A Landmark Achievement:
Design and Development of Insulator Hardwares, Conductor Accessories, clamps and connectors for 1200kv Transmission line & substation.
First to Supply 1200kV Transmission Line & Sub-Station Insulator Hardware Fittings, Clamps & Connectors for 1200kV National Test Station at Bina (MP) of Power Grid Corporation of India Ltd.
Power grid corporation of India ltd (PGCIL) took a visionary step of adopting 1200kv power transmission system as next higher system voltage after 765kv system for bulk power transmission keeping in view to the projected growth of capacity addition in power generation.PGCIL took a landmark decision of developing the entire range of equipment for substation and transmission line indigenously and created one project steering committee (PSC) and several working Groups (WG) equipment-wise.
Criteria for selecting partner:
Major criteria for selecting partners are as under :
- Resources – An organization with adequate technical personnel having full support of management
- Project Management – Good project management capability with team of capable project managers who understand intricacies of Electrical T&D
- Research and Development – An organization who put their thrust on continuous improvement and also adopts a DMAI (Define Measure Analyse Implement Control) approach of implementation
- Design & Drawing – Knowledge and expertise to understand General Information, Site/ Meteorological Data, Electrical clearances, Mechanical Strength, Voltage Gradient Homogenisation, Details of adjoining components viz. Insulators etc and bringing out best design skills to solve any challenge
- Hardware Fittings for Transmission Lines and Sub-stations – An organization who understands the T&D hardware and also has the capability to compile some more from a few other sources, based on the design parameters were selected.
In the nutshell the organization should own the entire responsibility of development and testing of the hardware for 1200kV.
PGCIL goes for Supreme :
Supreme & co.Pvt.Ltd (Supreme) during that period was mostly involved in lower voltage areas but were in the process of testing of 400kv line hardware fittings & accessories. Management of supreme being motivated with PGCIL’s move towards 1200kv development appealed for an opportunity of becoming an active associate of the relevent working groups as well as PSC with full commitment to develop such product in house and on their own investment for this national project of Bina Test Station.
Supreme was included in following working groups :
- WG for Transmission Line Insulator Hardware Fittings and Conductor Accessorie
- WG for Sub-station Hardware Fittings, Clamps & Connectors
- WG for Pre-fabricated Jumpers
Project management concept was introduced at the very outset. Bar Chart was prepared for all items for the purpose of Planning & Monitoring. Periodical internal review meetings were held to review progress, discuss challenges faced and to find and implement appropriate solutions.
Design & Drawing
Basic parameters were decided by PGCIL which included General Information, Site/ Meteriologcal Data, Electrical clearances, Mechanical Strength, Details of adjoining components viz. Insulators etc.
Hardware Fittings for Transmission Lines and Sub-stations
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;
- Steel Forging
- Fabricated Steel Plates
- Aluminum Tubes
The most important parameter for design of steel forging components is mechanical strength requirement which are derived from the E&M strength of Insulator as well as total string configuration. Primarily, Ball & Socket connections are used. Typically for 400kV system, 20mm B&S designation is used for 120kN and 160kN Insulators. PGCIL proposed to use insulator with EMF rating of 320kN and 420kN. These called for higher B&S designation of 24mm and 28mm Ball & Socket size made with chrome molybdenum alloy high strength steel designated as EN19 grade to withstand the Electro-mechanical strength requirement.
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 Temperature ranging from 975ºC.
