Hybrid Modular Multilevel Converter Research Articles

Hierarchical voltage balancing method for MMC based on DCSM

This paper presents a hierarchical voltage balance control method for a hybrid modular multi-level converter based on half-bridge submodule and dual-capacitor submodule. This voltage balancing method reduces the control complexity of the existing voltage balancing method of the central controller but does not significantly increase the control complexity of the submodule controller. In dual-capacitor submodule, only some logic operations are added. Furthermore, the proposed method reduces the hardware requirements and communication data between the submodule and central controllers. The reduced hardware requirements refer to the hardware circuits used to implement communication functions. The proposed method is also applicable to multi-level converter, which only includes half-bridge submodules, but requires two adjacent half-bridge submodules to use the same submodule controller. Simulation and experimental results validate the feasibility and effectiveness of the proposed hierarchical voltage balance control.

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.

Research on shutdown control strategy of hybrid DC system

The hybrid DC system combined with line commutated converter (LCC) and hybrid modular multilevel converter (HY-MMC), which is named LCC-HY-MMC, has technical advantages and economy. Shutdown is a key problem that any DC system needs to face. In order to ensure that the DC system and the sub-module capacitors of HY-MMC can be discharged quickly during shutdown stage, a complete shutdown control strategy is proposed in this paper. From the characteristics of shutdown, it can be basically regarded as three stages: DC energy feedback stage, controllable dissipation stage of sub-module capacitor energy and uncontrollable dissipation stage of sub-module capacitor energy. Finally, based on the 200 MW/ 200 kV digital simulation model built in the electromagnetic transient simulation software PSCAD/EMTDC, the effectiveness of the proposed method is verified.

An Energy Absorbing Method for Hybrid MMCs to Avoid Full-Bridge Submodule Overvoltage During DC Fault Blocking

The full-bridge submodules (FB-SMs) in hybrid modular multilevel converters need to absorb the enormous energy stored in dc side and arm inductors during dc fault blocking, which may lead to severe overvoltage. An energy absorbing branch (EAB) composed of metal oxide varistor (MOV) and thyristors is proposed in this letter. No extra power loss is produced by the EAB during normal operation. The EAB can absorb a part of the energy and reduce the energy absorbed by FB-SMs suffering from severe overvoltage by clamping the dc voltage. Thus, the maximum FB-SM overvoltage is reduced. Turning off the EAB several milliseconds after the blocking of the converter can accelerate the decaying of the dc fault in the transmission line and reduce the required energy volume of MOV with minimal effect on the maximum FB-SM overvoltage. The proposed EAB shows better technical and economic performance than existing methods. The proposed EAB is validated by simulations and experiments.

Analysis and Design of the Split-Capacitor-Based Sub-Modules Equipped for Hybrid Modular Multilevel Converter

The hybrid modular multilevel converter (MMC) is always featured and profited by the merits of the equipped power valves. Referring to this, a novel hybrid MMC topology equipped with the split-capacitor-based sub-modules (SCSMs) on the AC side is proposed. It aims to increase the utilization of the DC bus voltage with DC fault blocking capability. Especially compared to the hybrid MMC equipped with the full-bridge-based sub-modules (FBSMs) on the AC side, smaller power losses can be achieved for the proposed hybrid MMC, due to the reason that only one semiconductor device of the SCSM is inserted into the current flow route. Structurally, the proposed converter mainly consists of the half-bridge-based sub-module (HBSM) stacks and SCSM stacks. The HBSMs located on the DC converter side of the proposed hybrid MMC are in charge of exchanging active powers, while the SCSMs located on the AC converter side are in charge of shaping the circuit waveforms. Additionally, profited by the specific structure of the SCSM, the DC fault current could be cut off by imposing inversed voltages collected from the SCSM capacitor voltages on the uncontrollable diodes of the IGBTs. For the deep study, a detailed mathematical model and modulation control of the proposed hybrid MMC are analyzed. In addition, an analysis of the balancing control for SCSMs is also provided. Finally, the simulation and experimental results are proposed to verify the effectiveness of the theoretical analysis.

Voltage balancing control of hybrid MMC under over-modulation situations with optimal circulating current injection

The hybrid modular multilevel converter (MMC), applied in high voltage direct current (HVDC) transmission systems, can operate in over-modulation mode to handle the abnormal situations of reduced HVDC voltage due to the extreme atmospheric environment. However, it might suffer from severe voltage imbalance between the half-bridge submodule (HBSM) and full-bridge submodule (FBSM) capacitors when the HVDC voltage is lower than certain limitation. The approach of injecting the second-order harmonic circulating current (SHCC) will be effective to solve this problem. In order to minimize the power losses caused by the SHCC injection, an approach of optimizing the SHCC reference is proposed in this paper to realize the voltage balancing. For the hybrid MMC under the over-modulation situations, the critical states of voltage balancing between the HBSM and FBSM capacitors are derived at the first time. Based on this, the voltage feedback control is put forward to acquire the optimal reference SHCC accurately and flexibly. The effectiveness of the proposed approach of optimal SHCC injection is verified by the simulation and experiment results. Compared with the existing approaches of SHCC injection, it demonstrates that the proposed approach can achieve the lowest power losses with reliable voltage balancing.

