Placement Ques _ Electrical

8/12/2019 Placement Ques _ Electrical

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Some of questions are-

1. Distridution transformersare connected in configuration-

a. Star/delta b. delta/star c. star/star d. delta/delta2. In radial system, power is fed from-

a. From one end b. from two ends c.centre d. none

3. !at is capacitance grading-". !at is #oltage regulation-

$. Stinging c!art is used for- table is referred for sag calculation-

%. &or an e'isting transmissionline, t!e string efficiency is ()*. +ow if for same set up,dc is supplied, w!at will be t!e string efficiency-

a.()* b.more t!an ()* c. less t!an ()* d. 1))*

. sin effect depends on frequency-

a.directly proportional b. in#ersnaly proporsanal c.sqare of frequency d.none

(. coefficient of reflection of an open ended line is-

a. 1 b. -1 c.) d. none

. t!e power loss in an o#erloaded line is mainly due to-a. resistance b. capacitance d. inductance d. none

1).#oltage at generation stage is usually-a.11kv b.33# c.%%# d.22)#

11.0 fuses pro#ides best protection against-

a. s!ort circuits b. lig!tining c. sparking d. fire12. resistance switc!ing is used in case of-

a. air blast b. bul oil c. minimum oil d. all types of CB13. campared to ac, in dc corona loss is-

1".corona effect is more prominent in weat!erconditions-1$.in force #oltage analogy mass is analogus to-

a. 0 b. L c. d. current1%.p!ase margin of t!e system is used to-a. relative stability

b. absolute stabilty

c. time responsed. frequency response

1.wattmeter measures-

a. #olt ampere

b. #olt ampere reacti#ec. apperent power

d. true power

1(.inductance is measured by-a. wein bridge

b. sec!iring bridge

c. maxwell bridged. !ay bridge

1.form factor in ac means t!e ratio of-

a. pea to a#g. #alue b.pea to rms #alue c. rms to avg. value d. rms to pea #alue


2/34

2).in a single p!ase con#erter, t!e no. of S0 conducting during o#erlap

a.1 b.2 c. 3 d. "21.in a t!ree-p!ase full con#erter, output #oltage pulsates at frequency of

a. f b. 2f c. 3f d. %f

22.in a t!ree-p!ase full con#erter, e#ery S0 conducts for-a.%)deg. b. 12)deg. .1()deg. d.)deg

!o was 0amansay award winner in t!e yr 2))%

2. In t!e yr 2))% commonwealt! games are !eld ina4 Melbourne b45alasia etc

3. 6nder t!e c!ief 7udge of t!e supreme court, !ow may 7udges are t!ere

a42( b!" c42% d432

". In below names, w!ic! musician was deaf

Some names of t!e musicians are gi#en

$. !o was t!e first deputy prime ministerof India

a4morar7i desai b4c!aran c4s.pateletc

%. !o was t!e first woman prime minister in t!e world

#irimavo Bandaranaike

. !o is t!e writer of 8itan7ali

9ns: 0abindranat! ;agore

(. !o said first isan, =ai =a#an?

a47awa!arlal +e!ru b4ma!atma 8and!i c4Indira 8and!i d4 lal ba$adur s$astri@..

. !ere is t!e ABC !ead office locateda4 8ene#a b4 ietnam c4Singapore d4 vienna@@

1). In w!ic! state in India t!e sun temple locateda4 erala b45ad!ya Brades! c4 orissa d4 Andhra pradesh


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11 ow many spoes are t!ere in t!e as!oa c!ara in Indian flag

a!" b42% c42( d422 ans: 2"

12. !o was t!e present secretary for 6nited +ations

an >i-moon

13. !o was t!e only Indian !a#e t!e name #iceroy during t!e britis! rule

Some names of freedom fig!ters are gi#en.

1". ow muc! percent of forests are t!ere in India

a422* b41%& c42$* d43)*

1$. ;!e term


4/34

*+,-,0 L2,# )*B3,C4+, 456, 78,#4*2#

1. ;!e surge impedance of a 11) , 3-p!ase transmission line is "") o!ms. ;!e surge

impedance loading of t!e line is

a. 3 E11)42/"") 5b. E11)42/"") 5

c. E11)42/311)425

9ns.: Eb4

2.;!e capacitance and inductance per unit lengt! of a 3-p!ase line, operating at 11)

are .)1 microfarad and 2.$ m. ;!e surge impedance of t!e line is

Ea4 $) o!ms

Eb4 $)) o!ms

Ec4 2$) o!ms

9ns: Eb4

3. 9 long transmission line is energiFed at t!en sending end and is ept open circuited at

t!e recei#ing end. ;!e magnitudes of t!e sending end #oltage s and of t!e recei#ing end

