Magnetic Field Interference Among the Three Phases in a Common-Tank GIS With AMF VIs

In a common tank gas insulated switchgear (GIS) the 3 phases are very close to each other. For an eco-friendly high voltage GIS with vacuum switching technology inside, this posts not only dielectric challenges but also difficulty in the current interruption performance of the vacuum interrupters. It is known that the significant net transverse magnetic field (TMF) generated by the flow of current in paths nearby the interrupting contact gap causes deflection and concentration of the intense plasma to one side off the center of the contact arcing surface, compromising the proper operation of the axial magnetic field (AMF) in driving and keeping the arc diffuse. Using ANSYS Maxwell finite element analysis software, this work analyzes the magnetic fields, TMF and AMF, in a common-tank GIS with triangularly arranged 3 phases of AMF VIs, with two practical questions in mind. One is how much is the difference in the degree of this interference of magnetic fields among the three phases, for a single busbar GIS, vs for a double busbar GIS (that has 3 bus conductors parallel and in close proximity to the VIs). The other is whether this interference has any significant influence, on parameters such as the minimum arcing time, the likelihood of passing the three valid test shots in the T100a test duty, and therefore in deciding which phase, the middle or the 2 side phases, to select as the first-pole-to-clear in a 3-phase synthetic type test. It is found that, because the current flow in the bus conductor is always opposite in direction to that in the VI, for any of the 3 phases, there is no significant and systematic (–meaning across all phase angles) difference in the net TMF between the double busbar and the single busbar configurations. The results show that, the middle phase experiences a higher TMF than the two side phases, and when it is the first phase to clear, the 2<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> and 3<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> phases see a higher TMF than when one of the side phase is the first phase to clear.