221 / 2024-05-06 10:46:23

Magnetic Field Interference among the Three Phases in a Common-Tank GIS with AMF VIs

GIS,AMF

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.  Dielectrically, the other 2 phases, as well as the grounded tank, greatly depress the electric potential of the floating shield (or shields) of the vacuum interrupter (VI) under test, causing very high asymmetry, between the fixed and the moveable ends of the VI, in the electrical field distribution inside the vacuum arc chamber. On current interruption, it is known that the significant net transverse magnetic field (TMF) generated by the flow of current in all 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 the arc diffuse. Using commercial finite element analysis software (ANSYS Maxwell), 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 bus-bar GIS, vs for a double bus-bar 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. The results indicate that these parallel bus bars do increase the TMF very significantly, vs. without their presence, and that, in a synthetic test for T100a, selecting one of the two side phases, instead of the middle phase, to be the first-pole to clear may help reduce the effects of the TMF and thereby potentially enhance the likelihood for the CB to pass the three valid shots.