DC–DC converters with voltage boost capability are widely used in a large number of power conversion applications, from fraction-of-volt to tens of thousands of volts at power levels from milliwatts to megawatts. The literature has reported on various voltage-boosting techniques, in which fundamental energy storing elements (inductors and capacitors) and/or transformers in conjunction with switch(es) and diode(s) are utilized in the circuit. These techniques include switched capacitor (charge pump), voltage multiplier, switched inductor/voltage lift, magnetic coupling, and multistage/-level, and each has its own merits and demerits depending on application, in terms of cost, complexity, power density, reliability, and efficiency. To meet the growing demand for such applications, new power converter topologies that use the above voltage-boosting techniques, as well as some active and passive components, are continuously being proposed. The permutations and combinations of the various voltage-boosting techniques with additional components in a circuit allow for numerous new topologies and configurations, which are often confusing and difficult to follow. Therefore, to present a clear picture on the general law and framework of the development of next-generation step-up dc–dc converters, this paper aims to comprehensively review and classify various step-up dc–dc converters based on their characteristics and voltage-boosting techniques. In addition, the advantages and disadvantages of these voltage-boosting techniques and associated converters are discussed in detail. Finally, broad applications of dc–dc converters are presented and summarized with comparative study of different voltage-boosting techniques.

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Step-Up DC–DC Converters: A Comprehensive Review of Voltage-Boosting Techniques, Topologies, and Applications

Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.

Evolution of Topologies, Modeling, Control Schemes, and Applications of Modular Multilevel Converters

It is expected that in the near future the use of high-voltage dc (HVDC) transmission and medium-voltage dc (MVDC) distribution technology will expand. This development is driven by the growing share of electrical power generation by renewable energy sources that are located far from load centers and the increased use of distributed power generators in the distribution grid. Power converters that transfer the electric energy between voltage levels and control the power flow in dc grids will be key components in these systems. The recently presented modular multilevel dc converter (M2DC) and the three-phase dual-active bridge converter (DAB) are benchmarked for this task. Three scenarios are examined: a 15 MW converter for power conversion from an HVDC grid to an MVDC grid of a university campus, a gigawatt converter for feeding the energy from an MVDC collector grid of a wind farm into the HVDC grid, and a converter that acts as a power controller between two HVDC grids with the same nominal voltage level. The operation and degrees of freedom of the M2DC are investigated in detail aiming for an optimal design of this converter. The M2DC and the DAB converter are thoroughly compared for the given scenarios in terms of efficiency, amount of semiconductor devices, and expense on capacitive storage and magnetic components.

Comparison of the Modular Multilevel DC Converter and the Dual-Active Bridge Converter for Power Conversion in HVDC and MVDC Grids

This study presents a comprehensive review of high-power dc-dc converters for high-voltage direct current (HVDC) transmission systems, with emphasis on the most promising topologies from established and emerging dc-dc converters. In addition, it highlights the key challenges of dc-dc converter scalability to HVDC applications, and narrows down the desired features for high-voltage dc-dc converters, considering both device and system perspectives. Attributes and limitations of each dc-dc converter considered in this study are explained in detail and supported by time-domain simulations. It is found that the front-to-front quasi-two-level operated modular multilevel converter, transition arm modular converter and controlled transition bridge converter offer the best solutions for high-voltage dc-dc converters that do not compromise galvanic isolation and prevention of dc fault propagation within the dc network. Apart from dc fault response, the MMC dc auto transformer and the transformerless hybrid cascaded two-level converter offer the most efficient solutions for tapping and dc voltage matching of multi-terminal HVDC networks.

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Review of dc-dc converters for multi-terminal HVDC transmission networks

A dc–dc converter is essential for interconnecting HVdc networks of different voltage levels. This letter introduces a new family of dc–dc converters consisting of semiconductor switches and stack of submodules, which is named as hybrid-cascaded dc–dc converter (HCDC). This concept shows very attractive capital costs and low power losses. In this letter, the detailed operating principle of the HCDC is analyzed. Simulation results are obtained from PSCAD/EMTDC to verify the feasibility of the converters. The performances of the converters are evaluated in detail.

