of the proposed analogue sorting algorithm is evaluated with the simulation model of three phase five-level MMC in MATLAB/Simulink environment. The real time implementation of the proposed sorting algorithm with the SM capacitor voltage balancing strategy is implemented on Altera/Cylone - I (EP1C12Q240C8N) FPGA. A five-level continuous space vector pulsewidth modulation (CSVPWM) is realized on a PIC microcontroller (PIC18F452). A down-scaled model of single-phase five-level MMC is designed and constructed to investigate the reliable and stable operation of MMC with the proposed analogue sorting algorithm and SVPWM method. Simulation and experimental results are presented for validation. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has become one of the most attractive converters for high-power applications such as high-voltage dc (HVDC) converters, but also fusion devices’ power supplies. This converter, thanks to the discrete-leveled output voltage and its identical submodules (SMs) by which it is composed, represents a promising alternative to replace the flywheel generator (FG) that actually provides electrical power to Axially Symmetric Divertor Experiment (ASDEX) Upgrade device’s toroidal field (TF) coils. Due to the pulsed dc operation of these coils and their high-power needs (up to 150 MW) for each experiment, a small-scale version of MMC is under development with some differences compared with conventional ones: SM’s capacitors have been replaced with supercapacitor (SC) modules to increase the amount of available stored energy, while SMs belonging to different “rows” are interconnected to simplify their control and increase the reliability of the converter. This article shows the development and the operation of a single IGBT full-bridge (FB) SM highlighting advantages of this configuration and challenges of a full-scale converter: a proper power stage filter has been designed to protect SCs from voltage spikes and high-frequency current content generated during IGBT switching events, while SM’s control has been realized, thanks to the EtherCAT protocol, which ensures synchronization in case of SMs’ parallel operation and reliability of the future full-scale converter. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) have been a promising topology in medium-voltage motor drive applications over the past few decades. The main challenge that hinders the widespread use of the MMC in medium-voltage motor drives is the large voltage fluctuation under low-speed conditions. In this article, we propose a model-predictive control (MPC) for the MMC operating in a wide frequency range. Unlike conventional MPC methods, a novel cost function has been designed to realize the output current control and the voltage balancing control for both low- and high-frequency operations. Compared with the high-frequency circulating current injection method, the proposed method can easily achieve a tradeoff between the modulation range and the amplitude of injected common-mode voltage under different operating conditions. In addition, due to the exclusion of the switching process between low- and high-frequency operations, the control structure of this method is more straightforward. Finally, the experimental results verify the effectiveness and superiority of the proposed method. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) have been recognized as the most attractive technology for high voltage direct current (HVDC) applications during the last years. Their advantages include the ability to freely control the internally stored energy in the distributed submodules (SMs) and to independently control AC and DC powers. Recent studies showed that an MMC can take advantage of this degree of freedom to supply ancillary services with low energy requirements such as frequency response or power oscillation damping by connecting energy storage systems (ESSs) to its SM capacitors. This paper proposes a modular DC-DC converter as an interface between the energy storage element and the SM capacitor. As it can be built with switches allowing a high switching frequency, it is shown that the volume of the passive components can be reduced in comparison with interface converters known in the literature. The proposed control strategy avoids to transfer the harmonics of a SM to the ESS that prevents to drastically oversize the system. The proposed control method also provides an appropriate energy exchange without affecting the control philosophy of an MMC and it addresses balancing issues. Simulations and a downscaled prototype confirm the relevance of the proposed control. As the proposed interface converter is able to track the power reference with fast dynamics, it provides the same system-level advantages as other interface converters known in the literature but with a reduced volume. [ABSTRACT FROM AUTHOR]

