Research On Ground Electrode Current Of High Voltage Direct Current System In Ground Return Status

Along with the general policy "West to east power transmission, North-south power exchange and Nationwide interconnection", the High Voltage Direct Current(HVDC) transmission technology will take the key role in long distance, bulk power transmissions. But the HVDC transmission system maybe operated in ground return status when the system is in debug or failure. Great current flowing into earth through grounding electrode, resulted in the potential of earth increasing. It created DC bias current in transformer, done harm to AC transmission system and underground metal. DC bias magnetization will cause rapid increase of transformer excitation current and result in overheat, noises and vibration increase in magnetic circuits. In the case of direct current bias, there appear significant odd harmonic noises in the spectrum of transformer noises, the problems caused by which may make the related components in the transformer loose, then threaten the operation security of transformers. This dissertation aims to have a primary probe into questions mentioned above. First, a detailed explanation of HVDC Transmission System is presented. And then A three-layer earth model combining vertical layered and horizontal layered is established by analyzing the ground soil structure. Combining Quasi-Newton method, the parameters of soil structure obtained from unequally spaced Wenner four-probe test data is optimal estimated. According to this model, the ground current and voltage is calculated around the HVDC electrode site. An expression of soil conductive mechanisms was given using porous media and a discussion was given about the effect of DC current density on soil resistivity. Soil resistivity is determined by two parameters, the total of charges |z_i|C_i and ionic mobilityμ_i. The model describing this effect can be used to calculate the soil resistivity of HVDC grounding system on any current. The equivalent circuit can be worked out if adequate monitoring devices are installed on the transformers' earthed points, which are used to detect dc currents flowing in power grids. At last, The HVDC influence on the traditional electric network device and underground metal is detailed explaining.