Conventional Modular Multilevel Converter Research Articles

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.

An Enhanced Series-Connected Offshore Wind Farm (SC-OWF) System Considering Fault Resiliency

The series-connected offshore wind farm (SC-OWF) is a promising offshore wind generation solution to mitigate the need of centralized offshore high-voltage/power converter stations. Predominantly, researchers have focused on the steady-state operation and control of SC-OWFs, without considering the system-level characteristics and ability to ride-through dc side and ac network faults. This paper proposes an enhanced system for SC-OWF applications with fault-resilient capability, where comprehensive circuit configuration and protection strategies are articulated to minimize the negative effects caused by various types of dc and ac faults. For the offshore wind farm architecture, a grouping scheme is adopted where a substation based on disconnectors and diodes is proposed to realize prompt fault bypass/isolation and protection functions in the event of offshore system faults. Additionally, an onshore fault-tolerant modular multilevel converter (MMC) with modified dc-system-oriented control is employed to enable smooth and secure operation under steady-state and fault conditions. The proposed SC-OWF system is quantitatively substantiated by time-domain simulations where four ac/dc fault cases are considered, and the results consolidate the feasibility of the proposed configuration and control, indicating fault resilience of the SC-OWF system. Additionally, size, weight and cost estimations of the proposed offshore substation are presented and compared to a conventional MMC offshore station, to further highlight the merits of the proposed solution.

A modified modular multilevel converter to reduce the second order ripples in the submodule capacitor voltage: Design and analysis

SummaryThe modular‐multilevel converter (MMC) faces the problem of second order ripples in the sub‐module DC capacitor voltage during the low frequency operation at the load side. To sort out this problem, this paper demonstrates a three‐phase modified MMC that includes a novel circuit placed between upper and lower arms of each phase, where load is connected through proposed circuit. The aim of the proposed circuit is to reduce the magnitude of the ripples in each sub‐module's DC capacitor voltage; moreover, it also uses a less number of components compared with the conventional circuits. In order to keep the capacitor voltage at its rated value and to prevent the development of common mode voltage, the proposed circuit uses the high frequency‐based power transfer control channel for balancing power between the upper or top and lower or bottom arms. Furthermore, the mathematical analysis, operating principle, and proposed control is explained in detail for the conventional MMC and modified MMC. The simulation results have been obtained for the modified MMC with resistive load by using MATLAB and Simulink. An experimental test results have been verified with simulation results for the modified MMC topology.

A systematic review of Intelligent Fault-Tolerant Protection Scheme for Multi-terminal HVDC Grids


 
 
 Due to the advancement of power electronics devices and control techniques, the modular multilevel converter (MMC) has become the most attractive converter for multiterminal direct current (MTDC) grids thanks to its most relevant features, such as modularity and scalability. Despite their advantages, conventional MMCs face a major challenge with: i) fault-tolerant operation strategy; i) energy losses in conversion; iii) lack of DC fault handling capability. This paper provides a systematic review to identify the gaps in the literature about Intelligent Fault-Tolerant Protection Schemes for multi-terminal HVDC grids. Through the bibliometric analysis, it was possible to identify topics still to be developed within the four main clusters (Offshore wind farms, Wind turbines, Voltage Source Converters, and Wind power). The research topic opens three research paths: the first is the analysis of failures in HVDC (High Voltage Direct Current) grid equipment by the FDD (Fault Detection and Diagnosis) method; the second is failure analysis by the IFDD (Inverse Fault Detection and Diagnosis) method and the third is the possibility of interconnecting the different energy generation zones with different frequencies.
 
 

A hybrid arm‐multiplexing MMC for DC fault ride‐through without blocking

AbstractConsidering both the DC fault ride‐through (FRT) and lightweight requirements of modular multilevel converter (MMC), this paper proposes a hybrid arm‐multiplexing modular multilevel converter (HAM‐MMC). The phase leg of the proposed MMC topology is divided into upper, middle, and lower arms, configured with full‐bridge submodules (FBSMs), half‐bridge submodules (HBSMs), and FBSMs, respectively. The time‐division multiplexing of middle arms between upper and lower arms is achieved by introducing arm selection switches, which considerably enhances the submodule utilization and thus minimizes the submodule number. The structure and zero voltage switching strategy of arm selection switches are presented, and a modified sorting algorithm is developed for capacitor voltage balance. The negative level output capability of FBSMs in upper and lower arms allows the DC voltage to be reduced in response to DC faults. Compared with the conventional hybrid MMC composed of HBSMs and FBSMs in a 1:1 ratio, the HAM‐MMC has the same power quality and DC FRT capability, but uses 25% fewer capacitors for lightweight. Simulation results demonstrate that the proposed HAM‐MMC can smoothly ride through DC voltage dip without interrupting power transmission, and can also rapidly resume normal operation after a zero‐voltage disturbance.

