Research On Calculation Of DC Bias Current In Power Transformer And Its Influence On Loss Characteristics

In today’s society,people’s demand for electricity is increasing day by day,and the high-voltage direct current transmission project has been widely put into production due to the energy distribution characteristics of China.And the DC input into the ground generated by the DC system that is usually operated in a single pole is an important cause of the DC bias problem.Furthermore,a series of hazards such as increased vibration and noise,increased loss,and faulty operation of relay protection are caused to transformers and power systems.Therefore,it is necessary to pay attention to the problem of DC bias.To solve the problem of DC bias of the transformer,this paper focuses on three aspects: the generation of DC bias current,the loss characteristics of the transformer under DC bias,and the DC bias restraining measures.It is very important for the normal operation of the transformer and the safety of the power grid.First,the theoretical analysis of the generation mechanism of DC bias is given in this paper,and the calculation method of the equivalent resistance of the ground network is deduced.Using PSCAD/EMTDC to build an AC-DC hybrid power grid model,the DC current distribution and excitation current variation in the AC system under different operating conditions of the DC system are simulated and calculated,and the DC current in each substation is quantitatively calculated.Furthermore,it analyzes the distribution of the neutral point DC current under single-pole and double-circuit DC system unipolar operation,and obtains the relationship between the DC current of the substation and the position of the ground electrode and the distance from the ground electrode.The above calculation of DC bias current provides a basis for the subsequent influence of DC bias on transformer loss characteristics.Secondly,this paper uses the field-circuit coupling method and uses Ansys Maxwell to build the electromagnetic field calculation model of 220 kV transformer.The change of the electromagnetic characteristics of the transformer during no-load operation under DC bias is analyzed.The relationship between the neutral point DC current size and the magnetic flux density distribution,excitation current,and harmonic distribution is obtained.Then,the influence of DC bias on the loss characteristics of the transformer was studied,and the lossdistribution characteristics of the transformer core,pull plate,and clamp under the influence of DC current were obtained.Finally,considering the damage caused by DC bias,based on the advantages and disadvantages of the existing DC bias restraining measures,this paper proposes resistance-capacitance restraining method.This method has a fault protection function and effectively weakens the degree of DC bias in the transformer.Then,PSCAD/EMTDC was used to verify the DC restraining effect of the transformer under normal operation and single-phase ground fault of the system,and confirmed the effectiveness and the safety of the restraining device.