Capacitor Voltage Fluctuation Research Articles

Design of main parameter of modular multilevel converter applied in power grid simulator

Abstract In the context of application in a power grid simulator, we investigated the parameter design issues of several crucial components of the Modular Multilevel Converter (MMC) topology applied to the power grid simulator system. These components include the bridge arm inductance and submodule capacitance. Based on the fundamental circuit of the MMC applied to the power grid simulator system, we introduced an equal discharge time constant H. We analyzed the capacitor voltage fluctuation rate and the equivalent capacitance discharge time constant “H”, discovering that they still exhibit an inversely proportional relationship across different engineering scenarios. Consequently, a general method for determining submodule capacitance parameters was proposed, and in conjunction with previous engineering parameters, recommended values for an equal discharge time constant of 40 ms were provided. Furthermore, we introduced the concepts of double-frequency circulating resonant angular frequency and phase resonant angular frequency, outlining the principles for determining the bridge arm reactance values. Through the analysis of corresponding parameters in previous engineering studies, universally applicable recommended values for the bridge arm reactance parameters were proposed, considering the unit resonant angular frequency as the power grid frequency angular frequency 1.0ω.

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.

Synergizing Battery-Powered Permanent Magnet Synchronous Motor Control with Integrated Modular Multilevel Converter Systems

This proposed work presents a comprehensive examination of a modular multilevel converter (MMC) combined with a permanent magnet synchronous motor (PMSM) for battery energy management in applications. In order to provide effective energy transfer and dynamic control, the Modular Multilevel Converter serves as a bridge between the battery system and the Permanent Magnet Synchronous Motor drive. By cleverly controlling energy flow and minimizing losses during charging and discharging cycles, the combination of MMC with PMSM seeks to increase energy efficiency and prolong battery life. Since the energy source provides all of the power for the motor, it runs at a constant pace. When the accelerating mode finishes, the energy storage receives just enough power to maintain a steady voltage. The energy storage system utilizes the majority of the renewable power generated by the electric motor when it is in deceleration mode to raise the voltage in the system. During the subsequent acceleration mode, the stored energy might be released the MMC-BESS, which combines a modular multilevel converter with a battery energy storage system, is gaining popularity due to its high adaptability and dependability. This system integrates batteries into a sub module. Sub-modular capacitor voltage fluctuations are caused by the significant reactive power that is generated through the workings of modular multilevel converters. To reduce the fluctuation, large capacitance capacitors is required, which would significantly increase the system's capacity. The proposed approach means to upgrade energy proficiency and execution in electric vehicle applications. By joining the benefits of battery power with the flexibility of MMC frameworks, the arrangement offers upgraded control accuracy, decreased misfortunes, and further developed adaptation to non-critical failure. This combination works with consistent power the executives, guaranteeing ideal use of assets while meeting severe execution prerequisites. Through exhaustive examination and recreation, the adequacy of this incorporated framework is illustrated, displaying propelling the field of electric vehicle impetus and power conversion potential.

Suppression of the Capacitor Voltage ripple in Modular Multilevel Converter for Variable-Speed Drive Applications

The modular multilevel converters (MMC) utilization has brought about a transformative impact on high voltage direct current (HVDC) transmission relying on voltage-sourced converters (VSC). However, their application in medium-voltage (MV) variable-speed motor drives has not achieved broad acceptance due to the substantial voltage fluctuation in the capacitor, especially at lower frequencies. The present study introduces a hybrid MMC (HMMC) aimed at markedly limiting capacitor voltage fluctuations, particularly during low motor speeds. Vector control was used to achieve the required motor speed. The proposed HMMC validity was confirmed through the MATLAB/ SIMULINK environment. The results were compared with conventional MMC from the standpoint of the capacitor voltage fluctuation at low frequencies.

