Make your own free website on



Factors affecting Line design

Conductor size

Line resistance

Line inductance - one phase & 3-phase

Line capacitance, 1-phase & 3-phase

Effect of ground on capacitance of 3-phase line

Equivalent circuit for short transmission line

Equivalent circuit for medium length line

Long line equations

Equivalent circuit for a long


Surge impedance loading of lines

Ferranti effect

Advantages of bundled conductors

Disadvantages of bundled conductors

Factors affecting mechanical design of overhead lines and factors affecting span length

Corona , audio noise & radio interference

Insulators for overhead lines

 Sag and tension analysis of overhead lines

Distribution System Planning

Tests on Electrical Materials

Testing transmission line materials Indian Standards



 Internet websites: Simulator Line constants program



HOME takes you to the start page after you have read these Topics. Start page has links to other topics. --Simulator for transmission thermal limits

TOWERABC: Calculates line constants for overhead three-phase, single-circuit or double-circuit, transmission lines and produces contour plots of rms V, E, and H, plus sound. It can be used for 50/60 Hz, and also for harmonic frequencies.




1. The surge impedance of a 110 kV, 3-phase transmission line is 440 ohms. The surge impedance loading of the line is

  1. 3 (110) 2/440 MW
  2. (110) 2/440 MW
  3. (110) 2/ 3110) 2 MW

Ans.: (b)

2.The capacitance and inductance per unit length of a 3-phase line, operating at 110 kV are .01 microfarad and 2.5 mH. The surge impedance of the line is

(a) 50 ohms

(b) 500 ohms

(c) 250 ohms

Ans: (b)

3. A long transmission line is energized at then sending end and is kept open circuited at the receiving end. The magnitudes of the sending end voltage Vs and of the receiving end voltage Vr satisfy the following relationship

  1. Vs =Vr
  2. Vs is greater than Vr
  3. Vs is less than Vr

Ans: (c)

4. Voltage regulation of a short transmission line is

  1. always positive
  2. always negative
  3. either positive, negative, or zero

Ans: (c)

5. The capacitance of an overhead line increases with

  1. increase in mutual geometric mean distance
  2. increase in height of conductors above ground
  1. Both are true
  2. Both are false
  3. Only (i) is correct

Ans: (b)

6. Shunt compensation for long EHV lines is primarily resorted to

  1. improve voltage profile
  2. improve stability
  3. reduce fault currents

Ans: (a)

7. Series compensation is primarily resorted to

  1. improve voltage profile
  2. improve stability
  3. reduce fault currents

Ans: (b)

8. Fair weather corona loss may be computed using the empirical formula given by Peterson. According to Peterson's formula corona loss is proportional to

(a) f and V2

(b) f 2 and V

  1. f and V

where f and V are the system frequency and voltage respectively.

Ans: (a)

9. Bundled conductors are used in EHV lines primarily for

  1. reducing cost of the line
  2. reducing corona loss and radio interference
  3. increasing stability limit.

Ans: (b)

10. There are 20 discs in the string of insulators of a 3-phase 400 kV transmission line. String efficiency is 80 %. The maximum voltage across any disc is

  1. 25 kV
  2. 25/ 3 kV
  3. 25 3 kV

Ans: (b)

11. Two or three sheds or petticoats are provided in pin-type insulators in order to increase

  1. creepage resistance
  2. spark-over voltage(S.O.V)
  3. puncture voltage

Ans: (a)

12. Pin -type insulators are use up to

  1. 11 kV
  2. 33kV
  3. 132kV

Ans: (b)

13. Insulators used for transmission line at the dead -end tower are

  1. suspension type
  2. shackle type
  3. strain type

Ans: (c)


14. Economic studies have shown that D.C. transmission is cheaper than a. c transmission for lengths

a. below 300 km

b. beyond 600 km

c. beyond 1200 km

Ans. b


15.Transmission voltages in the range 230 kV-765 kV are known as

a.     High voltage

b.       Extra High Voltage

c.        Ultra High Voltage

Ans. b


16. Which one of the following statements is false?

As the transmission voltage increases,

a.        Corona loss decreases

b.       Conductor copper loss decreases

c.        Cost of insulators, transformers, switches & circuit breakers increases

Ans. c


17. The internal inductance of a solid conductor of radius r and carrying a current I is equal to

a.        0.5 I * 10-7 H/m

b.       0.5 I * 10-7 exp(-1/4)*r H/m

c.        0.5 * 10-7 H/m


Ans. c


18.       Which one of the following statements is not true?

a.        The GMD method of finding inductance does not apply to ACSR conductors

b.       Current density in ACSR conductors is uniform

c.        The GMD between two circular areas, each of different diameters, is equal to the distance between their centres.

