Performance Of Modular Multilevel Converter Research Articles

A capacitor voltage ripple suppression method employing bidirectional‐switching channel for the MMC driven pumped storage system operating at low‐speed stage

SummaryThe Modular Multilevel Converter (MMC) has gained the widespread application in the DC power transmission, owing to the advantages such as modularity and low output current harmonic content. However, its adoption in variable‐speed pumped storage systems remains limited due to the significant voltage fluctuations in its sub‐module capacitors during the low‐speed operation. In this paper, a capacitor voltage ripple suppression method employing bidirectional‐switching channel is proposed. It aims to improve the driving performance of MMC when the variable‐speed pumped storage motor operates at low speed. The proposed method creates bidirectional‐switching channels between the sub‐modules of the upper and lower bridge arms and moves the sub‐module capacitors that originally fixed to the upper and lower bridge arms into the channels. By controlling the bidirectional‐switching channel, the same capacitor can be put into the sub‐modules of different bridge arms. At the same time, a threshold switching control method is proposed to reduce the switching loss. Without introducing the common mode voltage, the capacitor voltage fluctuation can be substantially suppressed when the pumped storage motor operates at low speed. Experimental and simulation results verify the effectiveness of the proposed method.

Performance of Modular Multilevel Converter in Electric Vehicles Charging Station

The awareness of air pollution is the main reason for the development of Electric Vehicles. Hence, the many number of Electric vehicle connected to grid produce power quality problem in conversion of AC-DC. In this paper, to avoid this drawback, the MATLAB/SIMULINK model of Modular Multi Level Converter is implemented in charging station. MMC not only provide bidirectional flow but also improve power quality. The converter works on boost mode and buck mode. In G2V, the boost mode is for charging the battery of electric vehicle and buck mode is discharging the battery. For, the G2V, the buck mode reduces the voltage to charge the battery with current or voltage constant which is about the state of the charge of the battery. The out of the battery is pure dc with mmc which replace the filter also. It increases the life of the battery. The model of the MMC-based charger for electric vehicle was created in MATLAB/Simulink, and simulation results shows the output power of the MMC and circulation current is controlled.

Low-Frequency and Sub-Synchronous Power Oscillation Suppression and Analysis of Hybrid Line Commutated Converter for HVDC Grid

Building high-proportion renewable energy grid requires economical and reliable High Voltage Direct Current (HVDC) converters. Line Commutated Converter (LCC) is widely used because of its long transmission distance, low power loss, low manufacturing cost, and large transmission capacity, which modular multilevel converter (MMC) cannot achieve, but has commutation failure (CF) problems. Low-frequency and sub-synchronous power oscillation caused by CF will affect the safety of bulk power systems. This paper theoretically analyzes the power oscillation during CF and CF recovery, and then proposes a novel hybrid line commutated converter (H-LCC) based on integrated gate commutated thyristor (IGCT), which can effectively mitigate CF and suppress power oscillation. The principle of H-LCC suppressing power oscillation is analyzed. On this basis, the performance of LCC, MMC and H-LCC under the same fault conditions is compared by a simulation study, which proves that H-LCC can improve the security and stability of high-proportion renewable energy power systems.

Virtual synchronous generator‐based control of modular multilevel converter for integration into weak grid

AbstractIntegrating Power Electronics Converters (PECs)‐based renewable energy sources can establish a weak grid due to the substantial inertia reduction leading to severe frequency fluctuations. In this paper, a control strategy based on the Virtual Synchronous Generator (VSG) concept is designed for a three‐phase Modular Multilevel Converter (MMC) connected to a weak grid. The proposed VSG control approach improves the frequency response during transient mode, resulting in a 66.7% enhancement at the Point of Common Coupling (PCC). The proposed strategy is shaped within four control loops wherein the VSG‐dependent output current control possesses the duty of enriching the MMC output current tracking capability. Focusing on the MMC output current‐based transfer function, the effectiveness of this capability is guaranteed using the frequency domain analysis including the Nyquist and Bode diagrams. Moreover, a comprehensive discussion was carried out regarding the impact of VSG on the voltage of submodule capacitors and the circulating current. Simulation results in MATLAB/Simulink environment validate the accuracy and weak grid‐connected MMC performance under the proposed VSG‐based control technique. To further verify the superiority of the proposed strategy, two control techniques including Droop and SEBIR are compared with the VSG proposed controller.

