Desing & construction of 33 kv, 11 kv lines

DESIGN & CONSTRUCTION OF SUB TRANSMISSION, DISTRIBUION LINES & SUB-STATIONS

Er. K.V. Surya Prakasa Rao, Former SE/APTRANSCO

Design of Electrical Sub Transmission and Distribution LinesDesign aspects Power Distribution System is governed by Indian Electricity Rules. Desired features of Power Distribution System areSafety,Quality supply to consumersTo follow Standards in Design Construction, installation Protection, Operation and Maintenance of electrical equipment and lines, SubstationsTo lay down standard procedures and follow the same with accountability to people

Voltages at which power is to be Supplied L.T. single phase consumers – 240V L.T. three phase consumers – 415V H.T. consumers – 11,000V; 33,000V;

1,32,000V

Permissible Voltage Variations:

Up to 650V supply + 6% 11KV and 33KV + 6% ; - 9% 132 KV + 10.0%; - 12.5% 220KV + 5% Permissible Frequency Variation: + 3%

Pole sizes, spans and conduction sizes for the lines:

Type of Pole Length of Pole Voltage of Line Max Span (mts)

PSCC 9.1 meters 33KV 100

PSCC 8.0 meters 11KV 107

PSCC 8.0 meters 415/240V 67

Support Formation

Single Pole Support: 0 – 100 Deviation Double Pole Support: 100 - 600 Deviation Double Pole Support: 600 - 900 Deviation

Conductor: The table below gives sizes and other details of conductors standardized for use in

sub–transmission and distribution system for various types of lines. These are:ACSR – Aluminium Conductors Steel ReinforcedAAC – All Aluminium ConductorsAAAC – Aluminium Alloy Conductors

Voltage Class No. and Diameter of Wire Type of Conductor

Resistance / KM

33 KV Lines 1. 7/3.35 mm (50 mm2)2. 7/4.09 mm (80 mm2)3. 6/4.72 mm + 7/1.57 mm (100 mm2)

ACSR (AAAC)ACSR (AAAC)ACSR (AAAC)

0.5524 OHMS0.3712 OHMS0.2792 OHMS

11 KV Lines 1. 7/2.11 mm (20 mm2)2. 7/2.59 mm (30 mm2)3. 7/3.35 mm (50 mm2)

ACSR (AAAC)ACSR (AAAC)ACSR (AAAC)

1.394 OHMS0.9289 OHMS0.5524 OHMS

LT Lines 1. 7/2.11 mm (20 mm2)2. 7/2.59 mm (30 mm2)3. 7/3.35 mm (50 mm2)4. 7/2.21 mm (25 mm2)5. 7/3.10 mm (50 mm2)

ACSR (AAAC)ACSR (AAAC)ACSR (AAAC)AAC (AAAC)AAC (AAAC)

1.394 OHMS0.9289 OHMS0.5524 OHMS1.306 OHMS0.5524 OHMS

Stay Wire:

Stay Wires are used for anchoring of power line poles at dead ends and at angular locations. The individual wire used to form “stranded stay – wire” is to be of Tensile grade 4 having minimum tensile strength of 700 N / mm2 as per IS–2141. Three sizes have been standardized and are tabulated below:

No. of Wires & Const. Wire Dia (mm) Minimum Breaking Load of Single - Wire Before Stranding (KN)

Minimum Breaking Load of Line Stranded Wire (KN)

7 (6/1) 2.5 3.44 22.867 (6/1) 3.15 5.45 36.267 (6/1) 4.0 8.79 58.45

Factor of Safety:

The factor of safety adopted is 2 with the following working loads:Working Load on

PoleWind Pressure Zone (where to be used)

1.200 kg2.300 kg3.400 kg

50 kg / m2 75 kg / m2 100 kg / m2

Wind – Loads on conductors in Different Wind - Regions

Code SizeMm

Conductor

Wind – Load in Kg. per meter run of conductorWind Region 50 Kg/m2

W.R. 75 kg / m2

W.R.100 kg / m2

W.R.150 kg / m2

Dog 6/4.72+7/1.57

ASCR 0.472 0.757 0.943 1.415

Racoon 7/4.09 ASCR 0.409 0.613 0.818 1.227

Rabbit 7/3.35 ASCR 0.335 0.502 0.670 1.005

Weasel 7/2.59 ASCR 0.259 0.388 0.518 0.777

Squirrel 7/2.11 ASCR 0.211 0.316 0.422 0.633

Amt. 7/3.1 AAC 0.310 0.465 0.620 0.930

Gnat 7/2.21 AAC 0.221 0.331 0.442 0.663

Dog (equivalent)

7/4.26 AAAC 0.426 0.639 0.852 1.278

Racoon 7/3.81 AAAC 0.381 0.572 0.762 1.143

Rabbit 7/3.15 AAAC 0.315 0.472 0.630 0.945

Weasel 7/2.5 AAAC 0.250 0.375 0.500 0.750

Squirrel 7/2.0 AAAC 0.200 0.300 0.400 0.600

Ground Clearance: Clearance above ground of the lowest conductor (including

guard – wires) is to meet the following conditions:1. No conductor of an overhead line including service lines

erected across a street shall be at any part there of be at a height less than:

a) For low & medium voltage line (i.e. up to 650 volts) – 5.791 mts

b) For high voltage lines (up to 33 KV) – 6.096 mts2. Along a street:a) For low& medium voltage lines _5.486 mtsb) For high voltage lines(up to 33 KV) _ 5.794

mts 3. Elsewhere other than 1,2 abovea) For high voltage up to 11KV - 4.572 mtsb) For HV line above 11 KV but up to 33KV - 5.182 mts

Ground Clearance: Clearance from Building, Structure etca. The vertical clearance above building (from the highest

point) on the basis of maximum sag shall not be less than (2.439 m) 8 ft for low and medium voltage line (up to 650 volts) and (3.64 m) 12 ft for 11 and 33 KV line.

b. The horizontal clearance is to be 4 ft (1.219 m) upto 11 KV line and 6 ft (1.820 m) for 33 KV line.

i) It may be mentioned that for Railway crossing, rules as prescribed by Railways are to be followed

ii) Similarly the lines crossing or in proximity to the telecommunication lines, the overhead line is to be protected as per code of practice laid down by PTCC coordination committee

Construction of Lines

Construction Practice

The basic parameters for selection of materials and standards, once decided will help in adopting sound construction practices. Estimation of materials labour and transport facilities, having been assessed the materials should be made available at site, required working gangs (equipped with tools & tackles) should be formed as and when required.

The voltages of concern are: a) 33 KV sub-transmission line, b) ll KV Primary distribution line c) 415-240Volts Secondary distribution line

Survey of the Proposed Route of LineThe first step to be taken prior to the design or construction

of any line is to conduct reconnisiance survey of the country over which the line is to pass. Topo sheet map of the area which would indicate towns roads, streams/river, hills, railway lines, bridges, forest areas, telephones telegraph and power lines may be taken and the approximate route of the line marked on it. Before finalising the route, the following parameters should be kept in mind:

1. The shortest route practicable.2. As close as possible to the road for easy maintenance

and approach during the construction.3. Route in direction of possible future load.4. Angle points should be less.

The areas to be avoided as far as possible are:

(a) Rough and difficult country side.(b) Urban Development area.(c) Restricted access for transport vehicles.(d) Abrupt changes in line routes.(e) Difficult crossing - river, railway.(f) Proximity to aerodromes.(g) Natural hazards like steep valleys, hills,

lakes, gardens, forests, playgrounds, etc.

The route selected for a distribution line shall be such that it will give the lowest costconsidered over a period of years, consistent with accessibility for easy maintenance, etc. Thisincludes many considerations such as original cost, tree trimming and compensation, freedomfrom vehicular damages future development and availability for services.

The lines should be routed wherever possible to avoid natural obstacle such as steep hills or valleys', swamps, lakes, thick forests, rivers, etc. Lines should be so located at a safe distance from buildings and from possible fire, proximity to traffic and other hazards. Line shall not cross school play grounds, cemetery, except under special circumstances. Lines should be away from the buildings containing explosives.

Transportation contributes a major portion of construction cost. As such while finalising the route alignment, it may be ensured that due to transportation cost should be as low as possible.

Transport of RCC/PCC poles pose greater problem as they are generally heavier than other types of supports for same duty. The RCC/PCC poles are generally stronger on the longer axis than on shorter axis. Care should be taken on this aspect while handling, to prevent excessive stressing of the pole at the time of transporting. The unloading of poles from truck or trailer should also be done carefully. Suitable skid boards must be used and on no account, the poles be dropped. Several utilities have special trucks made with side loading arrangement for pole transportation or use trailers. It is preferably to provide a chain pulley block with a beam arrangement in the middle of the truck body to facilitate unloading/loading of poles. The poles should not be dragged on rough surface, but transported in small hand-cart

Detailed Survey

(a) Preliminary Walk Over survey (b) Detailed survey

Having provisionally fixed the route, on the survey map, a preliminary Walk Over survey is carried out, before conducting the survey with ranging rods. As far as possible, the line route is taken through areas with minimum tree growth. If there are alternative routes, all such routes are investigated for final evaluation of the most economic route.