Ans. b


19.       Expanded ACSR conductors are used

a.        To increase the tensile strength of the line

b.       To reduce corona loss

c.        To reduce I2R loss


Ans. b


20.       A conductor with 19 strands, each of same diameter and each having an inductance of L Henries is used for a transmission line. The total inductance of the conductor will be

a.        L/19

b.       L/361

c.        19L


Ans. a


21.       The line to neutral capacitance of single -phase line with conductors of radius 1cm and spaced 1m apart is equal to

a.        10-9/72 F/m

b.       10-9/36pF/m

c.        2 p210-7 F/m


Ans .a


22.       In a double-circuit line with hexagonal spacing ,

a.        The phases are balanced, but the conductors of each individual phase are not balanced.

b.       The conductors of each individual phase are balanced , but the phases are not balanced

c.        The phases, and the conductors of each individual phase are both balanced


Ans. c


23.       Which one of the following statements is true?


a.        Skin effect at 50 Hz is negligible for larger diameter conductors but becomes appreciable for smaller conductor.

b.       Skin effect at 50 Hz is negligible whatever the diameter of the conductor.

c.        Skin effect at 50 Hz is negligible for the smaller diameter conductors but becomes appreciable for the larger conductor conductor.


Ans. c


24.       Which one of the following statements is true?


a.        Resistance of a conductor decreases and the internal inductance increases as the frequency is increased

b.       Resistance and internal inductance of a conductor both increase with increase of frequency

c.        Resistance of a conductor increases and the internal inductance decreases as the frequency is increased


Ans. c


25.       The surge impedance of a double-circuit power transmission line is


a.        40 ohms

b.       200 ohms

c.        400 ohms

d.       800 ohms


Ans. b


26. The surge impedance of a telephone line is

a.        50 ohms

b.       75 ohms

c.        200 ohms

d.       400 ohms


Ans. b


26.       Bundle conductors are preferred in EHV transmission lines because


a.        It is easy to fabricate thin conductors and combine them to make a bundle

b.       Inductance of the line is reduced, and the corona loss, and radio & TV interference is minimized.

c.        Tower height is reduced and hence transmission cost is low.


Ans. b


27.       Inductive interference between power & communication lines can be minimized by


a.        Increasing the spacing of power line conductors

b.       Transposing power line conductors

c.        Transposing communication line conductors

d.       Either b or c.


Ans. d


28.       The percentage regulation of an overhead transmission line can be zero when the load power factor is


a.        Lagging

b.       Unity

c.        Leading


Ans. c


29.       Which one of the following statements is true?


a.        Skin effect increases the resistance of a conductor ,but proximity effect decreases the resistance

b.       Both skin effect and proximity effect increase the resistance of a conductor

c.        Both skin effect and proximity effect increase the internal inductance of a conductor


Ans. b


30.       A transmission line having parameters A1, B1, C1, D1 is in parallel with another having parameters A2, B2, C2, D2. The overall " A " parameter of the combination is


a.        A1A2 +B1C2

b.       (A1B2 +A2B1)/(B1+B2)

c.        C1+C2 + (A1-A2)(D2-D1)/(B1+B2)


Ans. b


31.       Disruptive critical voltage is


a.        Equal to

b.       Greater than

c.        Less than

visual critical voltage for corona on an overhead line.


Ans. c


32.       Corona loss


a.        Increases

b.       Decreases

c.        Does not change

the switching voltage on a transmission line.


Ans. b


33.       Handling of the telephone receiver may become dangerous due to


a.        Electromagnetic induction

b.       Electrostatic induction

c.        Both electromagnetic and electrostatic induction

from a power line.