The Hybrid-Cascaded DC–DC Converters Suitable for HVdc Applications

This paper introduces a modular multilevel dc/dc converter, termed the DC-MMC, that can be deployed to interconnect HVDC networks of different or similar voltage levels. Its key features include: 1) bidirectional power flow; 2) step-up and step-down operation; and 3) bidirectional fault blocking similar to a dc circuit breaker. The kernel of the DC-MMC is a new class of bidirectional single-stage dc/dc converters utilizing interleaved strings of cascaded submodules. The DC-MMC operation is analyzed and an open loop voltage control strategy that ensures power balance of each submodule capacitor via circulating ac currents is proposed. Simulations performed in PLECS validate the DC-MMC's principle of operation and the proposed control strategy. Experimental results for a 4-kW laboratory prototype illustrate the single-stage dc/dc conversion process for both step-down and step-up operating modes.

A Modular Multilevel DC/DC Converter With Fault Blocking Capability for HVDC Interconnects

This paper proposes an $m$ -port converter structure for high-voltage dc (HVDC) applications to facilitate power-flow routing between $k$ controlled dc buses and $m-k$ independent dc networks. Power-flow control is achieved via the injection of incremental dc voltages between the networks; thus, the converter structure is only rated for a fraction of the rated power and voltage of the connecting dc networks. Unlike previously proposed dc power-flow devices, these incremental dc voltages are generated without requiring power exchange with an external ac network. There are four variants of the structure, each offering unique advantages for deployment. These variants, and the modules that comprise them, are presented. A sample simulation case study is performed to demonstrate three-port bidirectional power-flow control between networks of similar voltage (495/500/505 kV). The proposed converter structure is shown to route power between the networks using modules with megavolt-ampere ratings of approximately 2% of the transmitted power. Thus, the proposed converter structure offers a highly cost-effective means of routing and controlling dc power flows within emerging HVDC grids.

A Multiport Power-Flow Controller for DC Transmission Grids

This paper proposes a hybrid electronic and magnetic structure named a variable-inverter-rectifier-transformer (VIRT) that enables a transformer with fractional and reconfigurable effective turns ratios (e.g., 12:0.5, 12:2/3, 12:1, and 12:2). This functionality is valuable in converters with wide operating voltage ranges and high step-up/down, as it offers a means to reduce the turns count and copper loss within the transformer while also facilitating voltage doubling and quadrupling. These properties are especially beneficial for miniaturizing the transformer stage in many power electronics applications, such as universal serial bus wall chargers. We introduce the principle of operation of the structure and present models for its magnetic and electrical behavior. The instance of VIRT described in this paper comprises four half-bridge switching cells distributed around a planar magnetic core and connected to two “half-turns” wound through that core. By controlling the operating modes of the half-bridge cells, we gain control over the flux paths and current paths used in the transformer, and this hybridization enables fractional and reconfigurable effective turns ratios. An experimental prototype integrates the VIRT with a stacked-bridge LLC converter to accommodate a widely varying input (120–380 $\text{V}_{\text{dc}}$ ) and output (5–20 $\text{V}_{\text{dc}}$ ), and VIRT is shown to be highly beneficial in keeping efficiency high over this wide output voltage range.

Variable-Inverter-Rectifier-Transformer: A Hybrid Electronic and Magnetic Structure Enabling Adjustable High Step-Down Conversion Ratios

This work proposes a three-port converter structure for high-voltage dc (HVDC) applications to facilitate power flow routing between a controlled dc bus and two independent dc networks. Control of line power flows is achieved via the series injection of incremental dc voltages between the networks, thus the converter structure is only rated for a fraction of the rated power and voltage of the connecting dc networks. Unlike previously proposed dc power flow devices, a significant advantage of the presented structure is that the incremental dc voltages are generated without requiring power exchange with an external ac network. As a result, the proposed converter structure offers a highly cost-effective means of routing and controlling dc power flows within emerging HVDC grids.

A three-port power flow controller for HVDC grids

Practical challenges in distributed generation and electric vehicles have motivated the rapid development of bidirectional multi-port dc-dc converters. This paper proposes a converter that not only can perform fast battery voltage balancing and limit ground leakage current, it also features low switching ripple and component count, providing significant cost savings from reduced filter requirements and improved efficiency. Experimental testing of a 3.3 kW prototype confirms the bidirectional power transfer capability and demonstrates above 99% converter efficiency over a wide range of input power.

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Mike Ranjram

Papers

A Modular Multilevel DC/DC Converter With Fault Blocking Capability for HVDC Interconnects

A Multiport Power-Flow Controller for DC Transmission Grids

Variable-Inverter-Rectifier-Transformer: A Hybrid Electronic and Magnetic Structure Enabling Adjustable High Step-Down Conversion Ratios

A three-port power flow controller for HVDC grids

A bidirectional multi-port DC-DC converter with reduced filter requirements