Modular multilevel converter (MMC) high-voltage direct current (HVdc) transmission is a promising method to integrate renewable energies. However, limiting the dc-link fault current and accelerating the fault-current clearance are critical issues for MMC-HVdc systems. In this article, we propose an air-core-transformer-based fault-current limiter (ACT-FCL) in cooperation with the dc circuit breaker to limit fault-current rising and decrease fault-clearing time. Compared with the conventional dc reactor, hybrid current-limiting circuit, current-commutation-based fault-current limiter (FCL), and magnetic coupling FCL, the ACT-FCL can increase the equivalent inductance during the dc fault stage to reduce the rising rate and peak of fault current. The ACT-FCL can also decrease the equivalent inductance during the fault-current-clearing stage to reduce the fault-clearing time. The simulations and laboratory experiments verify the effectiveness of the proposed ACT-FCL in the bidirectional HVdc systems. The ACT-FCL may be extended to more fields, such as dc microgrid and domestic dc transmission. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is widely used for high and medium voltage applications due to its several advantages over other multilevel topologies. The number of voltage levels that can be obtained at the ac output can be increased at the expense of a greater number of submodule (SM) capacitors. As the capacitor voltages need to be monitored for voltage balancing, the MMC system requires a large number of voltage sensors. This increases capital investment for the MMC system and failure points due to increased hardware interfaces. So far, a few works have been reported that uses observation techniques to either completely eliminate or reduce the number of sensors. This article presents a discrete-time sliding mode observer (DTSMO) for estimating the capacitor voltages and thus possibly eliminating all the voltage sensors of the SM capacitors. A switching function-based discrete mathematical model is used for the design framework of the proposed DTSMO. The errors in observation are taken as important performance indicators under both transient and steady-state conditions. It has also been observed that the proposed DTSMO delivers robust performance in presence of parametric uncertainty of capacitance values. The control algorithm is simulated in MATLAB/Simulink to validate the performance of the proposed observer. It is further validated through experimental results in a hardware prototype. [ABSTRACT FROM AUTHOR]