Ans. b


34.       Mutual inductance between a three -phase power line and a telephone line due to third harmonic current in the power line is

a.        The algebraic sum of mutual inductances from individual phase wires

b.       The arithmetic sum of mutual inductances from individual phase wires

c.        Zero


Ans. b


35.       Dielectric strength of mechanically sound porcelain is


a.        10 kV/cm

b.       22 kV/cm

c.        65 kV/cm

d.       100 kV/cm


Ans. c


36.       Dielectric strength of glass is


a.        22 kV/cm

b.       60-66 kV/cm

c.        140 kV/cm

d.       240 kV/cm


Ans. c



37.       Which one of the following statements is true?


a.        For 11 kV insulators, the ratio of wet spark-over voltage to working voltage is 8.3

b.       For 66 kV insulators, the ratio of dry spark-over voltage to working voltage is 8.3

c.        For 11 kV insulators, the ratio of dry spark-over voltage to working voltage is 8.3


Ans. c


38.       Which one of the following statements is true?


a.        Flash-over tests are performed on all insulators

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

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


Ans. none of the above


39.       Which one of the following statements is true?


a.        The spark-over voltage is less than the puncture voltage

b.       The spark-over voltage is greater than the puncture voltage

c.        The spark-over voltage is equal to the puncture voltage


Ans. a


40.       Which one of the following statements is true?


The sheds of an insulator should be shaped


a.        To conform to the electrostatic tube of force and the body should be shaped to conform to the equipotential surfaces

b.       To conform to the equipotential surfaces and the body should be shaped to conform to the electrostatic tubes of force

c.        To conform to the equipotential surfaces and so also the body


Ans. b


41.       In a suspension insulator, the mechanical stresses on the conductor are

a.        Increased

b.       Reduced

c.        The same


Ans. b


42.       The string efficiency of the insulator can be increased by


a.        Increasing the number of strings in the insulator

b.       Increasing the ratio , capacitance to earth/capacitance per insulator

c.        By the correct grading of various capacitances

d.       Decreasing the number of strings


Ans. c


43.       String efficiency of insulators for wet flash-over is

a.        Less

b.       More

c.        The same

as that for dry flash-over


Ans. b


44.       The potential across insulator discs can be equalized by having


a.        The same capacitance for each unit

b.       The highest capacitance for the lowest unit and decreasing progressively the capacitance of other units

c.        The lowest capacitance for the lowest unit and increasing progressively the capacitance of other units


Ans. b


45.       Grading ring serves the purpose of


a.        Equalizing the voltage distribution across discs

b.       An arcing shield

c.        Both equalizing the voltage distribution and acting as an arcing shield


Ans. c





Factors affecting Line design

         Voltage level

         Conductor type & size

         Line regulation & voltage control

         Corona & losses

         Proper load flow & system stability

         System protection

         Insulation co-ordination

         Right of way

         Mechanical design

         Sag & stress calculation

         Conductor composition

         Conductor spacing

         Insulator /conductor hardware selection

         Structural design

         Structure types

         Stress calculations


Conductor size

Gauge sizes decrease as the wire increases in size.

Number of strands = 3 n2 -3n + 1

where n = number of layers including the single central strand.

The following conductors are used.

AAC-all aluminum conductor

AAAC-all aluminum alloy conductor

ACSR-aluminum conductor steel re-inforced

ACAR-aluminum conductor alloy re-inforced


Line resistance

R = r l/A

R2/R1 = (T0 +T2)/ (T0 +T1)

R2 = Resistance at temperature T2

R1 = Resistance at temperature T1

T0 = Constant

= 234.5 for annealed copper of 100% conductivity

=241 for hard drawn copper of 97.3% conductivity

=228 for hard drawn aluminum of 61% conductivity

Skin effect is function of conductor size, frequency and resistance of conductor material.

Discuss the proximity effect, stranding and spiraling of conductors


Line inductance - one phase & 3-phase

Single-phase overhead line

Voltage drop in a single-phase line due to loop impedance

= 2 l (R + j w m 0 ln (Dm/Ds)/2p) I

l= line length, m

R= resistance of each conductor, m

Dm= equivalent or geometric mean distance (GMD) between conductor centres

Ds= Geometric mean radius(GMR), or self-GMD of one conductor

= 0.7788 r for cylindrical conductor

r= conductor radius

I = current

L= 2 x 10 -7 ln (Dm/Ds ) H/m

Three-phase overhead line (unsymmetrical spacing)

Dab +Dbc +Dca

Equivalent equilateral spacing=Deq = Dm = (Dab DbcDca) 1/3

In practice , conductors are transposed.

