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

Multiterminal high voltage direct current (MTDC) system can be either a series type or a parallel type. Series MTDC concept is essentially developed from the two- terminal HVDC but with some unique characteristics. In this paper, the AC fault ride through capability of series MTDC is discussed. The AC fault at inverter stations, which are connected to electrically separated AC systems, will be highlighted. For AC fault happens at one inverter station of series MTDC, the DC voltage of the corresponding inverter station may decrease to zero due to commutation failure, while the operation status of the other inverter stations (the healthy inverters) may still remain power transmission capability to some extent, which increases the availability of the series MTDC system. However, the current controller design, the voltage dependent current order limiter (VDCOL) logic and the AC system strength will impact on the dynamic performance. In this paper, a +800kV/3.2GW 4-terminal line commutated converter (LCC) based series MTDC model is established to study the dynamic performance during AC fault occurs at one inverter. The controller of the series MTDC is designed; the VDCOL logic for series MTDC is developed; simulation results of the dynamic performance during AC faults at the inverter station are presented.

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Dynamic performance of series multiterminal HVDC during AC faults at inverter stations

The invention provides a hybrid multi-terminal direct current power transmission system and an inverter station and a control method thereof. The hybrid multi-terminal direct current power transmission system comprises a rectification station comprising an LCC, at least two inverter stations comprising VSCs, a voltage detection component and a VSC control system on the side of the inverter stations. Under the condition that a detected direct current voltage value is lower than a predetermined value, a part of the VSCs are controlled to be switched from an inversion mode to a rectification mode and the rest of the VSCs are controlled to main the inversion mode and the VSC working in the rectification mode are controlled so that the direct current voltage thereof basically maintain the predetermined value. The detection for the drop of the direct current voltage of direct current transmission lines and the control of the operation modes of the VSCs of the inverter stations are performed on the side of the inverter stations, so that the rectification station and the inverter stations can separately take corresponding actions (decentralized control) so solve the problem that some converter stations cannot cut the power supply after the rectification station retreats from a hybrid multi-terminal direct current system without rapid inter-station communication.

Hybrid multi-terminal direct current power transmission system and inverter station and control method thereof

An objective of the invention is to provide a bidirectional power valve for current occurring in a high voltage DC conductor, control method therefor, hybrid multi-terminal HVDC System using the same The bidirectional power valve includes a first power diode arrangement of a first conducting direction, a second power diode arrangement of a second conducting direction; a mechanical disconnector, being connected with the second power diode arrangement in series; wherein: the first power diode arrangement and the series-connected second power diode arrangement and the mechanical disconnector are connected in parallel; and the first conducting direction of the first power diode arrangement and the second conducting direction of the second power diode arrangement are opposite to each other The current commutation and re-commutation can be achieved with less requirement of the timing accuracy of switching event which makes the usage of a mechanical disconnector and power diode feasible This will then result in a significant reduction of cost and power transfer losses

Bidirectional power valve and control method therefor and hybrid multi-terminal HVDC system using the same

The utility model provides a capacitor bank of direct current transmission hierarchical access system reactive compensation. The capacitor bank of the direct current transmission hierarchical access system reactive compensation includes: a switch a of an alternating current power grid A which can be electrically coupled with a low voltage level U1; a switch b of an alternating current power grid B which can be electrically coupled with a high voltage level U2; and a capacitor module. The capacitor module includes a first capacitor bank and a switch c; a first end of the first capacitor bank and a first contact end of the switch c are electrical coupled; a first end of a second capacitor bank can be grounded and switch and is electrical coupled with a second end of the switch c of the capacitor module; a second end of the second capacitor bank is electrically coupled with a second end of the first capacitor bank of the capacitor module; the switch a is located between the alternating current power grid A and the second end of the first capacitor bank of the capacitor module; and the switch b is arranged between the alternating current power grid B and the first end of the first capacitor bank of the capacitor module and the first contact end of the switch c. The alternating current power grid with different voltage levels can share the reactive compensation capacitor bank, and the investment in the direct current hierarchical access system can be reduced.

Chunming Yuan

Papers

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

Dynamic performance of series multiterminal HVDC during AC faults at inverter stations

Hybrid multi-terminal direct current power transmission system and inverter station and control method thereof

Bidirectional power valve and control method therefor and hybrid multi-terminal HVDC system using the same

Capacitor bank of direct current transmission hierarchical access system reactive compensation