Research On Transformer DC Bias Due To UHVDC Monopole Operation And Its Restraining

China's growth in electricity demand getting great, especially in economically developed coastal areas, but China's hydropower is mainly concentrating in the southwest, and thermal power is mainly concentrating in the northwest, the uneven distribution of energy and load impel China's west to east power transmission project. So vigorously exploiting hydropower and using UHVDC to transmit power to coastal economically developed areas is imperative. However, when the UHVDC transmission system works in mono-polar operation, DC flows through the neutral point and causes DC bias of transformer around UHVDC grounding polar, which will threaten the operation security of transformers. Therefore, it is important and necessary to research on transformer DC bias and it's restraining. Some key problems are reseached in this paper, such as the distribution of earth surface potentials and DC grounding current, the mechanism of transformer DC bias and its effect, transformer grounding via small resistance to limit DC bias current and so on.The DC grounding current flowing through transformer windings is decided by the distribution of earth surface potentials and DC grounding current around UHVDC grounding polar. Earth surface potentials distribution of some typical soil models are calculated in this paper. Different each soil layer's resistivity and thichness has different distribution of earth surface potentials. The relationship between each soil layer's resistivity and thichness and earth surface potentials is analyzed. And the distribution of earth surface potentials of vertical layered soil model is caculted too. Besides soil model, power network structure is anther important factor, effecting on the DC grounding current flowing through transformer.Transformer structures take a key part in the ability of enduring DC bias. Based on FEM, interior magnetic characteristics of three-phase main transformer, three-phase and three-leg transformer and three-phase and five-leg transformer are contrasted considering DC bias condition. According to the analysis result, the paper has drawn some conclusion about those transformers'performance of withstand DC bias. Choosing core diameters is related to the manufacturing cost of transformer and is one of important factors of transformer design. Some formulas of calculating transformer magnetic induction intensity is derived with different core diameters, which is help to improve the ability of transformer enduring DC bias.The mechanism of transformer grounding via small resistance to limit DC bias current is introduced in this paper. For partial grounding transformers, the over-voltages are caculated, caused by DC bias current, lighting, and short circuit fault. An analysis to effect about grounding resistance and DC bias current is given by used two-part network equivalent circuit. Transformer grounding via resistance is efficient way to limit DC bias current. But that could bring on DC bias of other neighbouring transformers. An improved Particle Swarm Optimization algorithm for two objective optimization problems (TOPSO) is presented. Based this method, a group of optimal resistance value could be found to inhibit DC bias of all object power grid transformers. And those resistance values could be as small as possible. To validate the approach, with TOPSO and NSGA-â…¡the network configuration of small resistances of all Luzhou power system transformers were calculated. The result shows that TOPSO is efficient and feasible for two objective optimization problems.