#oltage r satisfy t!e following relations!ip

a. s Gr b. s is greater t!an r

c. s is less t!an r

9ns: Ec4

". oltage regulation of a s!ort transmission line is

a. always positi#e

b. always negati#e

c. eit!er positi#e, negati#e, or Fero

9ns: Ec4

$. ;!e capacitance of an o#er!ead line increases wit!

i. increase in mutual geometric mean distance

ii. increase in !eig!t of conductors abo#e ground


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a. ot! are true

b. ot! are false

c. Anly Ei4 is correct

9ns: Eb4

%. S!unt compensation for long C lines is primarily resorted to

a. impro#e #oltage profile

b. impro#e stability

c. reduce fault currents

9ns: Ea4

. Series compensation is primarily resorted to

d. impro#e #oltage profilee. impro#e stability

f. reduce fault currents

9ns: Eb4

(. &air weat!er corona loss may be computed using t!e empirical formula gi#en by

Beterson. 9ccording to BetersonHs formula corona loss is proportional to

Ea4 f and 2

Eb4 f2

and

a. f and

w!ere f and are t!e system frequency and #oltage respecti#ely.

9ns: Ea4

. undled conductors are used in C lines primarily for

a. reducing cost of t!e line

b. reducing corona loss and radio interferencec. increasing stability limit.

9ns: Eb4

1). ;!ere are 2) discs in t!e string of insulators of a 3-p!ase ")) transmission line.

String efficiency is () *. ;!e ma'imum #oltage across any disc is


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a. 2$

b. 2$/3 c. 2$3

9ns: Eb4

11. ;wo or t!ree s!eds or petticoats are pro#ided in pin-type insulators in order toincrease

a. creepage resistance

b. spar-o#er #oltageES.A.4

c. puncture #oltage

9ns: Ea4

12. Bin -type insulators are use up to

a. 11

b. 33

c. 132

9ns: Eb4

13. Insulators used for transmission line at t!e dead -end tower are

a. suspension type

b. s!acle type

c. strain type

9ns: Ec4

1". Cconomic studies !a#e s!own t!at D.. transmission is c!eaper t!an a. c transmission

for lengt!s

a. below 3)) m

b. beyond %)) mc. beyond 12)) m

9ns. b

1$.;ransmission #oltages in t!e range 23) -%$ are nown as


7/34

a. ig! #oltage

b. C'tra ig! oltage

c. 6ltra ig! oltage

9ns. b

1%. !ic! one of t!e following statements is false

9s t!e transmission #oltage increases,

a. orona loss decreases

b. onductor copper loss decreases

c. ost of insulators, transformers, switc!es circuit breaers increases

9ns. c

1. ;!e internal inductance of a solid conductor of radius r and carrying a current I isequal to

a. ).$ I J 1)- /m

b. ).$ I J 1)- exp9)1/"Jr /m

c. ).$ J 1)- /m

9ns. c

1(. !ic! one of t!e following statements is not true

a. ;!e 85D met!od of finding inductance does not apply to 9S0 conductors

b. urrent density in 9S0 conductors is uniform

c. ;!e 85D between two circular areas, eac! of different diameters, is equal to t!edistance between t!eir centres.

9ns. b

1. C'panded 9S0 conductors are used

a. ;o increase t!e tensile strengt! of t!e line


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b. ;o reduce corona loss

c. ;o reduce I20 loss

9ns. b

2). 9 conductor wit! 1 strands, eac! of same diameter and eac! !a#ing an inductanceof K enries is used for a transmission line. ;!e total inductance of t!e conductor

will be

a. K/1

b. K/3%1

c. 1K

9ns. a

21. ;!e line to neutral capacitance of single -p!ase line wit! conductors of radius 1cmand spaced 1m apart is equal to

a. 1)-/2 &/m

b. 1)-/3%&/m

c. 2 21)-&/m

9ns .a

22. In a double-circuit line wit! !e'agonal spacing ,

a. ;!e p!ases are balanced, but t!e conductors of eac! indi#idual p!ase are notbalanced.

b. ;!e conductors of eac! indi#idual p!ase are balanced , but t!e p!ases are not

balanced

c. ;!e p!ases, and t!e conductors of eac! indi#idual p!ase are bot! balanced

9ns. c

23. !ic! one of t!e following statements is true

a. Sin effect at $) F is negligible for larger diameter conductors but becomes

appreciable for smaller conductor.


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b. Sin effect at $) F is negligible w!ate#er t!e diameter of t!e conductor.

c. Sin effect at $) F is negligible for t!e smaller diameter conductors but becomes

appreciable for t!e larger conductor conductor.