Model Predictive PI Circulating Current Control for Modular Multilevel Converter

Significant circulating currents in the modular multilevel converter (MMC) increase system losses and complicate heat-sink design. Conventional PI and PR controllers can achieve steady-state error adjustment, but are sensitive to parameter changes and model uncertainty, heavily relying on coordinate transformations and careful design of model parameters. Model predictive control (MPC) has the characteristics of simple design, good robustness, and excellent dynamic response; however, it encountered the complexity of adjusting weighting factors. This paper proposed circulating the current model predictive proportional integral control (MPPIC) method in abc reference frame. This hybrid control solution utilized the predictive model and traditional PI algorithm to combine the advantages of nonlinear and linear control. Compared with existing suppression methods, this method avoided complex mathematical operations and a selection of weight coefficients, was easy to implement, and can effectively suppress circulating currents under different modulation ratios. Simulations were conducted on MATLAB/Simulink to verify the effectiveness of the proposed control strategy. MPPIC can not only distinctly suppress the circulating currents, but also reduce the overall voltage fluctuation of sub-modules capacitors under different modulation ratios, and had almost no any adverse effect on the performance of MMC.

A simplified dual-stage model predictive controller for modular multilevel converters

The Modular Multi-Level Converter (MMC) topology is suitable for use in medium and high voltage applications. To cater to the multi-objective control requirements of the MMC, several Model-Predictive Control techniques have been developed. However, most of the available MPCs do not sufficiently address the simplification of the apparently complex switching logic of the MMC in regard to its practical implementation. To fill this gap, a modulated dual stage MPC is proposed here that controls the ac output current using a few simple steps, while maintaining sufficient control over the circulating current. The two MPC controller stages are used in a cascaded manner. The first stage calculates the voltage reference for grid current control and the second stage MPC selects the appropriate switching state to ensure circulating current control. The proposed technique uses carrier-based PD-PWM to retain its superior harmonic performance and ensures 2N+1 output pole voltage levels. While avoiding use of any cost function, a generalized fixed step approach is also presented that is independent of N value. The dual stage MPC presented in this paper is executed once in every switching time period to maintain a fixed switching frequency operation. Simulation and experimental studies show satisfactory performance of 25-level and 7-level MMC respectively, in a grid connected mode. Transient case study is also shown to demonstrate fast dynamic performance of the proposed controller.

Adaptive Balancing Strategy for Cell Capacitor Voltage in a Modular Multilevel Converter

Due to the modular structure and voltage scalable features, the Modular Multilevel Converter (MMC) has become an alternative converter for high and medium voltage-based transmission systems. Apart from this, MMC has edible features in controlling the real and reactive power for a high-voltage transmission system compared to the conventional converter. However, some of the technical challenges such as control complexity when subjected to more voltage levels, capacitor voltage balancing issues, increase in capacitor voltage ripples, DC fault handling, and circulating current affect the performance of MMC in various applications. In this paper, an adaptive balancing strategy is proposed for capacitor voltage balancing of each cell connected in the MMC. This approach balances the floating capacitor in a simplified manner and henceforth reduces the capacitor voltage ripples to the permissible limit. In addition, the circulating current present in the MMC circuit will be suppressed in comparison to the conventional method. The proposed strategy is investigated under various operating conditions to analyze the effective performance compared to the traditional technique. A comparative study of the proposed approach is presented with various parameters of MMC performed using MATLAB Simulink software.

High-Frequency Resonance Suppression Based on Unified MMC High-Frequency Impedance Model

High-frequency resonance (HFR) in the modular multilevel converter (MMC) has been reported in recent years frequently, yet, existing focuses on HFR are limited to the specific control strategy or grid condition, from which the conclusion may be not applicable if the control or grid changes. In this article, a unified MMC high-frequency impedance model (UHFIM) is proposed to fill this gap. By changing the gain of the model, UHFIM can reflect the high-frequency characteristics of the MMC under different control strategies in a unified form, which provides a theoretical basis for the general HFR suppression method. Analysis based on UHFIM reveals the worst cases of the HFR risk and the constraint of low-pass filter (LPF) order on the convergence of MMC high-frequency damping. In order to suppress HFR, this article proposes a joint design method for LPF and passive damper, which can completely remove the HFR risk from the entire high-frequency range without sacrificing too much dynamic performance of MMC, and it works no matter how the grid impedance varies. Besides, in the case that the damper cannot be installed, an alternative HFR suppression method based on delay time adjustment is also developed. Finally, the proposed unified model and the HFR suppression methods have been verified by simulation and experimental results.

