Eddy-Current Field Modeling And Losses Computation In Wound Core Of Tractive Transformer

There are 17 million transformers operating in Chinese power grid,with an annual power loss of 250 billion k W·h,exceeding the annual power generation of two Three Gorges hydropower stations.Because of interval impact characteristics of tractive load,the tractive transformer is mostly operated on no-load condition.It is always significantly concerned to decline the no-load loss of tractive transformer.Conventional laminated core in tractive transformer has the interlaminar joint to ensure the grain-oriented sufficiency of magnetic material.But as a result of the worse magnetic flux continuity,a few magnetic lines of force would inevitably overflow and probably enter the metal clamp,even bring about considerable energy dissipation.The largest characteristic of wound-core tractive transformer is through uninterrupted rolling into enclosed structure which eliminates the joint in corner region and makes the flux conform to magnetic orientation in arbitrary position.Thus core loss is further declined and primarily caused by eddy currents in electrical steels.To realize the design of large-scale wound-core tractive transformer better,the computational method to eddy-current losses is necessary to develop the targeted research.However,the present methods about eddy-current losses calculation have some limitations as:1)so-long multi-layers trapezoidal steel is consistently rolled into wound core,instead of traditional equal-width steel,which exhibits multilevel-circle section of gradual change with the increase of rolling ordinal numbers.Obviously,there is the complicated coupled relation on electrical performance between the laminations,which makes the conventional method be not appropriate anymore.So it is essential to rebuild the model and analyze by electromagnetic theory;2)the high-voltage large-capacity tractive transformer is made up of thousands of electrical steels.It would aggravate the non-uniform distribution of boundary electromagnetic quantities that is not considered by conventional method.Hence the eddy-current losses acquired by present method is less than the actual value;3)inrush current,direct-current bias and impact load with unbalanced property may lead to magnetic saturation of wound core.The no-load current is forced to be serious distortion and even emerge into additional losses under prominent nonlinear magnetic characteristic,where the original model to eddy-current field is no longer precise;4)it is lack of analytical formula or numerical method to eddy-current losses under fault situation based on thorough mechanism exploration.This paper has developed the intensive research to eddy-current field and its losses calculation of large-scale wound core of tractive transformer:1)Taking into account the grain-oriented magnetic property and geometric particularity of electrical steel,eddy-current losses formula of multi-layers trapezoidal steel is proposed,where the feature of important electromagnetic quantities is illustrated;2)The homogeneous concept for circle field region is proposed to solve the equivalent electromagnetic parameters in azimuthal coordinate combined with loss equivalence theory,then electromagnetic anisotropic homogeneous model on cylindrical coordinate is established;3)Divided-grade reluctance network model is created and relevance magnetic circuit length in different layers and boundary value of electromagnetic quantity is explored,then combined with decoupling analysis to frequency-division magnetization curve,the magnetic flux components individually by fundamental and harmonics are obtained,so the eddy-current losses analytical method of wound core that is uninterrupted rolling by fairly-long trapezoidal steel is proposed and eddy-current variety characteristic in wound core under abnormal condition,such as distorted excitation and overload;4)The boundary value of electromagnetic parameter in fault region is specifically defined by magnetic circuit analysis.Meanwhile,considering about extra eddy-current losses by contact resistance effect in actual lamination,the analytical model to calculate the eddy-current field on both-side burr effect is established,then computational method of eddy-current losses under local interlaminar fault is put forward.Finally,the fault simulation and no-load experiment are performed to prove the effectiveness and applicability of proposed method.