Browsing by Subject "low temperature creep"

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Analiza charakterystyk naprężeniowo-temperaturowych napowietrznych przewodów elektroenergetycznych ze stopów AlMgSi
(Data obrony: 2006) Smyrak, Beata
Wydział Metali Nieżelaznych
The dissertation is about alloy conductor (AlMgSi) creep process in overhead power lines. Based on theoretical analysis it was proved that rheological state of an overhead conductor depends on the instantaneous relation between its stress-temperature rheological equivalent (creep function) and stress-temperature equivalent of the span (conductor sag state equation). Elimination of inactive rheologically supporting steel cores in aerial power linę conductors and replacing them with highly resistant AlMgSi ałloy conductors is forced by the need to constantly increase current-carrying capacity of power lines. This solution incorporates a series of potential benefits (lower weight of conductors, ease of assembly, no corrosive contact with the steel core, favorable resistance). However, removing the steel core may result in rheological lengthening of a conductor, an in effect lowering of the tension stress and increasing sag. Although the creep process is an unequivocal function of stress and temperaturę, its intensity depends not only on their values, but, above all, on the gradients of the values. In particular, when analyzing the mechanical condition of the conductor, it should be taken into account that negative gradients both of stress and temperaturę may lead to weakening, and even loss of the material rheological activity. The performed analysis of the AlMgSi alloy wire creep process in variable stress conditions allows for using them to estimate real stress changes in aerial power linę conductors. It is especiałly significant to properly include the rheological term in the conductor sag state equation. Permanent length increases originating from the creep process cause further loosening of a conductor, which causes increased conductor sag and increases the danger of establishing contact with the ground. Therefore the knowledge of creep deformation value corresponding to the real working conditions of a conductor will enable not only determining the safe operating level of a power linę, but also using the existing stress reserve to dynamically control its current-carrying capacity. The knowledge of the duration period and the periods of occurrence of rheological inactivity over the whole exploitation period of the conductor is of fundamental importance. These values may be controlled through proper choice of the season in which the conductor is assembled, through an initial overstressing or through choosing the optimal rheołogical equivalent, represented by the $ n/\phi $ ratio. Proper use of this phenomenon forms the basis of modern assembly technology design of cables in power overhead linę spans, taking into account the need of dynamie current-carrying capacity control based on the knowledge of instantaneous position of the conductor above ground. In practice this problem is solved using sophisticated measuring techniques, including the CAT-1 monitoring system, which enables determining the instantaneous conductor temperaturę based on the knowledge of the tension forces in a conductor and weather conditions. This approach is meant to utilize the temperaturę margin of operation regime of a conductor, which is established based on the thermal resistance condition of the conductor material.