9ns. c

2". !ic! one of t!e following statements is true

a. 0esistance of a conductor decreases and t!e internal inductance increases as t!e

frequency is increased

b. 0esistance and internal inductance of a conductor bot! increase wit! increase of

frequency

c. 0esistance of a conductor increases and t!e internal inductance decreases as t!e

frequency is increased

9ns. c

2$. ;!e surge impedance of a double-circuit power transmission line is

a. ") o!ms

b. 2)) o!ms

c. ")) o!ms

d. ()) o!ms

9ns. b

2%. ;!e surge impedance of a telep!one line is

a. $) o!ms

b. $ o!ms

c. 2)) o!ms

d. ")) o!ms

9ns. b

2%. undle conductors are preferred in C transmission lines because


10/34

a. It is easy to fabricate t!in conductors and combine t!em to mae a bundle

b. Inductance of t!e line is reduced, and t!e corona loss, and radio ; interference is

minimiFed.

c. ;ower !eig!t is reduced and !ence transmission cost is low.

9ns. b

2. Inducti#e interference between power communication lines can be minimiFed

by

a. Increasing t!e spacing of power line conductors

b. ;ransposing power line conductors

c. ;ransposing communication line conductors

d. Cit!er b or c.

9ns. d

2(. ;!e percentage regulation of an o#er!ead transmission line can be Fero w!en t!e

load power factor is

a. Kagging

b. 6nity

c. Keading

9ns. c


a. Sin effect increases t!e resistance of a conductor ,but pro'imity effect decreases

t!e resistance

b. ot! sin effect and pro'imity effect increase t!e resistance of a conductor

c. ot! sin effect and pro'imity effect increase t!e internal inductance of a conductor

9ns. b


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3). 9 transmission line !a#ing parameters 91, 1, 1, D1 is in parallel wit! anot!er

!a#ing parameters 92, 2, 2, D2. ;!e o#erall L 9 L parameter of t!e combination

is

a. 9192 M12

b. E912 M9214/E1M24

c. 1M2 M E91-924ED2-D14/E1M24

9ns. b

31. Disrupti#e critical #oltage is

a. Cqual to

b. 8reater t!an

c. Kess t!an

#isual critical #oltage for corona on an o#er!ead line.

9ns. c

32. orona loss

a. Increases

b. Decreases

c. Does not c!ange

t!e switc!ing #oltage on a transmission line.

9ns. b

33. andling of t!e telep!one recei#er may become dangerous due to

a. Clectromagnetic induction

b. Clectrostatic induction

c. ot! electromagnetic and electrostatic induction

from a power line.


12/34

9ns. b

3". 5utual inductance between a t!ree -p!ase power line and a telep!one line due to

t!ird !armonic current in t!e power line is

a. ;!e algebraic sum of mutual inductances from indi#idual p!ase wires

b. ;!e arit!metic sum of mutual inductances from indi#idual p!ase wires

c. Nero

9ns. b

3$. Dielectric strengt! of mec!anically sound porcelain is

a. 1) /cm

b. 22 /cm

c. %$ /cm

d. 1)) /cm

9ns. c

3%. Dielectric strengt! of glass is

a. 22 /cm

b. %)-%% /cm

c. 1") /cm

d. 2") /cm

9ns. c


a. &or 11 insulators, t!e ratio of wet spar-o#er #oltage to woring #oltage is (.3

b. &or %% insulators, t!e ratio of dry spar-o#er #oltage to woring #oltage is (.3

c. &or 11 insulators, t!e ratio of dry spar-o#er #oltage to woring #oltage is (.3

9ns. c


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3(. !ic! one of t!e following statements is true

a. &las!-o#er tests are performed on all insulators

b. 0outine tests are performed on 1/2 percent of insulators supplied

c. Design tests are done on 1/2 percent of insulators supplied

9ns. none of t!e abo#e


a. ;!e spar-o#er #oltage is less t!an t!e puncture #oltage

b. ;!e spar-o#er #oltage is greater t!an t!e puncture #oltage

c. ;!e spar-o#er #oltage is equal to t!e puncture #oltage

9ns. a

"). !ic! one of t!e following statements is true

;!e s!eds of an insulator s!ould be s!aped

a. ;o conform to t!e electrostatic tube of force and t!e body s!ould be s!aped toconform to t!e equipotential surfaces

b. ;o conform to t!e equipotential surfaces and t!e body s!ould be s!aped to conformto t!e electrostatic tubes of force

c. ;o conform to t!e equipotential surfaces and so also t!e body

9ns. b

"1. In a suspension insulator, t!e mec!anical stresses on t!e conductor are

a. Increased

b. 0educed

c. ;!e same

9ns. b

"2. ;!e string efficiency of t!e insulator can be increased by


14/34

a. Increasing t!e number of strings in t!e insulator

b. Increasing t!e ratio , capacitance to eart!/capacitance per insulator

c. y t!e correct grading of #arious capacitances

d. Decreasing t!e number of strings

9ns. c

"3. String efficiency of insulators for wet flas!-o#er is

a. Kess

b. 5ore

c. ;!e same

as t!at for dry flas!-o#er

9ns. b

"". ;!e potential across insulator discs can be equaliFed by !a#ing

a. ;!e same capacitance for eac! unit

b. ;!e !ig!est capacitance for t!e lowest unit and decreasing progressi#ely t!e

capacitance of ot!er units

c. ;!e lowest capacitance for t!e lowest unit and increasing progressi#ely t!e

capacitance of ot!er units

9ns. b

"$. 8rading ring ser#es t!e purpose of

a. CqualiFing t!e #oltage distribution across discs

b. 9n arcing s!ield

c. ot! equaliFing t!e #oltage distribution and acting as an arcing s!ield

9ns. c

;AB
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Factors affecting Line design