Model predictive control has emerged as a promising approach to govern modular multilevel converters (MMCs), due to its flexibility to include multiple control objectives and simple design process. However, this control scheme presents relevant issues, such as high computational complexity and variable switching frequency. This work proposes a sequential phase-shifted model predictive control (PS-MPC) for MMCs. The key novelty of this proposal lies in the way the predictive control strategy is formulated to fully exploit a phase-shifted pulsewidth modulation technique through an appropriate choice of synchronized average models for each carrier. In this way, the proposed predictive controller obtains independent optimal modulating signals for each carrier in a sequential manner, by solving an optimization problem with reduced computational effort independent of the number of submodules. This sequential optimization allows one to formulate the optimal control problem to achieve multiple control objectives, similarly to the finite-control-set MPC (FCS-MPC). Nevertheless, the MMC governed with the proposed PS-MPC generates an output voltage with fix-spectrum and operates with an even power loss distribution among semiconductors in steady-state, outperforming the standard FCS-MPC strategy. Experimental results are provided to verify the proposed PS-MPC effectiveness when governing a three-phase MMC with four half-bridges per stack. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is attractive for high-voltage direct current (HVdc) applications. The dc line short-circuit fault is one of the key challenges for the HVdc system. In this article, a crossing thyristor branches (CTB)-based hybrid MMC (HMMC) with current interruption capability is proposed to protect the HVdc system under dc line short-circuit faults. In the CTB-HMMC, the thyristor branches are proposed to crossing-connect different arm inductors and each arm consists of unipolar full-bridge (UFB) submodules (SMs) and half-bridge SMs, where the CTB effectively reduces the UFB-SMs number in the arm of the HMMC with current interruption capability. The current interruption operations, including dc-side and ac-side current interruption, are also proposed for the CTB-HMMC in case of dc line short-circuit faults, where the proposed CTB-HMMC can interrupt the fault current with the advantages of fewer UFB-SMs in the arm, short current interruption time, low semiconductor cost, and low power losses. The simulation studies are conducted and a down-scale prototype is built, and the study results verify the effectiveness of the proposed CTB-HMMC and operation principles. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) has become popular for medium-voltage applications. During MMC operation, unavoidable circulating currents flow through its arms and cause extra losses in the converter system. To generate a reference for circulating current, different current injection methods to control the circulating current have been reported. The conventional current injection techniques used so far relies on a mathematical model that excludes the effect of arm inductor voltages in its design. For high-voltage applications with many submodules (SMs), the above exclusion does not make much difference. But for medium-voltage applications with few SMs, the applied arm inductor voltages may be significant for a given dc-link voltage. In this article, inductor voltages are included in the proposed circulating current model, and a new reference for circulating current is obtained resulting in reduced circulating current through MMC arms. The proposed method is analyzed in detail on both seven-level and nine-level three-phase MMC, where advantages of the 2N + 1 level phase disposition pulsewidth modulation (PD-PWM) scheme are retained. The simulation study is carried out in PLECS blockset to show the reduction in the rms magnitude of circulating current. The validation of the proposed technique in a lab-scale hardware prototype shows the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) in high-voltage dc applications usually adopt a distributed control architecture to manage a large number of submodules (SMs) through a communication network. The communication congestion and network disconnection might lead to communication interruption (CI) and eventually cause the system to malfunction. In this article, a resilient operation strategy is proposed and studied to ride-through the CI fault, in order to prevent frequent fault SM bypassing, replacement, or even system shutdown. The analysis of the MMC distributed control system with the presence of CI indicates that the insertion index of the faulted SM might become constant, which distorts the output current and results in overvoltage of the communication interrupted SM (CI-SM). The CI-SM capacitor voltage prediction can be used to determine the MMC safe operation period after CI occurs. During the safe operation period, the CI-SM power balance is sustained by utilizing prestored phase signals to generate a sinusoidal insertion index according to its capacitor voltage tracking error. Two operation modes are proposed and analyzed to ensure the MMC stable operation under various conditions. The system protection is sensibly used only if the CI duration exceeds a safe operation period, which avoids frequent SM cut-off. Good agreement of the CI-SM capacitor voltage is achieved between the theoretical and simulation results. The effectiveness and robustness of the proposed MMC resilient operation are experimentally confirmed. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) have attracted a great deal of research attention in medium-voltage motor drive applications. However, the excessive fundamental-frequency submodule (SM) capacitor voltage ripple at low speeds or during start-up poses a major technological challenge. This paper develops a new control strategy for SM capacitor voltage ripple suppression based on model predictive control (MPC). The system mathematical model with common-mode voltage (CMV) and the SM capacitor voltage ripple suppression models are established. Then, a cost function of the system is established, and the control system is designed. When compared to existing strategies, the modulation process, PI regulation process, and high frequency injection control are avoided due to the MPC strategy. The performance of the proposed approach is verified through MATLAB simulations and experiments on a MMC system. [ABSTRACT FROM AUTHOR]

Capacitance monitoring is one of the important issues for a modular multilevel converter (MMC) to obtain high reliability. Since pulsewidth modulation is usually implemented in the field-programmable gate array (FPGA), produced precise switching states in the FPGA cannot be directly accessed by the top digital signal processor (DSP) controller, which poses challenges to the existing capacitance monitoring methods based on precise switching states of the MMC. This article presents a submodule (SM) capacitance monitoring strategy for the MMC with a simple algorithm, where the fundamental frequency components of the SM capacitor voltage and current are extracted to estimate the SM capacitance based on reference, but not the precise switching states. The proposed scheme not only simplifies the implementation and calculation, but also avoids control limitations and heavy communication burden between the DSP and the FPGA. Besides, the proposed strategy effectively eliminates noise impact from sensors and increases accuracy. Simulation and experimental studies are implemented, and the results confirm the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

