Ans. 500 kV.
Q. 2. What is an electric power supply system?
Ans. The system of generation, transmission and distribution of electrical power is called the electrical power supply system.
Q. 3. What is the usable voltage for secondary distribution?
Ans. 415/240 V (415 volts for 3-phase loads and 240 V for single phase loads)
Q. 4. What is the difference between a feeder and distributor?
Ans. Feeders are the conductors which connect the stations in some cases generating stations) to the areas to be fed by those stations. Generally from feeders no tapping is taken to the consumers, therefore, current loading of a feeder remains the same along its length. It is designed mainly from the point of view of its current carrying capacity.
Distributors are the conductors from which numerous tappings for the supply to the consumers are taken. The current loading of a distributor varies along its length. Distributors are designed from the point of view of the voltage drop in them.
Q. 5. What is the legal requirement to which a distributor is subjected?
Ans. A distributor is subject to the legal requirement that the voltage at the consumer's terminals should be maintained within 36% of the declared (or rated) voltage.
Q. 6. Why is ac distribution considered superior to dc ?
Ans. Distribution by ac system is undoubtedly superior to that by de system as in ac system voltage control is easy by means of transformers.
Q. 7. List advantages of de system over ac system.
Ans. DC system has the following advantages over de system:
(i) Saving in conductor material,
(ii) No inductance,capacitance, phase displacement and surge problem,
(iii) No skin effect.
(iv) Less insulation requirement,
(v) No charging current,
(vi) Less corona loss and reduced interference with communication circuits,
(vii) Better voltage regulation,
(viii) No stability problems and synchronizing difficulties.
Q. 8. Give limitations of ac transmission over dc transmission.
Ans. AC transmission has got the following limitations over dc transmission.
(i) In case of overhead lines spacing between the conductors is to be kept more in order to provide
adequate insulation and to avoid corona loss,
(ii) More copper is required.
(iii) Greater the length of transmission line more is the capacitance of the line. There is a continuous loss on account of charging current even though the line is open.
(iv) The construction of transmission lines for ac systems is not so easy as for de systems.
(v) The alternators are to be synchronized before putting them into parallel.
(vi) The variation in speeds of alternators are to be controlled within very low limits.
Q. 9. What is the percentage saving in feeder conductor if the line voltage in a 2-wire de system be raised from 200 V to 500 V for the same power to be transmitted?
Ans. 84%.
Q. 10. List various systems of transmission of electrical power.
Ans. Though in practice 3-phase 3-wire ac system is universally used for transmission and 3-phase 4-wire ac system is used for distribution of electric power but for special purposes other systems may also be used. The various systems of power transmission are
(a) DC Systems :
(i) DC two-wire system.
(ii) DC two-wire system with midpoint earthed.
(iii) DC three-wire system.
(b) Single-Phase AC Systems:
(i) Single-phase two-wire system.
(ii) Single-phase two-wire system with midpoint earthed.
(iii) Single-phase three-wire system.
(c) Two-Phase AC Systems:
(i) Two-phase four-wire system.
(ii) Two-phase three-wire system.
(d) Three-Phase AC Systems:
(i) Three-phase 3-wire system.
(ii) Three-phase 4-wire system.
Q. 11. Why does the maximum voltage between each conductor and earth form the basis of comparison of volume of conductor material required in overhead system ?
Ans. In overhead system the conductors are insulated from the cross arms and supporting towers and as the towers and cross arms are earthed so the maximum voltage between each conductor and earth forms the basis of comparison of volume of conductor material required in overhead system.
Q. 12. Why does the maximum voltage between conductors form the basis of comparison of volume of conductor material required in underground system ?
Ans. In underground cables the maximum disruptive stress is between the two conductors of the cable; so the maximum voltage between conductors forms the basis of comparison of volume of conductor material required in underground system.
Q.13. What is the effect of voltage on transmission efficiency?
Ans. The transmission efficiency increases with the increase in transmission voltage.
(viii) No stability problems and synchronizing difficulties.
Q. 8. Give limitations of ac transmission over dc transmission.
Ans. AC transmission has got the following limitations over dc transmission.
