This study introduces a new type of end-face structure with polytetrafluoroethylene seals that improves the reliability of the synchronal rotary multiphase pump (SRMP). End-face models, including the relative motion and friction loss between the rotor and cylinder end faces, are established for both the original and the improved end-face structures. The end-face friction losses are analytically investigated based on the theoretical calculations. Due to the low relative velocity between the rotor and cylinder end faces, the end-face friction losses for both of the end-face structures are found to be insignificant compared to the total friction loss in the SRMP. The increased end-face friction loss caused by the polytetrafluoroethylene seals in the improved end-face structure is also negligible. Assessment of the friction losses of two end-face structures in high-speed and high-pressure applications shows that the improved end-face structure exhibits better performance than the original. The parametric analysis suggests that the wider but taller cylinder is helpful in reducing the end-face friction loss of SRMP.

In this paper, the effect of the line impedance difference between various inverters on power sharing with the traditional droop control method is fully analyzed. It reveals that the line impedance difference causes a significant reactive power error. An improved droop control method to eliminate the reactive power errors caused by the line impedance errors is proposed. In the proposed method, a voltage compensation determined by the actual reactive power error between the local inverter and the average one is added into the local voltage reference based on the CAN communication. Even when the communication is interrupted, the controller will operate with the last value of the average power, which still outperforms the traditional method. The effectiveness of the proposed control method is verified by simulation and experimental results, which show the proposed method possesses the better power sharing performance and dynamic response.

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Strong electric field will be produced in the earth around the grounding poles when HVDC transmission system operates in ground-return mode, which would have effects on the electric power equipments and the underground metal pipes nearby. This paper builds a model of multi-layer soil and a typical soil model on the basis of actual distribution of soil around the grounding poles, and then performs a calculation of earth surface potential distribution of typical soil model by finite element method. A comparison between the two different models is proposed to illustrate the different grounding characteristics of the DC earthed pole and different effects on the distribution of earth surface potential. The results show that, the ground resistance and the step voltage around the site are much affected by the soil resistivity where the grounding locates. While the ESP, which is within the range of tens of meters,is affected by the deep soil resistivity. Keywords-HVDC; finite element; earth surface potential; DC earthed pole I. II. E= J= J= E γ × ⋅ 0 ( 1) 0 ( 2) ( 3) 2 E= 0 V = -( 4) ( 5) γ x y z

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