A Trigger Signals Modification Scheme for Common-Mode Conducted EMI Reduction of Modular Multilevel Converter

Modular multilevel converter (MMC), due to its features of modularization, high efficiency, and so on, has gradually become the mainstream multilevel converter. However, it can induce large electromagnetic interference (EMI) because of the high-level <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dv/dt$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$di/dt$ </tex-math></inline-formula> during its commutation transient. Therefore, it is significant to analyze the characteristics of common-mode (CM) conducted EMI of MMC and propose a suppression scheme accordingly. In this article, considering the voltage to ground of submodule (SM) as a direct noise source, the relationship between CM current and equivalent noise source of one phase, as well as that between the equivalent noise source of one phase and switching action of each SM are deduced. Furthermore, a trigger signals modification (TSM) scheme is proposed, which can effectively suppress the CM conducted EMI on the dc side without obviously affecting the operating performance of MMC. In particular, this scheme manipulates the trigger signals already generated by the modulation and capacitor voltage balancing control, thus enabling good applicability. Finally, experimental results verify the validity of the analysis and the effectiveness of the proposed TSM scheme.

Small‐signal modelling considering internal perturbations for modular multilevel converters

Small-signal modelling of modular multilevel converters (MMC) has drawn lots of attention recently. Existing research mainly studies MMC terminal impedances by injecting corresponding AC-side and DC-side external small-signal perturbations. This paper focuses on the MMC internal small-signal perturbations, that is, perturbations in control signal and their effect. There are two kinds, that is, differential-mode and common-mode, of internal small-signal perturbations, which influence MMC external and internal state variables, respectively. Then, the existing MMC small-signal model is improved. With its help, accurate MMC control-signal to state-variables transfer functions are established, which can aid precise MMC controller analysis and performance optimization. In this paper, an analysis of MMC phase capacitor voltage regulation control is demonstrated as an example. The stability can be precisely ascertained based on the obtained transfer function. The effect of parameter variation of the proportional-integral controller is also studied. Moreover, an additional compensator is introduced in the existing control loop to attenuate resonance peaks in the transfer function, which mitigates the large voltage overshoot and makes the transient state much smoother. Simulation and experimental results support and well confirm the analysis of this paper.

Comparative Evaluation of Silicon and Silicon-Carbide Device-Based MMC and NPC Converter for Medium-Voltage Applications

The neutral-point-clamped (NPC) converter and the modular multilevel converter (MMC) are among the most popular converter topologies for medium- and high-voltage applications. This article investigates the impact of silicon-carbide (SiC) switches on the performance of MMC and NPC converters for medium-voltage (MV) applications. Furthermore, two different cases of the circulating current control schemes are considered for the MMC configuration. The experimental prototypes of three-level configurations for both of these topologies are developed to compare and analyze their performances. Furthermore, the double pulse test (DPT) setup is constructed to obtain the device loss characteristics of the switching devices. These characteristics are then used to compute the device losses using the derived loss models. Moreover, in order to generalize the results for wide power range of operation, two sets of devices are considered for the loss analysis of the considered converter configurations. The obtained results for conduction and switching losses are compared for both the converter configurations at various loading conditions. These results provide the operating range for the considered converters to achieve reduced converter losses. Finally, a comparative analysis for the selection of converter configuration based on the obtained results is presented.

A Novel Nearest Level Modulation Method with Increased Output Voltage Quality for Modular Multilevel Converter Topology

The modular multilevel converter (MMC) topology is gaining more interest because of its modular design, high efficiency, and scalable voltage levels in medium- and high-power industrial applications, where the nearest level modulation (NLM) method is frequently preferred. In this paper, a novel NLM method is proposed with increased output voltage quality for MMC topology. In conventional NLM (C-NLM), output voltage is obtained as N + 1 levels, where N is the number of submodules (SMs) per arm. The output voltage is increased to 2N + 1 levels by the proposed NLM method without using any additional SMs. The proposed NLM method is based on the offset term injection, which is optimally determined in terms of the best output performance of MMC. Also, trapezoidal reference signal is used instead of sinusoidal reference, which provides better output voltage quality and controls the modulation process. The proposed NLM method presents simple implementation as in the C-NLM, and it is implemented to the upper and lower arms of the MMC; then, arm voltages are successfully controlled. Furthermore, output voltage returns the value of zero in the C-NLM process for low-modulation-ratio applications in relatively small amount of SM usage of MMC design. However, the proposed NLM method gives promising results instead of zero voltage. In order to validate the superior performance of the proposed NLM method, a comparative study is presented with C-NLM and third-harmonic injected NLM method in terms of total harmonic distortion (THD) and magnitude of the output voltage and current. THD of output waveforms of MMC is significantly reduced, and DC voltage utilization is remarkably increased, thanks to the proposed NLM method. In addition, capacitor voltage balancing for the proposed NLM method is accomplished to keep the capacitor voltage of each SM of MMC constant. Simulation results are presented to verify the effectiveness of the proposed NLM method under various case studies. Finally, experimental validation is carried out using a field programmable gate array (FPGA)-based hardware implementation on the laboratory prototype to show the applicability of the proposed NLM method.