It may be mentioned here that the detailed survey can be carried out by theodolite and angle points can be fixed and marked with survey stones. A route map to a scale of 1 cm— 0.5 km can be prepared showing the various angles approach roads, near the line, routes detail of railways, communication lines, EHT line crossing, river crossing,, etc. but this is not necessary in case of small lines as the local staff usually is conversant with the topography and therefore marking of locations aligning the line with ranging rods is found to be satisfactory.

Right of Way(a) Once the route of the line is fixed approval has to be

obtained, From the railway authorities for Railway Crossings.

From the competent Forest Authorities for routing of the line in Forest areas.

From the State level Power Tele-communication Coordination Committee (PTCC).

(b) In addition if there are urban development Air-port and similar other areas falling in the route of the line, permission has to be obtained.

( c) Sometimes private gardens/orchards may fall on the route and require free cutting. The details of trees are to be marked. Compensation be got fixed from Revenue Authorities and paid to the owner

Pole Locations : In locating poles on lines, the following general principles are to be kept

in mind 1. Keep spans uniform in length as far as possible.2. Locate to have horizontal grade3. By locating the poles on high places short poles can be used and

will maintain proper ground clearance at the middle of the span. In extremely hilly or mountainous country, poles are located on ridges thereby increasing the spans without greatly increasing the pull on the conductor. This is possible because the sag can be made very large maintaining the required ground clearance.

4. Poles should not be placed along the edges of cuts or embankment or along the banks of creeks of streams

5. Cut-point for a section could be at 1.6 km length (except in special cases), where Double-pole structures are to be provided to take tension of the conductors. It may have been already estimated that 10 supports (locations) are mostly required in one km. length of H.T. lines and 15 supports for L.T. line

ConstructionThe construction activity of H.T. lines may b& divided as follows (1) Pit marking, pit digging. (2) Erection of supports and concreting. (3) Providing of guys to supports. (4) Mounting cross-arms, phi and insulators, and pin binding. (5) Paying and stringing of the conductor. . (6) Jointing of conductors. (7) Sagging and tensioning of conductors. (8) Crossings. (9) Guardings. (10) Earthings. (11) Testing and commissioning

For low tension lines the activities could be followed, with simplified procedure.

Pit Marking and Digging Procedure After surveying, the pole location should be marked with

the peg. The pits should not be too large than necessary, as otherwise, after erection of the 'pole and filing there remains a possibility of tilting of pole. For marking the pits, the dimensions of the pit and the centre to centre distance of pits are required. Pits having a dimension of about 1.2 m x 0.6 m should be excavated with its longer axis in the direction of the line. The planting depth should be about

1 /6 length of the support (1500 mm). Excavation is generally done by using pickaxe crow bars,and shovel, very hard or rocky soil may require blasting of rock by small charges of gunpower, etc.

Erection of Poles and Concreting

After excavation of pits is completed, the supports/ poles to be erected may be brought to the pit location by manual labour or by cart. Then the pole may be erected inside the pit.

Erection of poles can be done by using Bipod/ wooden horse made of 15 cm G.I pipe and 6 m long. The spread of the legs should be 10m. The tie wire for attachment of bipod to the pole is about 6 m long and is made of 7/10 SWG. (3.15 mm) Stay wire and this wire should be attached to the pole at 8 m. The pole is slid along the line route. The pole is tied with 3 ropes.The rope at the bottom prevents the pole from being dragged in the direction of the pull. To prevent the support from moving side in raising, two guy ropes are fixed on both sides and attached to temporary anchor. -

For smooth sliding and prefect placement of pole in the pit, an inclined trench having15.2 cm (6 in.) width and 10.2 cm (4 in.) length may be dug adjacent to the pit as shown below.A piece of M.S. channel may be placed in the inclined position at the other end of the pit for enabling the pole to slip smooth inside the pit. The following figure shows the procedure for erection of pole.

The trench would facilitate the pole to skid smoothly into the pit with jerks.

V3 from top of bipod' ' -- "' ;f»f y.|"V"J'.""'

Pully

The bipod is placed in position and attached to the pole by means of tie wire. The pull for lifting the poles is provided by rope pulley. When the pole has reached at an angle of (35° to 40°) the bottom holding rope is slowly released. When the pole assumes the vertical position, the holding ropes should be tightened.It should be ensured that the time of erection, four men are at the ropes and the supervisor should be at a distance for guiding correct position so that hi the event of breaking of rope, if pole falls, it will not result into an accident. Before the pole is put into RCC padding or alternatively suitable base plate may be . given below the pole to increase the surface contact between the poles and the soil. The padding will distribute the density of the pressure due to weight of the pole on the soil.Having lifted the pole the same should be kept in vertical position with the help of manila rope of 20/25 mm dia., using the rope as a temporary anchor. As the poles are being erected say from an anchor point to the next angle point, the alignment of the poles should De checked and set right by visual check. The vertical ties of the poles are to be checked with a spirit level. After the pole erection has been completed, and having satisfied that the verticality and alignments are all right, earth filling and ramming is to be done.In swamp and special locations, before earth filling, the poles are to be concreted up to the ground level of the pit.After poles have been set, the temporary anchors are to be removed.

Erection of DP Structure for Angle Locations For angles of deviations more than 10°, DP structure

may be erected. The pit digging should be done along the bisection of angle of deviation.

After the poles are erected, the horizontal/cross bracing should be fitted and the supports held hi a vertical position with the help of temporary guys of Manila rope 20/25 mm dial

Ensuring that the poles are held in vertical position (by spirit level) the concreting of poles with 1:3:6 ratio may be done from bottom of the support to the ground level. Before lifting the pole in the pit, concrete padding of not less than 75 mm thickness may be put up for the distribution of the loads of the support ojn the soil or anchor plate could be used. .

Concreting

The concreting mixture 1:3:6 ratio, would mean 12.8 bags of cement 100 Cft of 1 1/2 size gitti and 50 Cft of sand. It may be .noted that while preparing the concrete mixture large quantities of water should not be used as this would wash away cement and sand

33 KV Line(i) Provision at D.P. locations is for 6 guy-sets (20 mm Rod of turn-

buckle x 7/4 stay-wire, 8.5 kg in wt for each location). The quantity of concrete at the rate of 0.5 cum for D.P. locations and 0.3 cum for stay sets is 2.8 cum, irrespective whether D.P. locations are of P.C.C. poles or Rail pole.

(ii) Provision for 8 tangent locations in a section of 1 km is for 3 Guy-sets. These will require 0.9 cmt of concrete @ 0.3 cum per Guy-set. A prefabricated base plate is to be provided at the bottom of the P .C.C. support for uniform distribution of load. If this is not provided then provision at the rate OX)5.cm_of concrete per location for 8 locations should be made for casting the base-pad before erection of the P.C.C. support. Thus the total quantity of concrete required is 1.3 cum. Tangent locations are not concreted in several states but boulder 1 filling is carried out to economies. If P.C.C. pole tangent locations are to be concreted additional provision for concrete quantity is to be made. However Rail pole or joist tangent locations (if Rails or joists are used) should be concreted. Provision for tangent location's concreting is to be at the rate of 0.5 cum per location

llkVLine The guys are made with 7/3.15 stay wire (5.5 kg)

turnbuckle rod is of 16 mm dia. 6 Guy-sets are required at D.P. locations and 4 additional Guy-sets are required in a km for 8 tangent-locations. The quantity of concrete for Guy-sets is provided at the rate of 0.2 cum per Guy-set. D.P. locations of P.C.C. poles require 0.3 cum concrete per location. Boulder filling of tangent locations could be adopted. If concreting is done for tangent locations additional provision at the rate of 0.3 cum per location should be made. Base-pad is to be used if not additional provision for base^pad concreting should be made

LTLine

15 locations are there in 1 km. Provision for 9 guy-sets is made with 7/3.15 stay-wire (5.5 kg), the turn-buckle M.S. rod is of 16 mm dia. concrete quantity at the rate of 0.2 cmt per stay-set should be provided. Base pad should be used if not additional provision for base pad-concreting is to be made,

Providing of Guys to Supports In spite of careful planning and alignment of line route, certain situations arise where the conductor tries to tilt the pole from its normal position due to abnormal wind pressure and deviation of alignment, etc. When these cases of strain arise, the pole is strengthened and kept in position by guys. One or more guys will have to be provided for all support where there is unbalanced strain acting on the support, which may result in tilting/uprooting or breaking of the support.Guy brackets or clamps are fastened to the pole. The most commonly used form of guy is anchor guy. These guys are provided at (i) angle locations (ii) dead end locations (iii) tee off points (iv) steep gradient locations and (v) where the wind pressure is more than 50 kg sq. m.

The fixing of guys stays will involve (i) pit digging and fixing stay rod (ii) fastening guy wire to the support (iii) Tightening guy wire and fastening to the anchor. The marking of guy pit, digging and setting of anchor rod must be carefully carried out. The stay rod should be placed in a position so that the angle of rod with the vertical face of the pit is 30°/ 45° as the case may be.

G.I. stay wires of size 7/3.15 mm (10 SWG) or 7/2.5 mm (SWG 12), and 16 mm 720 mm stay rods are to be provided. For double pole structure (DP), four stays along the bisectional the each direction and two stays along the bisection of the angle of deviation or as required depending on the angle of deviation are to be provided.

