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

In This article, the state of the art in the variable-speed operation of hydropower plants is reviewed, with a focus on pumped-storage hydropower. Relevant literature is reviewed to address the benefits of variable-speed operation for both power systems and hydropower facilities. Two main configurations to enable variable-speed operation-the doubly fed induction machine (DFIM) and the converter-fed synchronous machine (CFSM)-are discussed and compared. The article addresses the technology and energy policies of the past, present, and future and points out how the motivation, services, value, and technology of variable-speed hydropower plants have been subject to considerable change.

Variable-Speed Operation of Hydropower Plants: A Look at the Past, Present, and Future

The dc-dc modular multilevel converter (dc-dc MMC) with a medium-frequency ac-link transformer is a promising candidate for the modern electric ship medium voltage dc power system to provide galvanic isolation, flexible voltage regulation, and fault ride through ability. However, the steady states and dynamics of such an isolated dc-dc MMC are rather complicated because it contains many state variables. Moreover, the on-ship requirements further complicate the system design. This paper thus proposes the modeling and control of such a dc-dc MMC. Based on the fact that such a dc-dc MMC is controlled by the phase-shifted operation, the fundamental period averaging (FPA) method is applied to obtain the steady-state and small-signal models, and based on the on-ship requirements, the modulation index control as well as the simple but efficient competitive dual closed loops (a voltage loop for normal condition and a current loop for overload condition) are designed. It is proved that the FPA method can preserve the necessary model information about the dc-dc MMC, and the proposed controllers can be well designed based on the steady-state and small-signal FPA models by compromising the on-ship requirements. The analysis and design results are verified by the simulations and experiments.

Modeling and Control of the Isolated DC–DC Modular Multilevel Converter for Electric Ship Medium Voltage Direct Current Power System

The dc–dc modular multilevel converter (MMC), which has originated from the ac–dc MMC circuit topology, is an attractive converter topology for interconnection of medium-/high-voltage dc grids. Proper operation of the dc–dc MMC necessitates injection of an ac circulating current to maintain its submodule (SM) capacitor voltages balanced. The ac circulating current, however, needs to be minimized for efficiency improvement. In addition, a unique type of imbalance amongst the SM capacitor voltages that is caused by dc power flow needs to be mitigated. This paper proposes a closed-loop control strategy for the dc–dc MMC to simultaneously regulate the dc-link currents, maintain the SM capacitor voltages balanced, and minimize the ac circulating current. Performance and effectiveness of the proposed control strategy are evaluated based on simulation studies in the MATLAB Simulink and experimentally verified on a laboratory prototype.

A Capacitor Voltage Balancing Strategy With Minimized AC Circulating Current for the DC–DC Modular Multilevel Converter

DC grid technology is regarded as a promising solution for future electric networks integrating a great amount of renewable energies. It calls for high-efficiency dc–dc converters with high voltage step-up ratio to interconnect medium-voltage (MV) dc distribution grids and high-voltage (HV) dc transmission grids. This paper presents an isolated bidirectional soft-switching dc–dc converter combining two-level converters in parallel on the MV side and a modular multilevel converter (MMC) on the HV side. A dedicated control method of the proposed converter is presented. By the proposed method, a certain reactive current is injected into the MV side by the MMC to ensure soft-switching on the MV side. The proposed converter presents low power-semiconductor total device rating and low semiconductor losses over a wide power range at variable input and output voltages. Simulation of a 50 kV/400 kV, 400 MW converter is conducted to evaluate semiconductor losses and verify the validity of this work.

A High Step-Up Ratio Soft-Switching DC–DC Converter for Interconnection of MVDC and HVDC Grids

High-voltage dc/dc converters play an important role in HVDC grids. Isolated modular dc/dc converters (IMDCCs) based on modular multilevel converter technology provide a good solution to high-voltage applications. In order to reduce the size of the system, the IMDCC is required to be operated with a high ac-link frequency, but this will lead to increased switching loss and thus degraded efficiency. This paper proposes a soft-switching operation scheme for such an IMDCC. In this scheme, a quasi-square-wave (QSW) modulation method is employed, where the chain-links generate quasi-square terminal voltages with reduced dv / dt . With such chain-link terminal voltages, the arm currents which provide good condition for the soft-switching operation of the QSW-IMDCC can be obtained. Since soft switching can be achieved for the power switches, the proposed scheme will suffer less switching loss, thus improving the efficiency of the converter. Moreover, a capacitor voltage-balancing control strategy is proposed. This strategy does not need any arm current sensors, thus reducing the cost. The proposed soft-switching operation scheme and capacitor voltage-balancing control strategy are verified by the simulation results.

Soft-Switching Operation of Isolated Modular DC/DC Converters for Application in HVDC Grids

SUMMARY Among various renewable sources, hydropower is still the cheapest clean energy. As most of the new constructed hydropower plants are located at remote areas, HVDC lines are usually employed for hydropower transmission. In this paper, hydropower plant with VSC HVDC unit connection is proposed, in which the generator(s) of a hydropower station is connected to a voltage source converter (VSC) station directly and the power is transmitted by DC line. By using VSC HVDC unit connection, the hydraulic turbine realizes variable speed operation and thereby improves the hydraulic turbine efficiency substantially. In the paper, the progress of hydropower unit connection is reviewed and a typical configuration of hydropower plant with VSC HVDC station unit connection is introduced; the advantages of employing VSC HVDC unit connection technology are presented. A calculation model is established to calculate the maximum hydraulic turbine efficiency. Finally, the turbine efficiencies are compared between the variable speed operation of hydropower plant with VSC HVDC unit connection and fixed speed operation of a conventional hydropower plant; the power frequency variation range during variable speed operation is analyzed.

Optimized Operation of Hydropower Plant with VSC HVDC Unit Connection

Grid-Oriented multiphysics model of Power-to-Hydrogen electrolyzers

Topology identification of low-voltage distribution network is beneficial to ensure the safe and reliable operation of distribution network and the asset management of utilities, which is the basis of distribution network system analysis. In this paper, the existing topology identification methods are divided into two categories. The first type is based on measurement data, including the correlation analysis, cluster analysis, linear programming and neural network. The second type is based on communication signal transmission and uses power line carrier communication for topology recognition. Finally, the existing methods are compared and concluded and the outlook of future work is given.

Xiaobo Yang

Papers

Soft-Switching Operation of Isolated Modular DC/DC Converters for Application in HVDC Grids

Optimized Operation of Hydropower Plant with VSC HVDC Unit Connection

Grid-Oriented multiphysics model of Power-to-Hydrogen electrolyzers

Overview of Topology Identification for Low-voltage Distribution Network