Transposition is carried out at switching stations

Average inductance per phase

L=2 x 10 -7 ln (Deq/Ds ) H/m


Line capacitance, 1-phase & 3-phase

Single-phase overhead line

Cab = 2 p e 0e r/ ln (D/r) (F/m)

The capacitance to neutral for a two- wire line is twice the line-to-line capacitance, Cab.

Three-phase overhead line

Line-to-neutral capacitance

Cn = 2p e 0e r/ ln (Deq/r) (F/m)

Charging current /phase =jv Cn Vph (A/m)



Effect of ground on capacitance of 3-phase line

The capacitance of a 3-phase transposed line considering ground effect is given by

Cn = 2p e 0e r/ [ln (Deq/r) -ln (h12 h23 h31/h11h22h33)] (F/m)

where h12= distance between conductor 1 and image of conductor 2, etc. Effect of ground is to increase the capacitance.



Equivalent circuit for short transmission line (up to 80 km)

Note that bold symbols indicate complex quantities.

Vs =Vr + Ir Z

Is = Ir = I

Draw a phasor diagram for a short line with inductive load and with capacitive load, using Vr as the reference phasor.

Show that

Vs = SQRT[(Vr + IR Cos f r +(or -) IX Sinf r) 2 + (IX Cos f r +(or -) IR Sinf r)2]

+ sign above is for lagging p. f

- sign above is for leading p.f

f r = angle between Vr & Ir

f s = angle between Vs & Is

d = f s-f r = load angle

tand = (IX Cos f r +(or -) IR Sinf r)/ (Vr + IR Cos f r +(or -) IX Sinf r)

Vs = AVr + BIr

Is = CVr +DIr

For a short line, A=1, B=Z, C=0, D=1

Line Efficiency (pu)= Vr I Cos f r/ Vs I Cos f s

Voltage regulation (pu)=(Vs-Vr)/Vr

= (VrNL- VrFL)/ VrFL

= [I(R Cos f r -(or+) XSinf r)]/ VrFL


Equivalent circuit for medium length line

A T or a p network is formed depending upon how the series impedance or the shunt admittance is lumped at a few points. See Fig.3

The ABCD parameters of the nominal-T network are:

A = 1+ZY

B = Z (1+ZY/4)

C = Y

D= A

The ABCD parameters of the nominal-p network are:

A = 1+ZY/2

B = Z

C = Y (1+ZY/2)

D= A

Nominal -T and Nominal-p networks are not equivalent electrically, as may be verified by using the Y-D transformation.

Voltage regulation (pu)= ((Vs/A) - VrFL)/ VrFL


Long line equations (above 240 km)

The solution of the voltage wave equation using the initial conditions is

V = (Cosh g x) Vr + (Z0 Sinhg x) Ir

I = (Y0 Sinhg x) Vr + (Cosh g x) Ir

g = sqrt (yz) = a + jb

a = attenuation constant pu length

b = phase-shift constant pu length

y = shunt admittance pu length

z = series impedance pu length

Z0 = surge impedance = sqrt (z/y); Y0 =1/Z0

Vs = AVr + BIr

Is = CVr +DIr


A = Coshg l

B = Z0Sinhg l

C = (1/Z0) Sinh g l

D = A

l= line length


Equivalent circuit for a long


The exact equivalent p circuit and the exact equivalent T circuit for a long line are shown in Fig.4

The elements of the p circuit are obtained from

Zp = B = Z0Sinhg l = (Z Sinhg l)/ g l

Yp /2 = (A-1)/B = ( Coshg l - 1)/ Z0Sinhg l = (tan(g l/2).Y/2)/(g l/2).

The elements of the T circuit are obtained from

ZT/2 = (A-1)/C = (Coshg l-1)/ ((1/Z0) Sinh g l)

ZT = 2 Z0 tanh (g l/2) = (Z tanh (g l/2))/ (g l/2)

YT = C= (1/Z0) Sinh g l = (Y Sinh g l)/ g l


Surge impedance loading of lines

Incident and reflected voltages on long lines

Vs = (1/2) (Vr +Ir Zo) ea l e jb l + ((1/2) (Vr -Ir Z0) e-a l e -jb l

Is = (1/2) (VrYo +Ir) ea l e jb l + ((1/2) (VrYo -Ir) e-a l e -jb l

The first and second terms in each of the above equations refer to the incident and reflected voltages respectively.

The wavelength is defined by

l = 2p /b

The velocity of propagation n of the waves is given by


n = l f

l = 6000 km at 50 Hz.