Control Design and Fault Handling Performance of MMC for MMC-Based DC Distribution System

Modular multilevel converters (MMCs) have emerged as a viable choice in future DC grid architectures due to their scalability to meet voltage level requirements. However, MMC-based DC distribution systems are at risk of short-term outages during the faults in either the DC or AC networks feeding the MMC, so it remains a challenge to accomplish AC and DC fault ride-through (FRT) capability in such applications. To ensure stable operations of the DC terminals, FRT strategies are required for the faults on both the AC and DC sides of the converter. This paper proposes a FRT strategy for the AC and DC side of the converter to ensure stable and economically viable operation of the DC distribution network. The asymmetrical faults in the upstream AC grids are managed by using the integrated energy of the MMC. Whereas, the DC FRT capability of the MMC is accomplished by changing the redundant submodules of the MMC to full-bridge submodules (FBSMs), thus allowing a DC FRT to be achieved by using DC circuit breakers that are low cost and reduced in size. Applying the proposed DC FRT strategy, which makes possible the use of low-cost and reduced in size DC circuit breakers in DC distribution, results in a reduction in the overall initial investments.

Analysis and Suppression of the Frequency-Aliasing Phenomenon Related to the Sorting-Algorithm-Based Voltage-Balance Strategy in an MMC System

The frequency-aliasing phenomenon, which can distort high-frequency harmonics in the analog input signal into low-frequency harmonics in the output digital signal, is widespread in the process of analog to digital conversions. The sorting-algorithm-based voltage-balance strategy is also such a conversion process. This strategy converts the analog-continuous-sinusoidal fundamental wave into the digital-discrete inserted-submodule number, deciding how many submodules are required to operate in modular multilevel converters (MMC). Thus, the frequency-aliasing phenomenon can occur in this strategy and deteriorate the performance of MMC as well. This article first proposes the mechanism of the frequency-aliasing phenomenon in the sorting-algorithm-based voltage-balance strategy. A quantitative analysis is then presented to indicate the root cause of the frequency-aliasing phenomenon in the voltage-balance strategy. This analysis also intuitively describes the frequency-aliasing phenomenon's effect, such as harmonic redistributions in the large-signal model, the phase delay in the small-signal model, and the associated deteriorations including the system stability's decrease and the power quality's reduction. Based on the above analysis, an optimal value design method is proposed to suppress the frequency-aliasing phenomenon by determining an appropriate voltage-balance frequency. A code restructuration method is further implemented to adapt the voltage-balance strategy to the optimal frequency and to avoid extra switching actions at the same time. The deteriorations caused by the frequency-aliasing phenomenon and the effectiveness of proposed methods are finally demonstrated via simulations experiments.

Design and Development of Experimental Based Phase Modulated Model Predictive Control for Torque Ripple Reduction of MMC Fed BLDC Motor

This article represents the torque ripple performance of modular multilevel converter (MMC) fed brushless dc (BLDC) motor using different current control technique. For reducing the ripple current in BLDC motor, a phase-modulated model predictive control (PMMPC) technique has been proposed. The stator ripple current is almost negligible using PMMPC. This PMMPC current control method is a significant minimization of torque ripple in BLDC motor. A comparative torque ripple behaviour of MMC fed BLDC motor has been done using phase-modulated model predictive control, model predictive control (MPC) and proportional integral (PI) control at different switching frequency. It has been observed that a PMMPC current control technique is more efficient as compared to the MPC as well as PI current control technique. It has also been observed that the torque ripple performance is improved while using PMMPC as compared to the MPC and PI controller. Simulation results have been verified with the help of experimental result and these results are obtained in good agreement to the simulated results.

