DESIGN CONCEPTS OF SUB-TRANSMISSION & DISTRIBUTION LINES

1. DESIGN CONCEPTS OF SUB-TRANSMISSION & DISTRIBUTION LINES

2. VOLTAGE SELECTION • Factors considered for voltage selection : • Length of line (voltage of the line is taken as 0.6kV per Km of the length of line) • b) Magnitude of Power to be transmitted • c) Cost of Terminal equipment • d) Economy consistent with the desired reliability.

3. PERMISSIBLE VOLTAGE VARIATION UNDER PEAK LOAD CONDITION ABOVE 33KV: - 12.5% to 10% UPTO 33KV : - 9% to 6% LT : - 6% to 6%

4. IN ORDER TO HAVE PERMISSIBLE VOLTAGE REGULATION AND ENERGY LOSSES, FOLLOWING LOADING CONDITION IS FOLLOWED: i) 33KV FEEDER : MAXIMUM DEMAND UP TO 6 MVA ii) 11KV FEEDER : MAXIMUM DEMAND UP TO 2 MVA iii) LT FEEDER : MAXIMUM DEMAND UP TO 160 KVA iv) SEPARATE 33KV & 11KV FEEDER SHOUDL BE INSTALLED FOR INDUSTRIAL CONSUMERS LENGTH OF 11KV LINE SHOULD BE LIMITED TO 15Km. FOR MORE LINE LENGTH, THE FEEDER SHOULD BE BIFURCATED

5. Conductor Selection • Type of conductor: • ACSR: High tensile strength, light weight giving small sags, longer spans are the advantages. Larger diameter increase pole loading due to windage necessitating heavier poles. • AAAC: Strong, durable, light weight & high conductivity. AAAC cannot take much tension compared to ACSR. Span length gets restricted. • Copper conductor: Costly & absolute. High conductivity, long life, less windage due to small diameter and thus lighter poles and high recap value.Less tensile strength causes small spans.

6. Conductor Selection (d) Galvonised steel conductor: Stranded conductors made of galvonised steel wires. Shorter life period. Size of conductor depends on (i) Power transmitted (ii) Length of line (iii) Permissible voltage regulation & (iV) Mechanical strength. The KW-KM that can be transmitted at a particular voltage with particular type of conductor are available. There are KW-KM charts for specific configuration of lines based on conductor cross section.

7. For Urban areas as a thumb rule: In order to meet future load demand, new 33kV Line should not be less than ACSR Dog (324A/65Sq.mm), new 11kV Line should not be less than ACSR Rabbit (208A/30Sq.mm) and for new LT Line, main branch line should not be less than ACSR Rabbit and spur lines with Weasel Conductor (150Amp/ 20Sq.mm)

8. STANDARDISED CONDUCTORS FOR DIST. LINES 33KV DISTRIBUTION LINES: ACSR Panther : 520A/130sq.mm Wolf : 430A/ 95sq.mm Dog : 324A/ 65sq.mm or equivalent AAAC. 11KV LINES: ACSR Dog : 324A/ 65sq.mm Raccoon : 270A/ 48sq.mm Rabbit : 208A/ 30sq.mm or equivalent AAAC. LT LINES: ACSR Dog : 324A/ 65sq.mm Raccoon : 270A/ 48sq.mm Rabbit : 208A/ 30sq.mm AAC Ant, Grasshopper or equivalent AAAC.

9. Span Length: • Span length depends on • Ease of construction and cost of line: • For heavier conductors, low tensile strength conductors, shorter poles, span length is reduced. Statutory clearances and sag-tension criterions determine span length. • (ii) Terrain Conditions : Hilly terrain or urban areas have more zig-zag route necessitating shorter average spans. Village areas can have straight lines with more span lengths as ground clearance requirement is less in field areas.

10. POLE SELECTION: • Poles are chosen with regard to local conditions and requirements. Usually steel, wood, re-inforced concrete(RCC) or Pre-stressed concrete(PCC) are used. PCC poles have been found to be most economical. • Length of poles depends on Voltage, clearance from ground, expected sag etc. • 9m and 9.5m PCC poles with factor of safety 2 are standardised for 33kv. 7.5m, 8m and 9m long PCC poles with factor of safety 2.5 have been standardised for 11kv & LT lines. contd..

11. POLE SELECTION: • Strength of pole should be able to meet the loads that are expected to act on the support during normal working conditions with specified factor of safety. • The Max. strength of pole is in transverse direction. The depth of plantation is 1/6th of the height of pole. The maximum load is considered at 600mm below the top of the pole.

12. POLE SELECTION: • Working Load on Pole depends on Wind Pressure Zones. Working Load Wind Pressure on PoleZone 200Kg 50 kg/sq.m 300Kg 75 kg/sq.m 400Kg 100kg/sq.m Foundation of poles is done based on the ground and local conditions. In cut points, low land and water submerssible areas concreting of the pole pit is done.