(i) In case of overhead lines spacing between the conductors is to be kept more in order to provide
adequate insulation and to avoid corona loss,
(ii) More copper is required.
(iii) Greater the length of transmission line more is the capacitance of the line. There is a continuous loss on account of charging current even though the line is open.
(iv) The construction of transmission lines for ac systems is not so easy as for de systems.
(v) The alternators are to be synchronized before putting them into parallel.
(vi) The variation in speeds of alternators are to be controlled within very low limits.
Q. 9. What is the percentage saving in feeder conductor if the line voltage in a 2-wire de system be raised from 200 V to 500 V for the same power to be transmitted?
Ans. 84%.
Q. 10. List various systems of transmission of electrical power.
Ans. Though in practice 3-phase 3-wire ac system is universally used for transmission and 3-phase 4-wire ac system is used for distribution of electric power but for special purposes other systems may also be used. The various systems of power transmission are
(a) DC Systems :
(i) DC two-wire system.
(ii) DC two-wire system with midpoint earthed.
(iii) DC three-wire system.
(b) Single-Phase AC Systems:
(i) Single-phase two-wire system.
(ii) Single-phase two-wire system with midpoint earthed.
(iii) Single-phase three-wire system.
(c) Two-Phase AC Systems:
(i) Two-phase four-wire system.
(ii) Two-phase three-wire system.
(d) Three-Phase AC Systems:
(i) Three-phase 3-wire system.
(ii) Three-phase 4-wire system.
Q. 11. Why does the maximum voltage between each conductor and earth form the basis of comparison of volume of conductor material required in overhead system ?
Ans. In overhead system the conductors are insulated from the cross arms and supporting towers and as the towers and cross arms are earthed so the maximum voltage between each conductor and earth forms the basis of comparison of volume of conductor material required in overhead system.
Q. 12. Why does the maximum voltage between conductors form the basis of comparison of volume of conductor material required in underground system ?
Ans. In underground cables the maximum disruptive stress is between the two conductors of the cable; so the maximum voltage between conductors forms the basis of comparison of volume of conductor material required in underground system.
Q.13. What is the effect of voltage on transmission efficiency?
Ans. The transmission efficiency increases with the increase in transmission voltage.
Q. 14. What are the various components of an overhead line ?
Ans. The main components of an overhead line are supports/towers, cross arms and clamps, insulators, conductors, guys and stays, lightning arresters, earth wire, V-guards, guard wires, phase plates, bird guards, danger plates, anti-climbing devices, vibration dampers etc.
Q. 15. What is the function of line support?
Ans. The function of a line support is to support the line conductors and keep them at a safe distance from the ground
Q. 16. What are the main requirements of line supports ?
Ans. The main requirements of line supports are high mechanical strength, lighter in weight, low maintenance cost, longer life, cheaper in cost, good appearance, easy accessibility for painting and erection of line conductors and immune to damage from lightning and fire.
Q. 17. What are four types of line supports ?
Ans. The four types of line supports are wooden poles, RCC or PCC poles, rail poles and steel structures or towers.
Q. 18. What are the drawbacks of wooden poles ?
Ans. The main drawbacks of wooden poles are comparatively smaller life, poor mechanical strength, tendency to rot below the ground level and requirement of periodical inspection.
Q. 19. How do tangent angle tower and deviation tower differ from each other?
Ans. Steel towers can be broadly classified as tangent towers and deviation towers. Tangent towers can be used for straight runs of the line and up to 2° line deviation from the straight run. The line is straight or along the tangent to the line route. In such towers the stress is because of the weight of the conductors, ice and wind loads. In addition extra forces due to break in the line on one side of the tower is also to be considered in the design of towers. The base of such a steel tower may be square or rectangular. Insulators used with such towers are suspension types.
For deviations exceeding 2º, special angle towers, sometimes called the deviation towers, are used. They are used where the transmission line changes direction. Such towers have broader base and stronger members as they are to withstand the resultant force due to change in direction in addition to the forces to which the tangent towers are subjected. Insulators used with such towers are of strain type. The cost of deviation tower is comparatively larger than that of a tangent tower because it is designed to withstand heavy loading as compared to standard or tangent tower.
