Modular Multilevel DC-DC Converter Research Articles

New analysis of VSC-based modular multilevel DC-DC converter with low interfacing inductor for hybrid LCC/VSC HVDC network interconnections

The integration of multiterminal hybrid HVDC grids connecting LCC- and VSC-based networks faces several technical challenges such as DC fault isolation, ensuring multi-vendor interoperability, managing high DC voltage levels, and facilitating high-speed power reversal without interruptions. The two-stage DC-DC converter emerges as a key solution to address these challenges. By implementing the modular multilevel converter (MMC) structure, the converter's basic topology includes half-bridge sub-modules on the VSC side and full-bridge sub-modules on the LCC side. However, while this topology has been discussed in the literature, its connection to an LCC-based network with controlled current magnitude lacks detailed analysis regarding operational challenges, control strategies under various scenarios, and design considerations. This paper fills this gap by providing comprehensive mathematical analysis, design insights, and control strategies for the modular DC-DC converter to regulate DC voltage on the LCC-HVDC side. Additionally, the proposed control scheme minimizes the interfacing inductor between the two bridges, ensuring uninterrupted power flow during reversal and effective handling of DC faults. Validation through Control-Hardware-in-the-Loop testing across diverse operational and fault scenarios, along with a comparative analysis of different converters, further strengthens the findings.© 2017 Elsevier Inc. All rights reserved.

Analysis and Design Guidelines of the Isolated Modular Multilevel DC–DC Converter With the Impact of Magnetizing Inductance

The Isolated Modular Multilevel DC-DC (IMM DC-DC) converter is one of the potential candidates for interconnection of medium and high voltage DC grids. One of the key components for realization of the IMM DC-DC converter is its transformer whose magnetizing inductance depends on the core material, dimensions, and airgap. Depending on the transformer design, the magnetizing inductance may vary over a wide range, adding to the complexity of the converter design. In this paper, the impact of the magnetizing inductance on the converter real and reactive powers, transformer primary and secondary side currents, arm current, differential-mode current, capacitor voltage ripple, and soft-switching operation is investigated. The presented analysis is validated by time-domain simulation in the Matlab/Simulink software environment and experiments on a scaled-down prototype.

Current Ripple Mitigation Strategy of Modular Multilevel DC/DC Converter for Battery Energy Storage System

For modular multilevel DC/DC converter (MDC) with conventional modulation strategies, the inductor current ripple will increase if DC/DC units' input voltages and/or output references are unbalanced. In this paper, a current ripple mitigation strategy is proposed for MDC battery energy storage system, which is based on harmonic model for ripple analysis using Fourier series. By varying the duties and phase-shifted angles simultaneously, multiple current harmonics are eliminated. Moreover, in order to freely adjust batteries' current under the condition of duties are calculated by ripple suppressing method in every modulation process, the control period is divided into several intervals and variables in each interval are separate. Through constraining every unit's average duty of all intervals, battery current is dominated under distribution scheme while achieving the goal of ripple mitigation. Experimental result on a four-unit MDC converter is presented to verify the feasibility and effectivity of proposed modulation method.

A hybrid DC-DC modular multilevel converter with capacitors parallel connectivity for arm energy balancing

The integration of renewable energy sources in Medium-/High-voltage DC grids has called up continuous research to develop DC-DC conversion systems. This paper presents a hybrid modular multilevel DC-DC converter topology which successfully overcomes the issue of capacitors voltage imbalance during DC-DC conversion. In the proposed topology, single MMC leg is employed with half-bridge submodules in the upper arm and full-bridge submodules in the lower arm along with soft-switched high-voltage valves. The proposed hybrid modular multilevel converter has inherited arm energy equalization with the help of the parallel connectivity of involved arms’ capacitors. During the equalization period, the series-connected upper arm capacitors are connected in parallel to the series-connected lower arm capacitors to transfer the energy between the involved arms which results in operating with balanced capacitors voltages. The proposed modular DC-DC converter is a bidirectional converter which allows power flow from high-voltage side to low-voltage side and vice versa, i.e., it can be operated successfully as a DC transformer. The operational concept of the proposed configuration, control strategy, and parameters design are presented. Simulation results are presented along with experimental results of a scaled-down prototype to validate the proposed approach. Simulation and experimental results show the viability of the suggested approach.

