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.

This paper proposes a frequency domain based methodology to analyse the influence of High Voltage Direct Current (HVDC) configurations and system parameters on the travelling wave behaviour during a DC fault. The method allows us to gain deeper understanding of these influencing parameters. In the literature, the majority of DC protection algorithms essentially use the first travelling waves initiated by a DC fault for fault discrimination due to the stringent time constraint in DC grid protection. However, most protection algorithms up to now have been designed based on extensive time domain simulations using one specific test system. Therefore, general applicability or adaptability to different configurations and system changes is not by default ensured, and it is difficult to gain in-depth understanding of the influencing parameters through time domain simulations. In order to analyse the first travelling wave for meshed HVDC grids, voltage and current wave transfer functions with respect to the incident voltage wave are derived adopting Laplace domain based component models. The step responses obtained from the voltage transfer functions are validated by comparison against simulations using a detailed model implemented in PSCAD TM . Then, the influences of system parameters such as the number of parallel branches, HVDC grid configurations and groundings on the first travelling wave are investigated by analysing the voltage and current transfer functions.

The Na-X (X = Si, Ag, Cu, Cr) systems have been critically reviewed and modeled by means of the CALPHAD approach. The two compounds, NaSi and Ag2Na, are treated as stoichiometric ones. By means of first-principles calculations, the enthalpies of formation at 0 K for the LT-NaSi (low temperature form of NaSi) and Ag2Na have been computed to be -5210 and -29821.8 Jmol-1, respectively, with the desire to assist thermodynamic modeling. One set of self-consistent thermodynamic parameters is obtained for each of these binary systems. Comparisons between calculated and measured phase diagrams show that most of the experimental information can be satisfactorily accounted for by the present thermodynamic descriptions

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Pole rebalancing in symmetrical monopolar HVDC grids is necessary to remove pole imbalances resulting from pole-toground faults. For selective protection employing DC circuit breakers, pole rebalancing considering backup protection operation in case of breaker failure has until now not been studied. This paper proposes fault clearing and post-fault restoration sequences including pole rebalancing to deal with DC-side faults, considering both primary and backup protection operations. The performance of the proposed sequences and the impact of the key breaker parameters on pole rebalancing are investigated in a four-terminal test system using PSCAD/EMTDC.