The active circulating current suppression may cause high-frequency current ripple in the dc link of modular multilevel converters (MMCs). In this article, a phase-shifted double-group multicarrier-based phase-disposition pulsewidth modulation strategy is proposed to eliminate the dc-link high-frequency current ripple of MMCs, where the fundamental frequency component and the second-order component of the arm reference are modulated, respectively, by two groups of carriers. By shifting the phase angles of double-group multicarrier in each carrier period, the high-frequency current ripple injected into the dc link of MMCs can be eliminated. The proposed strategy not only eliminates the dc-link current ripple, but also extends the active and reactive power region of MMCs in comparison with the existing method. Simulation and experimental studies are conducted, and their results verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

This article proposes a novel capacitor-voltage balancing strategy for the double-Y modular multilevel converter static synchronous compensator operated under unbalanced current conditions of the system. The proposed method, based on zero-sequence voltage injection, manipulates the current distribution between the converter arms to minimize the converter ratings. The proposed balancing strategy is compared with the classical approach, based on controlling the circulating dc currents. For the same unbalanced conditions, the proposed strategy leads to lower energy rating for the converter as compared with the classical one, whereas the semiconductor ratings remain the same in both strategies. Theoretical findings are verified through experimental results. [ABSTRACT FROM AUTHOR]

The half-bridge (HB) submodule (SM) based MMC requires a large number of sensors to measure capacitor voltages. In this letter, a double half-bridge (DHB) SM-based MMC is proposed, where each SM consists of two HBs. But only one voltage sensor is equipped in each DHB-SM for the capacitor voltage estimation, which is connected between the positive poles of two capacitors in the DHB-SM. A capacitor voltage estimation method is also proposed, which can estimate capacitor voltages in the proposed MMC with only a half number of voltage sensors in comparison with HB-SM-based MMCs. A DHB-SM-based three-phase MMC is simulated with power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) and a DHB-SM-based single-phase MMC prototype is built in the laboratory. Both the simulation and experimental results confirm the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this article, a new, simple, and generalized discontinuous pulse width modulation (PWM)-based submodule capacitor voltage balancing approach is proposed and generated within the carrier-based PWM for modular multilevel converters (MMCs). In this scheme, different variants of zero-sequence voltage (ZSV) component is injected into the pulse width modulator so that each arm of the MMC is virtually clamped to positive or negative dc bus during a specific interval to achieve discontinuous modulation. A simple approach based on the location of the voltage vector mapping to a two-level space-vector diagram is presented to determine various ZSV components. The effectiveness of the proposed scheme with a voltage balancing algorithm is demonstrated through PLECS simulations and experimentally verified using a hardware-in-the-loop-based system. The performance of MMC with different ZSV injections is holistically evaluated to determine the best possible sequence over a wide range of modulation index. [ABSTRACT FROM AUTHOR]

The submodule capacitor voltage sensing and balancing is necessary for proper operation of modular multilevel converters (MMC). Voltage controlled oscillator is used for capacitor voltage measurement in the reported literature where voltage sensor is required to be calibrated. In this article, a voltage sensing method that does not need calibration of individual voltage sensors is used for capacitor voltage measurement. Moreover, a simplified sorting algorithm is employed for capacitor voltage balancing. A method of sorting the capacitor voltages that selects the submodules to be inserted directly based on logic equations is used as compared to the conventional methods of arranging them in ascending or descending order. This facilitates to simplifying the sorting technique and the scheme is easy to implement using any controller. Furthermore, to simplify the overall MMC control, the decentralized control arrangement is developed. Fairly good capacitor voltage balancing is obtained. The proposed balancing and sensing schemes of submodule capacitors are validated on a three-phase seven-level MMC hardware prototype with six submodules per arm. The prototype is tested for different operating conditions such as precharging, circulating current control, and grid connected operation of MMC. [ABSTRACT FROM AUTHOR]