Coprime Polynomial Based Dual-Circulant Modulation for Inherent Submodule Voltage Balancing in MMDC

The modular multilevel dc-dc converters (MMDCs) have attracted much interest in medium voltage dc applications, but have to face the main issue of balancing submodule (SM) capacitor voltages. This paper proposes a dual-circulant modulation and proves it possessing the inherent balancing capability for arbitrary operation cases. High-speed communication for real-time sensor data transfer could be avoided, which reduces implementation costs and also enhances the system reliability. Two sets of circular switching patterns are preset and combined to complete the full constrains on SM voltages. The associated polynomial of circulant matrix is introduced and the coprime of polynomials demonstrates the full-rank feature of the extended switching matrix that promises the inherent balancing for any operation cases. Then the switching patterns are reallocated and optimized to keep the SM uniformity and reduce the capacitor voltage ripple at the same time. Full-scale simulations on MMDC models and down-scaled experiments on prototypes are both presented, which validates the inherent voltage balancing capability and the optimization of SM capacitor voltage ripple with the proposed dual-circulant modulation.

Dual-Circulant Modulation for Modular Multilevel DC/DC Converters With Inherent Balance Capability

Modular multilevel DC/DC converters (MMDC) have emerged recently for medium-voltage dc applications. Under single circulant modulation, the sub-module (SM) capacitor voltages in MMDC are found to be evenly divided only at some specific modulation coefficients. To obtain the inherent balance capability under all modulation coefficients and expand the application range of circulant modulation, a simple dual circulant modulation is proposed in this letter. On the basis of single circulant modulation, the self-balancing of the SM capacitor voltages is realized by using the additional switching patterns generated by a simple circulant permutation method. The theoretical analysis of the proposed method is presented, and the inherent balance capability of MMDC under the dual circulant modulation method is verified by experiments.

Multicell nonisolated resonant modular multilevel dc‐dc converter with self‐voltage balancing and step‐up conversion

SummaryWith the rapid development of renewable energy power generation technologies in high‐voltage direct‐current (HVDC) system, the high step‐up ratio dc‐dc conversion plays an important role to connect dc grids with different voltage levels. This paper presents a multicell nonisolated resonant modular multilevel dc–dc converter (MMDC) for medium‐ and high‐voltage applications. The converter exhibits expandability without using transformer. Moreover, by adding additional switched‐inductor (SL) cells, switched‐capacitor (SC) cells, and the upper arm submodules (SMs), the proposed converter can obtain high gain with low voltage stresses on switches and diodes. The operating states of SMs is regulated by the carrier phase‐shifted pulse width modulation (CPS‐PWM) to guarantee the number of inserted SMs is constant to support the high‐side load at any moment. The converter is operated in resonant mode with resonance between SM capacitors and arm inductor. The step‐up ratio could be regulated by adjusting the number of SL cells, SC cells, and upper arm SMs; thus, more flexible voltage regulation can be realized. In addition, the proposed converter has the voltage self‐balancing capability of capacitors in SMs and SC‐cells without additional balancing control strategy. The topology configuration, parameter design guideline, operating principle, and comparison with other MMDCs are presented. Finally, the theoretical analysis is verified by the simulation and bench‐scaled experimental prototype results.