Shared Redundancy Strategy to Improve the Reliability and Fault-Tolerant Capability of Modular Multilevel Converter

Utilizing redundant sub-modules is a well-known strategy to improve the reliability and fault-tolerant capability of the modular multilevel converter (MMC). Redundancy-based fault-tolerant strategy makes it possible to restore the full performance of the converter in the post-fault condition. The main drawback of this strategy is the high cost. In this configuration, each redundant sub-module (RSM) can only be utilized in a specific arm and it cannot be used in other arms. This limitation leads to inefficient utilization of the RSMs in conventional MMC. This paper proposes a shared redundancy strategy which allows RSMs to be used either in upper or lower arms. The derived reliability models in various case studies illustrate that proposed strategy significantly improves the reliability and useful lifetime of MMC. To investigate the effectiveness of the proposed strategy, simulation and experimental results are provided.

A Flying Capacitor Hybrid Modular Multilevel Converter With Reduced Number of Submodules and Power Losses

This paper presents a novel flying capacitor type hybrid modular multilevel converter (FC-HMMC) as an alternative to the traditional modular multilevel converter (MMC) for medium voltage (MV) applications. Combining the high voltage line-frequency switches with the low voltage high frequency submodule (SM) based chain-link, the proposed hybrid solution can balance the benefits from conventional multilevel converter and MMC. Compared to the HB-MMC, FC-HMMC can save 50% SMs, 25% devices, 37% capacitor and 20% conduction losses, hence improving the power density and power efficiency a lot. Besides, the dc and ac side is not coupled for this topology, which means the modulation index is not fixed. Another advantage is that the high voltage switch operates in soft-switching naturally at any modulation index or power factor, thereby reducing the power losses. Based on this characteristic, a unidirectional FC type hybrid modular multilevel rectifier (FC-HMMR) with only HV diodes is proposed as well. Finally, the feasibility of the proposed topology is validated by both simulation and experiments.

Low-Cost and Compact Asymmetrical Unidirectional-Current Modular Multilevel Converters

Although the modular multilevel converters (MMCs) that contain submodules (SMs) with negative voltage capability have various advantages, they usually demand many more semiconductors than the conventional half-bridge SM-based MMC (HB-MMC). This study proposes a unidirectional-current clamp-double submodule (UC-CDSM) by combining two unidirectional-current full-bridge SMs (UC-FBSMs) using a shared switching device. The sharing design enables the UC-CDSM-based MMC (UC-CD-MMC) to have 25% fewer switching devices compared with the UC-FBSM-based MMC. The quantity of switching devices is rather similar to that in a conventional HB-MMC while the UC-CD-MMC still retains the advantages, such as low capacitor usage, dc fault clearing capability, and wide-range dc voltage adjustability. Moreover, a unidirectional-current hybrid MMC composed of UC-CDSMs and UC-FBSMs (UC-HYB-MMC) is presented to further enlarge the adjustable range of dc voltage. Detailed comparisons indicate that the UC-CD- and UC-HYB-MMCs can reduce the valve costs by 32% and 25%, respectively, and volumes by 39% and 34%, respectively, compared with HB-MMCs. Simulation and experimental results verify the steady-state and dc-fault clearance of the proposed topologies, and that the capacitor voltages in the UC-FBSMs and UC-CDSMs are well maintained and balanced in the UC-HYB-MMC.

