Design & Development of 1200 KV Transmission Line & Sub-Station Insulator Hardware Fittings, Clamps & Connectors
Mr. H. K. Agarwal – CEO,
Supreme & Co. Pvt. Ltd.
Abstract – 1200kV Power Transmission System being indigenously developed in India under the aegis of PGCIL. So far, highest voltage at which the power is transmitted as 765Kv /800Kv Quad lines. The existing 400Kv line can transfer about 600 Mw power, 800Kv line can do between 1,200 Mw. In case of 1200kV Octagonal Bundle Conductor Transmission line the Power carrying capacity increase three folds as compared to 765 kV Quad Lines.
There is growing need for evacuation of large amount of power efficiently to distant load centers while reducing ROW requirements per mw of transmitted power and to cater to this need PGCIL has initiated steps to implement 1200kV Transmission line. Configuration for trial line is eight bundled conductors bersimis/moose both double circuit and single circuit. Special consideration w.r.t. design, manufacturing & testing of a) Insulator Hardware fittings, b) Conductor and earthwire accessories and c) clamps and connectors & substation’s were required and our experience in development of these modules are being shared.
Introduction
H.K.Agarwal – CEO,
Supreme & Co. Pvt. Ltd,
Kolkata, India
E-mail: [email protected]
K.Mukherjee- Technical Advisor, Supreme & Co. Pvt. Ltd,
Kolkata, India
E-mail:[email protected]
A.K.Pal – Senior General Manager,
Supreme & Co. Pvt. Ltd,
Kolkata, India
E-mail:[email protected])
P. Barua – General Manager
(Electro-Mechanical),
Supreme & Co. Pvt. Ltd,
Kolkata, India
E-mail: [email protected]
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 1200kV Power Transmission System for bulk power transmission. It was decided to develop the entire range of equipment for sub-station and transmission line indigenously and PGCIL created one Project Steering Committee(PSC) and several Working Groups(WG) equipment-wise.
Supreme & Co.(Supreme) signed an MOU with PGCIL on 12th November, 2008 to work in the following Working Groups for development of respective product:
- WG for Transmission Line Insulator Hardware Fittings and Conductor Accessories
- WG for Sub-station Hardware Fittings, Clamps & Connectors
- WG for Pre-fabricated Jumpers
I. INTENT OF PAPER
This paper is intended for sharing the approach and experience we underwent during developmental process with respect to design particularly for meeting electrical high voltage and eletro-mechanical criteria, augmentation of shop floor infrastructure & manufacturing/fabrication facilities, development of massive dies for Steel forging, Tube Bending & Aluminum casting, augmentation of Test facilities etc.
II. SYSTEM DESIGN
Various studies were conducted by PGCIL to determine the configuration of its 1200kV transmission lines & Switchyard. Studies included corona cage studies, air gap insulation studies, tests for voltage distribution on the insulator string, RIV & voltage gradient measurement. Based on the Studies the Conductor configuration, Air gap clearances and String configurations were finalized.
Bundle Conductor Selection – 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 & RIV were measured.
Eight bundle Bersimis with both 450mm and 550mm sub-conductor spacing was considered for 1200 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.
Selection of Insulation Levels
Lightning impulse withstand voltage and switching impulse withstand voltage levels were prime consideration for Insulation Co-ordination. With rise in system voltage, the ratio of Lighting Impulse Withstand Level (LIWL) and Switching Impulse Withstand Level (SIWL) to the rated voltage decreases, for example , LIWL/rated voltage for 400 kV, 765 kV and 1200 kV respectively is 4.15, 3.22 & 2.45 and SIWL/rated voltage for 400 kV, 735 kV and 1200 kV respectively is 3.06, 2.37 & 1.84.
Clearances
Based on experimental results conducted by PGCIL and CPRI, clearances stipulated by PGCIL are given below:
- Minimum Phase to Earth Clearance 8.30 m
- Minimum Phase to Phase Clearance 12.30 m
- Sectional clearance 10.5 m
- Phase to Phase distance (Pie Structure) 27.00 m
Technical parameters of 1200 kV Equipments
Based on the above studies, test results, all relevant technical considerations and discussions with representatives of manufactures, utilities as a part of working groups for individual equipments, major technical parameters have been identified by PGCIL(Ref-[1] & [2]) which are given at Annexure – I.
III. 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:
- Aluminum Tubes for CC/ Grading Ring
- Steel Forging
- Fabricated Steel Plates
1. 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 gradiant 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. Calculation enclosed at Annexure-II. 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. 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 being large in dimension was fabricated(bent) in two halves. Two numbers of Couplings to with inner dia maintained at close clearances to tube outer dia have been deployed at the two adjoining points of the two halves of CC ring. To facilitate smooth fitment, the couplings are tapered inside from centre to outermost point on either side. Opening up of CC ring has been arrested by two structural steel flats bolted with the bracket of CC ring and fastening both the halves of CC ring together.
