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Underground cable connection to GIS Equipment

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    Direct link of an oil or solid isolated underground cable will require a special enclosing joint to bridge the cable isolation to the gas isolation of the GIS equipment. This is the reason isolator cones are used and they are isolated with SF6 gas on the GIS equipment side and with oil or solid isolation fittings on the underground cable side. The internal isolator cones are pressure equipment made to defy the GIS equipment gas pressure, which is generally 0.6 MPa to 0.8 MPa. The copper conductor of the underground cable and the aluminum conductor in the GIS equipment are linked by an integrated conductor in the internal isolator cone. The outer enclosing is linked to the underground cable shield or in some situations high voltage cable shielding is not linked to the GIS equipment enclosure to avert induced currents in the cable shield, which may heat up the underground cable. Another reason for isolating the underground cable shield from the GIS equipment grounding is in the case of cathodic corrosion protection of the underground cable. In these situations an isolating ring is used at the GIS equipment.

    If such isolation rings are utilized, in the situation of disconnector switching in the GIS equipment, the generated high voltage transient over-voltages may induce lightning flashovers across the insulation ring. This creates noises and light flashing and may end in a staff accident.

    Generally, the lightings are not dangerous for the GIS equipment or the underground cable, but they are for staff around the GIS equipment. Hence, in such situations it is advised to use surge arresters across the insulation ring to get around the high voltage transients.

    Due to the high frequency of the transient voltage of several hundred MHz, it is essential to locate the surge arresters around the insulating ring. A minimum of four arresters are advised.

     

    Reference:

    Continuing Education and Development, Inc. 9 Greyridge Farm Court Stony Point, NY 10980

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