An Isolated Single Input-Multiple Output DC–DC Modular Multilevel Converter for Fast Electric Vehicle Charging

This article proposes an isolated single input-multiple output (SIMO) dc–dc modular multilevel converter (MMC) for fast charging of multiple electric vehicles (EVs). The proposed topology provides galvanic isolation and facilitates a high-voltage conversion ratio from medium voltage direct current (MVDC) to LVDC (EV battery voltage levels) via intermediate medium-frequency transformer. The proposed topology, as compared to conventional MMC, has reduced number of arms, semiconductor devices and has inherent dc fault blocking capability. Operation of the converter with medium frequency reduces the size of passive components present in it. The output voltage waveshape of the converter results in nearly sinusoidal wave being impressed across transformer's primary with very less <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv/dt</i> . Analysis of the converter in steady state and during fault conditions have been included in this article. The proposed converter is validated in simulation model and scale down laboratory prototype.

Diode rectifier‐based hybrid high‐voltage direct current converter for offshore wind farms

This paper presents a diode rectifier (DR)-based hybrid converter topology for high-voltage direct current (HVDC) transmission system with offshore wind farms (OWFs). The hybrid converter consists of a DR and an auxiliary converter. A high conversion ratio ac/dc converter is designed as the auxiliary converter, which can form the offshore grid voltage for OWFs to achieve black-start. Under steady state, all the active power output from OWFs is delivered through the DR. Meanwhile, the auxiliary converter is able to compensate the reactive power and harmonic currents of the system. Comparing with the conventional modular multilevel converter (MMC), the number of submodules (SM) of the proposed hybrid converter is significantly reduced, which decreases the construction cost and size of the system. The topology and operation principle of the hybrid HVDC converter are introduced. Then the corresponding control strategy is designed to achieve energy balance and power transmission. Based on the mathematic model, a design method for the parameters of the auxiliary converter is proposed. Finally, the feasibility of the proposed hybrid HVDC converter is verified by both the simulation and hardware-in-the-loop experiment results.

A Multiport Embedded DC Power Flow Controller for Meshed DC Distribution Grids

Ring and meshed dc distribution grids have attracted increasing interest lately. Their multipoint and multiterminal structure facilitates flexible operational control and reliable power supply across the network. However, the dc grids implemented with existing power converters lack sufficient control freedoms for proper power flow adjustment; therefore, it is necessary to develop additional dc power flow controllers (DCPFCs) to be added to these grids for improved power flow control capability. Specific to multiterminal dc networks built with modular multilevel converters (MMCs), this article proposes an embedded DCPFC (e-DCPFC) directly integrated in the MMC topology. Unlike previously proposed solutions, the e-DCPFC configuration is derived from the conventional MMC and does not need any isolation transformers or external power supply. Its other main features include modular construction, convenience for multiport extension and bidirectional power flow adjustment capability. In this article, the topology, working principle, and the coordinated control strategy for e-DCPFC and its host MMC are elaborated. Experimental results from a lab prototype verify the proposed topology and its performance under typical working conditions.

Control of a Waveshaper-MMC With Thyristor-Based Front-End Converter for Open-End Winding Variable Speed Medium-Voltage Induction Motor Drive

For variable frequency induction motor drive application, conventional modular multilevel converter (MMC) faces some issues, like high capacitor voltage ripple at low frequency, high circulating current, and high component count. This article proposes a novel configuration to address these issues, where a six-pulse thyristor-based controlled rectifier is combined with a waveshaper-based MMC (WS–MMC). Each phase of the converter consists of a WS and director valve, and then the phases are connected in series to create a common dc-link. The use of a thyristor-based controlled rectifier as a front-end converter increases reliability and reduces component count to a large extent. Unlike conventional MMC, circulating current does not exist in this converter, and the capacitor voltage ripple does not depend on the operating frequency. As a consequence, the current rating of the switching devices is reduced for the same power output, and capacitor voltage ripple at low frequency stays within the specified limit. Besides this, the component count of the proposed configuration is far less than the MMC-based topologies available so far. The operation principle, relevant control techniques, and analysis of the converter are discussed in detail in this article. The complete system is simulated in MATLAB/SIMULINK, and the performance of the control methods has been validated at different operating frequencies with various load torques. The results show satisfactory performance.