Q. 20. What are the main requirements of good conductor material used for overhead lines?
Ans. Main requirements of good conductor material used for overhead lines are high electrical conductivity, high tensile strength, low specific gravity, low cost, easy availability,
free from brittleness and the material should not react with
atmospheric condition.
Q. 21. What for ACSR conductor stands?
Ans. ACSR conductor stands for aluminium conductor steel
reinforced conductor.
Q. 22. Why are conductors for transmission line stranded ?
Ans. All conductors used for overhead transmission lines are
stranded so as to increase the flexibility. Solid wires, except
of smaller sizes, are difficult to handle when used for long
spans and tend to crystallise at the points of supports because
of swinging in winds.
Q. 23. Define term 'ACSR'. [Pb. Technical Univ. Dec. 2006)
Ans. ACSR conductor consists of a core of galvanised steel
strand surrounded by a number of aluminium strands.
Q. 24. Why are ACSR conductors preferred for transmission and distribution lines?
Ans. ACSR conductors are preferred for transmission and
distribution of lines because of the following facts:
(1) ACSR conductor being of high tensile strength and
lighter in weight produces small sag and therefore
longer spans can be used.
Because of the use of a larger span, the number of line
supports may be reduced by about 25%. Thus the
overall cost of supports, foundations, insulators and
erection is considerably reduced. In addition, there is
a substantial saving in maintenance cost. Further, since
most faults occur at the line supports, so the frequency
of occurrence of faults is automatically reduced.
(ii) The ACSR conductor has a largest diameter than any
another type of conductor of same resistance, so corona
losses are reduced.
(iii) In Another advantage with ACSR conductor is that,
because of reduced corona losses, critical voltage limit
of the conductor can be raised by 30 to 50% as
compared to copper conductors
Q. 25. Why is galvanized steel wire not suitable for EHT lines for the purpose of transmitting large amounts of power over a
long distance ?
Ans. The galvanised steel wire is not suitable for EHT lines for
the purpose of transmitting large amounts of power over a
long distance because of poor conductivity, high internal
reactance and eddy current and hysteresis.
Q. 26. What is the function of cross arms?
Ans. The function of cross arms is to keep the line conductors
at a safe distance from each other and from the pole.
Q. 27. What are the factors on which conductor spacings and
ground clearance depend ?
Ans. Factor affecting conductor spacings is voltage while factors affecting ground clearance are voltage and area (living or non-living, urban or suburban, along the streets or across
the streets, river crossings/hill crossings, railway track/
bridge etc).
Q. 28. Define term 'sag' in a transmission line.
Ans. The difference in level between the points of line supports
and the lowest point on the conductor is known as sag.
Q. 29. What are the factors affecting the sag in a transmission
line ?
Ans. Factors affecting the sag in a transmission line are weight
of conductor, span length (distance between the poles).
working tensile strength, ice coating, wind pressure and
temperature.
Q. 30. Why is it disadvantageous to provide either too high sag
or too low sag?
Ans. It is disadvantageous to provide either too high sag or too
low sag owing to the following facts. In case the sag is too high, more conductor material is required, more weight on the supports is to be supported, higher supports are necessary and there is a chance of greater swing-amplitude due to wind load. On the other hand in the case of too low sag, there is more tension in the conductor and thus the conductor is liable to break if any additional stress is to be taken, such as due to vibration of line or due to fall in temperature.
Q. 31. What is the effect of wind and ice on sag ?
Ans. Combined Effect of Wind and Ice. Due to weight of ice
deposits on the line, and the wind pressure, the mechanical
stress increases in the conductor and, therefore, the line
must be designed to withstand these stresses and tensions.
Under this condition, the weight of the conductor, together
with weight of ice acts vertically downwards while the wind
loading w, acts horizontally.
Q. 32. What is stringing chart?
Ans. The stringing chart gives the data for sag and tension to be allowed at a particular temperature. It is very useful while erecting the transmission line conductors for adjusting the sag and tension properly.
Q. 33. What are the causes of the failure of insulators ?
Ans. The insulator failure may occur due to cracking of porcelain, porosity, puncture, mechanical stresses, flash-
over etc.
