It is well known that the hot spot temperature represents the most critical parameter identifying the power rating of a transformer. This paper investigates the effect of the degradation of core magnetic properties on...It is well known that the hot spot temperature represents the most critical parameter identifying the power rating of a transformer. This paper investigates the effect of the degradation of core magnetic properties on temperature variation of aged oil-cooled transformers. Within this work, 2D accurate assessment of time average flux density distribution in an oil insulated-cooled 25 MVA transformer has been computed using finite-element analysis taking into account ageing and stress-induced non-uniform core permeability values. Knowing the core material specific loss and winding details, local core and winding losses are converted into heat. Based upon the ambient temperature outside the transformer tank and thermal heat transfer related factors, the detailed thermal modeling and analysis have then been carried out to determine temperature distribution everywhere. Analytical details and simulation results demonstrating effects of core magnetic properties degradation on hot spot temperatures of the transformer’s components are given in the paper.展开更多
The pseudo-equilibrium phase diagram and continuous cooling transformation diagram of an N-containing 8％ Cr roller steel were investigated by using thermodynamic calculation,differential scanning calorimetry,Ⅹ-ray d...The pseudo-equilibrium phase diagram and continuous cooling transformation diagram of an N-containing 8％ Cr roller steel were investigated by using thermodynamic calculation,differential scanning calorimetry,Ⅹ-ray diffraction,expansion method,and so on.Under equilibrium conditions,the main carbonitrides are MX,M7C3,and M23C6 types.The measured Ac1,Ac3,start temperature of martensitic transformation,and M7C3 transformation temperatures are 811,855,324,and 1100 ℃,respectively.Bainite appears at cooling rates ranging from 0.5 to 5 ℃/s and ferrite forms at grain boundaries at a cooling rate lower than 0.5 ℃/s.Finally,the effects of adding N and lowering the C content on workability and mechanical properties of common 8％Cr steel were discussed.展开更多
The occurrence of γ phase, a high\|pressure polymorph of olivine （α phase）, in the shock veins of Si\| xiangkou chondrite was due to a greater cooling rate （>10 000℃·s -1 ） in the veins. Because γ phas...The occurrence of γ phase, a high\|pressure polymorph of olivine （α phase）, in the shock veins of Si\| xiangkou chondrite was due to a greater cooling rate （>10 000℃·s -1 ） in the veins. Because γ phase partially reverted to β phase and no back\|transformation from β phase to α phase took place, the shock veins of Peace River chondrite with a cooling rate of 1 000\2 000℃·s -1 contain a great amount of β phase. In the shock veins of Mbale chondrite with a cooling rate of <500℃·s -1 , the majority of γ phase reverted to α phase. The heat dissipation in shock veins took place after a stage of shock compression of chondrite parent body, and the parent body was broken into fragmental pieces. Cooling rate in the shock veins constrained the back\|transformations of （Mg,Fe）\-2SiO\-4 high\|pressure polymorphs.展开更多
This study aims to analyze rapeseed oil’s effectiveness as a coolant of a high-voltage direct current(HVDC)converter transformer while comparing it with mineral oil.We employed a numerical model,assisted by Computati...This study aims to analyze rapeseed oil’s effectiveness as a coolant of a high-voltage direct current(HVDC)converter transformer while comparing it with mineral oil.We employed a numerical model,assisted by Computational Fluid Dynamics(CFD)tools,to simulate the convective heat transfer inside a HVDC converter transformer,which is an oil-directed air forced(ODAF)type.The model seeks to obtain the temperature and velocity fields at the critical points of the fluid domain.The simulation domain reproduces a 2D cross-sectional model in the center of the structure.The results show that rapeseed oil requires more time to gain heat from the core and windings due to its lower thermal conductivity.However,the average temperature of rapeseed and mineral oils will not significantly differ once the channel temperature reaches a stable value.It indicates that the rapeseed oil can effectively replace mineral oil under the same heating conditions.Also,a slower flow of the rapeseed oil allows the fluid to smoothly enter the cooling channel.Therefore,the rapeseed oil cools these channels by pushing the fluid in them and absorbs heat from the sources earlier than mineral oil.The improvement in cooling indicates that rapeseed oil is better than mineral oil as a coolant and electrical insulating fluid for power transformers.展开更多
文摘It is well known that the hot spot temperature represents the most critical parameter identifying the power rating of a transformer. This paper investigates the effect of the degradation of core magnetic properties on temperature variation of aged oil-cooled transformers. Within this work, 2D accurate assessment of time average flux density distribution in an oil insulated-cooled 25 MVA transformer has been computed using finite-element analysis taking into account ageing and stress-induced non-uniform core permeability values. Knowing the core material specific loss and winding details, local core and winding losses are converted into heat. Based upon the ambient temperature outside the transformer tank and thermal heat transfer related factors, the detailed thermal modeling and analysis have then been carried out to determine temperature distribution everywhere. Analytical details and simulation results demonstrating effects of core magnetic properties degradation on hot spot temperatures of the transformer’s components are given in the paper.