Decoupled Control for Double-T Dc-Dc MMC Topology for MT-HVdc/MVdc Grids

This paper proposes a decoupled control of a dc-dc modular multilevel converter (MMC) based on a double-T topology intended for multi-terminal high voltage direct current (MT-HVdc) transmission systems or emerging distribution systems operating in medium voltage direct current (MVdc). The aim of the proposed control strategy is to obtain an input current with reduced harmonic content and to eliminate the output ac common-mode voltage, which is not allowed in MT-HVdc systems. The control strategy consists of injecting two circulating ac currents and two dc currents that allow the energy balance between the arms of the converter and the general energy balance of the topology. The dc and ac currents are decoupled and allow control over load variations and reference changes in the dc-links. The proposed topology is mathematically modeled and the control method is then derived. Simulation results are presented to validate the proposed system.

Isolated Bipolar Modular Multilevel DC-DC Converter with Self-balancing Capability for Interconnection of MVDC and LVDC Grids

Isolated Bipolar Modular Multilevel DC-DC Converter with Self-balancing Capability for Interconnection of MVDC and LVDC Grids

The Resonant Modular Multilevel DC–DC Converter Adopting Switched-Inductor Cells for High Step-Up Ratio

With the prosperous development of distributed renewable energy power generation, high-voltage direct-current (HVDC) system has evolved into large-scale practical applications from the initial stage, and high step-up conversion dc–dc converters have become a core technology to promote the development of HVDC system. This article proposes a new resonant modular multilevel dc–dc converter (MMDC) based on the conventional boost converter. The single-switch and diode of the conventional boost topology are replaced by submodules (SMs), which are clamped by floating capacitors, and the inductor is replaced by switched-inductor (SL) cells. The phase-shifted pulsewidth modulation (PS-PWM) is applied to regulate the state of SMs, i.e., inserted state or bypassed state, to ensure that there is a constant number of SMs supporting the output voltage at any time. The converter is operated in resonant mode with resonance between capacitors of SMs and the upper arm inductor. The step-up conversion ratio is dependent on the inductors charging ratio, the number of SMs, and SL cells. Hence, more regulation schemes are available. The proposed converter possesses the advantages of MMDC and has the inherent-balancing capability of SM capacitor voltages without additional complex balancing control. The structure of the converter, operating mode analysis, parameter design, and comparison with other MMDCs are presented, which have been verified by simulated and experimental results.

A Modular Multilevel DC–DC Converter With Flying Capacitor Converter Like Properties

This article proposes a novel modular multilevel dc-dc converter with high power-density potential based on a cascaded string of voltage source submodules (VSMs) merged with a current source submodule (CSM) that is configured to rapidly invert the polarity of the current in the VSM string. The proposed hybrid converter combines the modularity advantages of the modular multilevel converter (MMC) and the high effective frequency advantages of the flying capacitor converter. Unlike other hybrid nonisolated MMC structures the proposed converter features a novel CSM stage enabling a near 50% reduction in string current, and the input source and output load to be commonly grounded. In addition, the proposed operational approach enables self-balancing of the VSMs and a simple overall control structure. This article introduces the main operating principles of this novel topology termed the current shaping modular multilevel forward converter along with a proposed control design. A peak efficiency of 97.1% is measured from a laboratory-scale prototype at 950-137V and 1.45 kW.

A Non-Isolated DC-DC Modular Multilevel Converter with Proposed Middle Cells

Unlike the modular multilevel converter (MMC) topology operated under the rectifier or inverter modes, control of the balanced state for the submodule (SM) capacitor voltage has emerged as the key issue for DC-DC MMCs. This is mainly caused by no balanced alternative powers being used for balancing SM capacitor voltages, which can be absorbed from the input or output DC sides of the converter. Typically, the alternative voltages and currents should be injected to achieve SM capacitor voltage balance in the DC-DC MMC. However, this solution is based on the cost of adopting the bulky LC filter components. For interconnecting different DC voltages in medium-voltage applications, this paper presents a non-isolated DC-DC MMC equipped with the proposed middle cells. It is intended to achieve DC voltage conversion without adopting bulky passive LC filters. On the one hand, the alternative currents, used for balancing the SM capacitor voltages, are arranged for flowing only within the phase legs of the proposed DC-DC MMC without disturbing the input current. On the other hand, through appropriate control of the middle cells, compensated components can be developed to eliminate the undesirable voltages on the output DC side. The middle cells of the proposed DC-DC MMC are supplied with the function of the active filter, which enables the DC-DC MMC system to escape the bulky LC components. Through theoretical analysis and a control strategy, the proposed DC-DC MMC has been analyzed comprehensively. Finally, the simulation and experimental results are verified to evaluate the effectiveness of the proposed DC-DC MMC.