Comprehensive Analysis and Optimal Control for Capacitor Voltage Fluctuation Reduction in Hybrid MMCs Under Different Over-Modulation Conditions

Comprehensive Analysis and Optimal Control for Capacitor Voltage Fluctuation Reduction in Hybrid MMCs Under Different Over-Modulation Conditions

Low-frequency operation control method for medium-voltage high-capacity FC-MMC type frequency converter

The application of modular multilevel converters (MMCs) to large drive systems is subject to severe low-frequency operation restrictions. The fluctuation of capacitor voltage and the high amplitude of common mode voltage in sub-modules is a thorny problem. In this paper, a novel fly-across capacitor modular multilevel converter topology is adopted to eliminate low-frequency voltage ripples by using the fly-across capacitor as a power transfer channel between the upper and lower bridge arms of the MMC. A novel finite compensation method is proposed—instead of the traditional full compensation method—that introduces a real-time variable limiting factor to change the amplitude of the mixed injected high-frequency differential-mode voltage and high-frequency differential-mode current and reduce the amplitude of the common-mode voltage on the AC side while lowering the current stress of the power devices. Finally, a complete system simulation model is constructed, and the topology with the proposed control strategy are verified to have good output characteristics under different operating conditions; good results are achieved in suppressing the sub-module fluctuation and common-mode voltage.

A selection principle of submodule switching state vectors for switching frequency reduction in voltage self‐balancing half‐bridge modular multilevel converters

AbstractThe problem of submodule switching frequency reduction in half‐bridge modular multilevel converters (HB‐MMCs) with capacitor voltage self‐balancing control is considered and explored in this paper. A selection principle of submodule switching state vectors is proposed based on the voltage self‐balancing switching state matrix, aiming to lower submodule switching frequency and device losses. The relationship between system stability and submodule switching signals is revealed according to the capacitor voltage self‐balancing characteristics, and the full‐rank constraints on the voltage self‐balancing switching state matrix are proposed. Considering the tradeoff between switching loss and capacitor voltage fluctuation, the evaluation indexes of voltage self‐balancing control effect are determined. The selection principle of submodule switching state vectors and the optimized construction method of switching state matrix are presented. Voltage self‐balancing HB‐MMC models are built in MATLAB/Simulink, and it is verified that the submodule switching state vector selection principle proposed in this study can effectively reduce switching frequency while meeting the evaluation requirements of practical engineering projects, so as to achieve the balance between switching loss and steady‐state capacitor voltage fluctuation.

A novel capacitor voltage fluctuation suppression method for the low-frequency operating stage of MMC driven pumped storage system

The pumped storage power plant, which has the energy conversion and storage capacity, is an important solution for improving the power grid absorption ability to the unstable and fluctuating new energy from wind farms, as well as photovoltaic power stations. The modular multilevel converter (MMC) is a competitive topology for driving the pumped storage motor due to the advantages of high output power level and low output current harmonics. However, the voltages of the sub-module capacitors fluctuate largely when the MMC is operating at low-frequency stage, which would lead to an abnormal start-up process or an unstable low-speed running state of the variable speed pumped storage system. In order to suppress the low-frequency voltage fluctuation of the MMC sub-module capacitors, this paper proposes a charge channel topology including in-phase and inter-phase channel between the sub-modules for transferring the capacitor charge. Then, the reverse fluctuating fundamental component and third harmonic circulating component, as well as the synchronizing fluctuating second harmonic component, of the sub-module capacitor voltages could be counteracted, respectively. Eventually, the start-up operation performance, as well as the low-speed operation performance of the variable speed pumped storage system are enhanced greatly. The effectiveness of the proposed low-frequency capacitor voltage fluctuation suppression method is verified through simulation results.

Port reduction operation of a three‐port nonagonal modular multilevel converter as soft open point

SummaryNonagonal modular multilevel converter (MMC) is a three‐port AC/AC converter promising as a soft open point (SOP) to connect different AC feeders or buses in distribution networks, owing to its flexible power control, transformerless structure, and low power component count and its capability of accommodating multiple AC ports of different frequencies. An SOP should be able to work under port reduction condition, which is often encountered when maintenance is required or the network structure is reconfigured. However, as a component reuse converter of circular structure, nonagonal MMC cannot turn off one or some of its branches to achieve this. To address this issue, a control scheme is proposed in this paper to enable nonagonal MMC to operate under port reduction condition by switching the nonagonal MMC to a hexagonal MMC, which is an AC/AC topology of a similar circular structure but has only two ports. Without inserting extra switches, the nine branches of nonagonal MMC is redistributed into six branches. By alternately operating the submodules, no submodules need to be shut down, reducing the submodule capacitor voltage fluctuation during port reduction operation. RTlab results verify the feasibility of the quasi‐hexagonal topology and the effectiveness of port reduction control.