Modeling and Control Method for a Three-Level Hybrid Modular Multilevel Converter

The modular multilevel converter (MMC) is one of the most favored topologies for medium and high voltage applications. However, the drawbacks, such as lots of capacitors and doubled number of devices, limit the power density. Recently proposed hybrid multilevel converters with high voltage switches and chain-links have shown the superior performance compared with the traditional MMC. Specifically, this article focuses on an emerging three-level hybrid modular multilevel converter, which can reduce half of submodule number. Based on the operating principle, a decoupling average model is derived to reveal the internal power relationship between the three-level stacks and chain-links. It can be proven the former plays a role to transfer a part of power from dc side to ac side directly. Therefore, the energy variation of chain-link is smaller, and the dc-link capacitance could be reduced. Besides, an improved trapezoidal arm current allocation is employed to solve the arm current distortion at the voltage zero-crossing point. In order to guarantee the arm current tracking performance, the variable sampling repetitive control based current loop designs are proposed. Finally, the experimental results of a 400-V prototype validates the proposed modeling and control methods.

Analysis and Design of the Energy Storage Requirement of Hybrid Modular Multilevel Converters Using Numerical Integration and Iterative Solution

Increasing the modulation index by utilizing the negative voltage states of full-bridge submodules (FBSMs) can greatly reduce capacitor usage of modular multilevel converters (MMCs), thereby optimizing the cost and volume. The hybrid MMC is composed of half-bridge submodules (HBSMs) and FBSMs, and the capacitor voltages of the two types of submodules (SMs) have different shapes as long as negative voltage states exist. This condition greatly complicates the analysis and design of the energy storage requirement of the hybrid MMC, which utilizes the negative voltage states of FBSMs to boost the AC voltage. A numerical calculation method for solving the capacitor voltages and designing the capacitances of FBSMs and HBSMs is proposed in order to accurately determine the minimum energy storage requirement considering the difference between the energy variations in FBSMs and HBSMs. In the numerical calculation, the energy storage and voltage of the arm are decomposed into FBSM and HBSM parts. According to the physical switching process, the output voltages of FBSM and HBSM parts are determined separately. The one-cycle waveforms of the capacitor voltages are then obtained by numerical integration of the power flows in FBSM and HBSM parts. An iterative solution procedure and the termination criterion that can ensure the accuracy of the obtained one-cycle waveforms are also proposed. Using the numerical integration and iterative solution procedure as the kernel algorithm, the proposed method can accurately analyze the capacitor voltages of the FBSMs and HBSMs and determine the minimum energy storage requirement of the hybrid MMC. Furthermore, the proposed method is applicable for various operating working conditions and various proportions of FBSMs. The simulation results verify the feasibility and accuracy of the analysis and design method.

Autonomous Control Based on Capacitor Energy Storage of Converter for DC Distribution System

The converter valve is the core equipment of the DC distribution systems. This paper proposes an autonomous control strategy for grid-connected and islanded operation of hybrid topology modular multilevel converter (MMC). The overall control structure is designed, including the outer loop for autonomous control and the inner current loop. Firstly, for the outer loop, it is proposed to use the equivalent capacitance storage energy of MMC power module to simulate the inertia of synchronous generator rotor to achieve the purpose of buffer power fluctuation. So, the outer loop provides system inertia. Second, the inner loop is designed for current tracking control and suppresses transient fault current effectively. It is proposed that the current inner loop reference value can be directly calculated through the grid-connected impedance matrix and voltage across it. And the small signal model of autonomous control for a hybrid MMC is established. Then the influence of the proposed control structure and control parameters on the impedance characteristics of the system is analyzed in detail. Finally, a hybrid MMC simulation model for DC distribution system is built based on PSCAD, simulating island operation, grid-connected operation mode and transient faults respectively for verifying the proposed strategies.

Real-Time Simulation of Hybrid Three-Level and Modular Multilevel Converter Based on Complete Equivalent Model for High Voltage Direct Current Transmission System

Real-Time Simulation of Hybrid Three-Level and Modular Multilevel Converter Based on Complete Equivalent Model for High Voltage Direct Current Transmission System

Comprehensive small-signal stability analysis for master hybrid MMC connected with strong and weak AC systems

The small-signal stability problem of power electronic converters has always been a critical concern for safe operation. This paper conducts a comprehensive analysis of the small-signal stability for the hybrid modular multilevel converter (MMC), which is promising in the application of future over-head-line high-voltage-direct-current transmission. The impacts of the strong and weak AC system connection are comparatively researched through eigenvalue-based analysis. The newly emerging factors that hamper the stability are identified for the weak AC system integration. The work in this paper provides valuable guidance for robust control parameter configuration of the hybrid MMC.