After concreting back filing and ramming must be done well and allowed 7 days to set. The free end of the guy wire /stay wire is passed though the eye of the anchor rod, bent back parallel to the main portion of the stay/guy and bound after inserting, the G.I. thimble, where it bears on the anchor rod. If the guy wire proves to be hazardous, it should be protected with suitable asbestos pipe filled with concrete of about 2 m length above the ground level, painted with white and black strips so that, it may be visible at night. The turn buckle shall be mounted at the pole end of the stay guy wire so fixed that the turn buckle is half way in the working position, thus giving the maximum movement for tightening or loosening.

Guy Strain Insulators Guy insulators are placed to prevent the lower part of

the Guy from becoming electricallyenergised by a contact of the upper part of the guy when the conductor snaps and falls on them ordue to leakage. No guy insulator shall be located less than 3.50 mm (vertical distance) from theground. ,

A type of insulators are to be used for L.T. Line-Guys One C type of insulator is to be used for ll KV Guys

Two C type of insulators are to be used for 33 kV.

Fixing of Cross-Anns & Top-brackets After the erection of supports and providing guys, the

•cross-arms and top-brackets are to be mounted on the support with necessary clamps bolts and nuts. The practice of fixing the cross-arms brackets before the pole erection is also there. In case, these cross are to be mounted after the pole is erected, the lineman should climb the pole with necessary tools. The cross-arm is then tied to,a hand line and pulled up bytthe ground man through a pulley, till the cross-arm reaches the line man. The ground man should station himself on one side, so that if any material drops from the top of the pole, it may not strike him. All the materials should be lifted or lowered through the hand line, and should not be dropped.

Insulators and Bindings

Line conductors are electrically insulated from each other as well as from the pole or tower by non-conductors, which we call 'Insulators'.

There are 3 types of porcelain insulator:1. Pin type2. Strain type 3. Shackle type

The pin type insulators are generally used for straight stretch of line. The insulator and its r. . pin should be mechanically strong enough to withstand the resultant force due to combined effect , of wind pressure and weight of the conductor in the span.

The strain insulators are used at terminal locations or dead end locations and where the angle of deviation of line is more than 10°.

The shackle type of insulators is used for LT. lines. The pins for insulators are fixed in the holes provided in

the cross-arms and the pole top brackets. The insulators are mounted in their places over the pins and tightened. In the case of strain or angle supports, where strain fittings are provided for this purpose, one strap of the strain fittings is placed over the cross-arm before placing the bolt in the hole of cross-arms. The nut of the straps is so tightened that the strap can move freely in horizontal direction.

Tying of Conductor on Pin Insulators Conductors should occupy such a position on the

insulator as will produce minimum strain on the tie wire. The function of the wire is only to hold the conductor, in place on the insulator, leaving the insulator and pin to take the strain of the conductor.

In straight line, the best practice is to use a top groove insulator. These insulators will carry grooves on the side as well. When the conductor is placed on the top groove, the tie wire serves only to keep the conductor from slipping out.

On corners and angles (below 5 deviation) the conductor should be placed on the outside of the insulators. On the far side of the pole, this pulls the conductor against the insulators instead of away from the insulator.,

Kind and Size of Tie Wire to be used In general the tie wire should be the same kind

of wire as the line wire i.e., aluminium tiewire should be used with aluminium line conductor. The tie should always be made of softannealed wire so that it may not be brittle and injure the line conductor. A tie wire should neverbe used for second time. Good practice is to use no. '6' tie wires for line conductor, (i) The lengthof the wire varies from 1 m for simple tie of a small insulators (Lt pin insulators) to 3 m (33 pininsulators)

Rule of Good Tying Practice (i) Use only fully annealed tie wire. (ii) Use that size of tie wire which can be readily handled yet one which

will provide adequate strength.(iii) Use length of tie wire sufficient for making the complete tie,

including an end allowance for gripping with the hands. The extra length should be cut from end ifthe tie is completed.

(iv) A good tie should(a) Provide a secure binding between line wire insulators and tie wire.

(b) Have positive contacts between the line wire and the tie wire so as to avoid shifting contacts.

(c) Reinforce line wire if the vicinity of insulator.(v) Avoid use of pliers.(vi) Do net use the wire which has been previously used.(vii) Do not use hard drawn wires for tying. Good helical accessories are available and can be used

Conductor Erection Paving and Jointing Conductor erection is the most important phase in construction. The

main operations are : (a) Transportation of conductor to works site. (b) Paying and stringing of conductor. (c) Joining of conductor. (d) Tensioning and sagging of conductor.

The conductor drums are transported to the tension location. While transporting precautions are to be taken so that the conductor does not get damaged/injured. The drum could be mounted on cable .drum support, which generally is made from crow-bar and wooden slippers for small size conductor drums. The direction of rotation of the drum has to be according to the mark in the drum so that the conductor could be drawn. While drawing the conductor, it should not rub causing damage. The conductor could be passed over poles on wooden or aluminium snatch block mounted on the poles for this purpose.

The mid span jointing is done through compression crimping or if helical fittings are used the jointing could be done manually. After completing the jointing, tensioning operation could becommenced. The conductor is pulled through come-along clamps to stringing the conductor between the tension locations. Sagging of conductor has to be in accordance to the Sag Tensionchart. In order to achieve it, it is preferred to pull the conductor to a tension a little above the theoretical value so that while transferring it from the snatch blocks to the, pit insulators and totake care of temperature variation proper sag could be achieved. Sagging for 33/11 kV line is mostly done by "Sighting". A horizontal strip of wood is fixed below the cross-arm on the pole at the required sag. The lineman sees from other end and the sag is adjusted by increasing or decreasing the tension. The tension clamps could then be finally fixed and conductor be fixed onpin-insulators. All fittings, accessories like guys, cross-arms, etc., could be checked as theyshould not have deformalities.

Sagging and Tensioning The conductor length in a section increases or decreases with variation in

atmospheric temperature. In summer when temperature is high the length increases due to expansion and in winter, when the temperature is low the length decreases due to contraction. With increase in length, the conductor becomes loose, sag increases and tension reduces, while in winter the sag decreases, tension increases. The conductor has to be properly keeping the required sag at the strung atmospheric temperature.

It is known that sag d = WI2/2T where I is half the span length, T is-tension in theconductor and W = V(w2+ww2), w is the weight of the unit length of the conductor actingvertically and ww is wind-pressure on the unit length of the conductor acting horizontally. If wedesign. The line for 75 kg/m2 wind-zonethen wind load on 1 m length of the conductor and 2/3projected Dia (Din mm) of the conductor ^

= [(2/3)x75]x(D/100)xlkg

The line has to be designed to withstand the above load as postulated by the I.E. Regulations. Hence it becomes necessary to calculate the tension and sag under conditions occurring at the time of erection. In practice the conductors are hung over Aluminium rollers and pulled up through ropes over snatch blocks for the required sag or tension and then transferred to the insulators. The tension is not measured as it requires elaborate arrangements and difficult to measure it accurately, the sag is only measured.

There are two important factors which vary the sag and tension : (i) Elasticity of the conductor and (ii) Temperature. Sag is directly proportional to Wand inversely proportional to T. If the length of the conductor increases due to temperature increase then sag will increase. This may be the case in summer, while it may be reverse in winter. The tension accordingly decreases or increases.

In order that the sag and tension values under varied working conditions may be kept according to the regulations, Sag-Tension charts are prepared for different spans and temperatures for ACSR, AAAC & AAC conductor.

Special Crossings

(A) In case the lines cross-over the other lines or buildings, safe minimum clearances are to be maintained as per IE Regulations. The clearances have been tabulated for this purpose under design aspects. These clearances should be maintained. The crossings could be for j

(i) Telephone/telegraph lines. (ii) Buildings. . (w) Lines of other voltages. (iv) Roads, streets, other than Roads/Streets.(B) River Crossing: Data for the highest flood-level should be obtained 'for

previous years. For medium voltage minimum clearance of 3 m be kept over the highest floor level. Double pole or 4 pole structure would be required to be specially designed, depending upon the span and conductor size for the river crossing. The structures should be located at such places that they could be approached under flood condition, also. The foundation of structures should be sound so that it may not get eroded or damaged due to rain water

Guarding Guarding is an arrangement provided for the lines, by

which a live conductor, when accidentally broken, is prevented to come in contact with other electric lines, telephone or telegraph lines, railway lines, roads, and persons or animals and carriages moving along the railway line or road, by providing a sort of cradle below the main electric line. Immediately after a live conductor breaks it first touches this cradle guard of G.I. wires before going down further. This, in turn, trips the circuit breakers or H.T7L.T. fuses provided for the H.T7L.T. lines, and the electric power in the conductor or the line is cut off, and danger to any living object is averted.

Guarding is not required for crossings of 66 kV and higher voltage lines where the transmission line is protected by fast acting, relay operated circuit breaker of modern design with a tripping time of even less than the order of 0.25 sec. from occurrence of fault to its clearance. For all other crossings, nice Railway Tele-communication lines and major road crossing guarding is essential

The minimum height between any guard wire and live crossing conductor shall not be less than 1 .5 m in case of a railway crossing.

The guarding consists of 2 G.I., bearer wires strung between the two line supports, and G.I. Cross-lacings connecting two-bearer wires at definite intervals. The bearer fixed to the guarding cross-arms on the line supports by means of threaded eyebolts for proper tightening. In minor L.T. Lines, only two guard-stirrups 600 mm long on either side are normally used withsingle G-.I. wire cross-lacing on either side, as a measure of economy. Due to electrification of railway-tracks nowadays, 1 1 kV & L.T. crossings have to be through under-ground cables.