When the line is terminated in its surge impedance Zr = Zo, there is no reflected wave. (Infinite line)


Surge Impedance Loading (SIL) of a transmission line

SIL = [Vr (L-L) (in kV)]2/Zo' (MW)

where Zo' = sqrt(L/C)

SIL is a measure of the maximum power that can be delivered over a line. The following factors affect the maximum power:

         Line length

         Terminal apparatus impedances

         All other factors that affect stability.

To increase SIL, kVr can be increased and Zo reduced by using series compensation.

The distinction between maximum power and SIL should be mentioned.


Ferranti effect

The parameter A = Cosh g l decreases with increase in line length. In such cases Vr is considerably greater than Vs, when the line is charged but unloaded. In underground cables, the effect is much more pronounced, even in short lengths. It is called the Ferranti effect. Discuss the effects of shunt compensation and reactive loading.


Advantages of bundled conductors

         Reduced line reactance

         Reduced voltage gradient

         Increased corona critical voltage, and therefore, less corona power loss, audible noise, and radio interference.

         Reduced amplitude & duration of high frequency conductor vibration


Disadvantages of bundled conductors

         Increased ice & wind loading

         Inspection more complicated ,spacers required

         Increased clearance requirements at structures

         Increased charging kVA which may be a disadvantage at light loads

Ds = GMR of subconductors

d = distance between two sub-conductors

Dsb = GMR of bundled conductor

Dsb = (Dd) 1/2 (For a 2-conductor bundle)

Dsb = (Dd2) 1/3 (For a 3-conductor bundle)

Dsb = (Dd3) 1/4 (For a 4-conductor bundle)

Average inductance per phase of a bundled conductor,

L= 2 x 10 -7 ln (Deq/ Dsb), H/m

Deq = (D12 D23 D31)

Dij = spacing between phase i and phase j


Factors affecting mechanical design of overhead lines

  1. Character of line route
  2. Right-of-way
  3. Mechanical loading
  4. Required clearances
  5. Type of supporting structures
  6. Conductor
  7. Type of insulators
  8. Joint use by other utilities

Factors affecting span length

  1. Character of route
  2. Proper clearance between conductors
  3. Permissible tensions under maximum mechanical load


There are five kinds of stresses on lines & supports

  1. Tensile
  2. Compressive
  3. Shearing
  4. Bending
  5. Twisting stress or torque


Sag and tension analysis of overhead lines


Required clearances:

The data for the following clearances of different voltage levels should be known.

1.        Clearance of conductors passing by buildings

2.        Minimum clearances of conductors above ground or rails

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

4.        Horizontal clearances at support between line conductors based on sags.


Sag and tension analysis:

Factors affecting sag are:

1.        Conductor load per unit length

2.        Span

3.        Temperature

4.        Conductor tension

5.        Level at supports


Conductor load depends on

1.        Weight of conductors

2.        Weight of ice or snow on conductors

3.        Wind blowing against wire


Effect of change in temperature:


If the conductor stress is constant and if the temperature changes, the change in length is

Dl = lo. a.Dt


Dt = t1-to= change in temperature

Dl = l1-lo = change in length

a = Coefficient of linear expansion of conductor per deg. C. If temperature is constant while conductor stress changes (i.e. loading), the change in length is

Dl = lo. DT/MA


DT =T1-To= change in tension in kg

M= modulus of elasticity of conductor

A = Metal cross-section of conductor.

Consider the following in sag & tension calculations:

1.        Supports at same level ( I) Catenary method ,( ii) Parabola method

2.        Supports at different levels (unsymmetrical spans)

3.        Effect of ice

4.        Effect of wind


Line location

1.        Profile & plan of right-of-way

2.       Templates for locating structures


These are used to provide the following

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

b.       Economic layout

c.        Proper grading of structures

d.       Prevention of excessive insulator swing or uplift at structures.





If an alternating potential is applied to two wires whose spacing is large in comparison with the diameter and the potential difference is gradually increased, a point will be reached when a faint luminous glow of violet colour will appear, and a hissing sound will be heard. This phenomenon is known as Corona. The formation of corona is accompanied by a loss of power. It causes non-sinusoidal nature of current and interference with neighbouring communication circuits.

Corona formation takes place due to ionization of a layer of air immediately surrounding the conductor. For air under ordinary conditions near sea level & without impurities, the value of potential gradient at which ionization takes place can be taken as 30kV/cm (peak).