Predictive Control of Modular Multilevel Converters: Adaptive Hybrid Framework for Circulating Current and Capacitor Voltage Fluctuation Suppression

Modular multilevel converters (MMCs) are widely used in voltage-sourced, converter-based high-voltage DC systems due to their modular design, scalability, and fault tolerance capabilities. In MMCs, multi-variable control objectives can be employed by using model predictive control (MPC) due to its fast dynamic response and ease of implementation. Nonetheless, conventional MPC techniques for MMCs have shortcomings, including high computational requirements, poor circulating current, and capacitor voltage fluctuation suppression. First, this study proposes an adaptive MPC technique that adapts the number of candidate combinations to the steady and transient states, significantly reducing the computational burden. Second, an improved hybrid combination of an MPC with a proportional-resonance (PR) controller enhances the circulating current and capacitor voltage fluctuation suppression performance. According to the phase difference between the circulating current and the capacitor voltage, the circulating current and capacitor voltage can be suppressed at different times by changing the circulating current reference of the PR controller. The switching frequency can be reduced by using the PR controller’s output to adjust the input submodule number instead of changing the duty cycle. The proposed techniques were validated by simulations and experimental case studies with a three-phase grid-connected MMC.

Integrated Control and Performance Analysis of High-Speed Medium-Voltage Drive Using Modular Multilevel Converter

The modular multilevel converter (MMC) is highly attractive to develop high-speed medium-voltage variable frequency drives (MV VFDs) due to its cascaded design, high-quality output, and strong reliability. However, high-speed MV VFDs have demanding requirements in capacitor voltage stabilization, current regulation, dynamic response, etc. The instability and distorted voltage/current could easily occur if MMCs are used as high-speed MV VFDs. This paper presents a high-speed VFD with SiC-based MMC feeding a permanent magnet synchronous machine. A new integrated control method is specifically proposed to address the aforementioned issues covering the low-/middle-/high-speed range. Performance analysis is conducted in terms of efficiency, power density, output current quality, and speed range. Steady-state and dynamic conditions are evaluated by a full-scale simulation model and a high-power SiC-based MMC testbed. Results show that the proposed method effectively removes the large capacitor voltage fluctuations, resonant circulating current, and saturated output voltage from zero to the highest speed. Compared with existing MMC VFDs of limited speed (< 4000 rpm), the SiC-based MMC VFD has three times higher speed (15000 rpm), improved efficiency (99.43%), high power density, and low current harmonic distortions.

Optimal Selection among Various Three-Phase Four-Wire Back-to-Back (BTB) Converters with Comparative Analysis for Wave Energy Converters

Wave energy converters are attracting attention as an energy source that can respond to climate change. In order to increase the energy efficiency of the wave energy converters, efficient power converters are also required. The efficient converters require operation at a low switching frequency, which increases the weight and volume of the passive components. Therefore, in this paper, the performance of various types of topologies is compared to select the optimal power converter for wave energy converters. In order to cope with the unbalanced operation and unbalanced load of renewable energy, in this paper, the topology of the four-leg type is analyzed centrally. In addition, the analysis was performed by applying the model predictive control that can quickly respond to the rapid energy change of wave energy. In addition, model predictive control was applied to the four-leg converter analyzed in this paper because it is suitable for application to atypical topologies. For performance analysis of various types of topology, the loss and efficiency of each converter were analyzed by applying a loss analysis model, and output current harmonics and leakage current characteristics, capacitor voltage fluctuation rate, etc., were additionally analyzed at various switching frequencies. In conclusion, the three-level four-leg converter showed up to 2.28% and 2.7% higher efficiency under balanced and unbalanced operating conditions.

MMC circulating current suppression strategy based on improved quasi-PR control

The internal circulation of modularized multilevel converter will affect waveform quality and increase system loss. In this paper, the basic principle and topological structure of MMC as well as the causes of the internal circulation are briefly analyzed. A method of harmonic component extraction based on the second-order generalized integrator and the suppression strategy of MMC circulation based on feedforward compensation quasi-proportional resonant control are proposed. The feasibility of this strategy is verified in MATLAB/Simulink, and the simulation results show that the circulation and capacitor voltage fluctuation are controlled within a reasonable range.

