Losses Research On Converter Transformer Under Effects Of DC Bias And Harmonics

In the high voltage direct current(HVDC)transmission system,the working nature of converter station results in occurrence of DC bias and rich harmonics in the converter transformer.Consequently,harmonics may increase the loss of converter transformer,cause the damage of electrical insulation and local overheating.At the same time,DC bias will bring a series of threats to normal operation of converter transformer.The possible threats include distortion of exciting current,increase of harmonics and loss,aggravation of vibration and noise,loose of core belt,bend of core limb,and movement of core pieces.During the process of product design,it is very essential to analyze and calculate converter transformer loss under conditions of DC bias and harmonics accurately,so that the harmonics and loss can be reduced and stability of converter transformer can be improved.Since magnetic properties of electrical steel sheet can be influenced by different excitation conditions,accurate analysis of converter transformer electromagnetic performance relies on the precise description of electrical steel sheet magnetic properties.This paper measured and analyzed magnetic properties of electrical steel sheet under DC bias,non-sinusoidal excitation and different temperatures.The measured data is applied to the loss calculation of converter transformer corresponding operating state.The influences of the electrical steel sheet magnetic properties variation on converter transformer magnetic field and the loss were discussed and analyzed under conditions of DC bias and harmonics.The main contents are summarized as follows.Firstly,this paper introduces measurement methods of electrical steel sheet magnetic properties.Based on the measurement system of magnetic properties imported from Germany,the magnetic properties of electrical steel sheet under AC/DC magnetic field,harmonic magnetic field and AC magnetic field with different temperatures were measured.The magnetic properties of the oil tank material Q235 B steel were measured under harmonic magnetic field,including B-H curves,B-P curves and hysteresis loops.The influences of the variation of temperature,DC bias current,harmonic number and harmonic content on the electrical steel sheet magnetic properties were analyzed in detail.Secondly,according to the product model data from TBEA Corporation,the 3D main magnet field simulation model of converter transformer was established by the commercial FEM software of MagNet.The magnetic property variation of silicon steel sheet may affect the distribution result of main magnetic field and core loss of converter transformer under different DC bias conditions.In order to investigate and discuss this problem,based on measured data in chapter II,this part considers magnetic properties without DC bias at 20? and with different DC bias at 80? in the transient FEM analysis by the field-circuit coupled method.Eventually,the results were compared with experiment results.Finally,the effectiveness of the improved calculation method in this paper was validated.Finally,according to the product model data provided by TBEA Corporation,the 3D leakage magnetic field simulation model of converter transformer was established by the MagNet.In order to study the influences of the magnetic properties variation of silicon steel sheet and Q235 B steel on converter transformer leakage magnetic field and structural part stray loss under the action of harmonics,based on measured data in chapter II,this part considers the electrical steel sheet magnetic properties without and with harmonics at 80? in the transient FEM analysis by the field-circuit coupled method.Then the stray loss in structural parts of converter transformer was calculated under the conditions of harmonics by utilizing the IEC 61378-2 standard.The above calculation results were contrasted with the experiment results,and it verifies the effectiveness of the improved calculation method in this paper.