Q. 34. How do the insulators fail ?
Ans. Electric failure of insulators may occur either by puncture
or flash-over. In the case of puncture the arc passes through
the insulator body while flash-over is caused by an arc
discharge between the line conductor and earth through air
surrounding the insulator.
Q. 35. How are insulators tested to determine flash-over
voltages?
Ans. For determination of flash-over voltage of a given insulator high voltage supply terminals are connected to the pin and insulator and the test voltage is gradually raised to a
specified value in approximate 10 seconds and is maintained
for one minute. The voltage is then gradually raised till a
flash-over takes place. The flash-over voltage depends
barometric pressure, temperature, humidity etc.
Q. 36. What is meant by birdage ?
Ans. Birdage means the short circuiting of the line conductor to earth through the large birds or similar objects.
Q. 37. What is string efficiency ?
Ans. The efficiency of an insulator string, called the string
efficiency, is the ratio of voltage across the whole string
and the product of the number of units and voltage across
the unit nearest to the line conductor.
Q.38 How are voltage distribution and the string efficiency
affected by rain ?
Ans. More uniform voltage distribution is obtained and the string efficiency is improved because of reduction in value of K
[when insulators are wet, mutual capacitance C increases
while the shunt capacitance C, (except for the unit nearest
the cross arm) remains constant)
Q. 39. What are the methods, that are used for improving string
efficiency?
Ans. Various methods used for improving string efficiency are
by using insulators with larger discs or by providing each
insulator unit with a metal cap, by using longer cross arms,
by capacitance grading or by static shielding,
Q. 40. What are the two functions that are performed by the grading ring?
Ans. The grading ring performs two functions viz equalizes the
voltage distribution across the insulator units and when used
in conjunction with arcing horn fixed at the top end of the
string serves the purpose of arcing shield and protects the
insulator string from flash-over.
Q. 41. What are the important properties that an overhead line
insulator must possess ?
Ans. The most important properties that an overhead line
insulator must possess are : High mechanical strength, high
relative permittivity, high insulation resistance, high ratio
of rupturing strength to flash-over voltage and capability
of withstanding temperature variations.
Q. 42. What types of insulators are used for overhead transmission and distribution lines ?
Ans. The insulators used for overhead line are pin type,
suspension type, strain type and shackle insulators.
Q. 43. What is corona ?
Ans. When the voltage between conductors of an overhead line
exceeds the disruptive critical voltage value, a hissing noise
accompanied by a violet glow appears. This phenomenon
is called the corona.
Q. 44. What are the various factors which affect corona ?
Ans. Factors affecting corona are (1) number of ions, size and
charge per ion, mean free path depending on atmospheric
conditions, (ii) line voltage, (iii) ratio of conductor spacing
to conductor radius, (iv) contour of the conductor surface,
and (v) state of the conductor surface.
Q. 45. What are the adverse effects of corona ?
Ans. Adverse effects of corona are (i) power loss but not very
important except under abnormal weather conditions,
(ii) corrosion due to production of ozone gas, and
(iii) interference with neighbouring communication circuits.
Q. 46. What do you mean by dielectric strength of air ?
Ans. The value of potential gradient at which complete disruption of air occurs, is called the dielectric strength of air.
Q. 47. What is meant by disruptive critical voltage ?
Ans. The disruptive critical voltage is defined as the minimum
phase to neutral voltage at which corona occurs.
Q. 48. What is visual critical voltage ?
Ans. Visual critical voltage is defined as the minimum phase to
neutral voltage at which glow appears all along the line conductors.
Q. 49. What are the various factors which affect corona loss?
Ans. The factors affecting corona loss are system frequency,
system voltage, air conductivity, air density, conductor
diameter, conductor surface condition, atmospheric
conditions and load current.
Q. 50. How can the corona effects be minimised ?
Ans. Corona effects can be minimised in a transmission line by
using large diameter conductors which may be accomplished by using hollow conductors with a hemp core, ACSR conductors or bundled conductors. Corona effects can also be reduced but to a limited extent, by increasing the spacing between the conductors.