Earthing Earthing shall generally be carried out in accordance with the requirements of

Indian Electricity Rules, 1956 and the relevant regulations of the Electricity Supply Authority concerned and as indicated below:

1 All metallic supports shall be earthed.2. For RCC/PCC poles the metal cross-arms and insulator pins shall be bonded

and 5 earthed at every pole for HT lines and at every 5th pole for L T lines.

3 All special structures on which switches, transformers, fuses, etc., are mounted should be earthed.

4 The supports on either side of the road, railway or river crossing should be earthed.

5 All supports (metal, RCC/PCC) of "both HT and L T lines passing through inhabited areas, road crossings and along such other places, where earthing of all poles is considered desirable from safety considerations should be earthed.

In special locations, railway and telegraph line crossings, special structures, etc., pipe/rod earthing should be done.

At other locations the coil earthing may be adopted. The coil earthing consists of 10m length of 8 SWG. G.I. wire compressed into a coil 450 mm length and 50 mm dia and buried 1500 mm deep.

Anti-climbing Devices

In order to prevent unauthorised persons from climbing any of the supports of HT & L T lines without the aid of a ladder or special appliances, certain anti-climbing devices are provided to the supports. Two methods generally adopted are (i) barbed wire binding, for a distance of 30 to 40 cm a height of 3.5 to 4 m from ground level, (ii) clamps with protruding spikes at a height of 3 to 4m.

Testing and Commissioning When the line is ready for energisation, jt should.be thoroughly

inspected in respect ofthe following.

1. Poles-Proper alignment, concerting and muffing.2. Cross-arms-Proper alignment.3. Binding, clamps and junipers - To check whether these are hi reach.4 Conductor and ground wire - Proper sag and to check whether there

are any cuts, etc.5 Guys : To check whether the Guy wire is tight and whether the Guy

insulators are hi6. Earthing System: To check whether the earthing connections of

supports and fittings are intact. Measure earth resistance with earth tester.

After the visual inspection is over and satisfied, the conductor is tested for continuity/ ground, by means of megger. At the time of testing through megger person should not climb on the pole or touch the guarding, conductor, guy wire etc.

(1) Before charging any new line, it should be ensured that the required inspection fee for the new line is paid to the Electrical Inspector and approval obtained from him for charging the line.

(2) The line should be energised before the authorised officer.(3) Before energising any new line, the officer-in-charge of the line shall

notify to the workmen that the line is being energised and that it will no longer be safe to work on line. Acknowledgement of all the workmen hi writing should be taken in token of having intimated them.

(4) Wide publicity by Tom-toming should be arranged hi all the localities through which the line, that is to be energised passes, intimating the time and date of energising and warning public against the risk hi meddling with the line.

(5) The Officer-in-Charge of the line shall personally satisfy himself that the same is in a fit state to be energised.

Functional Responsibility

A model for functional responsibility is attached for Divisions/Sub Divisions / individuals. This could be drawn on the basis of responsibility by the organisation (Department).

Sl.No

Particular of Works Sub Division Division

Constn Gang Head

AE (CSD) EE ( O & M) EE ST/RE (Const)

1 Preparation and sanction of estimate - - X -

2 Issue of Work Order - - X -

3 Survey and routing - X - Supervision

4 Forest Clearance - - - X

5 PTCC Approval - - - X

6 Marking of pole location - X - Supervision

7 Identity of materials - X - Supervision

8 Material lifting - X - Supervision

9 Entry of materials received in register - X - Checking

10 Shifting of material to the site - X - Supervision

11 Pit digging and pole erection X Supervision - Sample Check

12 Fixing of V cross arm top clamp and DC cross arm X Supervision - Sample Check

13 Fixing of insulators X Supervision - Sample Check

14 Fixing of stays /Guays X Supervision - Sample Check

15 Stringing of conductor X Supervision - Sample Check

16 Fastening of conductor with insulators X Supervision - Sample Check

17 Guardining at crossing X Supervision - Sample Check

18 Fixing of danger Board X Supervision - Sample Check

19 Pole painting and numbering X Supervision - Sample Check

20 Fixing of anticlimbing device X Supervision - Sample Check

21 Connecton of earth X Supervision - Sample Check

22 Test charge of the line X Supervision - Sample Check

23 Preparation of completion report X Supervision - Sample Check

24 Handling over to O &M X Supervision - Sample Check

Sl. No Particular of Works Sub Division Division

Constn Gang Head AE (CSD) EE ( O & M) EE ST/RE (Const)

1 Preparation, Sanction of estimate - - X -

2 Issue of work order - - X

3 Survey and routing X Supervision - Sample check

4 Marking of pole location X Supervision Sample check

5 Identity of materials - Sample check

6 Lifting of material - Supervision

7 Entry of materials to the site - check

8 Shifting of materials to the site - supervision

9 Pit digging and pole erection X Supervision Sample check

10 Fixing of LT cross arm with shackle insulators and earth

X Supervision Sample check

11 Fixing of stays /Guys X Supervision Sample check

12 Stringing of conductor X Supervision Sample check

13 Fastening of conductor with insulators X Supervision Sample check

14 Guardining at crossing X Supervision Sample check

15 Fixing of danger board X Supervision Sample check

16 Pole painting and numbering X Supervision Sample check

17 Earth connection X Supervision Sample check

18 Test charge of the line - Sample check

19 Preparation or completion report - Sample check

20 Handing over to ( O & M) - checking

REC has standardized the following sizes and types of supports for 11 KV and LT lines

Type Length Voltage Max Spn Type of With or without

earth wire

PCC 7.5 Mt 11 KV 107 Triangular Without earth wire

PCC 8.0 Mt 11 KV 107 Triangular With earthwire

PCC 7.5 Mt 415/240 V 107 Horizantal With earthwire

PCC 8.0 Mt 415/240 V 67 Vertical With earthwire

Stringing of the Line conductorREC has standardized the following sizes of conductors for 33 KV, 11 KV and LT lines

Voltage Class No. and Diameter of wire Type of conductor

33 KV Lines i) 7/3 .35 mm (50 mm2) Al ACSR

ii) 7/4.09 mm (80 mm2) Al ACSR

iii) 6/4.72 mm + 7/1.57 mm (100 mm2)

ACSR

11 KV Lines i) 7/2.11 mm( 20 mm2) ACSR

ii) 7/2.59 mm (30 mm2) ACSR

iii) 7/3.35 mm (50 mm2) ACSR

LT Lines i) 7/2.11 mm ( 20 mm2) ACSR

ii) 7/2.59 mm (30 mm2) ACSR

iii) 7/3.35 mm ( 50 mm2) ACSR

iv) 7/2.21mm (25 mm2) AAC

v) 7/3.10 mm (50 mm2) AAC

COIL EARTHING

Earthing Spiral

Bentonite power

PIPE/ROD EARTHING

COIL EARTHING

Earthing Spiral

Bentonite power

PIPE/ROD EARTHING

EARTHING ARRANGEMENT

Transformer Body /AB

Handle /Earth Terminal of Pole

11KV /433-250V Distribution Substation

Location Of Earth Pits And Connections

Note: 1. The connections To The Three_earth Electrodes

Should Be As Follows;(a).To one of The Earth Electrodes On Either Side Of

Double Pole Structure (X OR Y)(1).One Direct Connection From Three 11Kv Lightning

Arresters.(11).Another Direct Connection From The L.T Lightning

Arresters, If Provided(b).To Each Of The Remaining Two Earth Electrodes.(1). One Separate Connection From The Neutral (On The

Medium Voltage Side ) Of The Transformer(11). One Separate Connection From The Transformer. (111) One Separate Connection From The Earthing Terminal Of The pole2. 4mm (8Swg)G.I.Wire Should Be Used For Earth

Leads. All Dimensions in mm

6500mm 6500mm

6500mm

2400mm

J2

J2 J2

Construction Standard F-5

NeutralHV/LVLightening

arresters

33KV and 11KV Line Maintenance Required to minimize interruptions and improve efficiency of power supply.

OH lines be inspected periodically for maintenance purpose to detect any fault & repairs should be programmed.

OH lines be patrolled periodically at intervals (say 1month) at ground level while line is live.

Pre mansoon inspection should be programmed before mansoon.

Nature of faults- loose sags, snapping of conductors, tree branches touching line

conductors, tilting of cross arms, insulator failures (puncture) etc

1.Generation2.Transmission3.Sub transmission 4.Distribution

1. Power Station Stepup Sub Station2. Primary transmission line3. Grid Sub Station4. Secondary transmission line5. HV Sub Station Primary Distribution line6. Primary Distribution Line7. Distribution transformer Station8. Secondary Distribution Lines

Classification of Distribution System Type of Electric System -> AC or DC ; if AC

single Phase or Polyphase Type of Delivery System-> Radial, loop or

network; Radial Systems include duplicate or throw over systems

Type of construction: over load or underground

Principal features desired ->Safety, smooth and Even flow of Power ;

Economy,Primary Distribution.Secondary Distribution.

There are three different ways through which the primary distribution lines can be laid

1. The radial Primary circuit2. The loop primary circuit3. The ring main system

When power in supplied to the consumers through the secondary distribution system one of the following arrangements used

1.Radial System2.Looped System3.Net work system

The main purpose of planning is

1. to make the system economical while conforming to electricity rules of the country/state.