基金Sponsored by the Natural Science Foundation of Hebei Province for Distinguished Young Scholars(Grant No.E2011203131)the Natural Science Foundation-Steel and Iron Foundation of Hebei Province(Grant No.E2013203110)
文摘The pseudo-equilibrium phase diagram and continuous cooling transformation diagram of an N-containing 8％ Cr roller steel were investigated by using thermodynamic calculation,differential scanning calorimetry,Ⅹ-ray diffraction,expansion method,and so on.Under equilibrium conditions,the main carbonitrides are MX,M7C3,and M23C6 types.The measured Ac1,Ac3,start temperature of martensitic transformation,and M7C3 transformation temperatures are 811,855,324,and 1100 ℃,respectively.Bainite appears at cooling rates ranging from 0.5 to 5 ℃/s and ferrite forms at grain boundaries at a cooling rate lower than 0.5 ℃/s.Finally,the effects of adding N and lowering the C content on workability and mechanical properties of common 8％Cr steel were discussed.
文摘The occurrence of γ phase, a high\|pressure polymorph of olivine （α phase）, in the shock veins of Si\| xiangkou chondrite was due to a greater cooling rate （>10 000℃·s -1 ） in the veins. Because γ phase partially reverted to β phase and no back\|transformation from β phase to α phase took place, the shock veins of Peace River chondrite with a cooling rate of 1 000\2 000℃·s -1 contain a great amount of β phase. In the shock veins of Mbale chondrite with a cooling rate of <500℃·s -1 , the majority of γ phase reverted to α phase. The heat dissipation in shock veins took place after a stage of shock compression of chondrite parent body, and the parent body was broken into fragmental pieces. Cooling rate in the shock veins constrained the back\|transformations of （Mg,Fe）\-2SiO\-4 high\|pressure polymorphs.
基金This work was supported in part by the National Natural Science Foundation of China under Grant No.51806064 and No.51776066.
文摘This study aims to analyze rapeseed oil’s effectiveness as a coolant of a high-voltage direct current(HVDC)converter transformer while comparing it with mineral oil.We employed a numerical model,assisted by Computational Fluid Dynamics(CFD)tools,to simulate the convective heat transfer inside a HVDC converter transformer,which is an oil-directed air forced(ODAF)type.The model seeks to obtain the temperature and velocity fields at the critical points of the fluid domain.The simulation domain reproduces a 2D cross-sectional model in the center of the structure.The results show that rapeseed oil requires more time to gain heat from the core and windings due to its lower thermal conductivity.However,the average temperature of rapeseed and mineral oils will not significantly differ once the channel temperature reaches a stable value.It indicates that the rapeseed oil can effectively replace mineral oil under the same heating conditions.Also,a slower flow of the rapeseed oil allows the fluid to smoothly enter the cooling channel.Therefore,the rapeseed oil cools these channels by pushing the fluid in them and absorbs heat from the sources earlier than mineral oil.The improvement in cooling indicates that rapeseed oil is better than mineral oil as a coolant and electrical insulating fluid for power transformers.
