Transient And Steady State Performance Of The Electric contact

The behavior of electrical contact is well predicted by contact resistance, so its measurement is ,helpful for testing contact material under various conditions and for applications. The aging rate is increased rapidly with the increase in the voltage DROPacross the interface. Thus the a-spot temperature is a very important parameter with regard to aging of the contacts. Busbars are normally subjected to mechanical andthermal stresses at the busbar ends. Failure of high current busbar system normally occurs at the joints which represent the weakest points in the system. Improving the joint performance is of great importance to maintain the suitability of high current busbars.In this thesis, the effect of the joint length and the clamping force at different loading currents on the joint performance are studied. Both contact force and geometry of the joint play an important role in the electrical performance of aged contacts. The current-carrying areas of the interface between two meted electric conductors is usually sensitive. The fundamental mechanism behind the observeddynamic behavior of contact spots is very sensitive to the temperature. The life time of joints given by the time at the end of which either the insulation of the contact byoxidation is complete (break of contact) or when the softening temperature of the metal is reached. The factors affecting the joint life time are analysed. Also, the ageingmechanisms in the electrical contacts and an algorithm for their life time arehighlighted. The proper operation of the joints improves the continuity and reliability of the systems.The effect of a bad contact of the fuses on the phase balance of short cable connections, in which the resistance of the cables is negligible with respect to the contact resistance of the fuses, is studied. Also, the unbalanced condition whichresults from the unequality of the contact resistance of the three phase joints is analysed. Field experimental data for the effect of contact resistance on the currentdistribution of high voltage cable ends has been obtained. The main problem in highcurrent busbars is the temperature rise. This temperature rise has the bad effect oflimiting the current carrying capacity of these busbars. The evaporative coolingmethod of high current busbars is used for hollow busbar types to increase theampacity and to reduce thermal stresses at the busbar ends. By applying this method, the transient and steady state temperature of the busbars are decreased, and thus theampacity of the busbars will increase. Also, the power losses will decrease and energy can be save. On the surface of contacts which are expose to the atmosphere, the reaction with gases in the atmosphere produces contaminant films including oxides.The contact reliability is degraded by the contaminant films. Humidity in theatmospheric environment also influences the surface of the contacts. In this work,influence of humidity on the copper copper coated surface joints is studied. Oxidefilms on contacts create an excess of the electric resistance which can cause failures incontact applications. Coating of aluminium or copper busbars with different metals isone of the most common commercial practices used to improve the stability of theconnections and to prevent or delay the onset of oxidation particularly in the long term. Effect of coating material on performance of electric copper joints is presented.Also, effect of dust and coating on life time of joints is studied.