Research On Thermal Overload Protection Oriented To Temperature Rise Characteristics Of Traction Transformers

Traction transformer is the key electrical equipment in the traction power supply system of the electrified railway.And it’s very necessary for railway transport to make sure the security and reliability of the traction transformer’s operation.At present,China’s electrified railways are developing into higher speed and more load.The installation capacity of traction transformers,which was designed according to some economic factors such as cost and twopart electricity price system,cannot meet the increasing traffic volume of each railway.Highpower traction makes its thermal overload more frequent and serious,which makes the temperature rise endanger the safety of winding insulation.The current overheat or overload protection is not suitable for protecting traction transformers,while thermal overload protection can take into account the two principles of overload needs and protection for insulation.but there is no thermal overload protection and thermal model for traction transformers currently.Therefore,in order to meet the development needs of railway transportation,this paper conducts research on thermal overload protection based on the temperature rise characteristics of traction transformers.Firstly,based on the structures and load characteristics of the traction transformer,the theory of heat generation and heat transferring process of the heat source inside its solid-liquidgas structure is analyzed to study the thermal characteristics of the traction transformer.This part of the thesis makes three-dimensional multi-physics coupling models of transformers representing the typical structure of traction transformers with finite element analysis software ANSYS,including single-phase,unequal-capacity combined V/v,and three-phase iron core V/v.The loss simulation calculation is carried out by Maxwell,and then Fluent is used to analyze their steady/transient temperature field,flow field and temperature rise characteristics under different load or solar radiation.This part provides an intuitive reference and basis for establishing thermal models for calculating the temperature of traction transformers.Secondly,according to the temperature rise characteristics of traction transformer,the Susa thermal circuit is chosen to establish its thermal model.This paper improved and derived thermal circuit models of single-phase and common V-connected traction transformers by considering the influence of sunlight and multi-winding heating.After discussing the influence of the error of the parameters required in the thermal model,this paper give the estimation method of the non-nameplate parameters.Building the improved thermal circuit solution model of the test transformer through Matlab/Simulink,and comparing the IEC,IEEE guide method and the test data to verify the thermal model of this paper has higher accuracy.If the hot spot temperature is calculated by measuring the oil temperature,the calculation accuracy can be further improved.The comparison results show that the improved thermal circuit method can meet the requirements of thermal overload protection.Finally,considering the overload requirements and safety considerations of traction transformer,this paper studied the protection temperature-based criterion based on the thermal aging theory of insulation,and then designed three-stage thermal overload protection including trip at thermal limit(section I),alarm at high temperature(section II),and response to its life loss(section III),and made a detailed analysis of its setting and action principle.The protection simulation model was built to verify that the three-stage thermal overload protection designed in this paper can directly reflect the temperature state and life loss of the winding insulation,so it can protect the traction transformer more flexibly and reliably under complex traction loads than traditional overload and over-temperature protection.This part provides a reference scheme for solving the problem of insulation thermal protection under thermal overload of traction transformer windings.