This invention relates to a gas insulated hybrid transmission line with auxiliary shield for high voltage power application.
The recent trend in electric power consumption shows a steady increase and concentration in large cities and industrial centres. Since, power has to be transmitted from the remote power generating stations to load centres, there is a strong need for long distance power transmission line with large capacity and high reliability. The conventional overhead lines are prone to environmental problems. These environmental constraints have been overcome by transmitting power using under ground XLPE cables and jcomprtressed Gas Insulated Transmission Lines (CGITL). However, XLPE cables have a limitation of current capability and CGITL installations are expensive. The Hybrid Gas Insulated Transmission Line (HGITL) has combined features of XLPE cables and CGITL.
The basic structure of hybrid gas insulated transmission lines comprises a central conductor covered with XLPE solid insulation and housed in a grounded enclosure along with high pressure SF gas. In the HGITL structure, dimensions of the central conductor, thickness of solid insulation and the grounded enclosure are suitably designed for different voltage classes.
There are disadvantages with the conventional hybrid gas insulated transmission line (HGITL).One of the main disadvantage is that the XLPE covering in conventional HGITL is floating and maximum utilisation of the solid insulation is not possible.
Another disadvantage associated with the conventional HGITL is that due high gas pressure insulation reliability is below acceptable standard.
Yet another disadvantage associated with the conventional HGITL is that the leakage current is high.
Therefore, the main object of the present invention is to provide an improved Hybrid Gas Insulated Transmission Line for optimised insulation utilisation.
Another object of the present invention is to provide a gas gap which requires low gas pressure and reduces leakage current to acceptable values.
Yet another object of the present invention is to provide a high gas insulations reliability because of low operating stress.
A further object of the present invention is to provide a safe grounded enclosure of hybrid gas insulated transmission line.
Yet a further object of the present invention is to provide reduced working stress of XLPE insulation which depends on auxiliary shield voltage.Therefore, the possibility of long term insulation deterioration of the XLPE due to internal electrified stress is considerably less.
Still a further object of the present invention is to provide for Insulators, which support conductor and XLPE
structure operates at lower stress. Hence, the electrical design parameter of insulator is not critical like in CGITL.
According to the present invention there is provided a gas insulated hybrid transmission line with auxiliary shield comprising a central stranded HT conductor with XLPE solid insulation which operates at a designed potential, the said XLPE solid insulation is provided with an auxiliary shield and a grounded metal enclosure having a gas gap between said auxiliary shield and the grounded metal enclosure,characterized in that a conducting screen is provided over, the said HT conductor and said auxiliary shield is a conducting auxiliary shield to operate at an optimised potential and placed on the outer skin of said XLPE solid insulation and said gas gap is filled up with SF6 gas under low pressure.
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.
Figure 1 shows a cross section of a conventional HGITL.
Figure 2 shows a cross section of a HGITL with auxiliary shield as per invention..
In accordance with the present invention it proposes an improvement in construction of HGITL for optimised insulation utilisation. The basic structure of HGITL
comprises a central conductor (1) covered with XLPE solid
insulation (4). Unlike in conventional HGITL, where the XLPE insulation (4) covering is floating, in proposed improvement, it has been anchored to an optimised potential auxiliary shield (3), forcing maximum utilisation of the solid insulation. The gas gap (5) between the auxiliary shield and the grounded metal enclosure (6) requires low gas pressure and limits leakage current to comparable values. This new construction has been named as HGITL with auxiliary shield. The performance of HGITL with auxiliary shield has been investigated by analysing 245 KV system of this kind.
The gas insulation reliability in HGITL with auxiliary shield (3) is high compared to HGITL without auxiliary shield (3), because of low operating stress in the former construction. Further, the grounded enclosure (6) of HGITL with auxiliary shield (3) is safe,
because of low operating gas pressure. The leakage current in HGITL with auxiliary shield (3) is well below to the XLPE cable.
HGITL is proposed for optimized insulation utilization. In the new construction XLPE insulatation (4) operates at a designed potential (fixed/time varying), rather than at a floating potential, as in the case of conventional HGITL. This is achieved by introducing a conducting auxiliary shield (3) operating at an optimised potential, placed on the outer skin of the XLPE insulation (4). Since gas insulation operates at a lower stresses, mixture of SF6 with other gasses like nitrogen, carbon dioxide etc. can be used as insulation and also the cost of metal enclosures is reduced.The auxiliary shield voltage is applied to the gas and the particles operate in a low field intensity region. The particle problems related to the peak system voltage is reduced significantly.
The thickness of the XLPE insulation required is low and the system is made flexible for negotiating bends.
The invention described hereinabove is in relation to a non-limiting embodiment and as defined by the accompanying claims.

WE CLAIM;
1. A gas insulated hybrid transmission line with auxiliary shield
comprising a central standard HT conductor (1) with XLPE solid
insulation (4) which operates at a designed potential, the said XLPE
solid insulation is provided with an auxiliary shield (3) and a
grounded metal enclosure (6) having a gas gap between said auxiliary
shield and the grounded metal enclosure, characterized in that a
conducting screen (2) is provided over the said HT conductor (1) and
said auxiliary shield (3) is a conducting auxiliary shield to operate at
an optimised potential and placed on the outer skin of said XLPE solid
insulation (4) and said gas gap is filled up with SF6 gas (5) under low
pressure.
2. A gas insulated hybrid transmission line with auxiliary shield as
claimed in claim 1 wherein the SF6 gas (5) insulation is stressed by
lower potential.
3. A gas insulated hybrid transmission line with auxiliary shield as
claimed in claims 1 or 2 wherein mixture of said SF6 with other gases
like nitrogen, carbon dioxide etc. is optionally used as insulation.A gas insulated hybrid transmission line with auxiliary shield as
claimed in claim 1 wherein the said XLPE solid insulation (4) is
subjected to a fixed/time varying potential with the placing of
conducting auxiliary shield (3) on the other outer skin of said XLPE nsulation and operating at a optimised potential.
4. A gas insulated hybrid transmission line with auxiliary shield as
herein described and illustrated