Rural Electrification

Overview of topics

• Survey of OHL

• Design of Transmission and distribution system

• Cost Estimate

• Construction

• Some pictures

Softwares you need

• Autocad

• MHP(Microhydro) design aids

• MS Excel

• Optional:

• ETAP, Power world simulator, Netbas

T/D process

Survey Design Construction

Transmission and Distribution(T& D)

• Types

• Suspended Overhead on Poles (Overhead Line)

• Buried Cables (Underground Cable)

OH Line(Features)

• Less Expensive

• Frequent Maintenance Required

• Less cost per maintenance

• Does not look aesthetically good in dense locality

• Not preferred in areas which experience heavy snowfall

• UG Cable

• Costly Installation(if done per standard 6 times costly compared to OH)

• Less Maintenance

• Higher cost per maintenance

• Highly desirable in densely located areas

• Obvious choice for areas which experience heavy snowfall

• ACSR (Aluminium Conductor Steel Reinforced) conductors are the norms of the day(as against hard drawn copper which were used in earlier days)

Components used

• Transformers

• Poles

• Insulators

• Lightning arrestors and Earthing plates

Transformer

• ONAN type

• Step up transformer near to generating station of high KVA rating and stepdown of smaller KVA ratings near to load centers

• Sizing is done taking load demand and dividing by suitable power factor

Data sheet

Conductor

• ASCR type

• Mainly Dog, Rabbit, Weasel, and Squirrel

• If dog is on phase, rabbit is taken in neutral ;similarly if rabbit is in phase weasel in neutral and so on

Comparison between conductors

Poles

Voltage level

230V 400V 11, 33kV

Insulator types

Shackle Insulators of suitable voltage ratings upto 1000V OH Line

Shackle Insulators ---Supported(Clamped)by D-Iron set which is a clamp made out of galvanized steel

Shackle

S. No

Size

Dimensions

Weight Corresponding conductor

1 Small

55 mm x 55 mm

200 gm

Squirrel, service wire

2 Medium

75 mm x 90 mm

600 gm

Gopher, weasel and rabbit

3 Large

100 mm x 110 mm

1300 gm

Dog

Disc insulator

• Disc Insulators along

with suitable tension Set are used for deadening a line at 11kV or higher.

Pin Insulator

• Pin Insulators are used for straight runs of 11kV Line

Stay sets

• Stay sets to support poles

• Suitable Stay sets are provided at first pole, at line end, at all poles set at an angle and at every fifth pole even if the poles are in a straight line

• Two types 16mm dia for LT and 20mm dia for HT

Lightning arrestor and Earthing

• Lightning Arrestors to protect lines from Lightning

• 0.5kV ,ZnO (Gapless)Arrestors for 400Volt Line

• 12kV, 5kA ZnO(Gapless) Arrestors for 11kV Line

• Placed every 500m and on junctions and last(end) points

• Drop Out(DO) Fuse to Protect Transformers from Over Current

Earthing system

• 600mm*600mm*3.15 mm copper plate is used for earthing purposes(weighs around 10kg)

• Plate is connected with 8SWG copper wire(4.06dia)

• Earth Resistance value <5 Ohm is desirable but in no case should exceed 10 Ohm

Service wire

• Aluminum cable connecting from pole to home

• Generally flat twin sheathed cable is used

• Wrapped once or twice around the pole to reduce stress

• Distance is taken average of 30m

Survey

• Equipment required: • Total power station(TPS), GPS receiver or measuring tapes and

compass • Steps: • Start from generating station • Take the shortest and straight route for transmission line • Fix the position of transformer • Transformer should be positioned at the center of load as far as

possible so as to reduce the voltage drop • Measure distribution lines length in similar way • For distribution, within a radius of 30m service wire, otherwise

extend the conductor • Note all the lengths, nodes and drawings on your notebook

c

5 3

3

c

10

100HH

Talu dada

10HH

20HH

Tr3 Tr1 5

Tr2

Sunera

Dharan

N

Design of Transmission system

• Selection of voltage level

• 230/400/11,000/33,000Volts

• Balanced loading is considered

• Whole power system is radial

• Design Criteria of OH distribution line:

• Maximum allowable voltage drop at the farthest end shall not exceed 10%

• Find optimum line voltage

using 𝑉 = 5.5𝑙

1.6+

𝑃

100

0.5 where

l=length(kms) and P=Power in MW

• Take the nearest standard voltage

• Find line current using 𝐼 = 𝑃/ 3*V*pf

• Select the ambient temperature(25degrees)