13. GROUND CLEARANCES: LT11KV33 KV Ground Clearance across Street : 5.8Mtr 6.1Mtr 6.1 Mtr Ground Clearance along Street : 5.5Mtr 5.8Mtr 5.8 Mtr Ground Clearance in other areas: 4.6Mtr 5.2Mtr 5.2 Mtr Clearance in crossings in between LT, 11 KV and 33 KV is 2.44 Mtr Clearance from Buildings: VerticalHorizontal LT 2.5Mtr 1.2 Mtr 11KV 3.7Mtr 1.2Mtr 33KV 3.7Mtr 1.83Mtr

14. RIGHT OF WAY: • For 11 KV Line:- 7 Meter • For 33 KV Line:- 15 Meter

15. SAG – TENSION: • Sag is calculated by formula S=wl²/8T For calculating sag and tension, two sets of loading condition is considered. (a) Maximum wind pressure and minimum temperature. (b) Still air condition at maximum temperature. The wind pressure maps and temperature maps are available for determining above.

16. SAG – TENSION: Ruling (Equivalent) Span: Lr=√{(L1³+L2³+L3³+----)/(L1+L2+L3+---)} Having determined the ruling span and basic tension, the sag for any span may be calculated as S=(actual span/ruling span)²x Sr

17. Stay Wire & Guy Assembly: Guy assembly is needed for dead end and angular locations to counter balance the load on the supports due to pulling of the conductors. Anchor plate is buried and concreted. Anchor rod is binded with stay wire. The stay wire is fixed with Turn Buckle at the top. The load of stay wire is taken by adjusting the length through threaded Turn Buckle rod. Contd..

18. GUY STRAIN INSULATORS: Guy strain insulators are used for insulation of the bottom part (reachable part by cattle, human being) from the top in case of energisation of guy wire top portion. A - Type insulators : for 415/240 V C - Type insulator : for 11KV C - Type 2 – insulators :for 33 KV Type Min. failing load A 44KN C 88KN

19. EARTHING: • All metal supports, fittings etc. shall be permanently earthed through 8 SWG GI wire embedded in concrete during manufacturing. The ends of the wire are left projected from the pole to a length of 100mm at 250mm from top and 150mm below ground level. • Normally, Rail poles and Iron beams are earthed with Pipe Earthing for grounding of leakage currents and PCC/RCC poles are earthed directly with GI wire or with GI coils termed as Coil Earthing. The embedded ground wire in the PCC poles is attached to metallic supports on top of the poles.

20. GUARDING: Guarding is an arrangement provided for lines by which a live conductor, when accidentally broken, is prevented to come in contact with other electric lines, telephone lines, roads and persons or animals by providing a sort of cradle below the electric line. Guarding are made of GI wires which are earthed. As soon as broken wire touches it the line trips. Guarding is not required for crossing of 66kV and higher voltages.

21. ANTI-CLAIMBING DEVICES: In order to prevent unauthorized persons from climbing on HT & LT lines without ladder or special appliances, Anti Climbing Devices are provided. Anti-climbing Device is made of i) Barbered wire coiled around the pole for 40 – 80 cm distance at a height 3.5 to 4 meter from ground level ii) Clamps with protruding spikes at a height of 3 to 4 m.

22. CONFIGURATION: Typical pole configurations are i) Linear ii) Triangular. Linear configurations can be in horizontal type or vertical type. Low and medium voltage lines have linear configuration. Lines above 650V have normally triangular or Delta formation for single circuit lines.

23. RECONDUCTORING RECONDUCTORING RECONDUCTORING RECONDUCTORING RECONDUCTORING

24. Reconductoring: Replacement of conductor along with renovation of lines for replacement of defective/old parts.

25. Benefits of Reconductoring i) Enhancement of the service period of the line. ii) Better availability and reduction of breakdown and planned outages. iii) Better voltage regulation and reduced technical loss.

26. STEPS OF RECONDUCTORING (I) Survey of the line to be re-conductored. Survey includes identification of the defective parts like stay sets, poles cross arms, insulators etc. and also for HT & LT crossings for planning of the outages. (II) Making ready the non-shutdown activities like foundation for stays, shifting of poles and materials for re-conductoring. (III) Re-conductoring work for sections from cut point to cut point.

27. T&P’s required for Re-conducotring • ØCome-Along clamps • Ø Gas Cutter • ØWrench – Sets, Pulleys, Ropes, Hacksaw sets, • Aerial Rollers, Turn Table. • ØScaffolding, Flag. • Ø  Safety Equipments. • Tractor • Wire – less/Mobile sets.

28. PROCESS OF RECONDUCTORING 1. After electrical isolations and earthings of the line, fitters are deployed at both end cut-points and in between pin points. 2.Pin point fitters shall open the pin insulator binding of conductor and release the phase wires on aerial pulleys. Contd..

29. PROCESS OF RECONDUCTORING 3.Based on the tractor movement space, at one end of cut point the old conductor shall be pulled over the aerial pulleys by tractor and new conductor shall be tied with old conductor at the other end cut point so that the new conductor takes place of old conductor. 4.Tension fittings are then fitted. The defective materials like insulators,cross arms etc are replaced wherever necessary.

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