Calculation And Analysis Of DC Bias Of Three Phase Five Column Transformer

Owing to the advantages of low loss,small size and material saving,three-phase transformers have been used in large distribution networks to transmit electricity energy for users.The general distribution transformers mostly use three-phase three column transformers,while three-phase five column transformers are zero due to the existence of side column and upper and lower yoke The condition of the forming circuit of sequence current and zero sequence flux is provided.The zero sequence impedance of transformer is reduced and the zero sequence loss is reduced.This transformer is used in distribution network to meet the special electrical performance.At the same time,power transformer as an important equipment in the power grid to transfer voltage to transmit power,its safe and stable operation is directly related to the smooth operation of the whole power system and the quality of power transmission.According to the relationship between the voltage vector of high voltage transmission system and that of medium voltage or low voltage receiving system,the connection mode of transformer winding is determined,At the same time,due to the heart structure of three-phase five column transformer and the existence of zero sequence magnetic circuit,the magnetic circuits between three phases interact with each other,which is different from single-phase transformer and three-phase transformer There are great differences in three column transformers,so it is necessary to calculate and analyze the characteristics of three-phase five column transformer when DC bias occurs.In this thesis,the solid model of three-phase five column transformer is simplified to a certain extent,and the clamp and pad which have little influence on the magnetic field of the transformer are omitted.The finite element magnetic field model is established.According to the actual winding connection mode of the transformer,the equivalent circuit diagram is constructed,and the circuit equation matrix is listed.The parameters calculated in the magnetic field model are substituted into the circuit equation Iterative solution is carried out until it is stable.Because the actual winding resistance of transformer is very small,the time constant of circuit stability is very large.Therefore,in the calculation process,three phases of high voltage winding of transformer are connected in series with appropriate resistance to accelerate stability,reduce iteration times and improve calculation efficiency.After the calculation is stable,the voltage divided by the series resistance is compensated to both sides of the winding by the way of voltage compensation,so as to eliminate the influence of the artificial series resistance on the calculation results,such as the phase offset and amplitude reduction of the current waveform,and obtain more accurate calculation results after meeting certain compensation conditions.Three phase five column transformer has more side columns than three-phase three column transformer in structure.The magnetic flux not only flows between the three-phase cores,but also forms a flux loop with the side pillars close to the core,which leads to the difference of DC bias characteristics between single-phase transformer and three-phase three column transformer.Therefore,it is necessary to calculate the waveform of DC inrush excitation current when the three-phase five column transformer is no-load The waveform rule and harmonic distribution of three-phase excitation current are obtained by calculation and analysis,and the accuracy of the model is verified by comparing with single-phase transformer.The rated load is connected to the medium voltage side under load,and the transformer is a six winding coil.This thesis studies whether the primary and secondary current waveforms change under different DC biases,and whether the excitation current waveforms are different from those under no-load conditions.At the same time,due to the mutual influence between the three phases,there is a certain coupling relationship.This paper focuses on the analysis of several special moments when the excitation current waveform is distorted,and obtains the transformer magnetic field distribution at that time and more detailed solutions The change rule and reason of three-phase current under DC bias are explained.In this thesis,through the calculation and analysis of DC magnetic bias of three-phase five column transformer under no-load and load conditions,the waveform variation law of three-phase winding current and excitation current is obtained,and the magnetic field distribution of transformer is analyzed.The analysis results provide a basis for the model coupling and calculation of three-phase transformer and the influence of DC magnetic bias on transformer.