Study On Magnetic Bias Treatment Of Earth Electrode In North Zhejiang Converter Station Considering Geomagnetic Storm

With the wide use of large cross-section energy-saving conductors in China's 500 kV and above power grid,the problem of power grid transformer magnetic bias caused by earth electrode and geomagnetic storm is increasing.As GIC(Geomagnetically Induced Current)is becoming larger and larger,it is necessary to research how to comprehensively control geomagnetic bias caused by geomagnetic storms.The ±800kV UHVDC transmission project from Baihetan to Zhejiang Province is researched as the research object.The influence of magnetic bias current on the power grid under the condition of geomagnetic storm and monopolar earth operation is considered.The risk assessment of magnetic bias current on Zhejiang power grid is carried out,and the comprehensive bias has been put forward.The main contents and achievements in this paper are as follows:(1)Combined with the geological characteristics of Zhejiang Province,the block geoelectric structure model of Chenancun earth Electrode near area is established.By using the finite element method,the distribution of ground potential in the radius of 150km caused by 5000A grounding current of grounding electrode is calculated,and the distribution characteristic of earth potential in high resistance area,low resistance area and large water area is revealed.(2)By using GIC benchmark DC bias model and considering the capacitor isolation treatment device installed in the construction of West Zhejiang project,a multi voltage level DC model of Chenancun is built.Combined with the distribution of earth potential in each station,the grid model is simplified by the node voltage method,and the magnetic bias current in the transformer is calculated.Twenty four risk stations are calculated,and the characteristics of over standard stations are analyzed and summarized.The accuracy of the bias current calculation model is verified by the measured results of the West Zhejiang project.(3)Combined with the geomagnetic disturbance data of three geomagnetic storms,the maximum value of geoelectric field in seven areas of Zhejiang Province is obtained.The value of geoelectric field generated by geomagnetic storm on March 13,1989 in Wenzhou,Zhejiang Province is as high as 1.51v/km.Combined with the change of GIC current distribution of power grid before and after the bias treatment of Ginsi earth electrode.It is concluded that the capacitor isolation device will lead to the transfer of GIC current in power system;the risk of GIC bias of 500kV and above voltage level stations in Zhejiang power grid under three different intensity magnetic storms is evaluated,and 38 over standard stations are obtained.(4)The Gravitational Search Algorithm combined with weighting method is proposed.The stations of 220kV and above in Zhejiang province which need comprehensive treatment are given.Compared with the conventional cross line switching method,it shows that this method can effectively avoid the interaction between earth electrode and geomagnetic storm with better treatment effect and higher economy.The treatment can be used to guide the Baihetan-North Zhejiang project.The model and algorithm established in this paper are of great significance for the evaluation and treatment of DC bias.With the increasing scale of UHVDC and 500kV and above power system in China,the research results of this paper have reference value for the prevention of UHVDC bias and the site selection of new projects.