This paper provides an overview and comparison of the possible grounding and configuration options for meshed HVDC grids. HVDC grids are expected to play a key role in the development of the future power system. Nevertheless, the type of grounding and the base configuration for the grid have not yet been determined. Various studies related to multiterminal HVDC or meshed HVDC grids often assume one specific configuration and grounding scheme and take it for granted. However, as there exist a large number of options, an overview is needed to balance pros and cons. In this paper, the influence of the different grounding options on fault behavior is investigated for point-to-point connections. Furthermore, the impact of the grounding type on the system fault behavior is investigated with electromagnetic transient simulations. Next, the suitability of a configuration to serve as base configuration in a meshed DC grid is investigated and compared in terms of extensibility and flexibility. In this evaluation, the grounding type, the number and location of grounding points in a grid are considered as well. Finally, an overview of the most important conclusions is given in a summarizing table.

HVDC grids are considered to be essential building blocks for the future upgrade of the existing AC power system and as a means to transport the expected massive amounts of renewable energy from remote sources to the load centers. HVDC systems exist for over 50 years, yet meshed DC grids do not exist so far. For point-to-point HVDC connections, there is a certain freedom in choosing the configuration and earthing scheme. For a grid, different converter arrangements and earthing schemes can be considered. The choices made will influence how the grid will look like, the components in the grid and their rating, the operating principles, the protection philosophy, the degree to which the grid is extensible and the overall reliability and inherent redundancy. Clearly, it will influence investment and operating costs as well. This paper provides a qualitative overview of potential grid configurations for DC grids (symmetrical monopole, asymmetrical monopole, bipolar schemes, with and without metallic return and combined systems). The possible earthing options for a meshed HVDC grid are part of this discussion. The extensibility and reliability of the HVDC grid are specifically dealt with.

No abstract

With the increasing integration of renewable energies in the power system a thorough analysis of the adequacy of defence plans is needed. This paper focusses on the impact of a high penetration of photovoltaics in distribution systems on the under frequency load shedding schemes designed by the TSO. First an overview of the issues arising from a higher penetration of embedded renewable energy sources in the current UFLS scheme design is described. It will be shown that neglecting photovoltaics in load shedding scheme design can lead to adverse effects in system stability and can have a disproportional high social impact. Subsequently an overview of different methodologies to improve the UFLS scheme design is given. These methodlogies are then applied to the measurements of two feeders in the Belgian power system. From the evaluation of these results, it can be concluded that the proposed methodologies lead to substantial improvements in UFLS scheme adequacy.