Half-bridge Submodules Research Articles

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.

Design and Implementation of a High-Power DC/DC Converter for HVDC Interconnections With Wide DC Voltage Range and Fault-Blocking Capability

Highly efficient and lightweight high-voltage high-power dc/dc converter is one of the key equipments needed by future HVDC grid. In order to satisfy the technical requirements of interconnecting HVDC lines with different voltage levels, this article develops a thyristor-submodule hybrid dc/dc converter (TSDC) by combining the merits of low cost, low conduction loss, and high robustness of thyristors with the high controllability of the cascaded half-bridge submodules (SMs). The proposed converter presents the attractive features of efficient dc/dc conversion, reliable turn-on and turn-off of the thyristors, wide dc voltage operation range, and above all, the dc fault-blocking capability which avoids the installation of dc circuit breakers. The circuit structure, operating principle, and control schemes are discussed. The validity and effectiveness of the TSDC are verified by 150-MW, 100-kV/300-kV simulation case study as well as the experimental tests from a 4.5-kW, 300-V/450-V scaled-down prototype.

A High-Power Step-Up DC/DC Converter Dedicated to DC Offshore Wind Farms

In this letter, a high-power step-up dc/dc converter dedicated to dc offshore wind farms is proposed, with the combination of half-bridge submodules, thyristors, diodes, and transformer. Compared with the other promising dc/dc topologies, the device numbers, costs, losses, and energy storage requirement are greatly reduced. The operation principle is introduced. The effectiveness and validity are confirmed by simulation and experimental results.

Basic Topology, Modeling and Evaluation of a T-Type Hybrid DC Breaker for HVDC Grid

Hybrid circuit breaker (HCB) has shown good performance for interrupting dc fault current. However, a massive number of IGBTs in series is needed. These will lead to high costs and significant difficulty in balancing IGBT dynamic voltage. This paper proposes a bidirectional hybrid circuit breaker based on a cascaded half-bridge submodule structure to reduce the overall costs and implementation difficulty of HCB. The number of IGBTs is reduced by 50%, and the operating speed is fast enough. A model of a four-terminal modular multilevel converter (MMC) system with a proposed hybrid circuit breaker in different stages is developed to describe the fault blocking process more clearly. The performance of the proposed HCB is validated and compared with other HCB structure through both theoretical derivation and time-domain simulation.

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.

A smooth startup strategy for hybrid MMC based HVDC systems

• A semi-controlled charging method is presented for firing IGBTs. • A group modulation method for HMMC with unbalanced voltages is proposed. • An improved AC-side startup method is proposed to suppress the inrush current. 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.

Hybrid Modular Multilevel Converter for Variable DC Link Voltage Operation

Modular Multilevel Converter (MMC) secured its place in applications where high reliability, efficiency and effortless voltage/power scalability are of paramount importance. Without compromising these benefits, this paper proposes a novel control and appropriate design approach for Hybrid-MMC topology, comprising a mix of Full-Bridge and HalfBridge submodules in converter branches, as a grid-side stage of a back-to-back MMC, enabling operation at variable DC link voltage and arbitrary power factor. In the midst of transition to Renewable Energy Sources-dominated power systems, variable DC link voltage operation opens door to the use of Hybrid-MMC in retrofit of large Pumped Hydro Storage Plants to variable speed operation, where existing machines would not tolerate high common-mode voltage stress imposed by fixed-DC-link-voltage operated machine-side MMC stage. In this way, highly flexible grid-scale energy storage use of existing hydro capacities is enabled. Converter design approach and additional control layers have been developed and verified through a set of test scenarios. Sizing and operation at typical operating points are compared to equally-rated Full-Bridge-based MMC. Improvements over the existing Hybrid-MMC solutions include unity-power-factor operation at the entire attainable DC link voltage range and equal loading of upper and lower phase-leg branches regardless of the operating poin.

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.

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 Review of Multilevel Converters With Parallel Connectivity

Cascaded-bridge converters (CBCs) and modular multilevel converters (MMCs) enjoy growing popularity mostly due to modularity and scalability. Conventionally, their submodules allow only serial and bypass operation so that the use of low-voltage components for high-voltage output becomes possible. Dually, submodule parallelization adds the switched-capacitor behavior to CBCs/MMCs and has witnessed an upward trend in recent years. The salient advantages of parallel operation comprise sensorless voltage balancing, capacitance saving, current sharing, and system efficiency optimization. To capture the advancement in the field, this article reviews the state-of-the-art multilevel converters with parallel connectivity, covering various submodules, macrolevel circuit topologies, implementation challenges, and solutions, as well as control and optimization schemes. In particular, this article derives and classifies submodules as well as macro-level topologies according to basic H-bridge, asymmetrical half-bridge, and symmetrical half-bridge submodules. On top of that, this article introduces strategies for the simplification of submodules and the creation of novel topologies yet maintaining parallel connectivity. We highlight the role of graph theory in creating new analytic and synthetic methodologies for multilevel converters. In addition, this article discloses the relationship between multilevel converters with parallel connectivity and switched-capacitor converters.