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.

Modular Multilevel DC-DC Boost Converter for High Voltage Gain Achievement with Reduction of Current and Voltage Stresses

This paper presents a modular multilevel DC-DC boost converter for high voltage gain achievement with reduction of current and voltage stresses. Normally, conventional DC-DC boost converter (CDBC) has low voltage gain capability, higher current and voltage stresses which lead to high conduction loss of the semiconductor devices due to the circuit structure limitation. Therefore, 4-level synchronous modular multilevel DC-DC converter (SMMC) with Marx topology adaptation is considered to improve the limitation circuit structure of CDBC. Besides, the 4-level SMMC have high voltage gain achievement, it also has lower current and voltage stresses features. A 145 W and 48 V input voltage of 4-level SMMC has been designed and experimentally verified where the result is compared with CDBC. The results show that the CDBC required 0.76 of duty cycle while 4-level SMMC only require 0.5 duty cycle to achieve 200 V output voltage, respectively. Additionally, the current stress decreases by 75% on input inductor and 50% reduction from voltage stress of switching as compared to the CDBC. Consequently, the selection rating for the components can be decreased and higher efficiency can be obtained for the 4-level SMMC as compared to the CDBC.

Resonant Modular Multilevel DC–DC Converters for Both High and Low Step-Ratio Connections in MVDC Distribution Systems

DC transformers based on power electronics are key items of equipment for medium voltage dc (MVDC) distribution systems. Both high and low step-ratio dc-dc conversions are required to interface dc links at different voltages to form an integrated dc distribution system. The transformer-coupled resonant modular multilevel dc-dc converter (RMMC) is well-suited to high step-ratio connection between an MVDC network and a low voltage dc network but it is not suitable in low step-ratio conversion for linking two MVDC networks with similar but not identical voltages. This article presents a circuit evolution of the high step-ratio transformer-coupled RMMC into its low step-ratio transformer-less RMMC counterpart. These two RMMCs share the same structure and the same resonant process within the resonant submodule stack giving rise to the same operational advantages. Also, the circuit design experience can be readily transferred from one to the other. From these two base RMMC circuits, a family of RMMCs with further configurations is elaborated that provide a wider variety of connection options for MVDC distribution systems. In this circuit family, each high step-ratio transformer-coupled RMMC has a low step-ratio transformer-less RMMC counterpart and one can be transformed into the other via the circuit evolution process presented. The theoretical analysis for both high and low step-ratio RMMCs has been verified through full-scale simulations of medium voltage examples and further verified through down-scaled experiments on laboratory prototypes.

Modeling and Current-Mode Control of a DC–DC Modular Multilevel Converter

This article presents a dynamic model and a current-mode control for a dc-dc modular multilevel converter with the ultimate aim of expanding the converter to a three-leg configuration to reduce the filtering requirements of the dc ports. A complete model of the converter including a systematic approach for the control of the power throughput is developed. It is then shown that the expansion from a single-leg to three-leg configuration requires no additional modeling, and that the three-leg converter significantly alleviates the filtering requirements on both the high-voltage and low-voltage sides. The proposed model and control technique are verified using simulation results based on the MATLAB / Simulink software environment for both the single and three-leg configuration. Further verification is carried out through a single-leg 1-kW experimental prototype.

