Adrian Plesca
Gheorghe Asachi Technical University of Iasi, Blvd. Dimitrie Mangeron, 21-23, Iasi – 700050, Romania, E-mail: aplesca@ee.tuiasi.ro
Abstract
One of the most widespread applications of fuses in medium voltage distribution systems is the overhead lines protection, but unfortunately and very frequently the people on charge of the fuse selection leave aside important facts of the protection. Usually the fuse is selected taking into account only the selective coordination with other up-stream and down-stream protective devices. Due to this procedure, really the overhead line is not protected against its main problem, the overheating. This over-heating is the cause of the two bare overhead conductors application limits, a. the mechanical strength weakening (annealing), and b. the minimum distance to earth (maximum sag). The paper firstly presents a summarized study of the thermal behavior of bare overhead conductors, under steady-state condition and also for transient conditions after a step change in current, analyzing the heat losses and gains, due to conduction, radiation, convection, overcurrent flow, sun effect, etc. The conductor over-temperature for steady-state conditions can be easily assessed by using a simple exponential equation, after doing two o three iterative temperature estimations and the corresponding recalculations. This analysis allows the determination of the conductor rated current, based on the maximum steady-state temperature normally recommended by the conductor manufacturers. The relationship between the conductor temperature and conductor lengthening and thus sag, is shortly explained. The general equation for transient condition after a current jump is given, explaining the temperature dependence of part of its terms and the difficulties for finding the accurate solution. A simplified solution by linearizing the non-linear equation terms is proposed, which is of easy application and also allows determining for each current value the time needed to reach the limit temperature. With the described methodology a time-current characteristic for the conductor could be draw which can be directly compared with the fuse characteristic in order to assess the protected and not-protected zones. By applying the cooling equation, the over-temperature as time function was assessed, in order to calculate the useful life lost and the conductor creep. This simple methodology allows a more rational fuse selection for the bare overhead conductor protection.
website development: NAVDIH.NET General Sponsor: 
