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This paper proposes a new HVDC grid test system for electromagnetic transient analysis, suitable for HVDC power system studies ranging from protection to dynamic studies investigating converter behaviour and interactions. In the recent past research interest in HVDC grids has increased, leading to a multitude of studies concerning dc power flow and optimal power flow, dynamics and HVDC grid protection. However, each of these studies makes use of different grid topologies, configurations and transmission line parameters. In this paper, a standard HVDC grid test system is proposed and an implementation in EMT-type software is provided. The implementation in EMT-type software makes use of a frequency dependent cable model, continuous converter model and a reduced dc breaker model. By means of a protection study, the effectiveness and computational efficiency of the proposed HVDC grid test system is demonstrated. The model with its parameters will be made publicly available.

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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.

Pole rebalancing in symmetrical monopolar HVDC grids is necessary to remove pole imbalances resulting from poleto-ground faults. For selective protection employing DC circuit breakers, the interaction between DC circuit breakers and pole rebalancing methods have not been studied. This paper proposes new strategies for pole rebalancing methods to deal with DC circuit breaker operation in HVDC grids. A complete analysis of pole rebalancing using equipment at DC or AC side is performed for all stages of the fault clearing process. Based on the analysis, new control strategies are proposed to optimize the use of the pole rebalancing equipment. The proposed control methods are shown to enable the pole rebalancing equipment to meet the required high protection speed and low losses. Both DC and AC side equipment such as dynamic braking systems and AC groundings are investigated and proven to be applicable for pole rebalancing in selective protection strategies. The impact of the breaker technology on the interaction between DC circuit breaker requirements and pole rebalancing needs is investigated in detail. The conclusions are validated using EMTP simulation on a four terminal test grid.

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.

Abstract-Dc faults in HVDC grids lead to quickly increasing currents which should be interrupted sufficiently fast to prevent damages to power electronics components. Although several primary relaying algorithms for HVDC grids have been proposed, fast backup relaying algorithms are needed to ensure system reliability when primary protection fails. This paper proposes a local backup relaying algorithm for HVDC grids, which leads to a short delay between primary and backup protective actions. The proposed algorithm, consisting of breaker and relay failure subsystems, uses classifiers which detect primary protection malfunctions based on the voltage and current waveforms associated with dc breaker operation. The algorithm initiates detection of uncleared faults during primary protection operation, which results in accelerated actions by the backup protection after primary protection failure. The proposed algorithm is applied to a four-terminal HVDC grid. Study results show that the proposed algorithm accurately detects uncleared faults, identifies the source of primary protection malfunction and expedites backup protective actions by operating during the fault current interruption interval of the primary protection.

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