HVDC grids are a promising alternative for the expansion of the existing AC grid. They are interesting for the integration of large-scale renewable energy sources, connecting high power offshore windfarms, interconnecting new market areas, including asynchronous AC grids and off-grid communities, and they can even provide ancillary services to the AC system. It is expected to create a more reliable and flexible transmission grid using HVDC grids. However, their development will require, among other technologies, DC-DC conversion systems. Although the DC-DC conversion techniques are well understood in low and medium voltage, a specific review on the approaches for high voltage is needed. This paper presents an overview of the DC-DC power converters dedicated to HVDC proposing a classification based on their structure. Two large families are established: those which provide galvanic isolation, and those which do not. Several sub-families are also proposed. An overview of the main HVDC applications that can be targeted with each family is also presented, highlighting the main converter requirements for each application case.

A novel technique for monitoring the condition of metallized polypropylene film (MPPF) capacitors installed in the dc link of railway power trains is proposed in this paper. The technique is based on the capacitance estimation of the two different capacitors used in the dc link (dc filtering and 2F filter capacitors), and it is done without adding any extra sensor or filter to the respective traction drive scheme. Therefore, the proposed method is implemented using software but not additional hardware for the different trains. Two other important passive component parameters of the traction drive are also monitored, all of them by means of curve fitting based on the least-mean-square (LMS) algorithm. The proposed technique is implemented and validated at simulation and experimental levels, using MATLAB/Simulink and a 1.2-MW traction drive test bench. Very accurate results are obtained, and the predictive maintenance of MPPF capacitors for different topologies of railway power trains can be set through the proposed technique.

High‐voltage direct current (HVDC) is more and more often implemented for long distance electrical energy transmission, especially for off‐shore wind farms. In this study, a full DC off‐shore wind farm, which requires a high‐power and high‐voltage DC/DC converter, is considered. In order to reduce the size of the converter, the trend is to increase operating frequency. Silicon carbide (SiC) metal–oxide–semiconductor field‐effect transistors (MOSFETs) are becoming industrially available and give scope for the realisation of high‐performance DC/DC converters based on modular architectures. This study presents a prospective analysis of the potential of such devices in HVDC power systems. Considering the characteristics of Si insulated‐gate bipolar transistor and SiC MOSFET power modules, two DC/DC converter topologies are compared in terms of losses and number of components. In conclusion, a study of the efficiency based on converter energy loss is presented.

The connection of photovoltaic sources to a medium voltage dc collection network requires a dc-dc converter having specific grid-connected converter capabilities. This article presents the application of a phase-shifted full bridge (PSFB) converter for medium voltage dc collection networks suited to photovoltaic power plants. The unidirectional structure of the converter is beneficial in terms of cost and simplicity of hardware implementation of the MV circuit components. The design of the PSFB converter is presented considering ratings of 250 kW power, 1.2 kV input, and 20 kV output. A converter model and a novel PSFB input voltage control regulating the dc-link voltage are developed and verified. A tuning method is given taking into account the effect of different transformer leakage inductance values. The behaviour of the PSFB under the PV power production profile and MV network faults is studied in simulation and verified against a reduced-scale prototype with ratings of 30 kW and 350 V / 600 V operating at 20 kHz. The experimental verification of the control is performed considering a real PV power profile. The experimental results under faults show the same response as in simulations. The PSFB fault response is observed to be superior compared to other dc–dc topologies with capacitive output filter only, making it suitable for grid-connected operation.

This article presents the design, the fabrication, and the test of an isolated DC-DC converter for renewable energy applications. The converter is based on the Dual Active Bridge topology and uses silicon carbide power semiconductors and a medium frequency transformer. The design process covers hardware ranging from the semiconductor die to the complete power converter. For the control, a rapid prototyping approach was used. The experimental validation of the 100 kW prototype is presented.

The number of high voltage direct current (HVDC) links continue to increase over the years, most of them, for offshore applications or bulk power transmission over long distances. The present paper evaluates the possible development of a direct current (dc) grid in Europe given the present, and future, HVDC links. Eight potential cases for the interconnection between close links are suggested as starting scenario for a multiterminal network. The need of a dc-dc converter and its special requirements are evaluated in function of suggested interconnections. As an example, a case study is chosen to evaluate the behavior of an interconnection between line commutated converter (LCC) and voltage source converter (VSC) link using a front-to-front (F2F) isolated converter.