2. to minimize looses and maintain regulations within the permissible limits

For proper planning of a distribution system load survey and load fore casting of area are necessary.

In planning of an electrical distribution system it is necessary to know three basic things. The quanitity of the product or service desired ( per

unit of time) The quality of the Product or service desired The location of the market and the individual

consumers.

Connecting loads. Lighting loads Power loads Heating loads Electronic loads Consumer factors. Maximum Demand Demand factor Load factor Diversity factor Utilization factor Power factor

Over Head Lines:

The rules have seen framed for Safety Providing quality service to the people to lay down technical parameters and

specifications of materials to follow standards in construction, installation protection, operation & maintence.

to follow laid – down principles & procedures with accountability to people.

The main features fo O.H lines in the rules are

Supports - Factor of safety 2 to 3.5Conductors - Factor of safety 2.0Stay wires, Guard & Bearer wires - 2.5Wind load - 50 to 100 Kgs/m2 ( 150 Kg/m2)

Across Street Along Street Else where

i) up to 650V 5.791 mt 5.486 mt 4.572 mt

ii) 650 V to 33 KV 6.096 mt 5.791 mts 5.182 mt

Vertical clearances above buildings

Horizontal clearances

i) up to 650V 2.439 mt ( 8ft) 1.219 mt( 4 ft)

ii) 650 V to 33 KV 3.64 mt (12ft) 1.82 mt ( 6ft)

a)Ground clearances

Temperatures - Voltage Regulation - Frequency Variation - Maximum clearance between supports Earthing Lightning Protection Insulator & Insulator fittings Protection.

Planning the 33/11 KV Sub Station

Involves the following Steps:1. Tentative location based on available data of

the 11 KV Network2. Capacity of the Sub Station3. Selection of site

Orientation of the Sub Station Planning of the Sub Station

Main equipments of Sub Station are1.Structures2.Power Transformers3. A) Circuit Breakers

B) HT fuse (HG Fuses)4.Isolating Switches (Isolators)5.Bus Bar arrangements6.Insulators7.Lightening arrestors8.Instrument transformers

a) Current transformers b) Potential transformers

9.Control and relay panels with relays, meters etc.,10.Battery and Battery chargers11.Cables

i) Power Cables ii) Control Cables12.Earthing arrangement13.Station Transformer14.Control room15.Communication Equipment16.Fencing, Retaining wall17.Illumination, firefighting equipment, quarter

BASIC CONCEPT OF PLANNING

Awareness of the causes and their effects itself would reduce the system irregularities to some extent. All these difficulties ultimately lead to a low voltage profile in the system.

The poor voltage profile causes loss of equipments and energy. Thus, maintenance of the voltage profile to keep the consumer voltage at the declared level allowing the deviation within the permissible limits would keep the losses at control. The consumer voltage may be kept at the desired level by controlling one or more of the following variable on which it is dependent

1. The voltage received at the grid sub-station2. The range of tap changing gear available with the power transformers at

the grid sub-station3. The percentage impedance of the power transformer at the grid sub-station

4. The voltage drop in the sub-transmission line (33 K.V. or 66 K.V. lines)5. Tap available with the transformer at the primary distribution sub-station

(PDS) 6. The percentage impedance of the transformer at the PDS vii) vii) The

voltage drop in the primary transformers feeders7. The percentage impedance of the secondary distribution transformers and

taps available with them8. The voltage drop in the secondary distribution feeders 9. The voltage drop in the joints 10. Voltage regulators/booster and/or capacitors installed in the system

GUIDELINES FOR PLANNING

The following guidelines may be followed while planning secondary distribution system expansion or new:

1. Study the area carefully and estimate the load densities, present and future.

2. Select the transformer size and conductor size from the result of the optimization studies with due consideration for the existing sizes.

3. Estimate the present status of the system with respect of voltage regulation and losses

4. Estimate the optimal length of the feeders and the optimal loading limit.

5. Mark the feed area for each secondary distribution sub-station

6. Determine the load centre

7 In the case of the existing distribution system, if the transformer is not in the load centre, follow the following procedure:-

Check the length of the feeders from the load centre to the far-end points and compare with the optimal lengths. If the length is more than the optimal ones, do not try to shift the transformer to the load centre

Taking the optimal feeder length into consideration, divide the area into different feed area zones .

Determine the load centre of the different load Feed area zones, following the procedure suggested in appendix.

If the existing location of the transformer does not fall into one of these centres, then shift die transformer to the nearest centre and provide new transformers at the other centres.

Modify the feeder layouts, keeping them as straight as possible and the lengths within optimal limits

Operating instructions shall contain:

1. The designation of all the officers concerned to be intimated in the event of abnormal situation of any equipment /line and their telephone numbers.

2. General duties of the operator, the various checks he has to perform on various equipment in shift.

3. Telephone number, and addresses of 'Fire' 'Ambulance1 'Hospitals' in addition to the telephone number of all officers.

4. Single line layout of sub-station route map of each feeder indicating cut points, roads, crossing etc.,

5. Detailed break down operations of lines.

Example of Operation Instruction of Lines/Equipments CASE I 11 KV FEEDER

00.00 11 KV Feeder 'A' Trips note relay indications; Reset00.01 Charge the Feeder If OK, supply is restored temporary fault is

cleared If trips, note Line relay indications, reset00.03 Charge the Feeder, If OK, Temporary fault is cleared supply is restored, If trips, note the relay indications and reset Examine the switch yard for any visible fault, If no fault is noticed, open

line isolator.00.04. Charge lne OCB if OK, Trip the OCB, Close the line isolator charge the feeder. If OK Temporary fault is cleared, supply is 2: restored if trips, note the

relay indication & declare the feeder faulty. Inform all the officers concerned. if the breaker trips at the time of test charging the same, inform maintenance staff. In case, if at any time of test

charging the feeder, the power - transformer H.V. L.V. or group control or incoming trips, declare the feeder faulty

CASE II 00.00 33 KV feeder breaker (p) Trips at 132 KV sub station 'M'00.01 Note the relay indications, reset, charge the feeder, If OK supply is restored.

If trips note relay indications, reset 00.03 Charge the feeder If OK supply is restored If Trips note relay indication, open

line isolator.00.08 Charge the OCB, If trips, inform maintenance personnel for rectification If OK,

hand trip the OCB close line isolator charge the feeder. If OK supply is restored, if trips proceed as

follows: Contact Station 'A' Ask the operator to open incoming isolator and 33 KV out going line isolator Ask

him to examine the switch yard and report. If OK ask operator at 'A' to restore supply to the station 'A' and inform, If at the

time of charging any 11 KV feeder at station 'A' of the 33 KV breaker 'P1 trips, isolate the 11 KV feeder, restore normalcy. If operator 'A'

confirms that station 'A1 is normal, contact station 'B' operator, Ask him if the switch yard is normal. Ask him to open in-. coming 33 KV line isolator after 'B'-l is opened hand trip 'P' ask operator at 'A' to close outgoing isolator A-2.

Charge the feeder. If OK supply is restored If trips, declare 33 KV line between ASB is faulty

CASE III Power Transformer Trips on (A) Winding Temperature (B) Bucholtz Relay (C) Differential Relay (D) O/L Relay

A WINDING TEMPARATURE : Note the winding temperature is more than the set temperature? If so, is the transformer overloaded? If sot reduce the load on the transformer. .Are the cooler fans, oil pumps functioning satisfactorily? Is fuse blown out in Fan/Pump? If so, rectify. Ts there any shortive between the contacts of winding temperature relay due to vermin or ingress 'of moisture, if so take

remedial action. B Bucholtz Relay: Isolate the power transformer check bucholtz relay, is there any gas

collected, if so arrange for testing. If not check any shorting of the contacts Megger the power transformer close HV/LV breakers. If OK hand trip, inform maintenance personnel for check up

C Differential Relay Isolate the power transformer inform maintenance personnel for detailed

checkD O/L Relay.

Check if any feeder relays indication is received without the feeder breaker tripping isolate the feeder. Check the yard. If ok check is power transformer is overloaded.

GUIDELINES FOR ERECTION OF EQUIPMENT IN THE SUB STATIONS

Guide Lines of Erection of Power Transformers in 33/11KV Sub Stations

The erection of Power Transformers comprises of following Works :

Unloading of Transformer form Tractor Trailer/Lorry at the Sub Station.

Stacking aside wherever the Power Transformer plinth etc are not ready.

Moving the transformer on to plinth Assembly of all the mounting , accessories etc., Filling and topping up of transformer oil Oil circulation through filter if required. Earthing Jumpering

Unloading of Transformer form Tractor Trailer/Lorry at the Sub Station :. Generally the higher capacity Power Transformers are sent from the manufacturer duly dismantling, conservator tank Radiators, Piping etc. in either tractor trailor or lorry. For unloading the main tank from the vehicle we may use a suitable crane or do manually. When manual unloading is done, the following T & P and equipment are required.

