| With the rapid development of China’s power industry, people begin to pay moreand more attention to the high-voltage dc (HVDC)transmission technology. However,current of thousands ampere flow from the ground pole into soil in the HVDC unipolaroperation mode of the earth loop, resulting in range of adverse effects such as the nearsite surface potential increased, metal corrosion in the ground, the transformer dcmagnetic bias. Therefore, This article use Qianyang grounding electrode ofDeyang-Baoji±500kV HCDV as background, and study on the question thatgrounding current of the HVDC grounding pole brings to ac power system, in order toproposesing some reference suggestions for the routine maintenance of the line towergrounding and the transformer nearby, the new site selection. This article useTwo-dimensional axisymmetric finite element analysis model is established by usingparameters of QianYang grounding electrodes and soil in ANSYS softwave. Accordingto different soil layered resistivity and soil layered thickness, surface potential and stepvoltage near grounding electrodes is calculated. According to330kV QinYong lineparameters which is near the grounding electrode, π chain circuit model oftransmission tower lines is created in Simulink software, the laws of dc flow thougheach tower grounding is analysed by using the calculated surface potential dataanalysis.At last, the article uses ODFPS-700000/750transformer product-levelparameters to establish two-dimensional field circuit coupled model of a750kVautotransformer in the ANSYS MAXWELL finite element analysis software,considering the bias magnetic dc flow path and size, the iron core asymmetric saturatedis simulated. the main work and conclusions are as follows:①Under the dc system unipolar loop running mode, the surface potential startedto decrease rapidly, slow down gradually and finally tends to zero, the step voltage inthe the outer ring of grounding electrode is biggest. When the first soil floor’sresistivity is increases or the thickness is decreases, second soil floor’s resistivity isdecreases or the thickness is increases, the surface potential of the same radial distancewould be decreases. That is, the distribution of currents in the ground affected by thesoil resistivity and thickness of the first and second floor obviously. And the siting ofthe ground electrode should pay special attention to impacts of the first and middlefloor on the grounding performance. ②When the surface potential is unchanged, the outflow current of tower isdecreased as the ground resistance, dc resistance of grounding wire, the line verticaldistance is increases, and the line span and grounding current is decreases, under thesame surface potential. According to the current flowing through the metal andelectrochemical theory, calculation method of metal corrosion is derived, and330kVQinYong line tower grounding metal corrosion by grounding current is calculatedunder the most serious situation.③The excitation current and low harmonic also increases with increasing biasmagnetic dc of750kV autotransformer. Under the same bias magnetic dc current, theless the dc current outflow the middle voltage winding, the more unbalanced excitationcurrent of two transformer lord column, the bigger the degree of core magneticinduction asymmetry and the max change of magnetic induction in the same path. Thelargest core magnetic induction and the added magnetic induction appear in the ironyoke which is adjacent to return yoke At the same time, degree of core magneticinduction asymmetry of the lord column and return yoke is increase when the magneticbias dc current is increase,and degree of core magnetic induction asymmetry of ironyoke which is adjacent to lord column is always biggest. |
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