Concrete structures may suffer considerable restraint stresses during their hardening period. This is caused by several deformation impacts, especially temperature field changings due to hydration heat and volume chan...Concrete structures may suffer considerable restraint stresses during their hardening period. This is caused by several deformation impacts, especially temperature field changings due to hydration heat and volume changes due to autogenous shrinkage. Mainly affected are massive concrete members, but also the application of new concrete types or the erection of outstanding constructions requires further investigations in this context. 3D-FEM analyses of hydration heat induced temperature development in combination with the well known shrinkage give sufficient results for the deformation impact. The according elastic restraint stresses can be determined with consideration of the concrete’s rising elastic modulus and the restraint degree of the system. But due to duration of the heat flow process, the height of restraint stresses is strongly dependent from the viscoelasticity of the concrete. The viscoelastic effects consist of many components constituted by changing material properties influencing themselves. In practice, different simplified approaches are available for considering this in calculations. Their implementation in time step analyses is not generally admitted and requires expertise. In contrast, present research develops material models needing specific input parameters for every use case. This contribution focuses on a practicable approach considering the superposition of the viscoelastic behaviour of every stress increment in time step FEM analysis. The differentiation between the pure viscoelastic material behaviour (as it is given in the codes for idealistic conditions like creep or relaxation) and the according viscoelastic system response (addicted to the systems variable restraint degree) allows the transfer of this model into practice. As one application example of this approach, the compatibility check and the FEM-based recalculation of the monitoring program of a massive power plant slab will be presented.展开更多
在高压直流(high voltage direct current,HVDC)输电系统中,受铁心饱和及交直流场各种滤波器影响,相比常规变压器,换流变压器空载合闸时差动保护更容易误动,由于换流变压器工作环境的特殊性,在发生不对称故障时,可能在差动电流中存在较...在高压直流(high voltage direct current,HVDC)输电系统中,受铁心饱和及交直流场各种滤波器影响,相比常规变压器,换流变压器空载合闸时差动保护更容易误动,由于换流变压器工作环境的特殊性,在发生不对称故障时,可能在差动电流中存在较大的2次谐波,从而导致差动保护误制动。对换流变空载合闸和区内故障进行仿真分析,验证了2次谐波制动在HVDC系统变压器差动保护中确实有较大的局限性,同时具有误动和拒动的可能。经分析,由于变压器的铁心进入饱和需要一定时间,励磁涌流引起的虚假差动电流突变量出现时刻要比相电压突变量的出现时刻滞后一个时间差。据此,提出基于相电压突变量和差动电流突变量出现时差的变压器差动保护判据,运用该判据对各种换流变压器区内故障和励磁涌流进行判别。通过EMTDC进行了大量的仿真试验,结果验证了该判据的可行性和有效性。展开更多
文摘Concrete structures may suffer considerable restraint stresses during their hardening period. This is caused by several deformation impacts, especially temperature field changings due to hydration heat and volume changes due to autogenous shrinkage. Mainly affected are massive concrete members, but also the application of new concrete types or the erection of outstanding constructions requires further investigations in this context. 3D-FEM analyses of hydration heat induced temperature development in combination with the well known shrinkage give sufficient results for the deformation impact. The according elastic restraint stresses can be determined with consideration of the concrete’s rising elastic modulus and the restraint degree of the system. But due to duration of the heat flow process, the height of restraint stresses is strongly dependent from the viscoelasticity of the concrete. The viscoelastic effects consist of many components constituted by changing material properties influencing themselves. In practice, different simplified approaches are available for considering this in calculations. Their implementation in time step analyses is not generally admitted and requires expertise. In contrast, present research develops material models needing specific input parameters for every use case. This contribution focuses on a practicable approach considering the superposition of the viscoelastic behaviour of every stress increment in time step FEM analysis. The differentiation between the pure viscoelastic material behaviour (as it is given in the codes for idealistic conditions like creep or relaxation) and the according viscoelastic system response (addicted to the systems variable restraint degree) allows the transfer of this model into practice. As one application example of this approach, the compatibility check and the FEM-based recalculation of the monitoring program of a massive power plant slab will be presented.
文摘在高压直流(high voltage direct current,HVDC)输电系统中,受铁心饱和及交直流场各种滤波器影响,相比常规变压器,换流变压器空载合闸时差动保护更容易误动,由于换流变压器工作环境的特殊性,在发生不对称故障时,可能在差动电流中存在较大的2次谐波,从而导致差动保护误制动。对换流变空载合闸和区内故障进行仿真分析,验证了2次谐波制动在HVDC系统变压器差动保护中确实有较大的局限性,同时具有误动和拒动的可能。经分析,由于变压器的铁心进入饱和需要一定时间,励磁涌流引起的虚假差动电流突变量出现时刻要比相电压突变量的出现时刻滞后一个时间差。据此,提出基于相电压突变量和差动电流突变量出现时差的变压器差动保护判据,运用该判据对各种换流变压器区内故障和励磁涌流进行判别。通过EMTDC进行了大量的仿真试验,结果验证了该判据的可行性和有效性。