oltage le#el

onductor type siFe

Kine regulation #oltage control

orona losses

Broper load flow system stability

System protection

Insulation co-ordination

0ig!t of way

5ec!anical design

Sag stress calculation

onductor composition

onductor spacing

Insulator /conductor !ardware selection

Structural design

Structure types

Stress calculations

Conductor si:e

8auge siFes decrease as t!e wire increases in siFe.

+umber of strands G 3 n2-3n M 1

w!ere n G number of layers including t!e single central strand.


16/34

;!e following conductors are used.

99-all aluminum conductor

999-all aluminum alloy conductor

9S0-aluminum conductor steel re-inforced

990-aluminum conductor alloy re-inforced

;AB

Line resistance

0 G l/9

02/01G E;)M;24/ E;)M;14

02 G0esistance at temperature ;2

01 G0esistance at temperature ;1

;) G onstant

G 23".$ for annealed copper of 1))* conducti#ity

G2"1 for !ard drawn copper of .3* conducti#ity

G22( for !ard drawn aluminum of %1* conducti#ity

Sin effect is function of conductor siFe, frequency and resistance of conductor material.

Discuss t!e pro'imity effect, stranding and spiraling of conductors

;AB

Line inductance ) one p$ase ; ()p$ase

Single-p!ase o#er!ead line

oltage drop in a single-p!ase line due to loop impedance

G 2 l E0 M 7 ) ln EDm/Ds4/24 I

lG line lengt!, m
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0G resistance of eac! conductor, m

DmG equi#alent or geometric mean distance E85D4 between conductor centres

DsG 8eometric mean radiusE8504, or self-85D of one conductor

G ).(( r for cylindrical conductor

rG conductor radius

I G current

KG 2 ' 1) - ln EDm/Ds 4 /m

;!ree-p!ase o#er!ead line Eunsymmetrical spacing4

Dab MDbc MDca

Cqui#alent equilateral spacingGDeq G Dm G EDab DbcDca4 1/3

In practice , conductors are transposed.

;ransposition is carried out at switc!ing stations

9#erage inductance per p!ase

KG2 ' 1) - ln EDeq/Ds 4 /m

;AB

Line capacitance< 1)p$ase ; ()p$ase

Single-p!ase o#er!ead line

ab G 2 )r/ln ED/r4 E&/m4

;!e capacitance to neutral for a two- wire line is twice t!e line-to-line capacitance, ab.

;!ree-p!ase o#er!ead line

Kine-to-neutral capacitance

n G 2)r/ln EDeq/r4 E&/m4

!arging current /p!ase G7n p! E9/m4


18/34

;AB

,ffect of ground on capacitance of ()p$ase line

;!e capacitance of a 3-p!ase transposed line considering ground effect is gi#en by

n G 2)r/Oln EDeq/r4 -ln E!12 !23 !31/!11!22!334P E&/m4

w!ere !12G distance between conductor 1 and image of conductor 2, etc. Cffect of

ground is to increase t!e capacitance.

;AB

,=uivalent circuit for s$ort transmission lineEup to () m4

+ote t!at bold symbols indicate comple' quantities.

+sG+rM r>

s G r ?

Draw a p!asor diagram for a s!ort line wit! inducti#e load and wit! capaciti#e load,

using +ras t!e reference p!asor.

S!ow t!at

s G SQ0;OEr M I0 os r MEor -4 IR Sinr42 MEIR os r MEor -4 I0 Sinr42P

M sign abo#e is for lagging p. f

- sign abo#e is for leading p.f

r G angle between +r r

s G angle between +s s

G s-r G load angle

tanG EIR os r MEor -4 I0 Sinr4/ Er M I0 os r MEor -4 IR Sinr4

+sG +rM Br

sG C+rM0r
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19/34

&or a s!ort line, 9G1, BG>5

B G >E1M>5/"4

CG 5

0G

;!e 9D parameters of t!e nominal-networ are:

G 1M>5/2

BG >

C G 5E1M>5/24

0G

+ominal -; and +ominal)networs are not equi#alent electrically, as may be #erifiedby using t!e -transformation.

oltage regulation Epu4G EEs/94 - r&K4/ r&K

;AB

Long line e=uations Eabo#e 2") m4
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;!e solution of t!e #oltage wa#e equation using t!e initial conditions is

+G Eos! '4 +rM E>@ Sin!'4 r

I G E)Sin!'4+rM Eos! '4 r

? sqrt9y: ? A7

G attenuation constant pu lengt!