Hybrid Modular Multilevel Converter With Self-Balancing Structure

In the hybrid modular multilevel converter (MMC), the voltage imbalance between the capacitors of half-bridge submodule (HBSM) and full-bridge submodule (FBSM) would occur when the over-modulation index exceeds certain range, which is caused by the different charging/discharging properties of the HBSM and FBSM. An improved hybrid MMC with self-balancing branch (SBB) is proposed in this article to avoid the voltage imbalance under over-modulation situations. The SBBs could offer additional energy transmitting channel for each bypassed HBSM at the interval of negative arm voltage. Thus, the HBSMs could possess the same energy charging/discharging properties as the FBSMs, and then the voltage imbalance between the HBSM and FBSM capacitors could be avoided under any over-modulation indexes. The operation mechanism of the proposed SBB is revealed in detail, according to which the component parameter in the SBB could be selected. The effectiveness of the proposed hybrid MMC with SBBs is verified by the simulation and experimental results. Also, it shows that the proposed hybrid MMC could realize the voltage balancing under over-modulation situations with lower energy losses according to the comparisons with other MMC topologies.

MVDC Railway Traction Power Systems; State-of-the Art, Opportunities, and Challenges

Advances in voltage-source converters (VSCs), as well as their successful application in VSC-HVDC systems, have motivated growing interests and research in medium-voltage direct current (MVDC) traction power systems (TPSs) for high-speed rail (HSR) applications. As an emerging power-converter-based infrastructure, this study reviewed developments that shape two key evolving pieces of equipment—namely, high-power traction substation (TSS) converters, and power electronic transformers (PETs)—for MVDC TPS as well as prospects for smart grid (SG) applications in the future. It can be deduced that cost-effective and robust high-power TSS converters are available from hybrid modular multilevel converters (MMCs) for enhanced performance and fault-tolerance capability. In addition, silicon carbide (SiC) MMC-based PETs with input-series-output-parallel (ISOP) configuration are present for greater weight/size reduction and efficiency for MVDC rolling stock design. Finally, the implementation of a smart MVDC TPS incorporating a sophisticated railway energy management system (REM-S) based on the smart grid principles is feasible in the future, with numerous benefits. However, there are related challenges, like knowledge gaps on these technologies, the high costs involved, and lack of standardization to overcome to realize widespread future commercial deployment.

A smooth startup strategy for hybrid MMC based HVDC systems

Hybrid modular multilevel converter (HMMC) topology consisting of full-bridge submodules (FBSMs) and half-bridge submodules (HBSMs) is one of the promising technical solutions for long-distance high voltage direct current (HVDC) power transmission systems. However, the HMMC has very different charging circuit characteristics from the conventional half-bridge MMC (HB-MMC) topology and suffers from low and unbalanced precharge capacitor voltages and surge current when starting from the AC-side, due to its hybrid submodule configuration. In this paper, an AC-side startup scheme for the HMMC is proposed, including an additional semi-controlled charging process and an improved controlled charging method, which allows the HMMC to smoothly charge its submodule capacitors and establish the DC-link voltage without requirements of auxiliary sources. The improved controlled charging method is composed of a group modulation and a group voltage balancing control strategies, which enables the HMMC to operate stably with enhanced AC voltage output capability under the condition of low and unbalanced capacitor voltage in the HBSM group and FBSM group, thereby eliminating converter overmodulation and surge current during the startup procedures. In addition, DC-side startup strategy is also discussed in this paper. PSCAD/EMTDC simulation results based on a practical ±200 kV/750 MW HMMC-HVDC system demonstrate the effectiveness of the proposed method.

MMC-MTDC transmission system with partially hybrid branches

This paper proposes a hybrid submodule modular multilevel converter (MMC) topology which is suitable for multi terminal direct current (MTDC) transmission systems. Each arm of the proposed MMC topology consists of a half-bridge submodule (HBSM) branch and two parallel full-bridge submodule (FBSM) branches. Comparing with the conventional MTDC transmission system, the proposed topology can selectively block the DC fault current and isolate the corresponding fault line without expensive DC circuit breakers (DCCBs). Thus, the influence range of the DC fault can be reduced and the reliability of the power supply can be improved as well. The corresponding modulation and voltage balancing strategies are developed for the proposed hybrid MMC topology. The feasibility of the proposed topology and control strategy is verified in the MATLAB/ Simulink simulation.

