Dynamic Response Of Iced Overhead Electric Transmission Lines Following Shock Loads

The shock load, caused by ice or ice-related sources, is one of the common types of adverse loads, which threat the mechanical security of overhead electric transmission lines in cold regions. Thus, it is significant to study the dynamic response of lines following typical shocks, for the structural design of transmission lines, the development and optimization of de-icing techniques and procedures, and the safe operation of the electric networks.This paper focus on the key issue in the dynamic response analysis of transmission lines following shock loads, i.e. the modeling of the ice detachment process and the prediction of the detached ice amount. By finite element (FE) methods, the dynamic response and influence factor of transmission lines following three types of typical shocks (i.e. the mechanical de-icing shock, initial ice shedding shock and cable rupture shock) were studied. The main contents are as follows,(1) An ice detachment failure criterion, considering the adhesive force at the ice-cable surface and cohesive force within the ice, was proposed and implemented into the FE modeling of the ice detachment process and dynamic analysis of iced overhead lines following shock loads. The criterion was validated by reduced scale and full scale mechanical de-icing tests and compared with previous ice failure criteria. It is found that the proposed criterion provides a much better prediction of the ice shedding rates and the dynamic response of the lines, and successfully explains the phenomenon observed in the tests.(2) By revising the proposed ice detachment criterion and previous strain criterion, the eccentric ice deposit was modeled for the first time. The revised criteria were validated by full scale de-icing test, and were used to study the dynamic response difference of lines with eccentric and concentric ice. The results showed that the eccentric ice is easier to be de-iced (as observed in the test) than the concentric ice, and the dynamic response of lines with eccentric and concentric ice are significantly different. Thus, it is necessary to differentiate the dynamic response of lines with eccentric and concentric ice.(3) With the proposed ice detachment criterion, dynamic response of lines during the shock-wave de-icing procedure and the related influential factors were studied, such as the characteristics and applying locations of the impact loads, the span lengths and the elevation difference. It shows that large amplitude single impulse may cause damages to the cable, and it can be replaced by several small amplitude impulses or small amplitude but long duration impulse; the FE simulation confirms that the impact load should be applied next to the tower; the de-icing rate increases with the decrease of span length, while varies little with the change of elevation difference. Besides, a total of 16 schemes were proposed and compared numerically, and the best method was selected to suppress the vibration of lines during de-icing shocks.(4) The ice detachment criterion was revised and implemented into the dynamic analysis of transmission lines following initial ice shedding shock, by which the induced-ice-shedding effect was considered for the first time. The revised criterion was validated by full scale ice shedding test and by comparing with previous numerical simulation methods. The time-varying characteristics of the system mass, stiffness and damping and their influence on the dynamic response of iced lines following ice shedding were investigated. Also, the influence of initial ice shedding amounts and locations were studied. It is found that the previous methods, without considering the induced-ice-shedding effect, largely underestimates the adverse influence of initial ice shedding; Hence, there is obvious necessity to use the proposed method for dynamic analysis of transmission lines following ice shedding; with the increase of initial ice shedding amount and with the initial ice shedding location getting close to the span midpoint, more ice shedding is induced, which results in more dramatic vibration.(5) By introducing the ice detachment criterion into the dynamic analysis of iced transmission lines following cable rupture shocks, the induced-ice-shedding effect was considered. The applicability of the proposed criterion in cable rupture analysis was validated by full scale ice shedding tests. The influence of numerical simulation methods, ice and the cable-ground contact effect was investigated. The result shows that the traditional equivalent density method overestimates and the strain criterion underestimates the dynamic response of the lines following cable rupture, and the key parameters calculated by the present criterion lies in-between the results obtained with the former two methods. Besides, the maximum dynamic cable tensions and insulator axial forces are greater than their initial values, which is opposite for the results of iced line models considering the induced-ice-shedding effect. Also, it is found that the cable-ground contact effect has obvious influence on the dynamic response of bare cable line model, while it has little impact on the dynamic response of iced line models considering the induced-ice-shedding effect.