Calculation Of Converter Transformer Temperature Rise Under The Conditions Of DC Bias And Harmonic

In recent years,the development of China's HVDC project has been reached an unprecedented height.Converter transformer is an important equipment in the HVDC transmission system.The phenomenon of DC bias caused by DC power grid,geomagnetic storms and other factors increased gradually,coupled with the characteristics of DC transmission,so it will bear the AC voltage and DC voltage inevitably in its operation.For the converter transformer,its waveform of the exciting current will be distorted in the case of DC bias.The greater the bias current,the more prone to appear the tip wave current waveform which is severe saturation for semi-periodic,and make its leakage of the magnetic flux add,resulting in its loss increases and then causing the local overheating.The converter transformer's load current contains a large number of higher harmonic components led to the leakage of the harmonic magnetic field and high harmonic loss in the winding and metal structure.The fundamental harmonic loss and the higher harmonic loss work together to the structural parts of the transformer.If the distribution of this kind of loss is uneven and concentrated in a certain area of the structure,the local overheating will be caused and then the normal operation of the converter transformer will be affected.The local overheating will cause the aging of the insulation and then affect the service life.Therefore,it is important to calculate the temperature rise of the converter transformer under the two special cases.In this paper,the main work has the following two points:Firstly,the experimental method has been used to measure the magnetic characteristics of converter transformer's silicon steel sheet under the DC bias and harmonic conditions.Secondly,based on the principle of finite element simulation,the temperature field of an actual converter transformer are calculated under the different DC bias,the 5th and 7th harmonic conditions.The temperature field distribution under two circumstances are contrasted:(1)the nomal magnetization curve and the DC bias magnetization curves are employed respectively;(2)the nomal magnetization curve and the harmonic magnetization curves are employed respectively.The inhibiting effect of the 4mm copper shield for the leakage magnetic field caused by DC bias and harmonic are simulated and analysed.The experimental and simulation results show that there will be local overheating in the converter transformer under the conditions of DC bias and harmonic.And this kind of local overheating will have a strong impact on it's working life.The computation about temperature rise in the case of DC bias and harmonics in this paper lays the foundation for the further study of the temperature rise of the converter transformer.