An experimental study of breakdown and pre-breakdown phenomena in liquid nitrogen (LN2) is performed in this paper, in order to provide useful knowledge for the design of high voltage superconducting systems.
This paper proposes a novel pole-to-ground fault protection strategy for HVDC grids under symmetrical monopolar configuration employing low-speed DC breakers and pole rebalancing reactor (PRR) located at AC side to manage the rebalancing of the pole voltages. The first part of the paper is dedicated to the detailed description of the primary and backup protection sequences. In the second part, the proposed concept is validated through EMT simulations performed on a 4-terminal HVDC grid.
The present paper aims at describing the measurement methods applied to mock-up cables in order to evaluate their dielectrics. The focus is on leakage current and temperature measurements together with the processing based upon them to assist in the selection of the more appropriate material.
Effect of the Surge Arrester Configuration in MMC-HVDC Systems under DC and Converter Fault Conditions
Different surge arrester configurations are studied for a modular multilevel converter (MMC) in a symmetrical monopole configuration. Each configuration is analyzed under fault conditions including DC side faults and faults inside the converter station. The configurations considered are compared in terms of overvoltages, current levels and energy dissipation. It is found that the selection of the surge arrester parameters does not only depend on the overvoltage levels of the equipment, but also on the surge arrester configuration considered. The action of the surge arresters could result in higher longitudinal withstand voltage requirements of the arm reactors. Furthermore, special attention shall also be given to the possible low inductance loops created by the conduction of the arresters during a transient event.
Comparaison de mesures de conductivité obtenues par spectroscopie diélectrique et mesure de courant sous tension continue sur une résine époxy
De par sa facilité d’utilisation et sa capacité à isoler la permittivité complexe représentant les pertes dans un matériau tout en faisant un balayage fréquentiel, la spectroscopie diélectrique semble être une méthode de choix pour obtenir la valeur de conductivité ohmique d’un matériau. Cette méthode pourrait permettre de s’affranchir des temps conséquents de stabilisation généralement associés aux mesures de conductivité issues de courbes courant-tension effectuées suivant la norme IEC 62631-3. Par l’intermédiaire d’une comparaison entre ces 2 méthodes, les auteurs vont revenir sur les limites de chacune d’entre elle.
This paper proposes an in-depth analysis from the control point of view of dynamic models of a Modular Multilevel Converter (MMC) for high-voltage direct current (HVDC) application. Firstly, a generic method of analysis is presented for a natural arm-level state-space model. Its structural analysis highlights the decoupled nature of the MMC. Secondly, the well-known sum and difference of the upper and lower arm state and control variables is considered to obtain a (S/D) model. This transformation leads to a coupling between state and control variables and to an increase of the system complexity. Using the analysis results of the natural model and the (S/D) model, an original arm-modular control is finally proposed. The simulation results show the effectiveness of the proposed control, which is simpler to design compared to a conventional (S/D) control.
Power electronic traction transformers in 25 kV / 50 Hz systems: Optimisation of DC/DC Isolated Converters with 3.3 kV SiC MOSFETs
In AC electric trains, power electronic traction transformers (PETT) are multilevel single phase AC/DC converters connected to the AC medium voltage overhead line. For indirect topologies, DC/DC isolated converters are key elements of PETTs. This paper shows a method to design such DC/DC converters, and several variants are considered. Finally, the comparison results, in the case of a 25 kV / 50 Hz power supply and 3.3 kV SiC MOSFETs, show that the variant with a resonant AC link, with only one controlled bridge and a switching frequency lower than the resonant frequency, offers the best efficiency at rated power for a given volume.
A New Energy Management Control of Modular Multilevel Converters for Coping with Voltage Stress on Sub-Modules
This paper investigates the impact of the operating condition on the SM voltage ripples. In particular, it is revealed that under the classical control scheme where the Modular Multilevel Converter internal energy varies naturally with the DC grid voltage, the traditional sizing approach based on the analytical expression of instantaneous SM voltage may fail to respect the SM voltage constraint. To tackle this problem, this paper presents a solution by incorporating the advantages of the explicit energy management and the developed analytical expressions of the SM voltage ripple, which achieves a better utilization of the converter asset.
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 subfamilies 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.
This paper provides a fault analysis of bipolar overhead line based HVDC grids using HB-MMC converters. The impacts of fault type and fault resistance are shown and the physical behaviour of the transient fault return current is explained.