Interference with communication circuits may be due to both electromagnetic and electrostatic action, the former producing currents, which are superposed on the true speech currents, thereby setting up distortion and the latter raising he potential of the communication circuit as a whole.

Disruptive Critical Voltage

The maximum potential gradient , gr is maximum at the surface of the conductor is:

gr = V/(r ln (d/r)).

For visual corona at normal temperature & pressure,

V= 30 (r + 0.3 r) ln (d/r) kV (peak)

Conditions affecting corona:

         Line voltage

         Ratio d/r

         Contour of the surface

         State of the surface

Considering the above factors , the critical disruptive voltage to neutral becomes

Vc = m0g0 d r ln (d/r)

m0= irregularity factor

g0=disruptive critical voltage gradient for air in kV at NTP (21.1 kV/cm ,RMS)

d =air density factor =392 b/(273+t)

b=atmospheric pressure in cm of Hg

t=temperature in deg. C

The visual critical voltage is given by

Vv = m0g0 d r (1+ 0.3/sqrt(rd ))ln (d/r)


Power loss due to corona


Corona formation results in power loss. Peek's formula for corona loss is:

P= 241 [(f+25)l/d ]sqrt(r/d) (Vph - Vc)2 10-5 kW/ph

where Vph and Vc are the effective phase and critical disruptive voltages , f is the frequency of the system, l= length in km.

Peterson's formula for corona loss is :

P = 0.000021 f V2 F /[log10(d/r)]2

P = power loss in kW per km of conductor under fair weather conditions.

f = frequency, Hz

V = line to ground voltage

D = spacing between conductors

R = radius of the conductor

F = corona factor determined by test


Audio Noise


When corona is present on the conductors, EHV lines generate audible noise, which is especially high during polluted weather. The noise is broadband , which extends from very low frequency to about 20 kHz. Corona discharges generate positive & negative ions, which are alternately attracted & repelled by the periodic reversal of polarity of the a.c excitations. Their movement gives rise to sound-pressure waves at frequencies of twice the power frequency and its multiples, in addition to the broadband spectrum which is the result of random motions of the ions. Audible noise can become a serious problem from 'psychoacoustics ' point of view, leading to insanity due to loss of sleep at night to inhabitants residing close to an EHV line.


Radio Interference (RI)


Pulse type corona discharge from transmission line conductors gives rise to interference to radio broadcast in the range of 0.5 MHz to1.6 MHz.


Electromagnetic Effect


The emf induced in the communication circuit due to neighbouring power circuit depends on its distance with respect to the power line. The net emf induced due to electromagnetic coupling with a 3-phase line is small since the phasor sum of induced emfs tends to zero. However, the presence of certain harmonics would cause seriously high induced emfs. This problem is more serious these days since the power line current is not sinusoidal because of he use of static controllers.


Electrostatic Effect


The communication line may acquire dangerously high potential due to electrostatically induced charges. The interference between power & communication lines can be reduced considerably by transposing the conductors of both power & communication lines.

The communication line may require electrostatic shielding to overcome electrostatic interference.



Insulators for overhead lines


Materials & types of insulators

The insulators used in connection with overhead systems employing bare conductors are composed almost invariably of glazed porcelain. Glass has also been used for medium voltages . The porcelain used should be ivory white ,sound, free from defects and thoroughly vitrified .


There are three types of insulators for overhead lines:

1.        Pin-type

2.        Suspension type

3.        Strain type

a)       What is the difference between "puncture voltage "and "spark-over voltage?

b)       What is the difference between arcing distances under "wet "and 'dry' conditions?

c)       What is the "tracking distance?

d)       Tabulate the ratio of spark-over voltage to working voltage for different voltage levels.

e)       What are the merits of suspension insulator string?


         Each insulator is designed for a comparatively low working voltage, usually about 11 kV, and the insulation for any required system voltage can be obtained by using a "string' of such insulators.

         In the event of failure of an insulator, on unit , instead of the whole string, has to be replaced.

         The mechanical stresses are reduced.

         In the event of an increase in the operating voltage of the line , this can be met by adding the requisite number of units in each string.


What is the difference between suspension & strain insulators?


Potential distribution over a string of insulators


1.        Draw the equivalent circuit of string of three insulators.

2.        Show how would you determine the potential distribution across the above string.


Model questions


1.        " An insulator for overhead line should be designed so that it will spark-over before it will puncture". Why?