Cascaded Modular Multilevel Converter and Cycloconverter Based Machine Drive System

Low-speed drive is one of the challenges for modular multilevel converters (MMCs) due to large capacitor voltage fluctuation. In this article, a cascaded MMC and cycloconverter (CCV) based machine drive system is proposed to ensure stable operation of medium-voltage and low-speed machine. The MMC provides medium-frequency ac voltage for the CCV, and the CCV converts the medium-frequency ac input to low-frequency ac output required by the machine. Detailed analysis about MMC's operation frequency, device current stress, and submodule (SM) capacitance are given in this article. Proposed drive system can operate at zero/low frequency under rated load torque, while the SM capacitance is much smaller than that in existing methods. Simulation and experimental studies are conducted, and the results verify the effectiveness of the proposed system.

A modified submodule of modular multilevel converter using active power decoupling method for reducing capacitor voltage ripple under low‐frequency operation

AbstractIn this paper, a modified submodule (MSM) method is proposed to reduce capacitor voltage fluctuation by integrating a bidirectional buck‐boost type active power decoupling (APD) circuit into each SM for the low‐frequency operation of a converter system. The capacitor voltage fluctuation of the proposed MSM‐MMC can be significantly reduced compared to conventional SMs with the same capacitance by transferring the low‐frequency ripple power components to the auxiliary energy storage capacitor, resulting in a lower voltage fluctuation of the main capacitor in the SM, which requires a lower total SM capacitance. Additionally, a closed‐loop controller is proposed to control the APD capacitor voltages by inheriting the advantages of the repetitive controller and proportional‐integral (PI) controller to enhance the performance of SM capacitor voltage ripple suppression. The method can improve the start‐up performance of medium voltage drives without injecting high‐frequency components, which can avoid the common‐mode voltage imposed on the AC side of the system. The operating principle, auxiliary capacitor voltage control, and capacitor voltage ripple suppression control of the proposed MSM‐MMC method are analyzed in detail. The feasibility of the proposed method has been confirmed by simulation results based on case studies using MATLAB/SIMULINK software and further verified with hardware‐in‐the‐loop simulation (HILS) using the real‐time simulator OPAL‐RT platform.

Novel Voltage-Balance Control of Four-Level Hybrid-Clamped Converter With Open-Loop Optimized Common-Mode Voltage Injection

The four-level hybrid-clamped converter (HCC) is a new high-efficiency dc–ac converter. However, its dc-link capacitors face challenging voltage fluctuation issues at low frequencies, resulting in the distorted ac current and oversize converter volume. This letter presents a simple and effective control method of the HCC using open-loop common-mode voltage (CMV) injection. The mathematical model of the CMV and the capacitor voltage fluctuations is derived. The optimal CMV algorithm with the maximum suppression effect of capacitor voltage fluctuations is proposed as well as the implementation method. The microlevel mechanism of the proposed method in suppressing voltage fluctuation is clarified. The capability of the proposed method in improving dc voltage utilization is explored. Compared with existing methods, the proposed one reduces dc-link voltage sensors and computation complexity and mitigates CMVs at the load side. The optimal CMV is approximately 25% of the output voltage magnitude, and enhances the dc-bus voltage utilization by 12%. Results demonstrate that voltage fluctuations of HCC capacitors are reduced by over 60%. The proposed method has a solid dynamic balancing capability and works well for the wide-range frequency and modulation index.

Modified Carrier-Overlapped PWM With Balanced Capacitors and Eliminated Dead-Time Spikes for Four-Level NNPC Converters Under Low Frequency

Due to the lack of sufficient redundant states, four-level nested neutral-point clamped (4L-NNPC) converters suffer from severe flying capacitor (FC) voltage fluctuations under low fundamental frequencies, which restricts their applications in practice. To deal with this issue, a modified carrier-overlapped pulsewidth modulation (PWM) (COPWM) is proposed in this article, which could achieve zero average current flow through the two FCs over each carrier period. As a result, their voltages can be balanced naturally under steady states theoretically, and the low-frequency FC voltage fluctuation issue would no longer exist. In order to compensate for any nonideal factors in real industrial applications, an active control by dynamically adjusting the correspondence between carriers and switches is further applied for better control performance. A dead-time compensation scheme is proposed accompanied by this modified COPWM to eliminate the unexpected dead-time-induced voltage spikes of 4L-NNPC converters. The effectiveness of the proposed modified COPWM in regulating the FCs under low frequency and eliminating the dead-time-induced voltage spikes is verified by both simulation and experimental results.