Impedance Modeling and Analysis of Series-Connected Modular Multilevel Converter (MMC) and its Comparative Study With Conventional MMC for HVDC Applications

Similar to the conventional three-phase parallel-connected MMC (PC-MMC), the three-phase series-connected MMC (SC-MMC) is a strong potential solution for high-voltage direct current (HVDC) applications. The study of SC-MMC focuses on the control strategy, neglecting its small-signal dynamics, not to mention its stability when interacting with the AC grid. Work on small-signal modeling of MMC is limited to parallel-connected configurations. SC-MMC differs from PC-MMC in structure and harmonic content; hence, it is necessary to establish an accurate model of SC-MMC according to its own characteristics. This paper introduces the circuit and control of SC-MMC and compares it with PC-MMC. Multi-harmonic linearization is used to establish the AC impedance model of SC-MMC over the whole control system, including the third harmonic suppression controller. We take a PC-MMC station of the Wudongde HVDC project as a benchmark to explore the potential application of SC-MMC when constructing new HVDC stations to extract power from an existing DC transmission corridor. The strengths and weaknesses of SC-MMC in HVDC applications are discussed, and it is compared to PC-MMC with regard to the impedance characteristic.

PV/Battery Grid Integration Using a Modular Multilevel Isolated SEPIC-Based Converter

Photovoltaic (PV) plants can be built rapidly when compared with other conventional electrical plants; hence, they are a competent candidate for supplying the electricity grid. The output power of the PV modules can be used in plug-in electric vehicles (PEVs) DC charging stations to reduce the burden on the electricity grid, particularly during peak load hours. To integrate PV modules and electric vehicles (EVs) with the electricity grid, the modular multilevel converters (MMCs) topologies producing staircase voltage waveforms are preferred as they are able to deliver less total harmonic distortion (THD) and higher efficiency in addition to lower voltage stress on semiconductor switches. In conventional centralized MMC topologies, a direct connection to a high-DC-link input voltage is required which is not appropriate for PV plants. A new MMC topology for PV/EV/grid integration is proposed in this paper, where the individual PV arrays are directly connected to each phase of the AC grid to harvest the maximum available power point. A current-source converter (CSC) based on a single-stage isolated SEPIC converter is adopted as the submodule (SM) for the proposed MMC topology given its outstanding features, such as low input ripple current, high efficiency, high power factor, and flexible output voltage higher or lower than the input voltage. The single-stage SMs can operate in both DC/DC and DC/AC operating modes. Proper controllers for each mode of operation are designed and applied to supply constant current from either the PV modules or the battery cells by eliminating the second-order harmonic component. The performance of the proposed converter is verified by simulations and a downscaled prototype controlled by TMSF28335 DSP.

Overmodulation Operation of Hybrid Modular Multilevel Converter With Reduced Energy Storage Requirement

The hybrid modular multilevel converter (MMC) has the advantages of dc fault blocking and overmodulation capability as the promising candidate in future multiport HVdc systems. Especially, the overmodulation ability could increase the modulation index higher than 1 without voltage distortions on the ac side. However, the hybrid MMC under the overmodulation operations could suffer from the voltage deviations between the half-bridge submodule (HBSM) and full-bridge submodule (FBSM) capacitors. In this article, the structure of the self-balancing branches (SBBs) is introduced into the hybrid MMC to eliminate the aforementioned voltage deviations. As a result, the energy storage requirement (ESR) of the hybrid MMC under the overmodulation operations could be greatly reduced compared with the conventional hybrid MMC without SBBs. Besides, the voltage balancing strategy is improved to avoid the impulse current in the conduction path of the SBB, by which the current stress of the power devices could be decreased significantly. The simulations and experiments are carried out to verify the effectiveness of the proposed hybrid MMC. It shows that the ESR of the hybrid MMC could be reduced by 30% with decreased power losses under the overmodulation index 1.5 compared with the conventional hybrid MMC.

Optimization Scheme Based on High-Frequency Link Interconnection of Submodules

Submodule (SM) capacitor voltage ripple and imbalance are inherent problems with conventional modular multilevel converter (MMC). In addition, the large size capacitance in SM reduces the power density of MMC, and the life defect of electrolytic capacitor also decreases reliability of the converter. This article proposes a modified switched-capacitors based MMC (SC-MMC) topology with ripple-power decoupling channels. Without relying on any closed-loop control, the natural elimination of SMs ripple-power and automatic balance of SMs voltage can be realized simultaneously, which greatly increase the power density and simplify the control design of system. In this article, the ripple characteristic of the conventional MMC and the constraint on capacitance are studied, and an SMs ripple-power decoupling method is analyzed. The power characteristics and impedance constraint analysis of the switched-capacitors are described in detail and the configuration assessments including volume, cost, and efficiency of the SC-MMC are also analyzed. Finally, a down-scaled experimental platform is built, and the experimental results verify the correctness and effectiveness of the proposed scheme.

