Research On Excitation Characteristics And Component Loss For AC-DC Hybrid Operation Of Transformer

The operation mode of single-gigantic loop or bipolar unbalanced operation in high voltage direct current(HVDC)lead to potential difference between different locations,and solar activity generates the geomagnetic induced current(GIC),which will disturb the electromagnetic equipment such as transformer and iron core reactor grounded at neutral point.At the same time,DC-driven rail transit and a large number of non-linear power electronic components may produce direct current when running,which makes AC system contain high DC component,and lead to magnetic bias of electromagnetic equipment such as transformer.Local loss and temperature rise of transformer under DC disturbance are the research contents attracted much attention.In this paper,relying on the project of National Natural Science Foundation of China "Research on DC bias protection strategy of transformer based on identification of key parameters in AC-DC hybrid environment",the mapping relationship among electromagnetic characteristics,identification methods of key electromagnetic parameters,component loss calculation and abnormal characteristics of transformer in AC-DC hybrid mode is studied.Aiming at the problems of distortion of exciting current,increase of harmonic content,core excitation saturation and loss increase in AC-DC hybrid mode of transformer,the model of transformer's non-linear magnetic field and equivalent circuit is established.Then the electromagnetic coupling between magnetic field and circuit is realized by energy balance principle.Furthermore,constructing state equation of transformer in AC-DC hybrid mode based on electromagnetic coupling principle,the magnetic field are solved by finite element method and circuit are computed by high order numerical iteration method.Using port electrical information to study identification method of excitation current,then the excitation current is transformed by Fourier transform.The distribution of leakage and eddy current loss in transformer components under different operation modes,DC levels and load rates is further studied.The variation law of excitation current distortion,harmonic content,saturation state of core and components under different operation modes and different DC disturbances is deeply analyzed.On this basis,the intrinsic relationship between abnormal characteristic information is explored,and the excitation saturation state is characterized by harmonic content.A criterion of core saturation instability under DC disturbance of transformer is proposed,and a mathematical model of mapping relationship between excitation harmonic and component loss is constructed by fitting method.The validity of the identification method of transformer exciting current,the criterion of core saturation instability and the mapping method between abnormal characteristic information are verified by comparing the simulation calculation with the dynamic model experimental data,which provides a feasible method for judging saturation instability of transformer core and calculating component loss under AC-DC hybrid mode.