Q. 51. Distinguish between ac and dc resistance of a conductor,
Ans. The resistance offered to dc, by a conductor is known as
dc resistance while the resistance offered to ac is known as
ac resistance or effective resistance. For a given conductor
ac resistance is more than its dc resistance. This is because
an alternating current flowing through a conductor does
not distribute uniformly but tends to concentrate near the
surface of the conductor. The result is that the effective
area of the conductor is reduced causing an increase in ac
resistance.
Q. 52. What is skin effect ?
Ans. The phenomenon of concentration of an electric current
near the surface of the conductor is known as skin effect.
Q. 53. On what factors does the skin effect depend ?
Ans. The skin effect depends upon the type of material, frequency of current, diameter of conductor and shape of conductor.
It increases with the increase of cross section, permeability
and supply frequency
Q. 54. For determination of inductance of a circuit, the determination of flux linkages is essential. Why ?
Ans. It is well known fact that a current carrying conductor
is surrounded by concentric circles of magnetic lines.
In case of ac system this field set up around the
conductor is not constant but changing and links with
the same conductor as well as with other conductors.
Due to these flux linkages the line possesses inductance,
defined as the flux linkages per unit current. Thus for
determination of inductance of a circuit, determination
of flux linkages is essential
Q. 55. What is meant by GMR of a conductor ?
Ans. The product re-14 is known as GMR of a conductor and is
equal to 0.7788 times the conductor radius.
Q. 56. What is self GMD of a conductor consisting of seven
identical strands, each having a radius r?
Ans. 2.176r
Q. 57. What is a bundle conductor and why is it used ?
Ans. A bundled conductor is a conductor made up of two or
more conductors, called the sub-conductors, per phase in
close proximity compared with the spacing of phases
The use of bundled conductors per phase reduces the volt-
age gradient in the vicinity of the line and thus, reduces
the possibilities of the corona discharge.
Although the bundled conductors are used on EHV trans-
mission lines primarily to reduce corona loss and radio
interference, but they have several other advantages over
single conductors such as given below :
1. The bundle conductor lines transmit bulk power with
reduced losses, thereby giving increased transmission
efficiency.
2. Since the bundle conductor lines have a higher
capacitance to neutral in comparison with single
conductor lines, therefore, they have higher charging
current, which helps in improving the power factor.
3. Since by bundling, the self GMD or GMR is increased,
the inductance per phase, in comparison with single
conductor lines, is reduced. As a result reactance per
phase is reduced.
4. Since surge impedance of a line is given by Zo = root of L/C
and the bundle conductor lines have higher capacitance
and lower inductance in comparison with single
conductor lines, therefore, bundle conductor lines have
comparatively lower surge impedance with a
corresponding increase in the maximum power transfer
capability.
Q.58. What is bundling of conductors ?
Ans. The use of more than one conductor per phase is called the bundling of conductors.
Q.59. Differentiate between bundled and composite conductors.
Ans. The basic difference between a composite conductor and a bundled conductor is that the sub-conductors of a bundled
conductor are separated from each other by a constant
distance varying from 0.2 m to 0.6 m depending upon
designed voltage and surrounding conditions throughout the
length of the line with the help of spacers whereas the
wires of a composite conductor touch each other. The
bundled conductors have filter material or air space inside
so that the overall diameter is increased.
Q. 60. For what purposes bundled conductors are primarily used in transmission lines ?
Ans. The bundled conductors are used in EHV transmission lines primarily to reduce corona loss and radio interference.
Q. 61. What are the advantages of bundled conductors.
Ans. The advantages of using bundled conductors in EHT
transmission lines are : Reduced possibilities of the corona
discharge; reduced corona loss; reduced radio interference;
reduced losses resulting in increased transmission efficiency;
higher charging current resulting in improved pf; reduced
inductive reactance per phase; lower surge impedance
resulting in increased power transfer capability,
Q. 62. What is the effect of ground on the capacitance of a
transmission line ?
Ans. The presence of ground increases the capacitance of a
transmission line by a small amount.
Q. 63. Why the effect of ground on the line capacitance can be
neglected ?
Ans. The effect of ground on the line capacitance is very-very
small because the height of conductors is large as compared
to the distance between them.