To reduce stored capacitor energy in modular multilevel converters (MMCs), previous papers propose injection of harmonics in the arm circulating currents and zero-sequence voltages. While this approach is effective, previously utilized combinations of injected harmonics significantly increase converter current ratings. In contrast, the optimization approach developed in this article identifies harmonics that simultaneously achieve capacitor energy optimization and reduced overrating of converter currents. In addition, the proposed approach eliminates the possibility of convergence to local minima, which is a significant improvement on previously proposed nonconvex optimization approaches. Experimental verification of the proposed approach is performed on a low-voltage three-phase modular multilevel converter. [ABSTRACT FROM AUTHOR]

This article presents a simple pulse width modulator to achieve both $N$ + 1 and 2 $N$ + 1 modulation in a modular multilevel converter. The proposed implementation needs a single triangular waveform to control a modular multilevel converter with a large number of submodules. Hence, the proposed approach significantly reduces the required triangular carrier count and solves the practical implementation issues associated with the carrier-based pulse width modulation schemes in the digital control platforms. The performance of the proposed approach is validated with detailed experimental studies. The results show that the proposed strategy effectively generates a multilevel voltage waveform at the load terminals. Furthermore, the performance of the proposed approach is compared with the traditional phase-shifted carrier pulse width modulation schemes. In addition, it can be easily applied to the $n$ -level modular multilevel converter with any type of submodule configuration. [ABSTRACT FROM AUTHOR]

This letter proposes a novel capacitor condition monitoring method for modular multilevel converters (MMCs) based on reference submodules (SMs). It significantly enhances the capacitor monitoring accuracy by making full use of the SM voltage sensor measurement range. Accuracy comparison between the existing method and the proposed method is conducted to quantify the improvement of accuracy. Moreover, its operation principle and practical implementation considerations are presented. Finally, a three-phase MMC platform is built to experimentally verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This article proposes a mission profile-based reliability prediction method for modular multilevel converters (MMCs). It includes key modeling steps, such as long-term mission profile, analytical power loss models, system-level and component-level thermal modeling, lifetime modeling, Monte Carlo analysis, and redundancy analysis. Thermal couplings and uneven thermal stresses among submodules are considered. A case study of a 15-kVA down-scale MMC has been used to demonstrate the proposed method and validate the theoretical analysis. The outcomes serve as a first step for developing realistic reliability analysis and model-based design methods for full-scale MMCs in practical applications. [ABSTRACT FROM AUTHOR]

This article presents a new submodule (SM) circuit that facilitates the fault-tolerant capability for modular multilevel converters (MMCs) under various internal and external faults. The proposed SM circuit ensures continued operation of the MMC subsequent to multiple SM failures and symmetrical and asymmetrical grid faults without the need for additional hardware implementation. The proposed SM circuit can be simply integrated into conventional half bridge-based MMCs by using the switching signal adapter block. The switching signal adapter unit duly assigns a switching pattern to the SMs in each arm to obtain the most efficient current path and inherent capacitor voltage balancing features. A fully modular structure, enhanced internal fault management, external fault-handling capability, and ease of expandability are the key features of the proposed SM circuit. The configuration, modeling, and gating signal pattern of an MMC that adopts the proposed SM are discussed and analyzed in detail. Time-domain simulation studies using detailed nonlinear models validate the effectiveness of the proposed configuration and its principle of operation in an MMC-HVDC system under various fault conditions. In addition, hardware-in-the-loop test results confirm the feasibility of hardware implementation of the proposed SM circuit. [ABSTRACT FROM AUTHOR]

This letter proposes a new submodule capacitor condition monitoring method for modular multilevel converters (MMCs) based on the dc-side start-up procedure. During this process, the MMC can be simplified into an RC charging circuit, and the submodule capacitance can be estimated by investigating its relationship with the phase current and submodule voltage. Several practical issues related to the implementation, advantages, and limitations of the proposed method are presented as well. In order to validate the method, a series of experiments with different submodule capacitances are conducted based on a three-phase MMC platform. Experimental results confirm the effectiveness and feasibility of the proposed method. [ABSTRACT FROM AUTHOR]