2.        Why is wet S.O.V less than dry S.O.V?

3.        What is the effect of pollution on S.O.V?


String efficiency


= S.O.V of a string of n insulators/ ( n * S.O.V of one insulator)


The string efficiency depends on the ratio= capacitance per insulator/capacitance to earth.


Methods of improving string efficiency


The string efficiency can be improved by the following methods:


         By increasing the ratio


m = insulator self-capacitance/capacitance to earth


This would require long cross-arms and hence is not economical.


         Grading of the units.


This approach requires units of different sizes. Hence it is not generally preferred. The self-capacitance of the lowest unit has to be maximum and as we move upward , the self-capacitance should decrease progressively.


         Static shielding

The voltage distribution is controlled in this method by the employment of a grading or guard ring, which usually takes the form of a large metal ring surrounding the bottom unit and connected to the metal work at the bottom of this unit , and therefore to the line . This ring , or shield , has the effect of increasing the capacitances between the metal work and the line.


The string efficiency increases with the guard ring.


Here special features of the transformer bushing may be explained.


What is the effect of surface leakage resistance on the potential distribution across a string of insulators?


What is the effect of corona on string efficiency?



Distribution System Planning (Moduled.xls)


This Excel spreadsheet module demonstrates the basics of distribution system planning. We select the proper conductors and the numbers of shunt capacitors for compensation subject to the requirements on voltage regulation, losses and fixed and operating costs. We specify the customer demands either in power or in impedance. We specify the operating costs for losses. We also specify the capital costs for various conductor line building and for capacitor placements. We have to select the best combination of conductors and capacitors to minimize cost over a certain period, normally one-year


Tests on Electrical Materials


Type Tests Tests carried out to prove conformity with the specifications. These are intended to prove the general qualities and design of a given type of manufactured item.


Routine Tests-Tests carried out on each part/item manufactured to check parameters (as per requirements0, which are likely to vary during production.


Acceptance Tests- Tests carried out on samples taken at random from offered lot of manufactured item for the purpose of acceptance of lot.


Testing transmission line materials Indian Standards


ACSR Conductors for 400 kV and above IS: 398 (Part 5) 1992






Type Test



Visual examination

To verify good workmanship and surface finishing of the conductor


Measurement of diameter of individual aluminium & steel wires

To measure actual diameter of each strand to check that it is within specified limits


Measurement of lay ratio of each layer

To measure actual lay ratio of each layer strand to check that it is within specified limits


Breaking load test (on complete conductor)

To measure actual breaking load of complete conductor to check that it is within specified limits


Ductility test(for galvanized steel wires only). Torsion & elongation test

To record fracture of strand- number of complete twist shall not be < 18/16 for sample before /after stranding respectively

Elongation shall not be < 3.5%


Wrapping test

For no-break observation in aluminium/galvanized steel wire (strand) after wrap/unwrap process


Resistance test

To check resistance of aluminium strand at 20 0C


Galvanizing test (for galvanized steel wires only)

To check uniformity of zinc coating ( 4 dips of 1 min. each in CuSO4 solution of sp. Gravity 1.186). The weight of zinc coating shall not be less than specified value.


Surface condition test

To verify cylindrical shape and relative movement of strands under tension condition of 50% of ultimate breaking load of the conductor. (Applicable to conductors of nominal aluminium area 100 sq. mm and above)


Corona test

To check corona extinction voltage not less than specified value.


Radio interference voltage test

To check RI voltage level within limits.



ROUTINE TESTS: shall be same as Acceptance Tests and shall be carried out on each coil.


ACCEPTANCE TESTS: same as Type test Nos. 1-8 given above.

SAMPLING Criteria:






Earth Wires (Galvanized Strands for Earthing ) IS: 12776-1989

The tests under TYPE , ROUTINE, and ACCEPTANCE category are not specified in the Indian Standard. However, the following tests shall be carried out on the selected samples.






Breaking load test

To verify strength/measure ultimate breaking load of galvanized wires separately to be within limits


Elongation test

For elongation to be within limits


DC Resistance test

Actual resistance of wire to be within limits


Wrapping test

To verify capacity to withstand wire twisting. No break in wire after wrap/unwrap process


Galvanizing test

To check uniformity of zinc coating. No permanent copper deposition after 4 dips of 1 min. each in CuSO4 solution of sp. gravity 1.186. The weight of zinc coating shall not be less than specified value.