?p!ase-s!ift constant pu lengt!

y ? s!unt admittance pu lengt!

:G series impedance pu lengt!

>@ ? surge impedance Gsqrt EF/y4T )?1/>@

+sG +rM Br

sG C+rM0r

w!ere

G os!l

BG >@Sin!l

CG E1/>@ Sin! l

0G

lG line lengt!

;AB

,=uivalent circuit for a long

Line

;!e e'act equi#alent circuit and t!e e'act equi#alent ; circuit for a long line are s!own in&ig."

;!e elements of t!e circuit are obtained from

>G B ? >@Sin!l G E> Sin!l4/l
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5/2 G E-14/B ? 9 os!l - 14/>@Sin!l G EtanEl/24./24/El/24.

;!e elements of t!e ; circuit are obtained from

>4/2 G E-14/C ? 9os!l-14/ EE1/>@ Sin! l4

>4 ? ! >@ tan$ 9l/24 G E> tan$ 9l/244/9l/24

54 ? C? E1/>@ Sin! l G E5 Sin! l4/l

;AB

#urge impedance loading of lines

ncident and reflected voltages on long lines

+s ? E1/24 E+r Ar >o4 ele

7lA 9E1/24 E+r )r >@4 e

-le

-7l

s ? E1/24 E+r5o Arele

7lA 9E1/24 E+r5o )re

-le

-7l

;!e first and second terms in eac! of t!e abo#e equations refer to t!e incident and reflected #oltages respecti#ely.

;!e wa#elengt! is defined by

G 2/

;!e #elocity of propagationof t!e wa#es is gi#en by

G f

G %))) m at $) F.

!en t!e line is terminated in its surge impedance Nr G No, t!ere is no reflected wa#e. EInfinite line4

#urge mpedance Loading 9#L of a transmission line

SIK G Or EK-K4 Ein 4P2

/NoH E54

w!ere NoH G sqrtEK/4

SIK is a measure of t!e ma'imum power t!at can be deli#ered o#er a line. ;!e following factors affect t!e ma'imum power:

Kine lengt!


22/34

;erminal apparatus impedances

9ll ot!er factors t!at affect stability.

;o increase SIK, r can be increased and No reduced by using series compensation.

;!e distinction between ma'imum power and SIK s!ould be mentioned.

;AB

Ferranti effect

;!e parameter G os! l decreases wit! increase in line lengt!. In suc! cases r is considerably greater t!an s, w!en t!e line is c!arged but unloaded.

In underground cables, t!e effect is muc! more pronounced, e#en in s!ort lengt!s. It is called t!e &erranti effect. Discuss t!e effects of s!unt compensation

and reacti#e loading.

;AB

dvantages of bundled conductors

0educed line reactance

0educed #oltage gradient

Increased corona critical #oltage, and t!erefore, less corona power loss, audible noise, and radio interference.

0educed amplitude duration of !ig! f requency conductor #ibration

;AB

0isadvantages of bundled conductors

Increased ice wind loading

Inspection more complicated ,spacers required

Increased clearance requirements at structures

Increased c!arging 9 w!ic! may be a disad#antage at lig!t loads

Ds G 850 of subconductors

d G distance between two sub-conductors
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23/34

Dsb

G 850 of bundled conductor

Dsb

G EDd41/2

E&or a 2-conductor bundle4

Dsb

G EDd2

41/3

E&or a 3-conductor bundle4

Dsb

G EDd3

41/"

E&or a "-conductor bundle4

9#erage inductance per p!ase of a bundled conductor,

KG 2 ' 1)-

ln EDeq/ Dsb

4, /m

Deq G ED12 D23 D314

Di7 G spacing between p!ase i and p!ase 7

;AB

Factors affecting mec$anical design of over$ead lines

1. !aracter of line route

2. 0ig!t-of-way

3. 5ec!anical loading

". 0equired clearances

$. ;ype of supporting structures

%. onductor

. ;ype of insulators

(. =oint use by ot!er utilities

Factors affecting span lengt$

1. !aracter of route

2. Broper clearance between conductors

3. Bermissible tensions under ma'imum mec!anical load

4$ere are five kinds of stresses on lines ; supports

1. ;ensile

2. ompressi#e

3. S!earing

". ending

$. ;wisting stress or torque

#ag and tension analysis of over$ead lines

-e=uired clearances


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;!e data for t!e following clearances of different #oltage le#els s!ould be nown.

1. learance of conductors passing by buildings

2. 5inimum clearances of conductors abo#e ground or rails

3. rossing clearances of wires carried of wires carried on different supports

". oriFontal clearances at support between line conductors based on sags.

#ag and tension analysis

&actors affecting sag are:

1. onductor load per unit lengt!