Three-Phase Modular Multilevel Converter With Optimized Capacitor Sizing for Low-Voltage Applications

High capacitance requirement is a major concern of modular multilevel converters (MMCs) when applied to medium- or low-voltage systems. This article presents the design procedure and development of a modified MMC topology for low-voltage applications. With respect to the conventional MMC topologies, the proposed structure is designed and optimized to have a relatively low capacitance requirement. The switching states, modulation, control, and design procedure of the different parameters, including submodule capacitor and arm inductor, are investigated in this article. Moreover, the control scheme is presented to achieve regulation of the output voltage and the circulating current. The outcomes of this study are verified by simulation and experimental results.

Arm current reversal‐based modular multilevel DC‐DC converter

Nowadays, most of the converters used in high-power high-voltage (HV) applications are the conventional modular multilevel converters (MMC). However, in the case of DC-DC conversion, an imbalance of the capacitor voltages occurs and the conventional MMC fails to operate correctly. This paper introduces an arm current reversal-based modular multilevel DC-DC converter, which successfully provides balance among the capacitor voltages while operating in DC-DC conversion. The proposed configuration is used in medium voltage DC grids to feed DC loads or to interconnect between two DC grids of different voltage levels. The proposed converter is a two-stage DC-DC modular converter, which consists of a single-phase half-bridge MMC with half-bridge submodules (HBMMC) followed by a single-phase H-bridge MMC with half-bridge submodules (SMs). The operational concept of the proposed converter is based on reversing the arm current direction and reversing the output terminals with the help of the H-bridge MMC stage, which ensures the same direction of the voltage at the load terminals. The proposed converter provides a high conversion ratio, bidirectional power flow, simple architecture, and a simple control scheme. Detailed illustrations, analysis, and design of the proposed converter are presented. Besides, MATLAB-based and Opal RT-based simulation results and experimental results are presented to validate the proposed configuration claims.

A Hybrid Modular Multilevel Converter Family With Higher Power Density and Efficiency

Modular multilevel converter (MMC) has many advantages of scalability, reliability and easy implementation, but suffers from large number of switching devices and capacitors. To improve the power density and efficiency for medium voltage applications, this article introduces a hybrid modular multilevel converter (HMMC) family through the combination of active neutral-point-clamped converter and chain-link arm. Three topologies in HMMC family can effectively reduce chain-link voltage by half compared with traditional MMC. Single- and three-phase operation principles are presented to highlight this important features. Several critical metrics, such as component number, capacitor sizes, and semiconductor losses, are compared between the conventional MMC and HMMC topologies at three different medium voltage levels. The result indicates the best topology from the HMMC family can save around 30% devices, 50% total capacitor and 32% power losses compared with traditional MMC. A simple single-phase control method is also mentioned to balance capacitor voltage. Finally, a single-phase HMMC experimental prototype is built to verify the effectiveness of three topologies.

Modular Multilevel Converter With Embedded Energy Storage for Bidirectional Fault Isolation

Modular multilevel converter (MMC) has been widely used in the multi-terminal overhead line high-voltage direct current (HVDC) system due to its outstanding performance. However, the AC side and the DC side of MMC-HVDC have a strong interaction and will be affected by each other&#x0027;s faults, which brings the risk of system instability. In this paper, a modified MMC named active MMC with embedded energy storage in submodules (SMs) is proposed to isolate the impact of faults as a firewall. Firstly, the topologies of the active MMC and SMs with energy storage are introduced. Then the operation principle of the active MMC under different faults is analyzed in a multi-terminal MMC-HVDC system. Thirdly, the complete control scheme of the active MMC is proposed for bidirectional fault isolation. Finally, two main application scenarios of the active MMC are simulated in a three-terminal MMC-HVDC system and a particularly designed AC&#x002F;DC hybrid transmission system respectively in PSCAD&#x002F;EMTDC. The simulation waveforms of the active MMC are compared with the conventional MMC. Simulation results show that the active MMC can effectively protect the AC system and DC system from each other&#x0027;s faults.