Failure Rate and Economic Cost Analysis of Clamped-Single Submodule with DC Short Current Protection for High Voltage Direct Current System

Clamped-single submodule (CSSM) has DC short circuit current protection function to improve the safety and stability of high voltage, direct current (HVDC) system. In order to carry out the protection, it needs an additional number of insulated gate bipolar transistors (IGBTs) and diodes compared to the conventional half-bridge submodule (HBSM). In general, the failure rate tends to increase in proportion to the number of circuit components. Also, complex operation of the submodule may increase the failure rate, so accurate reliability analysis considering these points is required to apply CSSM in a practical HVDC system. We estimate the failure rate and the mean time between failures (MTBF) of CSSM using a fault tree. Fault-tree analysis (FTA) is possible to analyze the failure rate more accurately than the prior part count failure analysis (PCA) that considers only the number of parts, the type of parts, and the connection status of each circuit component. To provide guidelines for submodule selection under various conditions, we compare the economic cost of a CSSM with HBSM, FBSM, and clamped-double submodule (CDSM), and analyze the failure rate according to the voltage margin of the parts.

Experimental Validation of a Quasi-Z-Source Modular Multilevel Converter With DC-Fault Blocking Capability

This article considers the design methodology and the modulation of the quasi-Z-source modular multilevel converter (qZS-MMC) with half-bridge submodules (HBSMs) and evaluates its performance in the voltage boosting mode for medium voltage applications. The qZS-MMC consists of two qZS networks inserted between the two terminals of the dc input source and the dc-link terminals of an MMC, which allows the generation of an output voltage larger than the input dc voltage. Two modulation schemes have been analyzed based on a mathematical derivation for the converter internal voltages, currents, and stored energy. The qZS circuit is proven to provide the qZS-MMC with HBSMs to deal with dc faults. The experimental results validate the performance of the proposed modulation schemes and the dc-fault blocking capability of the qZS MMC. Finally, the losses of the qZS-MMC are compared against a standard MMC using full-bridge submodules that can also provide dc-fault capability. The range in which the qZS-MMC is more efficient has been identified. Furthermore, the qZS-MMC can provide a significant reduction in a number of semiconductor power devices with the same performance.

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.

Continuous operation of LVDC source/load under DC faults in MMC-DC distribution systems

• Reveal the mechanism of power imbalance of DC/DC converter under MVDC fault. • Analyze the LVDC voltage quality of DC/DC converter under MVDC fault. • Discuss the continuous operation capability of LVDC sources/loads under MVDC fault. • Propose a controllable storage unit to solve the non-continuous operating problem. With penetration and usage of renewable sources and loads based on power electronics increasing, the modular multi-level converter based direct current (MMC-DC) distribution system is getting broad attention. In MMC-DC system, DC/DC converters are employed to link the low voltage DC (LVDC) source/load and the medium-voltage DC (MVDC) distribution system. DC fault ride through capability at MVDC side is one of the technical difficulties, and has been the focus of previous studies. However, whether LVDC source/load could operate continuously under MVDC faults is still an open question. In this paper, key facilities, protection procedures and operating characteristics of LVDC source/load in two typical MMC-DC frameworks based on half bridge submodules (HB-MMC-DC) and full bridge submodules (FB-MMC-DC) are studied respectively. Analyses show LVDC source/load could operate continuously in HB-MMC-DC system with the help of DCCBs, but with voltage variation exceeding acceptable range, they fail to operate continuously in FB-MMC-DC systems. Therefore, a controllable storage unit is deployed at the LVDC side of the DC/DC converters to suppress voltage variation. The working principle and parameter design of the proposed solution are presented. Case studies are carried out in PSCAD/EMTDC for verification. Finally, comprehensive comparisons between the two MMC-DC systems are conducted.

Reduction of MMC Capacitances Through Parallelization of Symmetrical Half-Bridge Submodules

Modular multilevel converters (MMCs) enjoy the benefits of modularity and scalability. In particular, MMCs with symmetrical half-bridge submodules enable bipolar operation yet with a simple structure and low conduction losses. However, bulky dc capacitors in MMC submodules act as one major obstacle that retards the further improvement of system size, weight, and cost performance. Additionally, tight regulation and balance of dc capacitor voltages necessitate expensive voltage sensors paired with dedicated voltage controllers. This article proposes an effective hardware-based strategy that achieves MMC capacitance reduction through parallelization of symmetrical half-bridge submodules. On top of capacitance saving, the proposed strategy balances capacitor voltages in a sensorless fashion, which translates into the removal of voltage sensors and great simplification of control efforts. In addition, the proposed strategy allows fault-tolerant operation of MMCs. Finally, simulation and experimental results validate the effectiveness of the proposed strategy in capacitance reduction and capacitor voltage balancing of MMCs as static compensators.