Studies were conducted by M/s Deccan Enterprises to determine voltage distribution along insulator string. Polymer Insulator with Octagonal bundle conductor were considered with two types of outer Corona Control/ Grading rings viz., 700mm and 1100mm ring diameters. Comparative Results of studies are given in following Table-1:
| COMPARISION (TABLE 1) | |||
|---|---|---|---|
|
|
700mm |
1100mm |
Reference |
|
Along FRP Rod |
0.634kV/mm |
0.294kV/mm |
3 kV/mm |
|
Along SIR |
0.623kV/mm |
0.313kV/mm |
3 kV/mm |
|
On SiR |
|
|
|
|
First Shed(s) |
0.327 kV/mm |
0.321 kV/mm |
0.45kV/mm |
|
Second Shed(s) |
0.250 kV/mm |
0.248 kV/mm |
0.45kV/mm |
|
First Shed(b) |
0.416 kV/mm |
0.391 kV/mm |
0.45kV/mm |
|
Second Shed(b) |
0.409 kV/mm |
0.383 kV/mm |
0.45kV/mm |
|
Smaller Grading Ring |
0.84 kV/mm (max) |
0.8 kV/mm (max) |
(1.7 – 2.1 kV/mm) |
|
Bigger Grading Ring |
2.24 kV/mm (max) |
1.67 kV/mm (max) |
(1.7 – 2.1 kV/mm) |
Voltage Distribution pattern on whole string and near HV end indicated in Fig-1(a) & Fig-1(b) below :-
 |
 |
|
Fig-1 (a)
|
Fig-1(b)
|
Note: Surface Voltage Gradient has been reduced and achieved within limits by adopting higher diameter Corona ring of 1100mm when compared with reference values taken (Ref-[3]).
2. Steel Forging
The most important parameter for design of steel forging components is Electro-Mechanical(E&M) strength requirement which are derived from the E&M strength of Insulator as well as total string configuration. Primarily, Ball & Socket designation are selected. Typically for 400kV system, 20mm B&S designation is adopted for 120kN and 160kN Insulators. PGCIL have already introduced higher capacity insulators of 320kN and 420kN.
These call for higher B&S designation which resulted in use of 24mm and 28mm Ball & Socket size with chrome molybdenum alloy high strength steel designated as EN19 grade to withstand the Electro-mechanical 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 1050ºC 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.
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 yield point very close to breaking load which is not recommended under cyclic stresses arising out of vibration. 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.
The dimensions were validated by design calculations. 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
3. 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. 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.
I. SUB-STATION CLAMPS & CONNECTORS
Defining feature of a 1200kV connector which may be used as a connector, bus support or other device which, when installed on its conductor bus bar, 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 equipment were obtained from respective manufacturer through PGCIL. Sub-station 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.
Current carrying capacity for all items were validated by calculation and found safe. Corona & RIV performance has been 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.
Calculation methodology of effective electric contact area for gravity die cast substation connector has been attached at Annexure-III. An album containing photographs of Hardware Fittings String(with reduced number of Insulators), Clamps & Connectors along with photographs of various dies, new manufacturing equipment etc. can be viewed through our web link given below.
Air can act as an insulator till a voltage of 30 kV/cm. Above this voltage level, air ionizes into positive and negative ions. Thus, the primary factor causing corona is the voltage gradient around the conductors. Voltage gradient varies directly with the system operating voltage and inversely with square of the distance from the center of the conductor. Corona sometimes causes some unwanted electric signals which interfere with nearby Radio/ TV receptions and cause noise which is popularly known as radio noise or radio interference. These noises depend on frequency and decreases with increasing frequency.
Apart from radio interference corona also relates to considerable energy loss in the circuit, which is often termed as “CORONA LOSS”. In practice, the voltage gradient of the conductor at the operating voltage is determined for phase-to-phase spacing and the distance from the ground for the particular operating voltage.
V. CONCLUSION
Though the development of all fittings has been done to the extent described above yet continuous effort has to be done and after obtaining further test results, it is to be seen whether further optimization is possible.
VI. ACKNOWLEDGMENT
The authors convey their thanks to PGCIL for their continuous support, CPRI for their support in testing, Deccan Enterprises, all Equipment manufacturers, Tower & Sub-station erectors, all members of STC & WG and last but not the least all concerned persons in Supreme.
REFERENCES
- 1200kV National Test Station: Key Issues – I.S.Jha, Dr. S.K.Agarwal et al, Gridtech 2011
- 1200kV Transmission System and Status of Development of Substation Equipment/ Transmission Line materials in India – R.N.Nayak, M.C. Bhatnagar et al, 2nd International Symposium on Standards for UHV Transmission, 2009
- Electric Fields on AC composite Transmission Line Insulators -IEEE Transactions on Power Delivery, Vol-23,No-2, April 2008.
