3-phase encapsulated Pressurized Air Cables for 420 kV: Design, Optimization and Application

Pressurized air as insulation medium replaces SF6 gas and alternative PFAS gas mixtures. The dielectric design using air requires larger aluminium enclosure- and conductor diameters and therefore define its main characteristics: Very large conductor- and enclosure cross-sections, reduced capacitance, fire protection due to metallic enclosure, and reduced reactive power.

The outer dimensions of PAC busbar flanges nevertheless match the dimensions of traditional

SF6 busbar designs due to the proprietary, boltless flange design and up to 10 bar filling pressure.

Like gas-insulated lines and switchgear, PAC can be single-phase enclosed, or three-phase enclosed where all 3 phase conductors are arranged triangularly in the same enclosure. Threephase enclosed designs lead to a more compact overall arrangement, around 30 % less overall weight per meter and less induced enclosure current for the same current and voltage ratings – which makes them the dominant design up to 145 kV voltage range. No three-phase enclosed gas-insulated products are in use today for 420 kV, mainly due to manufacturing difficulties for the large enclosures as well as the complexity of three-phase spacers. The round shape of traditional GIL conductor tubes does not optimally use the available space inside a round enclosure tube. In this paper, a new conductor shape for three-phase enclosed pressurized air cables is proposed, which combines the boltless flange design with optimally shaped conductors in a triangular arrangement. Dielectric optimization using a generic optimization algorithm has been performed to find the optimum conductor shape for any given enclosure diameter and insulation level. Compared to round conductors in the same enclosure, the maximum dielectric stress can be reduced by ~3 % while doubling the conductor crosssection at the same time. This result enables compact, 3-phase enclosed pressurized air insulated busbars for rated currents up to 5000 A for transmission voltage levels up to 420 kV.

A comparison of technical parameters for the application of single-phase enclosed designs vs.

three-phase enclosed designs using the optimum conductor shape is presented. This comparison allows to choose the right system design for typical applications at 245 kV and 420 kV rated voltage.

It is concluded that the proposed new design using shaped conductors in combination with the boltless flange design enables compact, three-phase enclosed pressurized air insulated busbars with transmission capacity up to 3600 MW for a single system at 420 kV voltage levels inside a 700 mm enclosure diameter.