A Six-Arm Symmetrical Six-Phase Hybrid Modular Multilevel Converter With Unidirectional Current Full-Bridge Submodules

Multiphase converter complexity is one of the concerns that restrict the widespread utilization of multiphase machines. One of the developmental trends to alleviate the converter complexity is reducing passive and active component counts. In this regard, several examples have been reported in the literature for modular multilevel converter (MMC) topologies with a reduced component count for six-phase medium-voltage motor drives. A three-leg nine-arm MMC (9A-MMC) with half-bridge submodules has been proposed with a 25% reduction in the number of employed components, compared to the standard six-leg twelve-arm MMC, however with limited output voltage magnitude. To enhance the dc-link voltage utilization, hybrid designs of the 9A-MMC have been recently suggested with a combination of half-bridge and bidirectional/unidirectional current full-bridge submodules (FB-SMs) in each leg. For further structural reduction, this article proposes a two-leg six-arm hybrid MMC with half-bridge and unidirectional current FB-SMs for symmetrical six-phase machines. The proposed architecture achieves a further reduction in component count, reducing the implementation complexity without compromising the delivered power. Detailed illustrations, analysis, and operational concepts of the proposed topology are presented. To quantify its pros and cons, the proposed topology is assessed versus other existing alternatives in the literature. Simulation and experimental results are presented to validate the concept and effectiveness of the proposed architecture.

Single-end fault identification scheme for multi-terminal DC grid based on amplitude similarity of injection signal

Effective fault identification scheme plays an important role in the fault isolation stage and system recovery stage of multi-terminal DC grid. Therefore, an active detection based single-end DC fault identification scheme is proposed to improve the reliability and sensitivity of fault identification. The control strategy is suggested of the generation of sinusoidal signals at different frequencies by the hybrid modular multilevel converters. Based on the blocking effect of the fault branch on the injection signal, by introducing the improved correlation discrimination algorithm, a single-end fault identification scheme is proposed by using the amplitude difference between the measured and calculated values of injection signals. The proposed scheme can be used for fault isolation and adaptive system recovery. Intensive simulation studies prove the improved sensitivity, and reliability of the proposed scheme for the full length of transmission line without relying on DC boundary. Moreover, the scheme can identify the permanent fault during the system recovery stage.

Optimal Cell Utilization for Improved Power Rating and Reliability in a Grid-Scale Three-Phase Battery Energy Storage System Using Hybrid Modular Multilevel Converter Topology Without Redundant Cells

Redundant cells within a battery energy storage system (BESS) are an important aspect to be considered in a BESS planning, design, and operation in order to achieve high reliability. The conventional topology does not have the ability to take advantage of its idle cells, at least one-third of its cells are constantly idling in a typical three-phase BESS. This article presents a new topology for a three-phase grid-scale BESS, where it offers three ways to deal with its idle cells. First, the idle cells were eliminated from the BESS plant while still achieving the desired output power with reduced cost. Second, the idle cells were utilized where no redundant cell is required. Finally, they were utilized to meet the output power required during peak demand. Consequently, the overall construction cost of BESS plant, losses, size, and control complexity are reduced while the reliability is improved. The simulation results indicate that at least 140 out of 333 modules (3996 cells) are in an idle state during the BESS operation, with the three-phase output voltage achieved in both topologies. Using 333 modules, the three-phase output power has increased by 58.8% in the proposed topology (306 kW) compared with the conventional topology (180 kW).

Design and Simulation of Modular Multilevel Converter Fed Induction Motor Drive

Traditional modular multilevel converter (MMC) applications in medium voltage induction motor drive are difficult, particularly at low speeds because of the higher magnitude of the voltage ripple of the sub-module capacitor. This paper uses a hybrid MMC, particularly at low frequencies, to achieve a lower peak-to-peak voltage ripple of the sub-module capacitor. The vector control strategy with the closed-loop speed control indicates an accurate and wide-speed range. MATLAB / Simulink is used to simulate and obtain the simulation results of hybrid and traditional MMC with induction motor drive and compare from the standpoint of capacitor voltage ripple. The results are shown the reduction of peak-to-peak voltage ripple of the sub-module capacitor as the hybrid MMC is operated.

Design and Simulation of Modular Multilevel Converter Fed Induction Motor Drive

Traditional modular multilevel converter (MMC) applications in medium voltage induction motor drive are difficult, particularly at low speeds because of the higher magnitude of the voltage ripple of the sub-module capacitor. This paper uses a hybrid MMC, particularly at low frequencies, to achieve a lower peak-to-peak voltage ripple of the sub-module capacitor. The vector control strategy with the closed-loop speed control indicates an accurate and wide-speed range. MATLAB / Simulink is used to simulate and obtain the simulation results of hybrid and traditional MMC with induction motor drive and compare from the standpoint of capacitor voltage ripple. The results are shown the reduction of peak-to-peak voltage ripple of the sub-module capacitor as the hybrid MMC is operated.