Life-Cycle Expectation Using Fault-Tree Analysis for Improved Hybrid Submodule in HVDC System

An improved hybrid submodule employs a direct current (DC) short current protection function to improve the reliability of a high-voltage direct current (HVDC) system. However, it increases the number of circuit components to implement the protection. So, we need to evaluate the relationship between the protection function and the increased number of circuit components to assess whether the improved hybrid submodule (IHSM) is suitable to practical application or not from the viewpoint of reliability. Although conventional part count failure analysis considers the type and the number of parts, it cannot reflect the operational characteristics of the submodule. To overcome this problem, we design a fault tree that reflects the operational characteristics of IHSM and calculates the failure rate by using MIL-HDBK-217F. By part count failure analysis (PCA) and fault-tree analysis (FTA), we prove the high reliability of IHSM compared to half-bridge, full-bridge, and clamped-double submodules.

Experimental Validation of a Nested Control System to Balance the Cell Capacitor Voltages in Hybrid MMCs

In a hybrid modular multilevel converter (MMC), capacitor voltage balance between the Full-Bridge Sub-Modules (FBSMs) and Half-Bridge Sub-Modules (HBSMs) is only possible when the arm currents are bipolar. For a grid-connected MMC, operating at unity power factor, this is typically only achievable when the modulation index is less than 2. Previous control methodologies, based on open-loop feed-forward compensating currents, have been proposed to operate an MMC with a higher modulation index. However, these solutions do not minimize the compensating currents; they cannot compensate entirely for both the variations in the operating conditions and the parameters typically encountered in a real implementation; and they do not consider the actual capacitor voltage imbalance between the FBSM and HBSMs. In this paper, a new nested closed-loop control algorithm based on an outer voltage control loop with an inner current loop is proposed and experimentally validated. Feed-forward currents are still utilised in the inner loop, but they are calculated using a new optimising algorithm which minimises the required compensating currents. Moreover, to the best of our knowledge, this is the first work where explicit algebraic equations to calculate these compensating currents are provided. Experimental results to validate the approach, obtained with an 18-cell hybrid MMC, are presented and discussed in the paper.

Modified Nearest Level Modulation for Full-Bridge Based HVDC MMC in Real-Time Hardware-in-Loop Setup

Modular Multilevel Converter (MMC) is an emerging converter topology for medium and high voltage applications. Nearest level Modulation (NLM) is the conventional control topology used to control the MMC that produces the N+1 AC output waveform. In previous research work, the Modified NLM has been already proposed, producing a 2N+1 and 4N+1 output waveform while utilizing a half-bridge (HB) submodule (SM) topology. However, half-bridge-based MMC has a similar behavior as two-level Voltage Source Converter (VSC) and cannot block DC fault current in case of DC-side short circuit fault. So, in recent years, full-bridge-based MMC topology is preferably used by manufacturers as it has DC fault blocking capabilities. This paper presents the Modified NLM for Full bridge (FB) SM topology to take the critical benefits of FB SM topology and improve power quality. The proposed method is simpler to implement and produces a 4N+1 AC output waveform. The THD of the output voltage and current reduces to half compared to the conventional NLM method. The proposed method is verified using LabVIEW Multisim co-simulation and as well as real-time simulation.

Development of a Modular Educational Kit for Research and Teaching on Power Electronics and Multilevel Converters

This paper presents the detailed project of a modular educational kit to be used in research and teaching activities in the area of power electronics - with emphasis on applications in medium voltage multilevel converters. The half bridge topology has been chosen for the submodules, as they can be arranged together in a multitude of other topologies. A system composed by a mother board and daughter cards is presented. A DSP control card and a FPGA System-On-Module are inserted in the mother board in order to control the half bridge submodules. The targets of this project are applications in medium voltage - hence, the communication between the submodules and the FPGA/DSP is performed through optical fibers. Also a self power system is presented for the submodules, in order for them to operate at floating potentials. Experimental results have been presented for the most usual topologies of converters, including a Modular Multilevel Converter operating at 2000V.

Hybrid Modular Multilevel Converter Design and Control for Variable Speed Pumped Hydro Storage Plants

Hybrid modular multilevel converter control and design for pumped hydro storage plants is presented, addressing application-specific shortcoming of conventional back-to-back half-bridge modular multilevel converter: high common-mode-voltage machine stress. Common-mode-voltage-free operation in the entire or partial frequency range is enabled by variable DC link voltage control, through introduction of minimal full-bridge submodule share in hybrid active front-end converter stage. The developed generalized converter design approach for arbitrary DC link voltage range operation and additional internal energy balancing control layers enable down-to-zero DC voltage control. The results are verified through high-fidelity switched-model simulations of 6 kV converter, with 10/6 ratio of full-bridge to half-bridge submodules in active front-end. The analyzed hybrid active front-end stage benefits from lower converter losses for equal machine operation flexibility compared to full-bridge design, while trade-off between grid-side power factor range and full-bridge submodule share is offered within the design stage. Compared to the state-of-the-art, zero-to-rated DC link voltage operation is possible at lower full-bridge submodule share (62% against 75 %), at the penalty of reduced grid-side power factor. Alternatively, operation at higher full-bridge submodule share (62% against 50% existing solution) enables grid-side reactive power support over wide speed range, without branch current overload.