Three-Level SOC Equalization Control Strategy for MMC-BESS Based on Feedforward Sliding Window Integral Method

The modular multilevel converter of the battery energy storage system (MMC-BESS) not only is suitable for the large-scale energy storage and dispatching of AC and DC grids, but also has a strong ability to suppress power fluctuations caused by the new energy output or grid failures. When an asymmetric voltage or a sudden change in DC load occurs in the AC grid, in order to compensate for the power difference between AC and DC sides, the energy storage submodule of the MMC-BESS will have a large unbalanced charging and discharging current, destroying the equalization state of SOC and seriously affecting energy storage capacity utilization and battery service life. In order to deal with the above problems, in this paper, the characteristics of the power difference between the MMC-BESS phase unit and the upper and lower bridge arms are analyzed. It is found that when considering the fluctuation of the submodule capacitor voltage, the phase unit power has the fundamental frequency AC circulating current component, and the power difference between the upper and lower bridge arms has the DC circulating current component. Therefore, the three-level SOC equalization correction control strategy is proposed based on interphase, upper and lower bridge arms, and submodules, and the feedforward sliding window integral method is introduced into the SOC equalization correction control layer of upper and lower bridge arms, so as to achieve the purpose of more balanced and accurate power distribution among phases and among upper and lower bridge arms of each phase. The simulation results show that the MMC-BESS has effective compensation ability when there is a large power difference in the AC and DC power grid. Under the unbalanced working conditions of the three-phase power grid, the three-phase AC current can quickly reach equalization, and the total harmonic content is 1.38%, and the unbalance degree is 2.4%. Under the same operating conditions, compared with the traditional optimal one-third average method, the SOC equalization correction control strategy proposed in this paper has smaller submodule capacitor voltage fluctuation rate, harmonic distortion rate, and three-phase system circulating current. And the SOC of each phase has a faster equalization convergence speed.

Optimal Second-Harmonic Current Injection for Capacitor Voltage Fluctuation Reduction in Hybrid MMCs Under Grid-Side SLG Faults

The second-harmonic injection strategy for modular multilevel converters (MMCs) can effectively suppress the capacitor voltage fluctuation in steady states. However, the capacitor voltage fluctuation under grid-side single-line-to-ground (SLG) faults will be more severe. Moreover, there is no research on the mitigation of capacitor voltage fluctuation under grid-side SLG faults for the hybrid MMC with mixed half-bridge and full-bridge sub-modules. To address this issue for hybrid MMCs, an optimal second-harmonic current injection scheme for mitigating the capacitor voltage fluctuation in the hybrid MMC under grid-side SLG faults is proposed in this paper. The mathematical models of arm power fluctuations of the hybrid MMC are firstly developed under both steady states and grid-side SLG faults. Then, the principle and optimal strategy of the proposed capacitor voltage fluctuation strategy are presented. The capacitor voltage fluctuation under grid-side SLG faults can be reduced by suppressing the three-phase arm power fluctuations. A hybrid MMC model with the proposed scheme has been established in PSCAD/EMTDC to verify its effectiveness and feasibility. Simulation results show that the average voltage fluctuation reduction rate can reach 55.77% with the proposed strategy.

A Hierarchical Small-Signal Controller Stability Analysis Method for the MMCs

Differing from a two-level voltage source converter (TL-VSC), the internal dynamics of a modular multilevel converter (MMC) are much more complicated. Since the fluctuation of submodule capacitor voltage is unavoidable when transmitting power, the MMCs output voltages cannot track the references perfectly. The interaction between the time-varying capacitor dynamics and controllers may lead to self-instability of the MMCs. In order to address this self-instability issue, this paper first establishes the small-signal closed loop transfer function matrices (TFMs) of the external and internal control loop by the harmonic linearization method. Further, a novel design principle of MMC controllers is proposed. Then, the critical impact factors on the stability of circulating current controller and AC voltage controller are investigated based on the generalized Nyquist criterion(GNC). The proposed models and analysis methods are validated by time-domain simulations in PSCAD/EMTDC.