In order to improve the electric fuse features, a new concept of controllable fusing has been patented. The controllable fusing means the possibility of fuse to operate at certain time moments when an external command is activated. The experimental set up has been built on the basis of a virtual instrument application. Using this instrument, there is the possibility to acquire the current waveform which flows through the fuse and set the current threshold when a command signal is provided in order to blow the fuse on the basis of controllable fusing effect. Actually, there is implemented the time-current characteristic of the new fuse, and depending on the application the moment of circuit interruption can be delayed. It results an adjusted new time-current characteristic for the fuse.

Control Method for Electric Fuses with Controllable Fusing

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.

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Labview application to control a new type of high breaking capacity fuse

Results of a research program leading to the development of a new current limiting fuse device are described. The electrothermal properties of various metals were determined for optimal analysis. A new element meterial was developed. It is made from zinc and aluminum bonded together in a composite form. The composite fuse, rated general purpose, has a slow melting characteristic. A size improvement permits direct replacement for the expulsion fuse tube in distribution cutout supports.

A.C. Westrom

Papers

The Composite Fuse - A New Technology For Current Limiting Fuses