Wooden Sleepers 8’ to 12’ length, 12” width, 6” or 8” thick– 40 Nos 10 Ton tirfur with rope -1 20 ton winch with rope -1 5 Ton Chain Pulley block -1 2 Ton Chain Pulley block -1 Hydraulic Jacks 10 Tons Capacity - 4 Nos Wire rope – ¾” size - 20 Mtrs Manila rope of different sizes & lengths Crow Bars Rail Poles minimum 20 ft length - 4 Nos General T & P Wooden Packing pieces ¼”, ½” ,1”, 1 ½” , 2”, 4” thick – Set

Plat form up to the height if tractor trailor/ lorry is to be built up with wooden sllepers. By using hydraulic jacks, the main tank is to be lifted on all sides to a height so that the rail poles can be in serted at the bottom of main tank and main tank rests on rail poles The other end of poles are to be on the wooden sleeper plat form Now with the help of winch or tirfur the transformer main tank will be dragged on rail poles up to wooden sleeper plat form. When the main tank could be dragged correctly over the platform, with the help of hydraulic jacks the transformer main tank is raised slightly and rails are removed, The main tank is (lowered to one sleeper height by slowly removing the top sleepers on e after another). During removal of sleepers following step by step operations are done.

1. Keep two jacks under jack pads of transformers along the top sleeper ( Which is be removed one jack each on either and of sleeper)

2. Operate the Jacks so that lifting pad of jacks are tightly positioned under transformer jack pads

3. Now slowly pressurize jacks equally on both sides simultaneously so that one side of the transformer tank is raised slightly to enable to draw out the sleeper.

4. Now Place the wooden packing pieces one over the other by the sides of Jacks up to jack height

5. Now remove the sleeper slowly with out hitting the jacks6. Slowly lower the transformer tank, by releasing pressure in jacks slowly (both

simultaneously) and removing the packing pieces one after another7. Now remove the Jacks, when the side of transformer is securely resting on the next bottom

sleepers8. Now place the jacks on the other side of the power transformer tank and carry pout above

operation and remove other side sleeper also.9. After the transformer tank lowered to the height of one sleeper height, then sleepers are to

be placed along the rout to the plinth on which PTR is to be erected.10. ON the sleepers rail poles are to be kept duly inserting under the tank and transformer tank

is to be dragged close to the plinth.11. After dragging the transformer tank nearer to plinth the transformer tank is to be raised to the

level slightly above the plinth top level by using sleepers & Hydraulic jack12. Then the Power Transformer tank is to be dragged on to the plinth slowly with the help of rail

poles and winch tirfor.13. When the transformer tank is correctly positioned placed on the plinth further work is to be

taken up.

Assembling of Transformer fittings , Mountings

The radiator dummy plates are to be removed and ensured that no foreign material, moisture is accumulated in the radiators, the radiators can be fitted by using sleepers, Jacks/ Chain pulley block. The radiator valves shall be inclosed position only

Conservator tank is to be fixed by lifting the same suitably

Slicagel breather, vent pipe, Bucholtz relay thermometes are to be fixed on to the power transformer

Filling /toping Up of oil : Now New filtered tested Transformer oil is to be filled in to transformer through suitable clean pump & pipes slowly through one of the top valves while filling oil slowly open bottom valve and air releasing dummy of one radiator. When oil is filled up to top of radiator then close the air releasing dummy immediately open top valve of radiator. In the same way all the radiators are to be filled and conservator tank is filled up to 50% level approximately.

Then release the air once from all air releasing points. Checkup oil level in the OLTC unit Then earthing and jumpering is to be done as per

standards.

Erection of Breakers

Before Erection of Breaker, suitable plinths are to be constructed duly embedding the foundation bolts as per distances specified in the manual of the breaker

Once the curing period of plinth is completed and plinths are perfectly cured, first the mounting structures of the breaker is to be placed on plinth in position. Then the breaker is to be brought near to plinth on rail poles or MS Channels, lifted & erected with the help of chain pulley bock & ropes. During erection of Breaker for tying the Breaker to ropes lifting ring ears provided on the breaker are to be used but not bushings or bushing collar frames

CT base channels are to be fixed and CTs are to be positioned on the channels already fixed

Jumpering from Bus Isolators to Breaker; Breaker to CTs ; CTs to line Isolators is done with Panther Conductor through suitable clamps.

Double earthing of Breaker body & CT Body is to be done.

Erection of PTs, L Ass, CTs : Before erection of these equipment base

dimension, distances between mounting holes are to be noted.

Suitable holes are to be drilled in seating structure on which the equipment is to be erected.

The equipment is to be lifted by using lifting holes provided to the equipment with the help of chain pulley block manila rope

After positioning on the channels the base is to be fixed to base channel with suitable coated or GI bolts, Plain washers spring washers and double nuts

Earthing & Jumpering is to be done as per the standards.

EARTHING SYSTEM - MAINTENANCE

Earthing Systems:

Electrical earthing is designed primarily to render electrical installation safe. The purpose of earthing are :

1. Protection to the plant2. Protection to the personnel and3. Improvement in service reliabilityNon- current carrying parts with conducting surface such as tanks of Power

Transformers, and frame work of circuits breakers, structural steel work in switch yard instrument transformer cases, lightning arresters and armored cables armoring should be effectively grounded for protection of equipments and operating personnel. Earth connections of all equipments should be made in duplicate.

Connecting lead should have sufficient current carrying capacity.L A s should have independent earth electrode which should be inter connected

to the station grounding system.All paints, enamel, seals should be removed from the point off contact of metal

surfaces before earth connections are made.The resistances of earth system should not exceed 2 ohms for 33/11 KV Sub

Stations. But in the sub stations of Distribution companies Earth resistance Maximum of 1

Ohm is maintained.Suitable grounding mat should be provided in the sub station yard.

In a Sub Sta tio n the fo llo wing s ha ll be e a rthe d .

The neutral point of the systems of different voltages which have to be earthed.

Apparatus, frame work and other non-current carrying metal work associated with each system, for example transformer tanks, switch gear frame work etc.,

Extraneous metal frame work not associated with the power systems, for example, boundary, fence, steel structures etc.,

The earthing Means connecting of Electrical equipment, machinery or an electrical system with the general mass of earth is termed as earthing or grounding

FUNCTION OF AN EARTHING SYSTEM

The earthing system must provide an environment which is free from the possibility of fatal electric shock.

The earthing system must provide a low impedance path for fault and earth leakage currents to pass to earth.

The earthing conductors must possess sufficient thermal capacity to pass the highest fault current for the required time

The earthing conductors must have sufficient mechanical strength and corrosion resistance.

A Sub Station earthing system has to satisfy four requirements:

Earthing c a n be bro a dly d iv id e d a s :

System Grounding ( System Earthing) Equipment Grounding (Safety Grounding)

System Grounding:

It is a connection to the ground of a part of the plant forming part of the operating circuits for example the star point of the transformer or the neutral conductor. The grounding of the lighting, arrestors also comes under the head of system grounding. The provision of system ground reduces to considerable extent the magnitude of the transient over voltages and there by increases the life of electrical equipment besides minimizing the services interruptions.Thus the fundamental purpose of system ground is the protection of installation and improvement in quality of service. The system ground also will ensure the safety of the personnel to some extent, as it helps to clear the fault speedily.

Safety Grounds (Equipment Grounding)

It is a connection to the ground of non-current carrying parts of the equipments like Motors, Transformer Tanks, Switchgear enclosures, Metallic enclosures of all electrically operated equipments and also the installations used to carry/ Support electrical equipments. The frames of the equipments, if not earthed when come into contact accidentally with live parts will have potential with reference to the ground. The potential difference, when shunted between the hands and the feet of a person touching the frame, produces current through the body which can result in a fatality. By connecting body which can result in a fatality. By connecting the frames to a low resistance ground system, a sufficiently high current will flow into the ground when accidentally the live parts of the equipment / Machinery touch the frames, and consequently saves the operating personnel from fatal accidents.Thus the equipment grounding is basically intended to safeguard to a great extent from the hazards of touch voltages. The safety ground is so designed that the potential difference appearing between the frames and the neighboring ground is kept within safe limits.

Separation of system and safety grounds:

During ground fault conditions, the fault current flows via the system ground. When the system and safety grounds are inter connected, the fault current flowing (via) the system ground rises the potential of the safety ground. Also the flow of current to safety ground results in hazardous potential gradient in and around sub station. In view of the above it is some times suggested that separate system and safety grounds will avoid the danger arising due to potential gradients. The idea is that by connecting the system ground to a separate earthing system situated in a in accessible spot, the ground fault current does not flow through the safety ground. However, this separate system of grounds has many disadvantages and can be more hazardous as mentioned below

With separate grounds we can avoid danger due to potentials only for faults outside the stations.

Short circuit currents will be more if the fault occurs in the sub stations.

The resistance may be more and in some cases sufficient currents may not flow to operate the relays.

For effective separation of the earthing systems, the system ground shall be installed at a distance of at least twice the diagonal length of the sub station which is covered by safety grounding. The neutral of the transformer has to be connected to this remote earthing by means of insulated leads. Even with this arrangement one cannot always be sure about the complete isolation of the two systems and there is always a chance of inadequate electrical connection through buried neutral pipes etc., Hence, this is impracticable, complicated and costly. It is therefore a common practice to install a common grounding system and design the same for effective earthing and safer potential gradients.

System Earthing

System earthing is governed by provisions of Rule - Of I.E Rules, 1956. Unearthed systems have been tried and due to the phenomenon of Arcing Grounds associated with them, theses have been abandoned, excepting in a few cases of power station auxiliaries supply systems where other arrangements are made for indicating earth faults. In an ungrounded system the insulation of all the equipments, lines etc, will have to be much higher values as compared to those of equipments and lines of a grounded system. This aspect greatly reduces the costs and ensures more safety.

