Study On Influence Of HVDC Grounding Electrode On DC Bias Of Transformer

China's energy resources and power load are characterized by uneven distribution.HVDC technology has obvious advantages in long-distance and large-capacity transmission,and has advantages such as AC power transmission technology,small power loss,fast adjustment speed and reliable operation.Therefore,high-voltage direct current transmission is usually used to transmit electrical energy.When the DC system is in the case of overhaul,debugging or failure,a single maximal return line is adopted,that is,the earth is used as a return branch,and serves as a conductor of the transmission line.In this mode of operation,the DC current invades the AC system through the neutral point of the transformer,which will cause the iron core of the transformer to become severely saturated,and the excitation current is distorted,which may cause local overheating,damage the insulation or damage the structure of the transformer.Transformers are critical to the safe operation of the power system and are related to the stability of the entire power grid.Therefore,it is necessary to study the influence of the DC current of the DC grounding electrode on the surrounding transformer.In this thesis,the numerical calculation model of DC grounding electrode under complex soil structure is firstly established.The grounding performance of the grounding pole in the horizontal layered soil model and the vertical layered soil model soil is analyzed.Based on the dispersion mechanism of layered soil,the variation law of surface potential and grounding resistance is analyzed.Studies have shown that vertical layered soil has a significant effect on the ground potential.Secondly,combined with the substation circuit model to calculate the DC current flowing into the corresponding transformer;on the basis of this model,the influence of the position of the AC substation near the pole site on the DC current flowing into the transformer is analyzed,that is,the connection between the two substations and the ocean,mountains and other complexes are studied separately.The change of DCcurrent when the soil is parallel or vertical,and further analyze the influence of the distance between substations on DC current.The results show that under the three complex soils,the closer the substation is to the grounding pole,the larger the DC current flowing in,and the substation is in the marine soil,the effect is most obvious;when the substation connection is parallel with the mountains or the ocean,Effectively reduce the amount of DC current flowing in.Finally,the influence of the DC current of the high-voltage DC grounding pole on the excitation characteristics of the transformer and the internal magnetic field of the transformer is studied.The transformer circuit model and the three-dimensional magnetic field model are constructed by using the simulation software PSCAD and COMSOL to study the influence of DC bias on the transformer.The waveform characteristics and excitation current characteristics of the three-phase transformer output current and the variation law of the internal magnetic field of the transformer are obtained.The results show that different types of transformers are affected by DC bias magnetism,and the three-phase transformers have the weakest tolerance.According to the analysis of the internal magnetic field of the transformer,the magnetic induction strength of the core column is larger than that of other positions,and it is also seriously affected by the DC bias.