The ac-side cascaded H-bridge converter with a two-level (2L) main bridge has previously been proposed as a fault-tolerant converter for high-voltage dc. This paper explores the benefits of replacing the 2L bridge with a neutral point clamped (NPC) three-level bridge for medium voltage dc applications and defines the optimum operating conditions for this case. By modifying the topology to include an NPC main bridge, the peak stack voltage during normal operation is decreased considerably, which results in a 58% reduction in the required number of submodules (SM), thereby significantly increasing efficiency. However, this sacrifices the fault ride-through capability as the stacks are no longer able to support the ac voltage and thus two new SM topologies are proposed. The proposed topologies function as single full-bridges with two capacitors in parallel during normal operation. Under fault conditions, the SMs divide into two series-connected full-bridges to enable dc fault ride-through. Reverse-blocking integrated gate-commutated thyristors or ultra-fast mechanical switches are used to bypass the insulated gate bipolar transistors that are unused in normal operation and therefore the topology maintains a high efficiency. Simulation results are shown, and the proposed topologies are compared with the more conventional designs in terms of efficiency, energy storage requirement, and device count. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) are emerging as a promising topology for medium- and high-power applications. Aluminum electrolytic capacitors (AECs) are usually employed in MMCs as floating capacitors due to their high volumetric efficiency and low price. AECs gradually deteriorate over time due to electrolyte vaporization, and for this reason, they have been recognized as one of the most fragile components in the converter. As they continue to age, the AEC's capacitance decreases and its equivalent series resistance increases, which can result in an increase of submodule (SM) capacitor voltage ripple, power loss and could damage the operation of the MMC with prolonged use of aged capacitors. To prevent such damage, monitoring the health of capacitors is an important step to enhance the reliability of the MMC by predictive maintenance. This paper presents a failure prediction scheme for SM capacitors in the MMC by monitoring the SM capacitor voltage oscillations. Detailed simulation studies are carried out for a five-level MMC in the PLECS® platform and verified experimentally. The estimated capacitance through simulations and experiments is in close agreement to that value measured using the LCR meter. [ABSTRACT FROM AUTHOR]

This paper conducts a thorough analysis on the circuit dynamics of the single-phase direct ac–ac modular multilevel converter (MMC) to address the branch energy divergence problem in the railway power conditioning application scenario. Based on the analysis, a full degree of freedom (DOF) based control scheme is proposed. By controlling all the DOFs in a coordinated manner, extended branch power controllability can be achieved, which enables each branch's energy to be regulated individually without deteriorating the terminal currents. As a result, the control flexibility and reliability of the MMC can be effectively enhanced in the presence of asymmetric branch conditions. The proposed control scheme is implemented in a cascaded control structure with well-established algorithms. The frequency-domain stability analysis is also presented to guide controller parameter design. The effectiveness of the proposed control strategy is validated through the experiments under various conditions. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is attractive for medium- or high-power applications because of its high modularity, availability, and high power quality. In this article, the dc-link current ripple of the three-phase MMC derived from the phase-disposition pulsewidth modulation and circulating current control is analyzed. A variable carrier phase-shifted angle control strategy under the two-group carriers-based modulation is proposed to suppress the dc-link current ripple of the three-phase MMC. Through the regulation of the carrier phase-shifted angles under the two-group carriers-based modulation in each carrier period, the high-frequency current injected into the dc link from the three phases of the MMC can be counteracted, which suppresses the dc-link current ripple of the MMC with simple algorithm, and therefore improves the dc-link power quality. Simulations and experimental studies of the MMC are conducted, and the results confirm the effectiveness of the proposed control strategy for the MMC. [ABSTRACT FROM AUTHOR]

