Analysis And Application Of DC Bias In Axisymmetric Model Of UHV Transformer

With the large-scale construction of the UHV power grid in China,the importance of UHV Transformer in the power system is increasing.HVDC and geomagnetic storms cause DC bias of transformer,which threatens the safe and stable operation of UHV transmission system.Aiming at the computational complexity in the three-dimensional model of UHV Transformer DC bias,through analyzing the magnetic circuit structure characteristics of UHV Transformer,according to the actual parameters UHV Transformer,and the main column size and magnetic circuit average length without changing,The equivalent axial symmetry model of UHV Transformer is established through adjusting the cross sectional area of the side column and iron choke according to the of reluctance consistent principle.Further,the calculation problem is simplified as a two-dimensional finite element model,which can effectively reduce the scale of DC bias calculation.Considering the difference of magnetic flux leakage between the UHV Transformer under no load condition and the load condition,the DC bias calculation in two cases is carried out,and the results of the three dimensional model are compared to verify the validity of the axial symmetry two-dimensional model.In view of the characteristics of the field-circuit coupling principle in the calculation of DC bias for transformer,the efficiency of the axial symmetry DC-bias simulation model is further evaluated from two aspects of the magnetic field model and the circuit model.In the field of magnetic field calculation,the influence of the degree of different meshes on the calculation of DC bias is analyzed.The axial symmetry two-dimensional finite element model of UHV Transformer is small in scale,fast in magnetic field calculation and less affected by mesh size.In circuit calculation,the influence of high voltage side series resistance and time step on load DC bias calculation is analyzed.The calculation time of DC bias can be effectively reduced by increasing the series resistance and increasing the step length properly.Compared with the three-dimensional model,the axial symmetry two-dimensional model of UHV Transformer can carry out many cycles calculation in a relatively short time,which greatly reduces the computation workload of DC bias and improves the efficiency of DC bias.On the basis of effectiveness and efficiency,the axial symmetry model is applied to the DC bias calculation in the long transition process.The reason of the demand of the human series resistance is discussed,the relationship between series resistance and excitation current to reach a steady cycle through the calculation is obtained,and the DC bias of UHV Transformer under small series resistance of the high-voltage side is achieved,avoiding the voltage iterative compensation.Considering the large time constant of the DC-bias circuit model of the transformer under inductance load of the low-voltage side,the axial symmetry model is used to reduce the computation time of transient transition process,and to achieve the rapid calculation of DC magnetic bias in long transition process.