Study On Magnetic Field And Loss Distribution Of Transformer Under No-load Over-excitation

As one of the most important equipment in the power system,the normal operation of transformer is vital to the safe transmission of energy.Due to the current expansion of the size of the power system and the increase in the capacity of the single unit of the generator,the single installed capacity and working voltage level of the transformer are also continuously increasing,which brings new challenges to the manufacture and normal safe operation of the transformer.When the transformer is under no-load operation,due to the increase of the grid voltage or the decrease of the frequency,it can be in an over-excitation running state.If the duration of the transformer’s no-load over-excitation operation is long,the increase of core magnetic density will cause core high saturate and the loss will increase during the operation period.The core temperature rise will eventually cause the transformer insulation structure damage.This damage will affect the safe and stable operation of the power system.Therefore,it is of great practical significance and engineering application value to study the magnetic field distribution and loss of the transformer under no-load over-excitation.This thesis first calculates the no-load over-excitation current of the square core model and the typical transformer based on the measured data of the core magnetization performance and the magnetic circuit-circuit coupling method.The peak and effective values of the current calculation results are compared with the measured values,the accuracy of the analytical method has been verified.The no-load current harmonic content of the transformer under different no-load over-excitation multiples is analyzed,the change law of the no-load excitation current waveform and harmonic content is also obtained.Then,a three-dimensional transient field calculation model of a typical transformer is established using the Mag Net finite element field-circuit coupling method.The winding terminal voltage and harmonic content under no-load over-excitation are calculated and analyzed.With the increase of the over-excitation multiple,the variation law of the voltage waveform and harmonic content is obtained.The voltage waveform distortion is more serious at high over-excitation(1.2 times and above)operation,which has not met the national standard definition of sine wave.Finally,the three-dimensional transient magnetic field and core loss distribution of the transformer under no-load over-excitation are calculated and analyzed,with the increase of the over-excitation multiple,the distribution of the core magnetic flux density gradually increases and the local saturation of the core becomes more and more severe.In view of the serious distortion of the voltage waveform of the transformer under high over-excitation conditions,this paper compares and analyzes the advantages and disadvantages of the three original Steinmetz formula correction methods for calculating core loss,thereby determining that the Steinmetz waveform coefficient correction formula is more suitable for calculating the transformer core loss under no-load over-excitation.It provides a reference for the analysis of the electromagnetic thermal performance of the transformer under no-load over-excitation.