2. Span

3. ;emperature

". onductor tension

$. Ke#el at supports

onductor load depends on

1. eig!t of conductors

2. eig!t of ice or snow on conductors

3. ind blowing against wire

,ffect of c$ange in temperature

If t!e conductor stress is constant and if t!e temperature c!anges, t!e c!ange in lengt! is

l G lo. .t

t G t1-toG c!ange in temperature

l G l1-lo G c!ange in lengt!

G oefficient of linear e'pansion of conductor per deg. . If temperature is constant w!ile conductor stress c!anges Ei.e. loading4, t!e c!ange in lengt! is

l G lo. ;/59


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; G;1-;oG c!ange in tension in g

5G modulus of elasticity of conductor

9 G 5etal cross-section of conductor.

onsider t!e following in sag tension calculations:

1. Supports at same le#el E I4 atenary met!od ,E ii4 Barabola met!od

2. Supports at different le#els Eunsymmetrical spans4

3. Cffect of ice

". Cffect of wind

Line location

1. Brofile plan of rig!t-of-way

!. ;emplates for locating structures

;!ese are used to pro#ide t!e following

a. 5aintenance of proper clearance from conductor to ground and to crossing conductors

b. Cconomic layout

c. Broper grading of structures

d. Bre#ention of e'cessi#e insulator swing or uplift at structures.

;AB

Corona

If an alternating potential is applied to two wires w!ose spacing is large in comparison wit! t!e diameter and t!e potential difference is gradually

increased, a point will be reac!ed w!en a faint luminous glow of #iolet colour will appear, and a !issing sound will be !eard. ;!is p!enomenon is nown

as orona. ;!e formation of corona is accompanied by a loss of power. It causes non-sinusoidal nature of current and interference wit! neig!bouring

communication circuits.

orona formation taes place due to ioniFation of a layer of air immediately surrounding t!e conductor. &or air under ordinary conditions near sea le#el

wit!out impurities, t!e #alue of potential gradient at w!ic! ioniFation taes place can be taen as 3)/cm Epea4.

Interference wit! communication circuits may be due to bot! electromagnetic and electrostatic action, t!e former producing currents, w!ic! are

superposed on t!e true speec! currents, t!ereby setting up distortion and t!e latter raising !e potential of t!e communication circuit as a w!ole.


26/34

0isruptive Critical +oltage

;!e ma'imum potential gradient , gr is ma'imum at t!e surface of t!e conductor is:

gr G /Er ln Ed/r44.

&or #isual corona at normal temperature pressure,

G 3) Er M ).3 r4 ln Ed/r4 Epea4

Conditions affecting corona

Kine #oltage

0atio d/r

ontour of t!e surface

State of t!e surface

onsidering t!e abo#e factors , t!e critical disrupti#e #oltage to neutral becomes

c G m)g) r ln Ed/r4

m)G irregularity factor

g)Gdisrupti#e critical #oltage gradient for air in at +;B E21.1 /cm ,05S4

Gair density factor G32 b/E23Mt4

bGatmosp!eric pressure in cm of g

tGtemperature in deg.

;!e #isual critical #oltage is gi#en by

# G m)g) r E1M ).3/sqrtEr44ln Ed/r4

6ower loss due to corona

orona formation results in power loss. BeeHs formula for corona loss is:

BG 2"1 OEfM2$4l/PsqrtEr/d4 Ep! - c42

1)-$

/p!

w!ere p! and c are t!e effecti#e p!ase and critical disrupti#e #oltages , f is t!e frequency of t!e system, lG lengt! in m.


27/34


28/34

5aterials types of insulators

;!e insulators used in connection wit! o#er!ead systems employing bare conductors are composed almost in#ariably of glaFed porcelain. 8lass !as also

been used for medium #oltages . ;!e porcelain used s!ould be i#ory w!ite ,sound, free from defects and t!oroug!ly #itrified .

;!ere are t!ree types of insulators for o#er!ead lines:

1. Bin-type

2. Suspension type

3. Strain type

a4 !at is t!e difference between Lpuncture #oltage Land Lspar-o#er #oltage

b4 !at is t!e difference between arcing distances under Lwet Land HdryH conditions

c4 !at is t!e Ltracing distance

d4 ;abulate t!e ratio of spar-o#er #oltage to woring #oltage for different #oltage le#els.

e4 !at are t!e merits of suspension insulator string

Cac! insulator is designed for a comparati#ely low woring #oltage, usually about 11 , and t!e insulation for any required system #oltage

can be obtained by using a LstringH of suc! insulators.

In t!e e#ent of failure of an insulator, on unit , instead of t!e w!ole string, !as to be replaced.

;!e mec!anical stresses are reduced.

In t!e e#ent of an increase in t!e operating #oltage of t!e line , t!is can be met by adding t!e requisite number of units in eac! string.