Types of System Earthing:

Earthing through a resistance. Earthing through a reactance. Earthing through a Peterson coil Earthing directly or solid earthing.

Sub Station EarthingBecause of the difficulties and disadvantages involved in marinating the system grounding and safety grounding separately it is the common practice now to have a combined grounding system at the sub stations. Provision of adequate earthing in a sub station is extremely important for the safety of the operating personnel as well as for proper system operation. The Primary requirements of a good earthing system in a sub station are.The impedance to ground should be as low as possible. The impedance of the earth system shall not exceed the following limits in the sub stations

Power Stations 0.5 Ohms

Major Sub stations above 110 KV 1.0 Ohms

Minor Sub Stations below 110 KV 2.0 Ohms

Distribution Transformer Station 5.0 Ohms

Transmission line supports 10.0 Ohms

The Step and touch potentials should be within safe limits

Touch Potential :Touch potential is the potential difference between the ground surface potential where a person is standing and the potential of his outstretched hand (s) which are in contact with an earthed structure. It is normally assumed that a person’s maximum reach is 1.0 meter.Step Potential :Step Potential is the potential difference between outstretched feet, at a spacing of 1.0 meter without the person touching any earthed structureMesh Potential The maximum potential difference between the centre of a mesh in an earth grid, and an earthed structure connected to the buried grid conductors. It is worst case scenario of a touch potential.Transferred potentialThe transferred potential is a touch potential which is transferred some distance by an earth referenced metallic conductor. For example, consider a screened cable connecting two sub stations which are some distance apart. If a person disconnects the earthed termination at one end of a screened cable he may be subjected to the full ground potential rise occurring due to an earth fault. This can be a very high touch potential.

To keep the ground impedance as low as possible and also to have satisfactory step and touch voltages, an earthing mat will be buried at a suitable depth below the ground and it is provided with grounding electrode at suitable points. All the non-current carrying parts of the equipments in the sub stations are connected to this grid so as to ensure that under fault conditions, none of these parts are at higher potential than the grounding grid. Under normal conditions, the ground electrode make little contribution to lower the earth resistance; they are, however, desirable for marinating low value of resistance under all weather conditions, which is particularly important where the system fault currents are heavy.Earthing in a sub station must conform to the requirements of the Indian Electricity Rules and follow the directives laid down in section I and III of IS : 3043-1966. the earthing system has to be designed to have a low overall impedance, and a current carrying capacity consistent with fault current.

The factors which influence the design are: Duration of fault. Magnitude of the fault current. Resistivity of the underlying strata. Resistivity of the surface material Material of the earth electrode.

1. 100 X 16 mm and 75 X 8mm size MS steel flats are being ordered for forming the earthing system for EHT Sub station and 33/11 KV Sub Stations respectively

2. Earth mat shall be formed with the steel flats buried in the ground at a depth of 500mm.

3. The earth mat shall extend over the entire switchgear yard and beyond the security fencing of structural yard by at least one meter.

4. The outer most peripheral earthing conductor surrounding the earth mat shall be of 100 x 16 mm size MS flat.

5. The intermediate earthing conductors forming the earth mat shall be of 75 x 8 mm size flat.

6. All the risers used for connecting the equipment steel structures etc., to earth mat shall be of 50 x6 mm size excepting for earthing of L A s and transformer neutrals for which 100 x 16 mm or 75 x 8 mm size shall be used.

7. All Junctions (crossing of the steel flats while forming the earth mat and taking risers from the earthmat for giving earth connections to equipments, steel structural conducts, cable shearths shall be propersly welded.

8. Proper earthing lugs shall be used for connecting the earth terminals of equipments to the earthing steel flat.

9. Provisions shall be made for thermal expansion of the steel flats by giving suitable bends.

10. The earth mat shall be formed by placing 75 x8mm MS flat at a distance 5 meters along the length & breadth of the sub station duly welding at crossing.

11. All the equipments, steel structural, conduits, cable sheaths shall be solidly grounded by connecting to the earthing mat at least two places for each.

12. The ground mat of the switchyard shall be properly connected to the earth mat of the control house at least at two points.

13. welding is done shall be given a coat of black asphalitic varnish and then covered with hessain tape to avoid rusting.

14. All paints, enamel and scale shall be removed from point of contact in metal surfaces before applying ground connections.

15. The risers taken along the main switchyard structures and equipment structures up to their top) shall be clamped to the structure at an interval of not more than one meter with ground connectors.

16. 75 X 8 mm ground conductor shall run in cable trenches and shall be connected to the ground amt at an interval of 5 meters.

17. Grounding electrodes 2.75 Mtrs length 100 mm dia 9 mm thickness CI Pipes shall be provided at all their peripheral corners of the earthiong mat and also at Distance of 10 Mtrs along length & width of switch gearand in the entire switch yard.

18. The grounding electrodes shall be drived into the ground and their tops shall be welded to a clamp and the clamp together with the grounding shall be welded to the ground conductor.

19. The switchyard surface area shall be covered by a layer of crushed rock of size 25 x 40 mm to a depth of 100mm

20. Transformers and L A s and single phase potential transformer shall be provided with earth pits near them for earthing and these earth pits in turn shall be connected to the earth mat.

21. Power Transformers neutral shall be provided with double earthing. Neutral earthing and body earthing of power transformers shall be connected to separate earth electrode.

22. the entire earthing system shall be laid with constructional conveniences in the filed, keeping in view the above points.

23. The joints and tap-offs where welding is done shall be given a coat of black asphalitic varnish and then covered with hessain tape to avoid rusting.

THE PERMISSIBLE LIMITS OF STEP POTENTIAL AND TOUCH POTENTIAL SHALL BE

Maximum Acceptable step Voltage

Fault clearance times

Fault clearance times 0.2 Seconds 0.35 Seconds 0.7 Seconds

On soil 1050 V 600 V 195 V

On chippings 150mm) 1400 V 800 V 250 V

Maximum Acceptable Touch Voltage

Fault clearance times

Fault clearance times 0.2 Seconds 0.35 Seconds 0.7 Seconds

On soil 3200 V 1800 V 535 V

On chippings 150mm) 4600 V 2600 V 815 V

EARTH GRID- MATERIAL

S. No

Item Material to be used

1 Grounding Electrodes CI pipe 100 mm (inner dia) Meters long with a flange at the top

2 Earth mat 75 X 8mm MS Flat

3 Connection to between electrodes and earthmat

75 X 8mm MS Flat

4 Connection to between earth mat and equipment (Top Connections)

50 x 6mm MS Flat

The following are the minimum sizes of materials to used.

The size of trench for burying earth mat shall be 300mm X 500mm. The earth mat shall be buried in the ground at a depth of 500mm. The earth mat shall extend over the entire switch yard.

All junctions and risers in the earth flat shall be properly welded by providing additional flat pieces for contact between two flats

Provision shall be made for thermal expansion of steel flats by giving smooth circular bends Bending shall not cause any fatigue in the material.

After welding, the joints and tap offs shall be given two coats of Bitumen paint

Back filling of earth mat trench to be done with good earth, free of stones and other harmful mixtures. Back fill shall be placed in layer of 150mm, uniformly spread along the ditch, and tampered by approved means

EARTH ELECTRODESEarth electrodes shall be of CI pipe 100mm (inner dia) 2.75 meters long with a flange at the top and earth flat already indicated and shall be connected to earth grid in the Sub Station. All earth pits are to excavated and the preferred backfill is a mixture of coke and salt in alternate layers. A suitable size cement collar may be provided to each earth electrode. All bolted earth mat connections and strip connections to plant and equipment panel will be subject to strict scrutiny. Transformer Neutrals shall be connected directly to the earth electrode by two independent MS strips of 75 X 8mm. The transformer body earthing shall be done with 75 X 8mm flat. The independent connections of MS strips with earth mat shall be given on either side of the Transformer. All contact surface must be filled or ground flat ensures good electrical connection, and the contact surface shall be protected with a contact lubricant. Following this all connections shall be painted with heavy coats of bituminous black paint so as to exclude moisture.

EARTH GRID – WORK DETAILS

Neutral connection earth pipe shall never be used for the equipment earthing.

A separate earth electrode shall be provided adjacent to the structures supporting Lightning Arrestors. Earth connection shall be as short and as straight as practicable. For arrestors mounted near for protecting transformers earth conductors shall be connected directly to the tank.

An Earthing pad shall be provided under each operating handle of the isolator and operating mechanism of the circuit breakers. Operating handle of the isolator and supporting structures shall be bonded together by a flexible connection and connected to the earthing grid.

All equipment and switchgear etc., erected shall be earthed as per I.E Rules 1956.

