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Nowadays, the scale of power grid is remarkably increasing and there is a continuous reinforcement of AC–DC hybrid systems. Thus, the single-phase grounding fault currents on the AC grid-side of rectifier and inverter stations increase significantly, which may exceed the capacity limit of the circuit breakers. The large short-circuit currents would result in problems during equipment selection and threaten the safety of equipment and personnel. To solve such issue, the effect of the converter transformer on the single-phase grounding fault current on the grid-side is analyzed. The mechanism of high short-circuit current in DC near region is revealed. Also, the feasibility of neutral point earthing via small reactor at HVDC converter transformer to suppressing the single-phase grounding fault current is verified. In addition, the principle of selecting the suitable small reactor is summarized. The results show that the neutral point earthing via small reactor at converter transformer can effectively limit the single-phase grounding fault current on the AC-side. However, it would lead to an increase of overvoltage at the neutral point of HVDC transformer. Therefore, neutral grounding by small reactor might be implemented when the insulation level is satisfied.

A modification of the Adachi-Sugie-Lu (ALS) (1983) equation of state for better simultaneous representation of vapor-liquid equilibria, saturated liquid and vapor volumes, and vaporization enthalpies was presented. The Wong-Sandler mixing rule (1992) was extended for a general cubic equation of state, and its application was further extended to electrolyte solutions. A critical review of equations of state was made in order to identify important consideration in equation of state design. Mixing rules were briefly reviewed for the purpose of selecting a suitable mixing rule for the modified ALS equation. The models for electrolyte solutions were briefly reviewed for selecting a suitable model for calculating thermodynamic properties of electrolyte solutions by using the modified ALS equation. Monte Carlo computer simulation methods were reviewed as well since they were used in this work for the determination of equation parameters for ionic species. The modification of the ALS equation was based on an analysis of the Clapeyron equation, which describes the relationship between the vaporization volume change $\rm\Delta V\sb{vap}$ and enthalpy change $\rm\Delta H\sb{vp}.$ A new characteristic parameter, T$\rm\sb{r}(\Delta$Z = 0.5), was introduced for improving the performance of the equation on the simultaneous representation of volumetric and energy properties. Analysis of pure component data for 86 polar and non-polar fluids shows that a balanced and better representation of saturated vapor and liquid volumes is achieved. A new exponential form for $\alpha$(T) was proposed. The first derivative of this new $\alpha$(T) function is zero at the critical point, making the extension of $\alpha$ to the super-critical region easier and in a smooth manner. An extended Wong-Sandler mixing rule was proposed for its application to the modified ALS equation, which can represent most of cubic equations of state. The extended Wong-Sandler mixing rule with the modified ALS equation was evaluated using vapor-liquid equilibrium, volumetric and excess enthalpy data. Comparisons of several cubic equations of state with several mixing rules were made. This work gives better results for the simultaneous representation of vapor-liquid equilibrium and volumetric properties. For the simultaneous representation of VLE and excess enthalpy data, the results from this work are comparable with those obtained by using the PRSV equation (Stryjek and Vera, 1986) with the original Wang-Sandler mixing rule and better than those obtained by using other equations and mixing rules. The application of the extended Wong-Sandler mixing rule was further extended to electrolyte solutions by combining an activity coefficient model proposed by Chen et al (1982, 1986). The results obtained from the new approach for the calculated osmotic coefficients of aqueous electrolyte solutions are comparable with the original Chen's model (1986) but better than those reported by Zuo and Guo (1991), who obtained the values by means of a different cubic equation of state. The new approach was also applied to represent VLE values of ethanol-water-salt systems. The results show that the extension work is successful.