!at is t!e difference between suspension strain insulators

Botential distribution o#er a string of insulators

1. Draw t!e equi#alent circuit of string of t!ree insulators.

2. S!ow !ow would you determine t!e potential distribution across t!e abo#e string.

5odel questions

1. L 9n insulator for o#er!ead line s!ould be designed so t!at it will spar-o#er before it will punctureL. !y

2. !y is wet S.A. less t!an dry S.A.


29/34

3. !at is t!e effect of pollution on S.A.

String efficiency

G S.A. of a string of n insulators/ E n J S.A. of one insulator4

;!e string efficiency depends on t!e ratioG capacitance per insulator/capacitance to eart!.

5et!ods of impro#ing string efficiency

;!e string efficiency can be impro#ed by t!e following met!ods:

y increasing t!e ratio

m G insulator self-capacitance/capacitance to eart!

;!is would require long cross-arms and !ence is not economical.

8rading of t!e units.

;!is approac! requires units of different siFes. ence it is not generally preferred. ;!e self-capacitance of t!e lowest unit !as to be ma'imum and as we

mo#e upward , t!e self-capacitance s!ould decrease progressi#ely.

Static s!ielding

;!e #oltage distribution is controlled in t!is met!od by t!e employment of a grading or guard ring, w!ic! usually taes t!e form of a large metal ring

surrounding t!e bottom unit and connected to t!e metal wor at t!e bottom of t!is unit , and t!erefore to t!e line . ;!is ring , or s!ield , !as t!e effect of

increasing t!e capacitances between t!e metal wor and t!e line.

;!e string efficiency increases wit! t!e guard ring.

ere special features of t!e transformer bus!ing may be e'plained.

!at is t!e effect of surface leaage resistance on t!e potential distribution across a string of insulators

!at is t!e effect of corona on string efficiency

0istribution #ystem 6lanningE5oduled.'ls4

;!is C'cel spreads!eet module demonstrates t!e basics of distribution system planning. e select t!e proper conductors and t!e numbers of s!unt

capacitors for compensation sub7ect to t!e requirements on #oltage regulation, losses and fi'ed and operating costs. e specify t!e customer demands

eit!er in power or in impedance. e specify t!e operating costs for losses. e also specify t!e capital costs for #arious conductor line building and for

capacitor placements. e !a#e to select t!e best combination of conductors and capacitors to minimiFe cost o#er a certain period, normally one-year

4ests on ,lectrical Materials
http://www.geocities.com/cindulkar/MODULED.XLShttp://www.geocities.com/cindulkar/MODULED.XLS


30/34

4ype 4ests ;ests carried out to pro#e conformity wit! t!e specifications. ;!ese are intended to pro#e t!e general qualities and design of a gi#en type

of manufactured item.

-outine 4ests-;ests carried out on eac! part/item manufactured to c!ec parameters Eas per requirements), w!ic! are liely to #ary during production.

cceptance 4ests- ;ests carried out on samples taen at random from offered lot of manufactured item for t!e purpose of acceptance of lot.

4esting transmission line materials ndian #tandards

9S0 onductors for ")) and abo#e IS: 3( EBart $4 U12

456, 4,#4#

2o. 4ype 4est 6urpose

1 isual e'amination ;o #erify good wormans!ip and surface

finis!ing of t!e conductor

2 5easurement of diameter ofindi#idual aluminium steel wires

;o measure actual diameter of eac!strand to c!ec t!at it is wit!in specified

limits

3 5easurement of lay ratio of eac!

layer

;o measure actual lay ratio of eac! layer

strand to c!ec t!at it is wit!in specifiedlimits

" reaing load test Eon completeconductor4

;o measure actual breaing load ofcomplete conductor to c!ec t!at it is

wit!in specified limits

$ Ductility testEfor gal#aniFed steelwires only4. ;orsion elongation

test

;o record fracture of strand- number ofcomplete twist s!all not be V 1(/1% for

sample before /after strandingrespecti#elyClongation s!all not be V 3.$*

% rapping test &or no-brea obser#ation inaluminium/gal#aniFed steel wire Estrand4

after wrap/unwrap process

0esistance test ;o c!ec resistance of aluminium strand

at 2) )

( 8al#aniFing test Efor gal#aniFedsteel wires only4

;o c!ec uniformity of Finc coating E "dips of 1 min. eac! in uSA"solution of

sp. 8ra#ity 1.1(%4. ;!e weig!t of Finc

coating s!all not be less t!an specified#alue.

Surface condition test ;o #erify cylindrical s!ape and relati#emo#ement of strands under tension

condition of $)* of ultimate breaing

load of t!e conductor. E9pplicable toconductors of nominal aluminium area

1)) sq. mm and abo#e4


31/34

1) orona test ;o c!ec corona e'tinction #oltage not

less t!an specified #alue.

11 0adio interference #oltage test ;o c!ec 0I #oltage le#el wit!in limits.

-*842, 4,#4# s!all be same as 9cceptance ;ests and s!all be carried out on eac! coil.

CC,642C, 4,#4# same as 4ype test 2os. 1)' given above.

