So far, the highest voltage at which the power is transmitted is 765kV/800kV Quad lines. A 400kV line can transfer about 600 Mw power, 800Kv line can do around 1,200 mw. In case of 1,200kV octagonal bundle conductor transmission line, the power carrying capacity increases threefold as compared to 765kV Quad Lines.
In view of the projected growth of capacity addition in power generation and to take care of future increasing load demand, Power Grid Corporation of India Ltd (PGCIL) decided to adopt 1,200kV power transmission system for bulk power transmission. It was decided to develop the entire range of equipment for substation and transmission line indigenously and PGCIL created one Project Steering Committee (PSC) and several Working Groups (WG) equipment-wise.
Supreme & Co signed an MOU with Power Grid Corporation of India Ltd on November 12, 2008 to work in the following Working Groups for development of respective product:
1. WG for transmission line insulator hardware fittings and conductor accessories
2. WG for substation hardware fittings, clamps & connectors
3. WG for prefabricated jumpers
Various studies were conducted by PGCIL to determine the configuration of its 1,200kV transmission lines and switchyard. Studies included corona cage studies, air gap insulation studies, tests for voltage distribution on the insulator string, RIV and voltage gradient measurement. Based on the studies the conductor configuration, air gap clearances and string configurations were finalized.
Corona Cage Studies
PGCIL carried out these tests in a Corona cage at UHV test lab at Central Power Research Institute (CPRI), Hyderabad. Octagonal Bersimis ACSR (dia 35.1mm) bundle conductor was chosen. Three types of bundle arrangements, with different sub conductor spacing of 350 mm, 450 mm, and 550 mm were studied. The measurements were made with two nominal precipitation rates viz 57 mm/hr and 85 mm/hr. Corona inception voltage, corona loss, AN and RIV were measured.
Eight bundle Bersimis with both 450mm and 550mm sub-conductor spacing was considered for 1,200 kV system.
System Studies conducted with eight bundle Moose also gave good results and hence, in 1200kV National Test Station both Bersimis (D/C line) and Moose (S/C line) conductors were adopted.
Line Hardware Design
After obtaining such data from PGCIL and also compiling some more from a few other sources, the design parameters were selected and the aspect was categorized in three broad groups for:
1. Aluminum Tubes for CC/ Grading Ring
2. Steel Forging
3. Fabricated Steel Plates
Aluminum Tubes for CC/ Grading Ring
Corona Control (CC) rings are deployed in EHV/UHV level for dual purpose of reducing electrical stresses to ensure extinction of corona below a certain voltage level thereby reducing losses due to corona as well as taking care of voltage gradient along insulator string. The design of CC ring also ensures Radio Influence Voltage (RIV) under the acceptable limit. 1200kV system need special attention on the design aspect of CC Rings in view of the large dimension of the CC Ring to cover a large Hardware Fitting set. Surface Voltage Gradient on surface of conductor was calculated for Transmission Lines and Sub-stations and found within safe limit with respect to corona inception. Selection of CC ring has been done by covering a much larger diameter and effective surface area and hence performance with respect to Corona Inception and Losses will be within limit.
To achieve this, 100mm diameter 6063 Grade Aluminum Alloy extruded tube with 5mm thickness was selected.
The most important parameter for design of steel forging components is electromechanical (EM) strength requirement which are derived from the EM strength of insulator as well as total string configuration. Primarily, ball & socket (B&S) designation are selected. Typically for 400kV system, 20mm B&S designation is adopted for 120kN and 160kN insulators. PGCIL has already introduced higher capacity insulators of 320kN and 420kN. These call for higher B&S designation which resulted in use of 24mm and 28mm B&S size with chrome molybdenum alloy high strength steel designated as EN19 grade to withstand the EM strength requirement. Obviously, same designation was adopted for Ball and Socket components for respective hardware fittings for 1200kV.
