DC Bias Reactive Power And Loss Analysis Of Three Phase Five Column Transformer

When the DC transmission operates in monopole or the geomagnetic storm occurs,there is DC current flowing through the neutral point of the transformer.DC magnetic bias will cause a sharp increase in the degree of excitation saturation,a significant increase in the leakage flux,leading to serious distortion of the excitation current,an increase in reactive power,eddy current loss and overheating of structural components,and DC magnetic bias will seriously threaten the safe and reliable operation of the power system and transformer.Three-phase five-pole transformers are widely used in China’s distribution network and become important equipment in power supply stations and substations.Due to the existence of the side column of three-phase five-pole transformer,the magnetic circuit of the side phase and the middle phase is different.The magnetic flux of the side phase core can form a closed magnetic circuit through the upper and lower yoke of the core and the side column,providing a return channel for the intruded DC magnetic flux,further saturating the transformer core and endangering the normal operation of the transformer.Therefore,it is necessary to calculate and analyze the current,reactive power and eddy current loss of three-phase five-pole transformer when DC bias occurs.The thesis takes the three-phase five-column core transformer as the research object,and the research work is mainly carried out from three aspects:the current calculation under the different nature of the transformer DC bias load,the comparison of reactive power,and the calculation of the tank eddy current loss.Based on the actual physical structure of the three-phase five-pillar transformer,the physical model is partially simplified,ignoring the structural components that have a small impact on the excitation current calculation,and a field circuit coupling calculation model for the three-phase five-pillar transformer is established.The inductance and current parameters of the transformer are calculated using the finite element method,and the DC bias calculation results are obtained considering the pure resistance,impedance,and resistance capacitance loads on the load side.Due to the occurrence of different harmonics in the transformer after DC bias,the impedance values under each harmonic component are different for three different types of loads,resulting in different ways of distorting the winding current after the occurrence of bias.By analyzing the winding current and excitation current of a complete cycle after the end of the oscillation process under different DC current sizes,the DC bias variation rules for three different types of loads are obtained.This thesis analyzes the change rule of reactive power of three-phase five-pillar transformer when DC bias occurs,and provides a calculation method for reactive power of three-phase five-pillar transformer,and analyzes its reactive power characteristics.Analyze the change rule of reactive power in each phase as the DC current increases under no-load and pure resistance rated loads on the load side,analyze the reason for the difference in the change of three-phase reactive power,and fit the reactive power characteristic curve to obtain the three-phase electromagnetic change rule of the three-phase five-column converter.In order to analyze the eddy current loss in the oil tank of a three-phase five-pillar transformer,a three-dimensional eddy current loss calculation model for the three-phase five-pillar transformer is established.Since the material used in the transformer oil tank is low magnetic steel and its thickness is greater than the penetration depth of the magnetic field,it is necessary to use a progressive mesh generation method for the oil tank wall.Using an eddy current field calculation model,the distribution of eddy current loss and leakage magnetic field in the oil tank of a three-phase five-pillar transformer under DC bias is calculated.By simulating and calculating the eddy current losses in the oil tank of a three-phase five-pillar transformer with different degrees of magnetic bias,it is found that the eddy current losses of the three-phase five-pillar transformer continue to increase as the degree of magnetic bias deepens.Based on the core structure and magnetic circuit characteristics of a three-phase five-column transformer,the reasons for the variation of the tank eddy current loss under magnetic bias are analyzed.The results of this study can provide a reference for the operation and design of three-phase five-pillar transformers under DC bias.