Influence of Short-Circuit Duration on Dynamic Stresses in Substations
The short-circuit peak current and the electromagnetic short-circuit peak force appear at about 10 ms after the short-circuit inception. However, the maximum value of dynamic stress due to the electromagnetic forces may occur much later and depends on the fundamental frequency of the structure. A decrease in dynamic stress can be obtained even if the short-circuit duration is much longer than 10 ms. In the presentwork, the dynamic stress on stiff conductors and insulators during the short-circuit, as well as, the subsequent free oscillation after fault clearing, are analytically determined and the influence of short-circuit duration on the dynamic stress in certain cases of busbar structures is investigated. Furthermore, the shortest short-circuit durationwhichresults in maximum dynamic stress is given for the entire range of the relevant mechanical fundamental frequencies.
The Response of Substation Bus Systems to Short Circuit Conditions Part I: A Comparison of Design Methods
The four methods proposed in the literature for the structural design of substation bus systems to withstand short circuit loads are compared in the case of a single, simple element of a substation. These comparisons show that the continuous, dynamic design method, which treats the distributed mass of each element, will provide more reliable designs than will the methods presently in use. They also suggest that this method will provide more economical alternatives for renovation of existing substations, where the cost penalty can be especially severe if the traditional static method is used. It is concluded that the development of the continuous dynamic method for use on a routine basis is warranted, in spite of its computational complexity.
Effect of stress conditions on collapse deformation behavior of a rockfill material
A set of saturated and dry-saturated large-scale triaxial tests were conducted in order to explore the effects of stress conditions on the saturation-induced collapse deformation behavior of a rockfill material. Specimens were tested in different confining pressures and for dry-saturated tests the specimens were submerged in various shear stress levels. The results indicate that in all of the dry-saturated tests a sudden reduction in the volume of the specimens was observed during the submerging process. The ratio of the minimum axial strength of a submerged specimen (at the end of the saturation process) to the shear strength of the specimen before saturation is defined as the coefficient of stress recovery, Csr. Results of the dry-saturated tests showed that this ratio increases as the confining pressure increases and decreases by increasing the shear stress level. According to the results of dry-saturated tests the values of internal friction angle reduction caused by saturation (Δϕc), the ratio of the elasticity modulus of the material after saturation to its elasticity modulus in dry condition, i.e., Ewet/Edry, and also the saturation induced sudden volumetric strain (εvc) decrease as the confining pressure increases. However the shear stress level does not have any meaningful effect on the variation of Dfc, Ewet/Edry and (εvc).