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

The increasing penetration of renewable energy sources (RES) poses a major challenge to the operation of the electricity grid owing to the intermittent nature of their power output. The ability of utility-scale battery energy storage systems (BESS) to provide grid support and smooth the output of RES in combination with their decrease in cost has fueled research interest in this technology over the last couple of years. Power electronics (PE) is the key enabling technology for connecting utility-scale BESS to the medium-voltage grid. PE ensure energy is delivered while complying with grid codes and dispatch orders. Simultaneously, the PE must regulate the operating point of the batteries, thus for instance preventing overcharge of batteries. This paper presents a comprehensive review of PE topologies for utility BESS that have been proposed either within industry or the academic literature. Moreover, a comparison of the presently most commercially viable topologies is conducted in terms of estimated power conversion efficiency and relative cost.

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A Review of Power Electronics for Grid Connection of Utility-Scale Battery Energy Storage Systems

The nonresonant single-phase dual-active-bridge (NSDAB) dc–dc converter has been increasingly adopted for isolated dc–dc power conversion systems. Over the past few years, significant research has been carried out to address the technical challenges associated with modulations and controls of the NSDAB dc–dc converter. The aim of this paper is to review and compare these recent state-of-the-art modulation and control strategies. First, the modulation strategies for the NSDAB dc–dc converter are analyzed. All possible phase-shift patterns are demonstrated, and the correlation analysis of the typical phases-shift modulation methods for the NSDAB dc–dc converter is presented. Then, an overview of steady-state efficiency-optimization strategies is discussed for the NSDAB dc–dc converter. Moreover, a review of optimized techniques for dynamic responses is also provided. For both the efficiency and dynamic optimizations, thorough comparisons and recommendations are provided in this paper. Finally, to improve both steady-state and transient performances, a combination approach to optimize both the efficiency and dynamics for an NSDAB dc–dc converter based on the reviewed methods is presented in this paper.

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Overview and Comparison of Modulation and Control Strategies for a Nonresonant Single-Phase Dual-Active-Bridge DC–DC Converter

Provided is an electric power converter. A First and second series circuits of switching devices and a series circuit of capacitors are connected in parallel. Between the connection point in the first series circuit of switching devices and the connection point in the series circuit of capacitors, a first bidirectional switch is connected and, between the connection point in the second series circuit of switching devices and the connection point in the series circuit of capacitors, a second bidirectional switch is connected. The connection point in the first series circuit of switching devices is connected through a reactor to an input-output common connection line connected to one end of an AC power supply and the other end of the AC power supply is connected to the connection point in the series circuit of capacitors.

Electric power converter

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

The potential of medium-voltage dc (MVDC) grids in various applications has been under investigation by different researchers. In several works, the advantages of implementing MVDC technology into collector grids of wind and photovoltaic (PV) parks are pointed out. The main driver is that the grid-side inverter of the distributed generators as well as lossy grid filters and bulky 50 Hz transformer become obsolete. However, MVDC grids are a promising alternative to established ac grids not only for power generation. Within this work, the advantages regarding system efficiency and investment costs of using MVDC for the distribution of electrical energy is presented. Conventional low-voltage ac-grids and possible medium-voltage ac grids are also analyzed to compare the different implementation approaches.

Medium-voltage DC distribution grids in urban areas

The in-field validation of wind turbines (WTs) behavior is very time consuming and cost intensive, especially when fault ride-through tests are conducted. Full-size wind-turbine test benches allow a realistic operation of WTs in an artificial environment. Due to the independency of wind and grid conditions, the cost and duration of the test program and certification can be reduced. This paper presents the development of 4 MW full-size wind-turbine test bench following a multiphysics power hardware-in-the-loop concept. Different from other test facilities, the presented test bench utilizes a direct-drive prime-mover concept combined with a nontorque-load (NTL) unit to enable the application of highly dynamic stresses at the rotor hub and additionally a power-electronics-based grid emulator. Through the measurement results of the test programs conducted on the test bench, the capability of the test bench in replicating the field conditions including voltage dips is demonstrated. In addition, a time consuming efficiency measurement can be performed with the reduced duration on the test bench. This shows another main benefit of the test bench compared to the conventional test method for WTs.

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Development of a 4 MW Full-Size Wind-Turbine Test Bench

The integration of storage systems into the grid is becoming increasingly important due to the growing amount of volatile power sources. This paper shows a theoretical approach for designin...

Development of a Modular High-Power Converter System for Battery Energy Storage Systems

An auxiliary resonant-commutated pole (ARCP) ensures soft switching in the entire operation range of a three-phase dual-active bridge dc-dc converter. This work evaluates the design and the resulting boost in system efficiency for different semiconductor materials and devices. Afterwards, a full-scale medium-voltage prototype of an ARCP is constructed. The subsequent measurements are presented within this work, before the economic feasibility is discussed.

Marco Stieneker

Papers

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

Medium-voltage DC distribution grids in urban areas

Development of a 4 MW Full-Size Wind-Turbine Test Bench

Development of a Modular High-Power Converter System for Battery Energy Storage Systems

Ensuring soft-switching operation of a three-phase dual-active bridge DC-DC converter applying an auxiliary resonant-commutated pole