Study On Thermal Time Constant And Load Characteristics Of Epoxy Resin Cast Dry Transformer

Epoxy resin cast dry-type transformer has many advantages,such as fire prevention,disaster prevention,maintenance-free,etc.It is widely used in load center and some special site.Overheat of the hot spot temperature in the winding and aging of the insulation are the main reasons for dry-type transformer failure,which is closely related to the selection of insulation materials and the design of temperature distribution of the winding.The thermal time constant reflects the speed of transformer winding temperature change when the load changes.It is very important to determine the hot spot temperature and aging rate of the dry-type transformer with the transient load,especially under short-term emergency overload,and it is the basis of the load guide and the determination of the overload curve.The traditional epoxy resin cast dry-type transformer is researched in this paper.The calculation principle and influencing factors of the thermal time constant of the casting winding is analyzed.Through the tests of the model winding and whole machine,the thermal time constant of the heating process at different load rates and different locations of the measuring points are tested and researched.The thermal time constant of the heating process at each measuring point under different load rates is obtained.The thermal time constant of the heating process at different measuring points is obtained.The average temperature rising and cooling thermal time constant of winding in cooling process is obtained by converting resistance measurement in cooling process.The relevant data are analyzed and compared,and the relevant laws are deduced and analyzed,so as to determine the influencing factors of thermal time constant of transformer.In order to get the relationship between the average temperature constant of winding conductor and the change of winding with current further,this paper innovatively proposes that the high voltage winding of epoxy resin cast dry-type transformer should be directly applied by DC power supply,and the input DC voltage should be changed to keep the DC current of winding constantly.According to Ohm's law,the winding should be converted from the relationship between DC voltage and DC current.According to the relationship between resistivity and temperature of metal material,the average temperature of winding could be calculated,and the curve of average temperature rise with time can be obtained directly.The thermal time constant of average temperature rise of winding can be calculated after fitting.This method can effectively avoid the influence factors such as reactive power,directly obtain the winding conductor temperature rise,simple and intuitive,and more directly reflect the relationship between the winding thermal time constant and load rate.Based on the research of thermal time constant,the relationship between thermal life aging rate and hot spot temperature of dry-type transformer windings is studied,and considering the influence of thermal time constant on hot spot temperature change when load changes,the curve of thermal life loss with load rate could be calculated.The relationship between thermal time constant and aging rate is calculated and analyzed through with a practical example,and the relationship between thermal time constant and hot spot temperature is calculated and analyzed by using EXCEL calculation tables to guide practical application.