The series bridge converter (SBC) is a modular multilevel converter (MMC) recently developed to enhance power density in high-voltage high-power applications. The MMC is a well-established solution, widely researched, and exploited in practical HVdc connections, thanks to its high power quality and high efficiency. However, the main limitation of the MMC is the relatively large energy storage, also due to the fact that power ripples in the submodule capacitors include a component at the fundamental ac frequency. As a result, volume becomes critical in applications such as offshore or city center in-feeds, where space is restricted and expensive. The SBC offers a more compact footprint by exploiting a series connection on the dc side and by operating the submodules with rectified waveforms, thus moving the minimum component of the instantaneous power to twice the ac fundamental and reducing capacitors size. The drawback of the converter is a more complex energy control compared to the MMC. This paper proposes the first experimental validation of the SBC, using a 2-kW laboratory-scale prototype. Since the basic converter design has been discussed in previous papers, the focus of this paper is on converter control design and experimental validation. [ABSTRACT FROM AUTHOR]

Modular multilevel converters (MMCs) can be configured to perform ac/ac conversion, which makes them suitable as railway power supplies. In this paper, a hierarchical control scheme for ac/ac MMCs for railway power supplies is devised and evaluated, considering the requirements and the operating conditions specific to this application. Furthermore, admittance models of the ac/ac MMC are developed, showing how the suggested hierarchical control scheme affects the three-phase and the single-phase side admittances of the converter. These models allow for analyzing the stability of the interconnected system using the impedance-based stability criterion and the passivity-based stability assessment. Finally, the findings presented in this paper are validated experimentally, using a down-scaled MMC. [ABSTRACT FROM AUTHOR]

Various methods have been presented in the past to emulate the electrical behavior of modular multilevel converters (MMCs). To meet the demands for the reliability aspect study of MMCs, this paper proposes a viable setup to emulate the thermal behavior and to investigate its feasibility for reliability testing and thermal model validation of the power modules in the MMC. The proposed mission profile emulator has three distinctive features: 1) capable of emulating and measuring the thermal stresses of power modules; 2) capable of implementing practical switching profile as a full-scale MMC; and 3) having significantly reduced requirement for dc power supply compared to existing setups used for electrical studies. Theoretical discussions, and simulations as well as the experimental results are presented to demonstrate the capability of the mission profile emulator both electrically and thermally. Moreover, this paper is accompanied by a video demonstrating how to measure the junction temperature of the power devices using the optical fiber and the thermal camera. [ABSTRACT FROM AUTHOR]

In this paper we present an approach for real-time simulation and Hardware-in-the-Loop (HIL) testing of Modular Multilevel Converters (MMCs) that rely on switching models while supporting system level analysis. Using the Latency Based Linear Multistep Compound (LB-LMC) approach, we achieved a 50 ns simulation time step for systems composed of several MMC converters and for converters of various complexity. To facilitate system level testing, we introduce the use of a serial communication-based (Aurora) interface for HIL testing of MMC converters and we analyzed the effect that communication latency has on the accuracy of the HIL test. The simulation and HIL results are validated against an MMC laboratory prototype.

One of the next challenges for modular multilevel converters (MMCs) is how to the size the critical components to their limits with reduced design margins, while at the same time fulfilling the reliability target. To do it confidently, better thermal modeling with quantitative uncertainty analysis is necessary to support the decision-making during the product design stage. Regarding the conversion of long-term mission profiles into thermal profiles for reliability evaluation, this paper proposes a computationally efficient thermal modeling method for insulated-gate bipolar transistor modules in MMCs. The proposed method considers the impact of the inherent thermal unbalance and has minimum computation with quantitative error analysis. Finally, both simulations and experiments have verified the theoretical results. [ABSTRACT FROM AUTHOR]