#M6L2D Criteria

5anufacturer s!all normally tae samples of indi#idual wires for t!e acceptance tests 1-( abo#e before stranding of not less t!an 1)* ofwire coils.

9lternati#ely, if desired by t!e purc!aser at t!e time of placing an order t!at t!e tests be carried out in presence of t!e purc!aserWsrepresentati#e. Samples s!all t!en be obtained by cutting 1.2 meters from outer end of finis!ed conductor from not more t!an 1)* of t!e

drums.

oils offered for inspection s!all be di#ided into two equal lots , t!e number of lots being equal to t!e number of samples to be selected , a

fraction of a lot being counted as a complete lot . Ane sample coil s!all be selected at random from eac! lot.

;AB

Cart! ires E8al#aniFed Strands for Cart!ing 4 IS: 12%-1(

;!e tests under ;BC , 0A6;I+C, and 9CB;9+C category are not specified in t!e Indian Standard. owe#er, t!e following tests s!all be carried out on t!e selected samples.

2o. 4est 6urpose

1 reaing load test ;o #erify strengt!/measure ultimate breaing load of gal#aniFed wires

separately to be wit!in limits

2 Clongation test &or elongation to be wit!in limits

3 D 0esistance test 9ctual resistance of wire to be wit!in limits

" rapping test ;o #erify capacity to wit!stand wire twisting. +o brea in wire after

wrap/unwrap process

$ 8al#aniFing test ;o c!ec uniformity of Finc coating. +o permanent copper deposition after "dips of 1 min. eac! in uSA"solution of sp. gra#ity 1.1(%. ;!e weig!t of Finc

coating s!all not be less t!an specified #alue.

% ;orsion test ;o #erify capacity to wit!stand torsion of wire. +o brea in wire after process.

0C=C;IA+ 0C-;CS;: If test sample fails any of t!e tests, t!ree furt!er samples from same lot , out of w!ic!, one sample from same drum of original

sample , be selected and t!e tests repeated on all t!ree samples.


32/34

6orcelain 0isc nsulators for *ver$ead Lines wit$ 2ominal +oltage E 1@@@ +< # (1)1%'%

456, 4,#4#

2*. 4ype test 6urpose

1 isual e'amination &or wormans!ip /surface defects2 erification of dimensions &or ensuring dimensions as per requirement

and appro#ed drawing

3 isible disc!arge test &or measuring #isual corona

" Impulse #oltage wit!stand test ;o c!ec ability of t!e insulator !ousing to

wit!stand #oltage stresses under dry and wetconditions

$ et power frequency #oltagewit!stand test

;o c!ec ability of t!e insulator !ousing towit!stand #oltage stresses under wet

conditions

% ;emperature cycle test ;o c!ec capability of t!e insulators to

wit!stand t!ermal stresses Clectro-mec!anical filing load test ;o c!ec capability of t!e insulators to

wit!stand under combined electrical and

mec!anical stresses

E&or string insulator units U;ype only4

( 5ec!anical failing load test ;o c!ec capability under mec!anical

stressesE&or string insulators of ;ype 9 and t!ose of

;ype to w!ic! electro-mec!anical failing

load test is not applicable, and for rigidinsulators only4

2" ours mec!anical strengt! test ;o c!ec capability to wit!stand t!eelectrical stresses E for insulators- ;ype

only4

1) Buncture test ;o c!ec capability to wit!stand t!eelectrical stresses Efor insulators U;ype

only4

11 Borosity test ;o confirm non-porous nature of product

12 8al#aniFing test ;o determine t!e uniformity and t!icness ofFinc coating

+A;CS:

1. 9 radio interference test is under consideration

2. ;ype tests are normally carried out once and unless ot!erwise agreed to, test certificates gi#ing results of type tests , made on not less t!an two

insulators identical in all essential details wit! t!ose to be supplied, are regarded as e#idence of compliance. ;!e tests s!ould be carried out in t!e

order mentioned below:

a4 An bot! insulators: ;ests 1-%


33/34

b4 An first insulator: ;ests ,,(,11

c4 An second insulator: ;ests 1) 12.

3. ;ype tests s!all be carried out and certified by t!e manufacturer or by an agreed independent aut!ority.

-*842, 4,#4#

+o. 0outine test Burpose

13 isual e'amination &or wormans!ip /surface defects

1" 5ec!anical routine

tests

; o confirm wit!standing mec!anical stresses during normal

conditions E for string insulator units only4

1$ Clectrical routine tests ; o confirm wit!standing electrical stresses during normal

conditions E for ;ype string insulators and rigid insulators4

CC,642C, 4,#4# 4est 2os. !< G< %< < '< 1@< 11< 1!

#M6L2D C-4,- # (1< 1%'

F*- 0M,2#*2# ; 4,M6,-48-,C5CL, 4,#4#

Lot si:e< 2 First sample

si:e< n1

#econd

sample si:e

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