Q. 64. What is meant by transposition of line conductors ?
Ans. Transposition of line conductors means changing the
positions of the three phases on the line supports twice
over the total length of the line. The line conductors in
practice, are so transposed that each of the three possible
arrangements of conductors exist for one-third of the total
length of the line
Q. 65. Why transposition of conductors in a three-phase transmission lines is essential ?
Ans. The inductance and capacitance of each phase will be
different in case of conductors of three-phase line being
spaced irregularly. The apparent resistance of the conductors
is also affected on account of transfer of power between
the phases, which occur due to mutual inductance. Thus
all the three line constants are affected by irregular spacing
of the conductors in a 3-phase line.
Also, due to unsymmetrical spacing, the magnetic field
external to the conductors is not zero, thereby causing
induced voltages in adjacent electrical circuits, particularly
the telephone lines, that may cause disturbances in the
telephone lines,
Transposition of line conductors in transmission lines is
essential to counter the unbalancing effect, mentioned
above, of irregular spacing of line conductors.
Q. 66. What are the factors which govern the performance of a
transmission line ?
Ans. The factors governing the performance of a transmission
line are series resistance R and inductance L. shunt capacitance and conductance G. However, the effect of
conductance is negligible.
Q. 67. How do you classify transmission lines?
Ans. Transmission lines are classified depending upon the manner in which capacitance is taken into account.
Q. 68. What are the causes of voltage drop and line loss in a
transmission line ?
Ans. The voltage drop in a transmission line is due to its
resistance and inductance whereas the line loss is due to
its resistance.
Q. 69. What is voltage regulation of line ?
Ans. Regulation. When the load is supplied, there is a voltage
drop in the line due to resistance and inductance of the
line and therefore, receiving-end voltage VR is usually less
foi than sending-end voltage Vs. The voltage drop i.e.,
difference of sending-end voltage and receiving-end voltage
expressed as a percentage of receiving-end voltage is called
the regulation
When the load is thrown off i.e., when the line is supplying
no load, the receiving-end voltage becomes equal to
sending-end voltage and therefore, regulation can be defined
as below:
Regulation is defined as the change in voltage at the
receiving (or load) end when the full load is thrown off.
the sending-end (or supply) voltage and supply frequency
remaining unchanged. It is usually expressed as a
percentage of receiving-end voltage.
The lower the voltage regulation, better it is, because low
voltage regulation means little variation in receiving-end
voltage due to variation in load current.
Knowledge of voltage regulation helps in maintaining the
voltage at the load terminals within prescribed limits by
employing suitable voltage control equipment.
Q. 70. Explain the effect of load power factor on the transmission efficiency.
Ans. To deliver given power at a given voltage at consumer end,line current is inversely proportional to the power factor of
the load. The current, hence, the power loss in the line goes on increasing as the power factor of the load goes on decreasing. Thus efficiency of the transmission line decreases with the fall in power factor and vice versa.
71. What do you understand by the Ferranti effect?
Ans: The phenomenon of rising in voltage at the receiving end of the open-circuited or lightly loaded line is called the Ferranti effect.
72. How does rise in voltage at the receiving end of an open-
circuited line depend upon the length of the line and system
operating voltage ?
Ans. Rise in voltage at the receiving end of an open-circuited
line is proportional to the square of the length of the line
and varies directly in proportion to the system operating
voltage.
73. How do the terms ‘impedance drop', 'voltage drop' and
'voltage regulation', in connection with transmission lines
differ?
Ans. The impedance drop in a line is the voltage drop in the
line impedance and is equal to the phase difference between
the sending-ending voltage and the receiving-end voltage.
The voltage drop in a line is the arithmetical difference
between the sending-end and receiving-end voltages.
The voltage regulation is defined as the change in voltage
at the receiving-end when full-load is thrown off, the
sending-end voltage being held constant.
Q. 74. Why in medium and long transmission lines, regulation is greater than voltage drop ?
Ans. The regulation is greater than voltage drop in a medium/
long transmission line due to rise in voltage owing tº
Ferranti effect
Q. 75. What are the units of generalised constants of a transmission line ?
Ans. Generalised constants A and D are dimensionless whereas the units of generalised constants B and C are ohms and siemens (mho) respectively.
Q. 76. What is the range of surge impedance for an overhead
transmission line ?