Torsion test

To verify capacity to withstand torsion of wire. No break in wire after process.


REJECTION & RE-TEST: If test sample fails any of the tests, three further samples from same lot , out of which, one sample from same drum of original sample , be selected and the tests repeated on all three samples.



Porcelain Disc Insulators for Overhead Lines with Nominal Voltage > 1000 V, IS 731-1989





Type test



Visual examination

For workmanship /surface defects


Verification of dimensions

For ensuring dimensions as per requirement and approved drawing


Visible discharge test

For measuring visual corona


Impulse voltage withstand test

To check ability of the insulator housing to withstand voltage stresses under dry and wet conditions


Wet power frequency voltage withstand test

To check ability of the insulator housing to withstand voltage stresses under wet conditions


Temperature cycle test

To check capability of the insulators to withstand thermal stresses


Electro-mechanical filing load test

To check capability of the insulators to withstand under combined electrical and mechanical stresses

(For string insulator units Type B only)


Mechanical failing load test

To check capability under mechanical stresses

(For string insulators of Type A and those of Type B to which electro-mechanical failing load test is not applicable, and for rigid insulators only)


24 Hours mechanical strength test

To check capability to withstand the electrical stresses ( for insulators- Type B only)


Puncture test

To check capability to withstand the electrical stresses (for insulators Type B only)


Porosity test

To confirm non-porous nature of product


Galvanizing test

To determine the uniformity and thickness of zinc coating




1. A radio interference test is under consideration


2.        Type tests are normally carried out once and unless otherwise agreed to, test certificates giving results of type tests , made on not less than two insulators identical in all essential details with those to be supplied, are regarded as evidence of compliance. The tests should be carried out in the order mentioned below:

a)       On both insulators: Tests 1-6

b)       On first insulator: Tests 9,7,8,11

c)       On second insulator: Tests 10 &12.

3.        Type tests shall be carried out and certified by the manufacturer or by an agreed independent authority.





Routine test



Visual examination

For workmanship /surface defects


Mechanical routine tests

T o confirm withstanding mechanical stresses during normal conditions ( for string insulator units only)


Electrical routine tests

T o confirm withstanding electrical stresses during normal conditions ( for Type B string insulators and rigid insulators)



ACCEPTANCE TESTS: Test Nos. 2, 6, 9, 7, 8, 10, 11, 12





Lot size, N

First sample size, n1

Second sample size, n2

Permissible failure, a

First rejection number, r1

Second rejection number, r2

Up to 1000






1001- 3000












10001 & above







Lot size, N



First sample size, n1

Second sample size, n2

First sample size, n1

Second sample size, n2

Up to 1000





1001- 3000










10001 & above





NOTE: The samples selected shall be divided approximately into three parts and subjected to the applicable tests in the following order.

Parts of sample

Tests on string insulator units

Tests on Rigid insulators

Type A

Type B

Type A

Type B

First & second part

5, 6,8,11 &9

5, 6,7 or 8 (whichever is applicable), 9 (when specified) &11

5, 6, 8, & 11

5, 6, 8, & 11

Third part

5, 6, 10 & 12

5, 6, 10 & 12

5, 6, 10 & 12

5, 6, 10 & 12






Other relevant Indian Standards are the following:


AAA Conductors [All Aluminium Alloy Stranded Conductors ] IS: 398 (Part 4) 1994

ACSR Conductors IS: 398 (Part 2) 1994

Pre-formed armour rods for conductor IS: 2121 (Part I) 1981

Repair Sleeve and Mid-span joints for conductors IS: 2121 (Part II) 1981

Stockbridge Vibration Dampers for overhead Lines IS: 9708-1993

Spacer and Spacer Dampers for Twin Horizontal Conductors IS:10162-1982

Earth-wire accessories- Suspension and Tension Clamps; Mid-span Compression Joints; Repair Sleeves and Flexible Copper Bonds IS2121(Part-3)-1992

Cross-by Clips IS2121(Part-3)-1992

H Frame/Tower Structural Steel[rail, Girder, Angle etc.] IS:2062

Stay wires[Hot-dipped galvanized Stay Brand IS:2141-1992

Porcelain Insulators-Bus Post for system nominal voltage > 1000 V IS: [2544-1973]

Step Bolts and Their Nuts for steel structures IS: 10238 1982