- Specification IS 800
- Specification IS 2062
- Electrical Contracts Principles and Applications -edited by Paul G. Slade
ANNEXURES
Annexure – I
Annexure – II
Annexure – III
Annexure – I
Major Technical Parameters
Annexure – II
Calculation of Conductor Surface Voltage Gradient
Method obtained IEE Power Series 17, “High Voltage Engineering and Testing”. Appendix 3.4
E=(V/ö3)*(b/(r*ln((a/Re)*2h/ö(4h2+a2)))
Where,
b=(1+(n-1)r/R)/n
Re=Rn*ö(nr/R)
R=S/(2*Sin(p/n))
E= Conductor Surface Voltage Gradient ( kV/cm)
V= Rated Voltage ( kV )
b= Factor for Multiple Conductors
r= Radius of Conductor ( cm )
R= Outside radius of bundle ( cm )
Re=Equivalent Radius of bundle conductor ( cm )
S= Distance between Component conductor centers ( cm )
a= Phase Spacing ( cm )
h= Height of conductor above ground ( cm )
(This value is taken as the distance between phase connection/bus bar to the metalwork at earth potential)
n= Number of component conductors in bundle
Line Calculations
|
V=1200 kV Therefore, b= 0.1485 Re= 699.38 cm E= 9.99 kV/cm |
R=S/(2*Sin(p/n))=45.7/(2*Sin(p/8)) b=(1+(n-1)r/R)/n Re=Rn*ö(nr/R) Therefore, |
Substation Calculations
|
V=1200 kV Therefore, b= 0.1485 Re= 699.38 cm E= 9.209 kV/cm
|
R=S/(2*Sin(p/n))=45.7/(2*Sin(p/8)) b=(1+(n-1)r/R)/n Re=Rn*ö(nr/R) Therefore, |
Twin Tube Calculations
|
V=1200 kV Therefore, b=1 Re= 95.603 cm E= 5.09 kV/cm
|
R=S/(2*Sin(p/n))=45.7/(2*Sin(p/2)) b=1 ( For Tube Conductors ) Re=Rn*ö(nr/R) Therefore, |
Both above values found to be below negative corona onset value of 20 kv/cm.
Annexure – III
Calculations of effective electric contact area for gravity die cast substation connector
Calculations for effective contact area-
As a thumb rule the longitudinal contact provided is 6 to 8 times of the conductor /bus bar thickness. Similarly it is ensured that physical contact which a connector makes wit bus bar conductor covers at least 60 percent of the circumferential area
It is known that all solid surfaces are rough on micro scale and such surface when viewed micro logically have peaks and valleys whose shape, height, separations etc is a function of many variables . The effective electrical contact comes from interfaces only when metal to metal contact spots are produced . In a typical connection , the area of actual electrical contact is only a fraction of total physical contact . These contact spots are also known as ” a’ spots and are empirically observed to be between 1 to 2 percent of the physical contact area .
Now for our calculations if we assume the conductor to have a dia of 32 mm , it will have total circumference of 100 mm . 60 percent of this is 60mm
Longitudinal contact is taken at 32 x6 = 192 mm which makes the total contact area for each conductor of the bundle 11520 sq mm.
This taken for 8 conductors of the bundle is 92160 sq mm.
Ampacity of lm6 gravity die cast alloy is taken at 10 amps per sq mm Effective ampacity can be taken at 0.1 amps per sq mm considering effective electrical contact area to be 1 percent of the physical contact area
Thus the total ampacity of each bundle is 92160 x 0.1 amp which is 9216 amp. Thus the ampacity wise the design is safe by a long margin
Our Experience after charging of 1200 kV Line at Bina ( M.P.)
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, sub-station insulator hardware fittings & 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. At last the efforts and time put in to the project yields its fruit on 27.01.2012 , when the Test line for 1200 Kv was charged in presence of all the company delegates and the PGCIL project steering committee, who took an active part in designing and developing the project.
This project may be regarded as a golden project which will change the Power transmission sector in the world.
Mechanical Performance :
After installation of all the Transmission Line and Sub-Station Insulator Hardware Fittings, Clamps & Connectors it was observed that the Physical & Mechanical properties of the fittings , clamps and connectors are very satisfactory . The performance of the products were good and as per our expectations. There was no discrepancy or any misfit of our products with any other products which been used by other members of the work group as our products were made keeping in view the compartibility with other products and as per the specified guidelines very strictly.
Electrical Performance :
The major concern in these kind of UHV lines is the Corona effect which leads to a huge loss of power on transmission, so we had taken utmost care while designing the Corona rings . So corona rings were our major concern and special attention was taken to design and manufacture such big corona rings to cover a big dimension of hardware fittings. And another concern was there that should be free from any sharp edges.
After installation of the corona rings it was observed that the CC effect was well under control as well as it took care of voltage gradient along insulator string. So Surface Voltage Gradient has been reduced and achieved within limits by adopting higher diameter Corona. Again the design of CC ring also ensured Radio Interference Voltage (RIV) under the acceptable limit.
Supreme’s experience – Working with PGCIL and Other Work Group Delegates.
It was an extreme honor to work with a team of people with extreme excellence in their respective fields and through professionals. The Project steering committee and the work groups were very cooperative and courageous to wards the project and it was the collective effort of these people who made the project a success and may be this project indicates the golden era of Indian power sector has arrived. So with this article Supreme and Co. (P) Ltd. is taking the opportunity to thank all the Working Committee members, Pr