SELECTION OF SITE FOR SUB STATION

While selecting site for Sub Station the following points should be kept in view.1.The Sub Station should be as near the load centre as feasible.2.The Sub Station should be far away from the obstructions to have permit easy and safe approach of HV over heads transmission lines.3.The Sub Station should be easily accessible to the road to facilitate transport of equipment4.As far as possible near a town and away from built up areas5.Sufficiently away from the areas where military rifle practices are held6.The Site should have as far as possible good drinking water supply7.The Sub Station should not be located within two miles of any aerodrome8.The site selected should have sufficient area to properly accommodate the Sub Station equipment, Structures, Buildings and also future extensions.9.SITE REQUIREMENTS OF SUBSTATIONS:-10.33KV SS -1Acer,132KV SS-2to5Acers,220KVSS-5to20Acers,11.400KVSS-50to75Acers,

DESIGN & LAYOUT

The factors that determine the final layout of Sub Station are

1.No, of incoming and out going feeders2.Expected loads demand on Sub Station3.Soil resistivity4.Facilities of Operation and Maintenance.5.The normal weather conditions 6.Expected fault levels at the busbars

Selection of Arrangement of Switching of Functions

Relation of the Sub Station to the system as a whole i.e the effect of a disturbance at a Sub Station on continuity Power Supply in the System.

Relation of the individual switching functions to the other system feeders & functions

Immediate as well as long range adaptability to system requirements

Evaluation of Bus Bar fault contingency. Degree of switching flexibility to be provided for

equipment and line maintenance purposes.

P & T LINE CROSSINGS:Posts & Telegraphs Department has to give clearance

for the crossing arrangement of power line with P & T lines. A detailed sketch showing profile of crossing span, angle of crossing and electrical clearance shall accompany the proposal along with the prescribed questionnaire duly answered. Clearance for the power lines will be given if the following conditions are fulfilled.

(i) The angle of crossing of the power line with the P&T line is not less than 60o

(ii) The nearest power conductor shall be away from the telecom line by not less than the distances tabulated below under maximum sag conditions.

For 132 KV Lines: 2.744 Mtr. (9feet).For 220 KV Lines : 4.572 Mtr. (15 feet)For 400 KV Lines : 4.881 Mtr. (16 feet)

RAIL CROSSINGSClearance is to be obtained from the Railway Authorities

for the proposed power line crossing railway track. A sketch showing full particulars such as Vertical Clearance of the lowest power conductor over the railway track, angle of crossing and the shortest distance from the railway track from the nearest tower shall accompany the proposal for railway crossing. The prescribed questionnaire duly answered and Factor of Safety Calculations shall also be sent along with the proposals for railway crossing.

Clearance for the railway crossing will be accorded if the following conditions are fulfilled. The power line shall cross the railway track at an angle not less than 60o.

The crossing span shall not exceed 80% of the normal design span.

The minimum clearance of the lowest power conductor over the railway track shall be as per the statement 2-1 on page II-4.

The minimum distance between footing of the nearest tower and the railway track shall not be less than 1.5 times the height of the tower.

The Railway crossing span shall be strung with double tension hardware fitted to the towers on either side of crossing span with dead end towers.

The foundations, structures and power conductors and earth-wire are all provided to have a minimum Factor of Safety of 2.00 under normal conditions and 1.50 under Broken Wire Conditions.

The towers shall be earthed by providing two earth pits on either side.

The clearance over the Railway Track and the bottom most conductor for different Transmission lines shall not be less than the distances below under max. sag conditions.Statement 2-1:For 132 KV Lines : 14.60 Metres.For 220 KV Lines : 15.40 MetresFor 400 KV Lines : 17.90 Metres.

STRUCTURES DESIGN LOADS: WIND PRESSURE ON STRUCTURES: In regions other than coastal regions 125

Kg/Sq. m. on 1.5 times the projected area of members of one face for latticed structures and other non-cylindrical objects and on single projected area in the case of other structures. In coastal regions the wind pressure may be assumed as 260 Kg./Sqm.

WIND PRESSURE ON CONDUCTORS : In regions other than coastal and hilly regions, 75 Kg/Sq.m on two thirds projected areas. Coastal areas 125/150 Kg. / Sq.m. In hilly regions 90 Kg. / Sq.m

Maximum tension per conductor of transmission line conductors strung from terminal towers to station structures or strung buses :

1) 33 KV and 11 KV 450 Kg.2) 66 KV – 450 Kg.3) 132 KV and 220 KV – 900 Kg.4) 400 KV – 1000 Kg.5) Ground wire Tension – 450 Kg. MAXIMUM SPANS OF LINES ADJACENT TO STATIONS 33 KV and below – 60.00 m 66 Kv and above – 150.00 m UPLIFT OF ADJACENT SPANS: Maximum slope (mean of the 3-phases) at the point of attachment 1 : 8 above

horizontal.

FACTOR OF SAFETY: FOR STEEL: 2.0 based on maximum loading

conditions (on elastic limit for tension members and crippling load for compression members).

FOR R.C.C. : 3.5 on ultimate breaking load FOR SAFETY AGAINST OVER TURNING:

Steel–2, R.C.C – 2.0

Busbars: The substation busbars can be broadly classified in

to the following three categories: Out door rigid tubular busbars Outdoor flexible ACSR or aluminum alloy busbars Indoor busbarsBusbar of Outdoor Switchyard:These are of following forms: 1.Tubular aluminum conductors are supported on post

insulators made of porcelain. These are bolted to get extended lengths.

2. ACSR/AAC conductor is supported at each stringing point on strain insulator .Such flexible busbars are used for long spans with (beams and columns) support structures at each end. In some cases the strain insulators may be supported on the walls of power house or valve house buildings.

Rigid Tubular Busbars:Commonly used Sizes of Aluminum Pipes:

System VoltageNominal Dia .(External / Internal) mm

11 kV, 33 kV & 66 kV 42/35

132 kV60/5288/78

220 kV

101.6/90.1101.6/85.4114.3/97.2114.3/102.3

400 kV114.3/97.2114.3/102.3127/114.5

Voltage Class 430 kV / 230 kV

Size 4" IPS (EH)

Outer diameter 114.30 mm

Inner diameter 97.20 mm

Thickness 8.51 mm

Cross sectional area 2825.61 sq.mm

Type of designation 63401 WP as per IS:5082

Tensile strength 20.5 kg/sq.mm

Weight 7.7 kg /m

Current rating at 75º C 3000 Amps

STRUNG BUS BARSThe materials in common use for strung type Bus

bars are ACSR conductors. Bundled conductors (two or four) are used where high ratings for bus bars are required. The size of conductors commonly used are:

11 KV - 61/3.18 mm ZEBRA Twin33 KV - 61/3.18 mm ZEBRA Twin132 KV - 61/3.18 mm ZEBRA Single/Twin220 KV - 61/3.53 mm MOOSE Single/Twin/Quadruple

61/3.8 mm400 KV - 61/3.53 mm MOOSE Quadruple

System Voltage

Conductor SizeEquivalent

Aluminum area (Sq.mm.)

Current Carrying

Capacity at 75oC

11kV, 33 kV & 66 kV

61/3.18mm ZEBRA ACSR 420 737

37/3.00mm PANTHER ACSR 200 487

37/2.79mm LYNX ACSR 180 445

132 kV 61/3.18mm ZEBRA ACSR 420 737

37/3.00mm PANTHER ACSR 200 487

220 kV 61/3.18mm ZEBRA ACSR 420 737

61/3.53mm MOOSE ACSR 520 836400 kV 61/3.53mm MOOSE ACSR 520 836

Strung Busbar:Commonly Used Sizes of Conductor:

STANDARD BAY WIDTHS IN METERS:11 KV - 4.733 KV - 4.766 KV - 7.6132 KV - 12.2 & 11220 KV - 17.00 400 KV - 27.00

STANDARD BUS AND EQUIPMENT ELEVATIONS Rated voltage (KV)

Equipment live terminal elevation in meters

Main Bus / Buses elevation in metres

Take-off elevation in metres

Low High

11 & 33 2.8 to 4.0 5.5 to 6.5 9.0 6.5 to 8.566 4.0 6.0 to 7.0 9.0 to 10.5 9.5

132 3.7 to 5 8.0 to 9.5 13.5 to 14.5 12.0 to 12.5220 4.9 to 5.5 9.0 to 13.0 18.5 15.0 to 18.5400 8.0 15.5 -- 23 M

STRINGING TENSIONS:The insulators, bus bars and connections should

not be stressed to more than one fourth of the breaking load or one third of their elastic limit whichever is lower.

CLEARANCES: The following are the minimum clearances for out-door equipment and rigid conductors in air.

Rated voltage (KV)

BIL Basic insulation level (KV)

Minimum Clearance between

Phase to phase spacing in isolators and switches

Phase to phase (mm)

Phase to earth (mm)

11 75 400 310 610 920

33 170 400 320 760 120

66 325 750 630 1530 2140

132 550650

13501600

11501380

2140 3050

220 9001050

23002700

19602300

3400 4000

400 14251550

40005200

35003640

Normally adopted phase spacings for strung bus are indicated below:

11 KV - 1300 mm33 KV - 1300 mm66 KV - 2200 mm132 KV - 3000/3600 mm220 KV - 4500 mm400 KV - 7000 mm.

a) The minimum clearance of the live parts to ground in an attended outdoor sub-station and the sectional clearance to be maintained between live parts in adjacent sections for safety of persons while working with adjacent sections alive are given below:

Voltage rating (KV) Minimum Clearance to ground (mm.) Sectional Clearance (mm.)

11 3700 2600

33 3700 2800

66 4600 3000

132 4600 3500

220 5500 4300

400 8000 7000

The bottom most portion of any insulator or bushing in service should be at a minimum height of 2500 mm above ground level.

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Desing & construction of 33 kv, 11 kv lines

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