Current Interruption in Atmospheric Air
The principal function of air-break disconnect switches in high voltage power systems is to provide electrical and visible isolation of a part of the system. The isolation can be related to normal day-to-day operation of the system such as to repair or carry out maintenance on transmission lines and station equipment. Because the switches are operated under energized conditions, they are required to interrupt the currents associated with the switching event. The currents in question are of low magnitude and can be inductive (unloaded transformer magnetizing current), capacitive (bus or short line charging currents) or loop currents (current transfer between parallel buses with a substation or between parallel lines). The opening operation results in free burning arcs in air between the fixed and moving switch contacts and rules with respect to current interruption capability are empirical. Practice, particularly in North America, has been based on the ‘classic’ AIEE 1950 paper by Andrews, Janes and Anderson which related magnetizing and loop current magnitudes to the reach of the extreme point of the arc, all based on actual field tests. Later studies at BC Hydro and the Eindhoven University of Technology showed that the work of Andrew’s et al, however well-intentioned, was greatly flawed. The above-noted later studies starting in 1999 showed that very distinctive and unique features exist for each of the three current type arcs in terms how they evolve and interact with the circuit. The presentation will use mainly video material to explain the nature of the switching event when switching out unloaded transformers, breaking small capacitive currents and for loop switching. The use of auxiliary interrupting devices will also be discussed.
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