The invention relates to a hybrid gas insulated transmission line for supply of power from the remote power generating stations to various load centers. There is always a strong
need for long distance power transmission line with large

capacity and high reliability.

There are disadvantages with the conventional overhead lines. These overhead lines are prone to environmental problems
To overcome the environmental problems underground cables like XLPE cables and compressed gas insulated transmission lines (CGITL) are being used.
There are disadvantages with the XLPE cables for transmission of power. The XLPE cables have a limitation of current capability.
There are also disadvantages associated with the CGITL installations being cost intensive are non viable for long distance transmission of power.

Therefore, the main object of the present invention is to propose a hybrid gas insulated transmission line (HGITL), which has the combined features of XLPE cables and CGITL.
Another object of the present invention is to propose H6ITL with lower potential and lower operating stress of the gas
insulation.
Yet another object of the present invnetion is to propose a HGITL having better gas insulation reliability resulting in
material saving.
According to the present invention there is provided a hybrid gas insulated tranmission line comprising a central stranded HT conductor, said stranded HT conductor is covered with cross linked polyethylene (XLPE) solid insulation and housed in a metal enclosure, characterized in that, said metal enclosure is filled with SF6 gas insulation, said SF6 gas insulation being stressed by a lower potential compared to compressed gas transmission insulated lines and said XLPE solid insulation is provided with a conducting screen over its outer surface and said enclosure being suitably grounded.

The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawings.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a cross-sectional view of the hybrid
gas insulated transmission line.
In accordance with the present invention it proposes a hybrid gas transmission line which combines the feature of XLPE cables and compressed gas insulated transmission line (CGITL).
The basic structure of (HGITL) comprises a central stranded HT conductor (1) covered with XLPE solid insulation (2), housed in a grounded (6) metal enclosure (3) filled with SF6 gas insulation (4). The conducting screen (5), of few mm, which is at floating potential is provided over the outer skin of XLPE (2). Here the XLPE insulation (2) operates at

a potential depends on dimensions of the central conductor (1), insulation (XLPE) (2) thickness and enclosure (3) diameter. This design is designated as HSITL is shown in Figure 1. In the above HSITL of the central conductor (1), thickness of solid insulation (2) and grounded (6) enclosure (3) are suitably designed for different voltage classes.
In this design of H6ITL by providing the conducting screen over the XLPE solid insulation (4) the line operates at a lower potential compared to similar size of XLPE or CBITL. This is because the insulated gas and the solid insulation share the system voltage. Thus the insulated gas is responsible for only a part of the voltage. Consequently the SF6 gas insulation (4) is stressed by the lower potential compared to CGITL.
Further, due to lower operating pressure in this design, the gas insulation C4) reliability is higher compared to CBITL, because of lower operating stress. The insulation (4) requirements (gas pressure and density) are proportionally reduced, resulting in material saving.

It is also possible to use mixture of SF6 gas with nitrogen carbon dioxide as insulation (4).
The current capacity of HGITL is also increased compared to XLPE and CGITL.
The working stress of XLPE insulation (2) is reduced compared to XLPE cable. Therefore, the occurrence of long term deterioration of the XLPE insulation (2) in HGITL due to internal electrified stress is considerably lower than for XLPE cable. This enhances the life and reliability of the equipment and at the same time current carrying capacityof HGITL is more than XLPE and CGITL cables.
The invention described hereinabove is in relation to a non-limiting embodiment and as defined by the accompanying claims.

WE CLAIM:
1. A hybrid gas insulated tranmission line comprising a central
stranded HT conductor (1), said stranded HT conductor is covered
with cross linked polyethylene (XLPE) solid insulation (2) and
housed in a metal enclosure (3) characterized in that, said metal
enclosure (3) is filled with SF6 gas insulation (4), said SF6 gas
insulation (4) being stressed by a lower potential compared to
compressed gas transmission insulated lines and said XLPE solid
insulation is provided with a conducting screen (5) over its
outer surface and said enclosure being suitably grounded (6).
2. A hybrid gas insulated transmission line as claimed in claim ,
1, wherein mixture of said BF6 with other gases like nitrogen,
carbon dioxide is optionally used as insulation.
3. A hybrid gas insulated transmission line as claimed in claim 1, wherein said grounded metal enclosure (3) and the conducting screen (5) over the outer surface of the XLPE insulation which operates at a potential depending on the dimension of the central conductor (1), thickness of insulation (2) and diameter of metal enclosure (3).

4. A hybrid gas insulated transmission line substantially as herein described and illustrated with the accompanying drawings.