Analysis On Magnetic Field And Loss Characteristics Of Converter Transformer Under DC Bias

As the exchange and rectification component between the AC and DC power grid,the safe and stable operation of converter transformer are becoming one of the hot topics for engineering designers with continuous construction of HVDC transmission network.As converter transformer is located at the connection of AC/DC power network and affected by the operation mode of DC grid,it leads to serious harmonic and is susceptible to DC bias effect.With the excitation current distortion of converter transformer under DC bias,the core is over saturated and the magnetic flux leakage is increased,which results to the losses of core tie-plate,clamp and tank increased,and easily causes local overheating problems.Therefore,the study of magnetic field and losses characteristics of converter transformer has vital theoretical and practical significance.In this paper,the thorough researches and systematic investigations are proposed from three aspects,including the calculation of excitation characteristics,the modeling and calculation of losses and the magnetic-thermal properties of the converter transformer,and the main researches are as follows:(1)A single-phase three-column transformer is used as the prototype to calculate the excitation current through the finite element method and the dichotomy method,the dichotomy approach is determined to calculate the excitation current of converter transformer under DC bias through comparing the simulation results with the experimental ones.Then no-load excitation properties of an actual converter transformer are analyzed,and Fourier decomposition of obtained currents are calculated to study the spectrum characteristics of no-load excitation current under DC bias.The mentioned calculation and analysis lay the foundation of accurate calculation on the magnetic field and the losses distribution of the converter transformer under DC bias.(2)A three-dimensional calculation model of harmonic loss of converter transformer under DC bias is established based on surface impedance boundary method,and the ZZDFPZ-321100/500 converter transformer is analyzed and studied.The error between calculated harmonic losses of converter transformer under rated operation and results of experimental measurement can meet the engineering requirments,and it is less than the calculated error of analytical method.On this basis,the characteristics of leakage magnetic field and harmonic losses of converter transformer under different DC bias current are studied,which provides a more efficient and reliable means for the calculation of the losses of converter transformer and the optimized design of products.(3)Aiming at the problem of temperature rise of structure parts of converter transformer,the core tie-plate test model of magnetic leakage and temperature rise is designed,and simulation and experimental studies are conducted.After which,a numerical calculation method based on magneto-thermal-fluid coupling is proposed.By experimental verification,the calculated error of magnetic field and temperature rise are less than 5% and 10% respectively.Subsequently,this method is applied to analyze and calculate the temperature rise of structural parts of converter transformer under DC bias.Through the coupling numerical calculation on the structure parts of converter transformer without DC bias current and with different bias current,the law of the temperature rise of structure parts following changed DC bias current is obtained,which provides an effective analysis to calculate the magneto-thermal issues of structure parts of converter transformer more accurately.