• Select the allowable range of temperature and find the resistance at new temp value

• Use the relation: R𝑡 = 𝑅20 ∗ ((1

α+𝑡) /(

1

α+𝑡20))

Where R20= resistance of conductor at 20degrees

• Calculate the total resistance multiplying by length of line

• Find efficiency using relation:ῃ =𝑃

𝑃+3∗𝐼2∗𝑅∗

100

• Model the transmission line as short and find the receiving end parameters

•𝑉𝑠𝐼𝑠

=1 𝑍0 1

∗𝑉𝑟𝐼𝑟

Vs

Z

Vr

Is Ir

Distribution line voltage drop calculation

I1,ф1 I3,ф3 I2,ф2

Z Z Sending end voltage (V)

V1 V2 V3

Phase Current(A) Voltage drop (cV)

3phase P/(sqrt3*V*pf) Sqrt3*I*Z*L

1 phase P/(V*pf) 2*I*Z*L

Final design

c

89/5/3/sq

2/0.5/1/sq

1/1/3/sq

c

10/5/3/sq

100HH

10/1/3/dog Talu dada

10HH

20HH

11/0.4 0.4/11 89/1/3/dog

11kV 0.5kV

A/B/C/D A-power(kW) B-length(km) C-phase D-conductor type HH-House holds

lightning arrestor

Typical drawings

Cost Estimation

• Conductor estimate • Add 10% for sag obtained from design length • Calculate the length of phase conductors and neutral

• Pole estimate • For HT lines, divide length by 50m to get no. of 8m steel tubular poles • For three phase section, divide length by 35m to get no. of 7m wooden poles • For single phase section, divide length by 35m to get no. of 6m wooden poles

• Transformer estimate • Calculate the cost of no. of transformers in the project

• Insulator estimate • HT • Multiply no. of steel tubular poles by 3 to get number of pin insulators • LT • For 7m poles multiply by 4 to get number of shackle insulators • Care must be taken to get medium and large size shackle insulator • Multiply no. of 6m poles by 2 to again obtain shackle insulator

• Pole mounted substation • Each pole requires 6 disc and 6 pin insulators, 3 DO fuse, 3 lightning arrestors, 1 earthing set

and 1MCCB

• Double station • Each station requires 4 steel tubular poles, 6 disc insulators and 3 pin insulators

• Lightning arrestors • HT arrestors: • Divide the HT straight length by 500 and multiply by 3 o obtain no. Of lightning arrestors • LT arrestors: • Divide the LT straight length by 500 to obtain no. Of lightning arrestors. If it’s a 3phase

system multiply by 3 to get no. of lightning arrestors . • If it’s a single phase system only 1 LA is connected and thus no. is obtained

• Stay set • On every first, fifth and last pole, it should be provided. Simply, divide total no. of tubular

poles by 5 which gives the no. of HT stay set • Similarly, divide total no. of 6m and 7m poles by 5 to get no. of LT stay set

• Earthing set

• Divide the HT straight length by 500 to obtain no. of earthing sets

• Divide the LT straight length by 500 to obtain no. Of earthing sets

• Service wire

• Multiply house hold number by 30m and per metre cost to get service wire cost

• Multiply all numbers by respective cost to get total transmission cost estimate

Sample of cost estimate

Construction

• Choose a straight short line • Calculate the number of poles required for the

given length and mark it through pegs • The poles should be buried 1m with bitumen

paint • Fix D-iron and insulators • Insert the pole and check the verticality with a

plumb bob • Commence unrolling of conductor and install

using manual wire puller

Unrolling

Method of joining

Earthing

Lightning arrestor installation

Salient features

• Low voltage transmission system is still popular in under-developed and developed countries where reach of national grid is difficult

• Off grid settlement or decentralized generation

• Help in fights on poverty elimination and sustainable development which is the mean motto of UNDP, Practical Action, GIZ etc

Pictures

References

• MHP design aids, Pushpa Chitrakar, GIZ Nepal

• Microhydro Design manual, Adam Harvey

• Mini grid manual, Allan Inversin

• www.etap.com

• www.aepc.gov.np

• Jayaram Karkee, Minigrid Engineer, Resource management and rural Empowerment Center

• Transmission and Distribution Electrical Engineering Third edition, Dr C. R. Bayliss CEng FIET and B. J. Hardy ACGI CEng FIET

• A text book on power systems, BR Gupta