Ans. 400-600 ohms
Q.77. What is the range of surge impedance in case of underground cables ?
Ans. 40–60 ohms
Q. 78. What is surge impedance loading with respect to an
overhead transmission line ?
Ans. Surge impedance loading (SIL) or natural load is the load
that can be delivered by a line having no resistance and
the loading being driven at unity power factor.
Q. 79. How does synchronous phase modifier differ from a
synchronous motor?
Ans. Synchronous phase modifier differs from an ordinary
synchronous motor in as much as they are built for the
highest economical speeds, and provided with smaller shafts
and bearings and special attention is paid for securing a
high overall efficiency,
Q. 80. How does the reactive power of a transmission line vary
for given values of V, VR and X ?
Ans. For fixed values of Vs, VR and X, the reactive power is
proportional to the magnitude of voltage drop in the line (.e., Vs - VR).
Q. 81. Why high and extra voltages are adopted for transmission of bulk power over long distances ?
Ans. This is because the maximum power transferred over a
given line increases with the increase in Vs and VR:
Q. 82. What is meant by receiving-end power circle diagram ?
Ans. The circle diagram drawn with receiving-end true and
reactive power components as the horizontal and vertical
ordinates is called the receiving-end power circle diagram.
Q. 83. What is meant by sending-end power circle diagram ?
Ans. The circle diagram drawn with sending-end true and reactive power components as the horizontal and vertical ordinates is called the sending-end power circle diagram.
Q. 84. What is universal power circle diagram ?
Ans. In the ordinary receiving-end power circle diagram, if the
receiving-end voltage is kept constant, the centre of the
circles is fixed and for different sending-end voltages circles
are drawn; these will be concentric but if the receiving-end
voltage is not constant the circles will not be concentric.
Similar is the case with the sending-end power circle
diagram. When sending-end voltage is not constant, its
centre will also vary.
To overcome the above limitation the universal power circle
diagram is drawn. In this diagram each distance on the
original diagram is divided by where V "reference or base voltage" and B is the generalised circuit
constant. This gives dimensionless units.
Q. 85. For what purpose metallic sheath is provided in underground protect it against ingress of moisture, gas or other damaging cables ?
Ans. The metallic sheath is provided around the insulation to
liquids (acid or alkalies) from the soil and atmosphere,
Q. 86. Why is a layer of bedding provided over the metallic sheath in underground cables ?
Ans. A layer of bedding is provided over the metallic sheath for
the protection of metallic sheath against corrosion and from
mechanical injury.
Q. 87. Why are copper conductors not used in power cables ?
Ans. Copper conductors are not used in power cables because
of scarcity of copper.
Q. 89. What are the main requirements of insulating materials used for underground cables ?
Ans. The main requirements of the insulating materials used for underground cables are high insulation resistivity, high
dielectric strength, good mechanical properties, non-
hygroscopic, non-inflammable, immune to attacks by acids
and alkalies.
Q. 90. Why VIR cables preferred over paper cables for smaller
installations ?
Ans. VIR cables are preferred over paper cables for smaller
installations as they are smaller in size, stronger, durable
and economical.
Q. 91. Why single-core cables are usually not provided with
armouring ?
Ans. Single-core cables are usually not provided with armouring in order to avoid excessive loss in the armour
Q. 92. Give classification of underground cables.
Ans. According to voltage, the cables may be classified as:
(1) Low voltage (LT) cables for operating voltage up to 1,000 V.
(ii) High voltage (HT) cables for operating voltage up to
11,000 V.
(iii) Super-tension (ST) cables for operating voltage up to
33,000 V.
(iv) Extra high tension (EHT) cables for operating voltage
up to 66,000 V.
(v) Extra super-voltage cables for operating voltage
beyond 1,32,000 V.
A cable may be a single-core or multi-core depending upon
the type of service for which it is intended to be used. A
multi-core cable may be two-core, three-core or four-core.
For 3-phase, 3-wire systems either three single-core cables
or three-core cables can be used depending upon the operating
voltage and load demand. For 3-phase, 4-wire systems 4-core
cables may be used.
Q. 93. List three groups of underground cables used.
Ans. The underground cables may be grouped into three groups as (i) LT cables, (ii) HT cables and (iii) EHT cables.