All forged components were subjected to Heat Treatment (Normalizing) in furnace at 850ºC with a soaking period determined according to the thickness of the product. Major dimensions were initially selected based on previous experience of design used for EHV segment and extrapolating the higher mechanical strength withstand ability as well as addressing certain aspects arising purely due to levels of mechanical stresses so significantly higher that mere extrapolation would not have sufficed. All components need to have required yield, shear and bearing strength and these have been validated by design calculations. Moreover, design has been based on mechanical strength values applicable under normalized condition avoiding use of hardened and tempered steel. Ductility was key consideration as parts must undergo macro deformation before failure. Use of fasteners with Grade higher than 8.8 was avoided since higher grade fasteners have lesser margin between yield point to breaking point. Fasteners with grade higher than 8.8 are also susceptible to hydrogen embrittlement. Stress concentration may also arise due to various local changes in geometrical shape, sharp bends or abrupt changes in cross-sectional area. Stress concentration at roots and run outs of the thread are avoided by use of thread rolled bolts. Providing higher edge distance and generous radius at the edges and may mitigate other stress concentration related problem. Thereafter, forging components were revalidated by testing on Tensile Testing machine as per PGCIL standard technical specification i.e. 67% proof load (5min) & 100% UTS(1min). All components Test Reports are available with Technical Department of Supreme.
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 will be 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 were 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. All Yoke Plates have been revalidated after final fabrication through Mechanical Testing as per PGCIL standard technical specification i.e. 67% proof load (5min) & 100% UTS(1min). All components Test Reports are available with Technical Department of Supreme.
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 grading along insulator string. The design of CC ring also ensures Radio Interference Voltage (RIV) under the acceptable limit. 1200kV system needs 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. This will be reconfirmed through tests. To achieve this, 100mm diameter 6063 Grade Aluminum Alloy extruded tube with 5mm thickness was selected. Bending of such tubes called for new and higher capacity bending machines along with new dies which were developed in-house. CC ring for Tension Hardware is of so big dimension, it was fabricated (bend) in two halves. Two numbers of Couplings having inner diameter near to tube outer diameter have been deployed at the two adjoining points of the two halves of CC ring. To facilitate good fit, the couplings were tapered inside from centre to outermost point on either side. Opening up of CC ring was arrested by two structural steel flats bolted with the bracket of CC ring and fastening both the halves of CC ring together.
Sub-station Clamps & Connectors :
Design concept & Philosophy
According to our design criterion, the corona performance of the connectors must be better than that of the conductors as the conductors are designed to control surface voltage gradients below the negative corona onset level.This is achieved by providing smooth contours on all surfaces and eliminating all projections.In compression elements ,tapers are provided to keep the voltage gradient at a level lower than that of the cables. 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 that are 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 AS-6 as per IS-617. Drawings of various equipment were obtained from respective manufacturer through PGCIL. Sub-station layout drawing developed by Larsen & Toubro was also obtained. Current carrying capacity for all items were validated by calculation and found safe. Corona performance will be validated by testing at laboratory/ test station.Due care has been taken regarding smoothness of surface, providing generous radius at all edges, adopting low corona Dome nuts etc. Ensure that no sharp corners like bolt head or threads project out of the clamp body. 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. After the results or corona testing is obtained, if required corona shelding rings will also be designed and improved. The 1200kV Project Team of Supreme have relentlessly worked with full commitment and still continuing the same. As on date about 11 Items have been manufactured and dispatched to PGCIL Test Station at Bina.
One photograph containing 400kV and 1200kV hardware fittings gives a comparative idea of the huge size of string in case of 1200kV level.
On the first attempt, Supreme developed a pre-fabricated jumper with eight numbers of tubes to form an octagonal structure. However, comparison of conductor and bus bar surface voltage gradient of “8 Bundle Moose” & twin tube of 100 mm (As shown in Annexure D) shows that a compact pre-fabricated jumper comprising 2 tubes spaced 457mm apart of dia 100mm with cross section area of individual tube 4 times as that of “moose” ACSR can be safely designed.
Though the development of all fittings has been done to the extent described above yet continuous effort is to be done and after obtaining further test results, it is to be seen whether further optimization is possible.
Keshab Mukherjee (Technical Advisor), A.K.Pal (Senior General Manager), P. Barua (General Manger –Electro Mechanical)
Supreme & Co. Pvt. Ltd.