High Impedance Autotransformer Loss Calculation And Temperature Field Study

Auto-transformers have been widely utilized in power systems(especially in ultrahigh voltage power grids)for their superior characteristics of low material consumption,small volume,light weight,low loss,and the potential to be developed to large capacities.However,due to the complex electromagnetic structures of the autotransformers,high voltages and large currents both exist at the same time during operations,it's of important theoretical and engineering significance to calculate the magnetic field leakages and short-circuit impedance,analyze the losses in structural parts,and study the heat generation and dissipation capacities of auto-transformers.In this paper,the electromagnetic loss and temperature rise characteristics of autotransformers are systematically researched based on an OSFSZ-250000 k VA/220 k V oilimmersed auto-transformer prototype.The main research contents are as follows:(1)Based on flux leakage method and field-circuit coupled method,the shortcircuit impedance model for auto-transformers is established,and the influences of winding structure forms on short-circuit impedance are analyzed.A three-dimensional(3D)nonlinear eddy current field model in frequency domain considering the skin effect is established and the magnetic leakage fields and stray loss distributions in winding areas and structural components are analyzed.The calculation results are compared with the experimental values to verify the methodologies.(2)Since the local overheating phenomena may be caused by the magnetic flux leakage concentrations,the influences of the transverse vertical shielding,longitudinal shielding,and tank pressure plate embedded shielding structural parameters(shielding plate length,thickness,distance between the shielding plates,etc.)on the shielding effectiveness are analyzed,and the influence rules are summarized.A novel oil-tank magnetic shielding structure is proposed to further enhance the shielding effectiveness,and Taguchi method is used to optimize the structural parameters to minimize both the eddy-current losses in the oil tank and the material consumption of the shielding plates.The researches in this chapter can give certain guidance for the design of transformer shielding structures.(3)The electromagnetic-thermal coupled model for the structural parts of transformers based on the blocking theories is established for the precise prediction of the structural component temperature rises.The temperature rise distributions of the oil tanks,yoke clamps,and pressing plates with traditional and proposed shielding structures are analyzed and researched to validate the shielding effectiveness in the thermal aspect.The calculation results are compared with experimental ones to verify the accuracy of the simulations.(4)The 3D fluidic-thermal coupled model of the transformer winding regions is proposed based on finite volume method(FVM),and the oil flow and temperature rise distributions are analyzed considering the axially unbalanced loss distributions of the windings.The methodologies are validated by experimental results.The influence of different inlet velocity/temperature,different number of pads and other factors on the distribution of winding oil flow and temperature rise are compared and analyzed.It provides the basis for oil circuit and thermal design of transformer winding area.