آنالیز بر روی واکنش دینامیکی لاین پیش نمونه از رساناهای های شکسته سرد

A full scale transmission line section of three continuous spans was established. With the consideration of the equivalent mass of the accreted ice, steel cables are used to simulate the iced conductors. For different types of conductors and ice thickness, broken conductor experiments were carried out. Under different broken cases, time histories of the tensions and displacements at the middle of conductor spans were measured. The first order damping coefficients of the line section for different broken cases were calculated. The Fourier transform of the experimental time history of the conductor tensions was completed. The dynamic impact factors of the conductor tensions were determined. The experimental results show that the impact effect is more significant for the location nearer to the break point. The dynamic impact factors decrease with the increase of the ice thickness, and the impact factors of conductors without accreted ice are much higher than those of conductors with accreted ice. With the increase of the ice thickness, the initial tensions before break as well as the ratios of the residual static tensions to the initial tensions increase. Nearly all the first peak tensions are close to the initial tensions for the broken cases with accreted ice. The damping coefficients determined by the experimental identification were applied to the finite element analysis (FEA) model. The stiffness of the accreted ice as well as the contact effect between the conductors and the ground are considered in the FEA model. The numerical simulations were performed for different broken cases. Both the residual static tensions and the first peak tensions by the numerical simulations were well agreed with the experimental values. The maximum differences are 5.6% and 12.9% respectively.

Accreted ice, galloping and strong wind can cause break of conductors or ground wires in transmission lines. In the broken process, high longitudinal unbalanced tension will be produced, which can cause transient impact on transmission towers. The stresses and displacements of transmission towers increase significantly. The tower will be collapsed and even a major collapse with cascading effect happens. Many Studies have been focused on load values and dynamic responses of the tower-line system by numerical and experimental methods in recent years. The numerical studies are mainly by two methods. The first is the static calculating program based on the equilibrium conductor length method [1] and [2], and the impact effect cannot be considered. The second is the dynamic analysis by implicit or explicit method [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] and [17], the dynamic responses from the broken conductors can be calculated by nonlinear transient analysis. Alan B. Peabody and G.McClure [11] have modelled the EPRI-Wisconsin line with broken conductors using ADINA. It shows that the first peaks of the insulator tensions were modelled accurately in time and magnitude. The second peaks were modelled accurately in time. However, the magnitudes were larger than those measured during the tests. A damping ratio of 0.5% was used in this study. Some scale and prototype tests for broken conductors have been carried out. Broken tests of conductors and insulators were completed in the Wisconsin tests line by Alian H.Peyrot et al [18]. Longitudinal loads and the impact effect on the transmission line were studied. John D.Mozer et al. [19] carried out a simulating broken test of a three span conductor- ground wire-steel pole system. The broken tensions of the conductors and the ground wires as well as the strains of the steel poles were measured, and the impact coefficients of the support structures from broken wires were determined. Geometrical proportion of the test model to the real 345kV transmission line is 1:30. The length of the middle span is 9.753m. The conductors and the ground wires were substituted by copper wires. The wire broken test was initiated by cutting a short length of string which had been inserted in the wire span to be broken. Dynamic responses of the pole-line system as well as the dynamic impact factors were determined. China Electric Power Research Institute (CEPRI) carried out a simulating broken test for a ±800kV tangent tower [20]. The span length of the broken span and the unbroken span are 55m and 103m respectively. Geometrical proportion of the model tower to the prototype tower is 1:2. The initial tension of the conductors is a quarter of the real tension. Broken of the bundle conductor was simulated by changing the conductor tensions. Displacements and strains of some typical members of the tower were measured. North-East Electric Power Design Institute of China completed a prototype broken test in a transmission line section [21]. The line section has seven continuous span conductors and the span length is 450m. Dynamic strains and displacements at the suspension points were obtained. It shows that the peak values of the dynamic strains usually occur with the second waves. The second peak values sustains for about 0.5s to 1.0s. Ratios of the peak value to the stable value are from 1.95 to 2.74. Heavily accreted ice is one of the main reasons causing conductor ruptures. After the serious ice disaster in South China in 2008, in order to ensure the anti-bending and anti-torsion capacity of transmission towers, the Technical Code for Designing of Overhead Transmission Line in Medium & Heavy Icing Area [22] was revised, and the icing rates were regulated for the calculation of broken tensions. In the past studies, the effect of the accreted ice was almost not considered in the broken analysis and experiments. Furthermore, the damping coefficients of the conductors were not measured in the experiments and an approximate value was assumed in the dynamic analysis. In this paper, a full scale transmission line section of three continuous spans was established. For different types of conductors and ice thickness, broken conductor experiments were carried out. Under different broken cases, time histories of the tensions and displacements at the middle of conductor spans were measured. Damping coefficients of the line section for different broken cases were calculated. Dynamic impact factors of the conductor tensions were determined. Damping coefficients by the experimental identification were applied to the FEA model. The numerical simulations were performed for different broken cases. Both the residual static tensions and the first peak tensions by the numerical simulations were calculated and compared with the experimental values. Moreover, according to the equilibrium conductor length theory, A MATLAB program was compiled to calculate the residual static broken tensions. The calculated results by MATLAB program were compared with the FEA values and the experimental results.

In this paper, experimental and numerical studies on the dynamic responses of a prototype line section with broken conductors were carried out. The numerical results were compared with the experimental results. Some conclusions were drawn as follows. (1) The impact effect is more significant for the location nearer to the break point. The dynamic impact factors decrease with the increase of the ice thickness, and the impact factors of conductors without accreted ice are much higher than those of conductors with accreted ice. The dynamic impact factors of the iced conductors are in the range of 1.47 to 2.98. (2) With the increase of ice thickness, the initial tension increases and the ratio of the residual static tension to the initial tension increases. The first peak tensions of the iced conductors are almost near to the initial tensions. (3) The first peak values of the conductor tensions decrease with the increase of the ice modulus. While the total tensions of the conductor and the accreted ice vary with a relatively little extent. (4) The conductor-ground contact effect has little effect on the first peak value and the residual value. For the second peak value, the conductor tension with contact effect is higher than the value without contact effect by about 17.5%. (5) The peak values decrease with the increase of the damping ratio. When the damping ratio is up to 0.10, the second peak value cannot be found obviously. The residual static tension varies little with different damping ratios. (6) The residual static tensions and the first peak tensions basically agree with the experimental values. The maximum error percents are 5.6% and 12.9% respectively.