Discontinuous Conduction Mode Operation of the Current-Shaping Modular Multilevel DC–DC Converter

The Current Shaping Modular Multilevel DC-DC Converter (CS-MMC) is a recently proposed class of modular multilevel dc-dc converters for dc distribution grid applications consisting of a single string of cascaded voltage source submodule cells (VSMs), a current source submodule (CSM) and notably no series inductor. In the CS-MMC, since it is the CSM that shapes the string current and there is no series inductor, this frequency can readily be in the medium-frequency range, enabling VSM cells to be realized with low cell capacitance. In the previous work, only continuous conduction mode (CCM) operation of the CS-MMC had been considered which led to several limitations including elevated switching losses and low utilization of the semiconductor devices in low output voltage applications. In this work, discontinuous conduction mode (DCM) operation is proposed for the CS-MMC which addresses these limitations. In traditional dc-dc converters, DCM is determined by the inductor current ripple. However, unique to the CS-MMC, DCM is determined by the VSM capacitor voltage ripple. In this work, the proposed DCM operational approach is presented along with its analysis and control. Both simulation and experimental results from a laboratory scale converter system are provided for validation.

A Transformerless DC–DC Modular Multilevel Converter for Hybrid Interconnections in HVDC

In this article, a modular multilevel dc-dc converter philosophy is proposed which can form an effective interlink to exchange power between line commutated converter (LCC) based high voltage direct current (HVdc) link and voltage source converter-based HVdc grid of same or different voltage levels. The converter exhibits symmetric four quadrant operation irrespective of the voltage polarity in the LCC-based HVdc link. In the event of reversal of power flow by reversing the voltage polarity in the LCC-based HVdc link, the power flow direction/magnitude through the interlink can be made to remain unchanged without the need of any extra switching arrangement. The working principle is explained analytically and its operation is validated by simulation as well as experimental results.

A Step-Up Nonisolated Modular Multilevel DC–DC Converter With Self-Voltage Balancing and Soft Switching

Evolving from the popular modular multilevel ac-dc converter, the single-stage nonisolated modular multilevel dc-dc converter (MMDC) is advantageous for medium- and high-voltage applications. However, exploiting ac circulating power to balance the submodule energy, when utilized for high step ratio applications, existing MMDC topologies suffer from circulating current through the arms and large filter inductor at the low-voltage side. To overcome these issues, this article presents a new power transfer mechanism to balance the submodule energy automatically by reconstructing the half-bridge submodule into a quasi-resonant circuit. Based on this submodule structure, a new MMDC topology for step-up applications is proposed. Compared to the existing MMDCs, the proposed one offers the following advantages. First, the common-mode circulating current through the lower and upper arms is avoided. Second, the self-balancing of the capacitor voltages is guaranteed by the proposed modulation method to insert and bypass adjacent submodules in a complementary manner. Third, the soft-switching operation is achieved for the majority of the switches to alleviate switching losses. Fourth, the voltage stress across the input side inductor is limited to the submodule voltage, thereby reducing the size of the inductor. Simulation analysis and experimental results verify the performance of the proposed MMDC.

The Current Shaping Modular Multilevel DC–DC Converter

The recently proposed modular multilevel dc-dc converter (M2DC) and the HVDC-dc auto transformer (HVDC-AT) enable direct power transfer between dc networks at low-to-moderate step ratios using only a single power electronic conversion stage. In both converter structures, an internal circulating sinusoidal ac current is established to balance the energy within the converter. In practice, however, the frequency of the circulating ac current is limited to a few hundred Hz, requiring significant ripple power to be filtered by large voltage submodule (VSM) capacitors. In this article, a novel energy transfer mechanism for interfacing dc networks is introduced wherein the frequency of the ac currents can be increased by one to two orders of magnitude, thus enabling a commensurate reduction in VSM capacitor size and cost. This is achieved by eliminating the ac chokes within the modular converter structure. Instead, a current source submodule (CSM) is employed to shape the circulating current, leading to a current of nearly square-wave shape with a frequency in the kHz range equal to the switching frequency of the CSM. This article describes one particular topology based on this current shaping mechanism, namely a high step down ratio dc-dc converter. The viability of the mechanism is demonstrated via simulation and experimental results from a laboratory scale implementation.