This called for development of full range of forging dies, socket cutters, set of gauges etc. Matter was taken up immediately and fresh design was made for dies. New die blocks were imported and die sinking was done at pre-approved vendor’s works under constant supervision of Supreme’s technical personnel.
All steel forging component dies were thus specially manufactured and components were forged with newly developed set of forging dies made of imported die steel and forged in drop hammers of 1T, 1.5T & 2T capacities (depending on the size of product) at red-hot condition with temperature ranging from 975ºC to 1,050ºC and then flashes were removed at trimming presses with the help of newly developed trimming dies. Components were allowed for natural cooling in open air.
Fabricated Steel Plates
Design of plates for 1200kV Hardware Fittings also called for innovation owing to number of sub-conductors which was decided as 8(eight) per phase based on system consideration. In the case of Suspension Hardware Fittings, the challenge lied in development of a single piece Yoke Plate to support all 8 sub-conductors at same vertical plane from 8 different points(holes) but keeping sub-conductor spacing i.e. distance between adjacent sub-conductors identical which should be 457mm. Eight Armour Grip Suspension Clamps supporting each sub-conductor were attached with the Yoke Plate at the hole points as mentioned above.
Design parameters viz. thickness, edge clearance etc. of Suspension Yoke Plate was selected based on previous experience of 400kV strings and subsequently validation was done through calculation of Tensile, Bearing, Shearing Strength of Plate and Bearing Strength of Bolt as per guidelines of IS 800. Concept of design followed was generally same as has been done and described above for steel forging. As far as material is concerned, low carbon structural steel as per IS 2062 Grade-A has been adopted. Holes in plates in the subject case may be susceptible out of unbalanced load sharing and hence edge distance has been kept more than the guideline of IS-800. All hole edges have been countersunk for relieving of stress, bolt dimension selected to ensure avoidance of contact of load-bearing surface with threaded part.
Similar design philosophy has been adopted for all other yoke plates, links, flats deployed for other hardware fittings viz. tension etc.
Substation Clamps & Connectors
Defining feature of a 1,200kV connector which may be used as a connector, bus support or other device which, when installed on its conductor busbar, does not generate corona or noise at nominal voltage. Thus, UHV connector design primarily factors the voltage gradient at the surface of the conductors to determine the Corona inception and extinction voltages apart from the necessary electrical and mechanical performance requirements as that of the LV/MV/HV connectors.
For spacers, the critical regions are the edges of the bundle. This is because of the shielding the bundle provides to the parts within it. Hence, the parts at the edges of the bundles are provided with meticulous finishes at the factory to ensure corona-free operation.
All Clamps and Connectors have been manufactured by Gravity Die Casting of Aluminum Alloy Grade A-6 as per IS-617 or fabricated from aluminium alloy extrusions. Drawings of various equipments were obtained from respective manufacturer through PGCIL. Substation layout drawing developed by Larsen & Toubro was also obtained.
The major challenge was development of all casting dies which are of huge dimension and called for augmentation of handling facilities. This was done in-house successfully to cope up with the requirement. All casting dies were indigenously manufactured and all components were produced by gravity die casting at aluminum casting plant of Supreme.
The Experience: Supreme is proud and honored to be a part of the project and an active member in the work group which took the responsibility of designing and developing the transmission line, substation insulator hardware fittings and connectors.
Supreme has put its years of experience in developing the above mentioned products which was a great challenge to manufacture due to the complexity of the design, huge physical dimensions of the products & parts, high electrical and mechanical requirements and above all the production constraints.
Supreme accepted this as a challenge and worked with great commitment and focus for the development of the product and made sure the products meet all the electrical, mechanical and dimensional requirements perfectly in order to make the project a grand success.
[This article has been excerpted from a technical paper authored by Supreme & Co on its role in supplying 1,200kV hardware for Power Grid Corporation of India Ltd’s UHVAC test station at Bina. For more details, please contact Harish Kumar Agarwal, CEO, Supreme & Co (firstname.lastname@example.org) or visit www.supreme.in]