Fast Calculation Of DC Bias Of UHV Transformer Based On Equivalent Inductance Surface

With the continuous deepening of the research and prevention of DC bias of transformers,an accurate and efficient calculation method of DC bias is urgently needed.In this paper,the low-voltage windings of UHV transformers are taken as an example to perform the dynamic inductance parameter change law and fast calculation of DC bias.Research.The field-circuit coupling method is one of the commonly used methods for numerical calculation of DC bias.This method has high calculation accuracy and low speed.Considering the sensitivity of the excitation current to the saturation saturation of the excitation current and the stability of the amplitude of the load current,the current and load currents were selected to form the current plane.Select appropriate points on the current plane for sampling,obtain the primary and secondary winding currents through the current correspondence relationship,and solve the magnetic field by finite element to obtain the self-inductance and mutual-inductance parameters.According to the different saturation levels of the magnetism,the sampling density was reasonably selected,and the surfaces of the parameters such as self-inductance and mutual inductance were finally obtained.Based on this,the laws of self-inductance and mutual inductance under DC bias were analyzed.Starting from the circuit model of the transformer,the mutual inductance circuit is decoupled to obtain a T-type transient equivalent circuit.The T-type transient equivalent circuit is obtained by linearly changing the mutual inductance circuit.The equation of the T-type transient equivalent circuit is derived.The simulation results show that the T-type transient equivalent circuit and the mutual inductance circuit are in the calculation accuracy and convergence.Speed and calculation stability are equivalent,and the T-type transient equivalent circuit instead of the mutual inductance circuit for the field-circuit coupling method calculation is reliable and effective.Based on the T-type transient equivalent circuit,the equivalent inductance parameters are defined:equivalent leakage inductance on the primary side,equivalent leakage inductance on the secondary side,equivalent excitation inductance.The equivalent inductance parameter can reflect the change law of the transformer affected by DC bias..Based on the sampling calculation and the definition formula of the equivalent inductance parameter,the equivalent inductance parameter surface is obtained,and the variation law of the equivalent inductance parameter affected by the DC bias is analyzed,which provides a reference for the related protection of the transformer.By interpolation of the equivalent inductance surface,the equivalent inductance parameters under the specific excitation current and load current can be obtained,which replaces the process of solving the transformer inductance parameters by the finite element method,which greatly reduces the time cost and computing resources required to obtain the inductance parameters..An interpolation equivalent inductance surface method for calculating DC bias is proposed.This method improves the conventional field-circuit coupling method and greatly improves the calculation speed of DC bias.The simulation results show that as long as the sampling density is reasonable,the interpolation equivalent inductance surface method has high calculation accuracy and fast calculation speed,which can shorten the calculation time of UHV transformer DC bias.The effect of the sampling density of the equivalent inductance surface on the calculation accuracy is analyzed in detail,and the sampling density is divided into three parts:the sampling interval of the large field current,the sampling interval of the small field current,and the sampling interval of the load current.Influence on the calculation accuracy of interpolation equivalent inductance surface method.The interpolation equivalent inductance surface method improves the calculation speed of DC bias,and provides a fast calculation method for the research and prevention of DC bias.