The stability of a modular multilevel converter connected to an ac grid can be assessed by analyzing the converter ac-side admittance in relation to the grid impedance. The converter control parameters have a strong impact on the admittance and they can be adjusted for achieving system stability. This paper focuses on the admittance-shaping effect produced by different current-control schemes, either designed on a per-phase basis or in the $dq$ frame using space vectors. A linear analytical model of the converter ac-side admittance is developed, including the different current-control schemes and the phase-locked loop. Different solutions for computing the insertion indices are also analyzed, showing that for a closed-loop scheme a compact expression of the admittance is obtained. The impact of the control parameters on the admittance is discussed and verified experimentally, giving guidelines for designing the system in terms of stability. Moreover, recommendations on whether a simplified admittance expression could be used instead of the detailed model are given. The findings from the admittance-shaping analysis are used to recreate a grid-converter system whose stability is determined by the control parameters. The developed admittance model is then used in this experimental case study, showing that the stability of the interconnected system can be assessed using the Nyquist stability criterion. [ABSTRACT FROM AUTHOR]

Connecting a modular multilevel converter to an ac grid may cause stability issues, which can be assessed by analyzing the converter ac-side admittance in relation to the grid impedance. This paper presents a method for calculating the ac-side admittance of modular multilevel converters, analyzing the main frequency components of the converter variables individually. Starting from a time-averaged model of the converter, the proposed method performs a linearization in the frequency domain, which overcomes the inherent nonlinearities of the converter's internal dynamics and the phase-locked loop used in the control. The ac-side admittance obtained analytically is first validated by simulations against a nonlinear time-averaged model of the modular multilevel converter. The tradeoff posed by complexity of the method and the accuracy of the result is discussed, and the magnitude of the individual frequency components is shown. Finally, experiments on a down-scaled prototype are performed to validate this study and the simplification on which it is based. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is attractive for medium- or high-power applications because of the advantages of its high modularity, availability, and high power quality. Reliability is one of the most important challenges for the MMC consisting of a large number of submodules (SMs). The capacitor monitoring is one of the important issues in the MMC. This paper proposed a reference submodule (RSM)-based capacitor monitoring strategy for the capacitance estimation in the MMC, where the capacitances in the monitoring SMs can be estimated based on the capacitor voltage relationship between the RSM and the monitoring SMs. The proposed monitoring strategy does not rely on the information of all capacitor voltage and current, which effectively simplifies the algorithm for capacitance estimation. The simulation studies with the time-domain professional tool PSCAD/EMTDC are conducted and a down-scale MMC prototype is also tested in the laboratory with the proposed capacitor monitoring strategy. The study results confirm the effectiveness of the proposed capacitor monitoring strategy. [ABSTRACT FROM AUTHOR]

This paper proposes a hybrid modular multilevel dc–dc converter with dc fault ride-through capability. In each phase, the lower arm and shunt arm are composed of half-bridge submodules and full-bridge submodules (FBSMs) respectively, and the upper arm is composed of both types of submodules. The shunt arm eliminates the large ac filter required at the output of dc side. Moreover, it provides the bidirectional fault ride-through capability together with the FBSMs in the upper arm. Required kVA rating of the switches in the proposed topology is analyzed and compared to other topologies. With the optimal number of FBSMs in the upper and shunt arms designed by overmodulation, the proposed topology has a reduced kVA rating of switches, especially for the voltage conversion ratio above 0.54. A control strategy is presented to control the ac circulating current and balance the energy between the three arms. The performance of the proposed topology is validated by real-time simulation and a lab-scale test bench. [ABSTRACT FROM AUTHOR]

This paper develops a modulation based fault-tolerant (FT) strategy for restoring the operation of three-phase modular multilevel converters (MMCs) with faulty switches. This FT strategy is based on a proposed modified space vector modulation (SVM) technique that generates balanced line-to-line (line) voltages even in the case of a fault occurrence. In the postfault operation, the proposed strategy is able to restore the fundamental amplitude of the line to the neutral (load) voltages to that of the normal operation condition with a slightly increased voltage stress over the switches in the faulty phase. In this paper, first a brief background about MMCs and SVM technique is provided. Then, the proposed FT strategy and the modified SVM technique are presented. Finally, several simulation and experimental results are provided to validate operation of the proposed strategy. [ABSTRACT FROM AUTHOR]