Computation Of Electromagnetic Characteristics And Eddy Current Loss For A No-load DC-biased Ultra High Voltage Transformer

In recent years,with the rapid spread of EHV and UHV DC transmission lines at home,the convenience of electricity transmission has achieved,while the effect of DC-biased problem caused by DC transmission on the earthing transformer in the AC system is increasingly frequent and severe.UHV transformer is one of the core components of UHV power grid and its operation reliability and safety is very important to the normal running of the power grid.Based on the ideas of field-circuit coupling method to calculate the bias current,the field-circuit coupling calculation process and the structure character istics of the UHV transformer has been analyzed detailed,in view of the problems arised from field-circuit coupling algorithm application in DC-biased calculation for UHV transformer and the actual parameters of UHV transformer,transformer core winding model is set up and regular mesh unit division method is used in establishing DC-biased magnetic field calculation model,so field model node unit scale is reduced effectively;According to the electrical connection diagram of UHV transformer,through the way of tandem fixed resistance and compensating voltage to solve the problem of longevity of transient process;Using the fourth order runge kutta method to solve time-domain current,while estimated current of the intermediate point should be returned to the field model,in order to reduce the inductance fluctuation,improve the calculation accuracy.Through the three steps to build the field-circuit coupling calculation model,suitable for no-load current calculation of DC-biased UHV transformer.In this thesis,on the basis of the model to further simplify the calculation process,improve the efficiency of computing,by getting UHV transformer inductance-current curve and exciting current peak-DC curve,we put forward a kind of real-time model for DC-biased exciting current calculation.Compared with the time-domain magnetic-circuit coupled method,the real-time calculation is proved to be exact.Through selecting the optimal iterative algorithm and applying a variable step strategy,we can get the corresponding waveform of exciting current with the dc bias in the millisecond time.The real-time model has widespread application prospect in online assessment on electric parameter perturbation due to the DC-biased problem of UHV transformer,preventing refusing action or incorrect tripping of power system relay protection and other electrical equipment safety operation and so on.Established the eddy current loss calculation model for DC-biase UHV transformer.First,we get a series of bias current by means of the real-time algorithm then,we can calculate the eddy current effect,study the eddy current loss of middle plate,the column pull board,clip pieces,copper shield and tank,which provides a reference for further research on temperature rise effect and evaluation index of DC-biase UHV transformer.