Research On Losscharacteristics Of Transformercore Under Distorted Fluxconditions

With the rapid development of ultra-high voltage direct current(UHVDC)transmission technology,the waveforms of voltage and current in power system are distorted and serious harmonic pollution is formed in power grid.Converter transformer and smoothing reactor are important equipments in UHVDC transmission system.Under load conditions,the current in the windings of converter transformer and smoothing reactor is often accompanied by high-order harmonics.The existence of these harmonics increases the loss,temperature and noise of the transformer,which leads to the danger of local overheating and affects the normal operation of the transformer.Therefore,it is necessary to study the loss characteristics of transformer cores and structural under distorted flux conditions.Accurate core loss measurement is the basis of core loss calculation and analysis.Based on the core model of product-grade transformer,a magnetic performance test platform which can generate arbitrary harmonic excitation waveforms is built.Using the experimental scheme with controllable flux density,the effects of harmonic phase,harmonic content and harmonic order on hysteresis and loss characteristics of cores are investigated in detail.Based on the experimental data obtained from the magnetic performance test platform,firstly,the classical Steinmetz formula and Bertotti loss separation model under standard sinusoidal excitation are identified and validated;Secondly,based on the classical Steinmetz formula and Jordan formula,the magnetic loss of distorted flux is calculated by Fourier decomposition method.And through theoretical analysis and deduction,a method for calculating core loss under distorted flux action based on classical Steinmetz formula is proposed,that is,using equivalent frequency processing technology.At the same time,based on the loss separation theory,considering the influence of the waveform characteristics of distorted flux on magnetic loss,an improved Bertotti loss model is established by introducing correction factors to each part of core loss.Finally,in order to deal with the permeability and conductivity,a three-dimensional core simulation model is established by using the equivalent homogenization method.Considering the influence of the magnetization characteristics in weak sectors,the calculation error of low magnetic density end of core is reduced.And an effective method for core loss calculating under distorted flux condition is proposed by Mag Net,a software which is used for numerical analysis of electromagnetic field.The results and conclusions obtained in this paper have important engineering significance and application prospects for the optimal design of transformers and product heating analysis.