Study On The Core Magnetic Loss Of Harmonic Transformer Based On Steinmetz Model

In recent years,with the development of high-voltage and ultra-high voltage DC transmission,deep coupling of AC and DC,and the wide application of power electronic equipment,the electromagnetic environment in the power grid is more complex,and the harmonic pollution is more and more serious.As one of the core power equipment in power grid,transformer is responsible for the transmission and transportation of electrical energy.Core is the key component of transformer,and its loss value accounts for a large proportion in the total loss of transformer.Therefore,accurate calculation of core loss under complex harmonic excitation is very important to optimize the structure of large power transformer and improve the transmission efficiency of power system.The core of transformer is composed of tens of thousands of silicon steel sheets.The magnetic properties of silicon steel sheets are nonlinear and anisotropic,which makes it difficult to analyze the magnetization and loss characteristics of the core.At the same time,the magnetization and loss characteristics of transformer core under complex harmonic excitation are different from those under sinusoidal excitation.Based on the above two points,this paper analyzes the magnetization and loss characteristics of transformer core under harmonic excitation,and carries out specific work for the calculation of core loss.The main work of this paper is as follows:1.The measurement of magnetic properties of silicon steel is the basic work of core loss calculation.In this paper,a magnetic properties measurement system of silicon steel sheet based on Epstein square circle method is built to measure the magnetic properties of iron core materials.The traditional core process coefficient is introduced to make up for the influence of the difference between the magnetic properties of single silicon steel sheet and laminated core on the loss calculation.At the same time,considering the uneven distribution of the magnetic flux density in the "joint area" and "column yoke area" of the core,the traditional core process coefficient is improved.2.A test platform of core magnetic loss under harmonic excitation is built to analyze the magnetization and loss characteristics of the core under harmonic excitation,and further investigate the influence of harmonic characteristic quantities(harmonic content,harmonic order and harmonic phase)on the magnetization and loss characteristics of the core of the transformer.3.The applicability of the original Steinmetz equation in the calculation of core loss under harmonic excitation is analyzed.The modified Steinmetz equation,the generalized Steinmetz equation,the improved generalized Steinmetz equation and the wave coefficient method are theoretically derived.For the calculation of core loss under complex harmonic excitation,the accuracy of the above formula is examined in combination with the improved core process coefficient.Based on the calculation results of the improved generalized Steinmetz equation,the influence rules of different harmonic content,harmonic order and harmonic phase on the magnetic loss of transformer core are summarized.4.The method of homogenization is used to model the laminated core,and considering the magnetization characteristics of the weak magnetic region of silicon steel material,the accurate solution of the electromagnetic field of the laminated core under harmonic excitation is realized.In order to consider the influence of local hysteresis loop on core loss,a calculation method of core loss considering local hysteresis loop is proposed by using the finite element software Mag Net and the improved core process coefficient.The calculation results of this method are compared with those of the improved generalized Steinmetz equation.