Application of modified triangular injection technique on MMC to decrease ARM current

Abstract In the area of the converter, the main focus of research is going on to improve the performance of Modular Multilevel Converter (MMC). This paper discusses the triangular injection modulation structure as well as the capacitor voltage balancing approach of the submodule (SM) for MMC converters to maintain a minimum harmonic profile as well as to achieve a lower arm current. To identify the value of Total Harmonic Distortion (THD) and arm current as a minimum in comparison with some well-known modulation methods from literature, a converter structure was developed in MATLAB Simulink environment, and the simulation results showed that the value of arm current and THD profile of MMC in case of this method was lower in comparison with typical modulations. A small prototype of the converter driven by the triangular injection method has been presented to confirm this method's validity.

Switching Signals Based Condition Monitoring for Submodule Capacitors in Modular Multilevel Converters

The capacitors are one of the most important com- ponents in modular multilevel converter (MMC). Due to the large numbers of capacitors applied in MMC, the primary technical challenge about the capacitors is reliability. To identify the abnormal capacitors and improve the performance of MMCs, the capacitor condition monitoring strategy is proposed in this brief. In the monitoring algorithm, the relationship of the capacitances in an arm is deduced. The actual and nominal sums of switching signals for submodules (SM) are used to estimate the capacitances. Furthermore, a compensation for dead time is added to enhance the monitoring accuracy. The proposed condition monitoring algorithm not only simplifies the calculation of capacitances with high accuracy, but also has no adverse influence on the normal operation of MMCs. The feasibility of the proposed strategy is verified by the simulation results in MATLAB/Simulink.

A Review of Modular Multilevel Converters for Stationary Applications

A modular multilevel converter (MMC) is an advanced voltage source converter applicable to a wide range of medium and high-voltage applications. It has competitive advantages such as quality output performance, high modularity, simple scalability, and low voltage and current rating demand for the power switches. Remarkable studies have been carried out regarding its topology, control, and operation. The main purpose of this review is to present the current state of the art of the MMC technology and to offer a better understanding of its operation and control for stationary applications. In this study, the MMC configuration is presented regarding its conventional and advanced submodule (SM) and overall topologies. The mathematical modeling, output voltage, and current control under different grid conditions, submodule balancing control, circulating current control, and modulation methods are discussed to provide the state of the MMC technology. The challenges linked to the MMC are associated with submodule balancing control, circulating current control, control complexity, and transient performance. Advanced nonlinear and predictable control strategies are expected to improve the MMC control and performance in comparison with conventional control methods. Finally, the power losses associated with the advanced wide bandgap (WBG) power devices (such as SiC, GaN) are explored by using different modulation schemes and switching frequencies. The results indicate that although the phase-shifted carrier-based pulse width modulation (PSC-PWM) has higher power losses, it outputs a better quality voltage with lower total harmonic distortion (THD) in comparison with phase-disposition pulse width modulation (PD-PWM) and sampled average modulation pulse width modulation (SAM-PWM). In addition, WBG switches such as silicon carbide (SiC) and gallium nitride (GaN) devices have lower power losses and higher efficiency, especially at high switching frequency in the MMC applications.

Improved Indirect Model Predictive Control for Enhancing Dynamic Performance of Modular Multilevel Converter

Model predictive control has become a tremendously popular control method for power converters, notably a modular multilevel converter, owing to the ability to control various objectives at once with a particular cost function and prominent dynamic performance. However, the high number of submodules in cascaded control means that the model predictive control for the modular multilevel converter suffers from a computational burden. Several approaches focused on reducing the computational burden based on limiting the number of possible switching states (possible choices) to be evaluated at each sampling instant. The dynamic performance of the modular multilevel converter is degraded in a transient state, despite the reduced computational burden. This paper presents an improved indirect model predictive control method to reduce the computational burden and enhance the dynamic performance. The proposed approach considers the steady-state and transient state individually and applies a different range of choices for each specific case. The range of choices during the steady-state is limited in order to reduce the computational burden without deteriorating the output quality, whereas the number of choices will be increased during the transient state to guarantee dynamic performance. The results that were obtained by implementing an experiment on a laboratory setup of a single-phase modular multilevel converter are presented in order to verify the proposed approach’s effectiveness. From the experimental setup, the computational time in the proposed approach was reduced by about 75% when compared with the conventional indirect model predictive control, whereas keeping fast dynamic performance.

Discontinuous Space Vector Modulation Schemes for Modular Multilevel Converters

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.