Q. 94. What are voids ?
Ans. Empty spaces in cables are called voids. The formation of
voids in cables causes unequal voltage stresses (which may
exceed the safe limits) and also temperature rise due to
leakance current.
Q. 95. How void formation can be eliminated in cables ?
Ans. In EHT and super-voltage cables useful for 132 kV and
above, the formation of voids are eliminated by increasing
the pressure of compound in it.
Q. 96. Give the limitations of 3-core belted type cable.
Ans. The belted type construction is not suitable for the cables
used for voltages above 22 kV because of development of
both the radial and tangential stresses. The tangential
stresses act along the layers of insulation. The electric
resistance, therefore, dielectric strength of the impregnated
paper is much higher across the layers than along the layers.
The leakage current on account of tangential stresses along
the impregnated paper insulation causes power loss at the
centre filling and local heating resulting in breakdown at
any moment. Further owing to non-homogeneity of dielectric
in belted construction, when cables are loaded and unloaded
Some portions of the dielectric are less stressed whereas
Some portions are overstressed resulting in formation of
vacuous spaces and voids. These vacuous spaces are ionised
when voltage is applied and ultimately deteriorate the cable Q.
insulation.
Q. 97. On what factors does the insulation resistance of a cable
depend ?
Ans. The insulation resistance of a cable depends upon core
radius, insulation thickness, length of cable and resistivity
of insulating material.
Q. 98. How does the insulation resistance of a cable vary with its length ?
Ans. Insulation resistance of a cable varies inversely as the length of the cable.
Q. 99. On what factors does the capacitance of a single core
depend ?
Ans. The capacitance of a single core depends upon its length,
relative permittivity of its dielectric and the ratio of core
diameter to sheath inner diameter.
Q. 100. How does electric stress vary in the dielectric of a cable ?
Ans. The electric stress in the dielectric of a cable varies
inversely as the distance from the centre of the cable and
hence minimum at the sheath and maximum at the
conductor surface.
Q. 101. For what value of Rir the minimum value of stress is
obtained in a cable ?
Ans. For value of Rr = e i.e., 2.71828 the minimum value of
stress is obtained in a cable.
Q. 102. What do you understand by the term grading of cables ?
Ans. The process of achieving uniformity in dielectric stress is
known as grading of cables.
Q. 103. What is the modern practice adopted to avoid grading of
cables ?
Ans. The modern practice adopted to avoid grading of cables is
to employ oil-filled or gas-pressure cables,
Q. 104. Name the sources of heat generation in cables,
Ans. Within the cables the sources of heat generation are
(i) I2R losses in the conductor
(ii) Dielectric losses in the cable insulation,
(iii) Losses in the metallic sheath and armourings,
Q. 105. Why loss angle of a cable should be very small ?
Ans. As the dielectric losses occurring in a cable are very small,
the value of loss angle 8 is small.
Q. 106. How does dielectric loss vary with the change in voltage, frequency of supply and capacitance of cable ?
Ans. The dielectric loss in a cable varies in proportion with the
capacitance of a cable and frequency of supply and varies
as the square of the voltage.
Q. 107. Mention important factors to be considered while selecting a cable.
Ans. The factors to be considered in the selection of a cable are
continuous current rating, voltage grade, permissible voltage
drop and short-circuit current rating in case of HT cables
only) and operating cost.
Q. 108. What is the cause of underground cable failure?
Ans.
1. The most common point of failure is at the cable sealing box mostly due to poor workmanship of the cable jointer when the end was sealed. Damage caused to the insulation on account of improper handling at the time of laying will also ultimately result in a failure.
2. Another very common cause is the mechanical catering of the lead sheathing of a cable, such as by a crowbar, especially in industrial installations where excavation and building operations are carried on in areas having several underground cables.
3. Although the lead sheathing of underground cables has a very high degree of resistance against corrosion, but in spite of it certain soils are chemically active and may cause severe pitting and corrosion.
4. The cable may also get damaged due to vibration fatigue or overheating.
5. Where a cable is laid vertically up posts or walls, the impregnating oil is likely to leak from the cable box at the lower level due to the hydrostatic pressure.