| Along with the large-scale construction of HVDC project, the negative influence such as electromagnetic effect, thermal effect, electrochemical effect and so on which are brought by DC unbalanced operation can’t be ignored. Distribution of DC earth surface potential will induce step voltage, which could affect human and animal safety. Overflow and divergence of DC grounding current will induce electrochemical corrosion, which could shorten the life of pipe network. Shunt of DC grounding current will induce magnetic bias effect, which could aggravate the loss of equipment. Severely, it will threaten the safe and stable operation of AC-DC interconnected system. Because the number of grounding electrode is rather more in the UHV large sending end area, DC grounding current is larger. With the complicated and diversified geological condition, the above problem must be more outstanding. Therefore, studying the distribution characteristics of DC grounding electrode’s current filed, realizing the influence degree on AC power grid while the HVDC was operated in single-pole mode and reducing the protection cost for the operation of DC system in large sending end area have important scientific significance and project value.DC Grounding electrode current field was the research object. The difference in geological structure and the shape of DC grounding electrode were considered. The electric field division model of circular current source was established. The calculation method for distribution of DC earth surface potential was put forward. The characteristics of step voltage around DC grounding electrode was studied. The soil structure simplification in the calculation of distribution of DC earth surface potential was discussed. The results show that the calculation formula for distribution of DC earth surface potential is infinite series. The circular mirror image in the complex stratificated medium contains two kinds. One is the same with that in the horizontal stratificated medium. Another is the product of the first one and the reflection coefficient with coordinate translation. The Hummel method can be used to simplify the soil structure and improve the calculation speed of DC earth surface potential.Based on the infiltration and diffusion model of DC grounding current, the calculation formula for overflow and divergence of DC grounding current was put forward. The overflow of DC grounding current on inner and outer cirque surface of DC grounding electrode was studied. The divergence of DC grounding current at different directions in the earth was studied. The main influencing factors were discussed. The results show that the electric field force produces pounding effect in the center of cirque. When observation point is close to DC grounding electrode, the pattern of electrode has a big effect on the overflow and divergence of DC grounding current. When observation point is far from DC grounding electrode, soil structure has an effect on the overflow and divergence of DC grounding current. When the distance between DC grounding electrodes in the send area and receive area is short, the interaction of two electrodes should be considered.The interconnected power system model was established while the HVDC was operated in single-pole mode. The characteristics of shunt of DC grounding current via AC power grid were studied. The component principle of equivalent parameters of DC grounding current pathway was studied. Based on the characteristics which the resistivity varied with the soil-layer depth, the calculating method for the shunt of DC grounding current was discussed. The results show that the shunt of DC grounding current is related to the number of earth layers, the resistivity and the thickness of each layer. The relationship between shunt of DC grounding current and the geographical position can be expressed approximately by discrete logarithmic function. DC current source and the complex earth stratification resistance can be equivalent to the voltage source and internal resistance. The calculation method proposed in this paper has been proven in good agreement with the actual.In accordance with the characteristics of UHV large sending end area, the influence of different arrangement modes, operating conditions and sharing schemes of multiple DC systems’grounding electrodes on DC grounding current field was studied. In order to economize land resource and reduce the costing for protection, the arrangement means of multiple grounding electrodes exist in the same area are discussed. |
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