Transmission system

2. TRANSMISION,DISTRIBUTION
& UTILISATION (15EE52T)

3. UNIT- 01
TRANSMISSION SYSTEM
CO-01- Explain the basic elements of transmission system,
types of transmission system

4. • What is Transmission Line?
Transmission line is the long
conductor with special design (bundled) to carry
bulk amount of generated power at very high
voltage from one station to another as per
variation of the voltage level.

5. Electric power transmission is the bulk
movement of electrical energy from a generating
site, such as a power plant, to an electrical
substation. The interconnected lines which
facilitate this movement are known as a
Transmission network.
This is distinct from the local wiring between
high-voltage substations and customers, which is
typically referred to as Electric Power
Distribution.

6. A.C Transmission and Distribution system
Single Line Diagram

8. • Generating Station(G.S) :
It is the voltage actually generated by alternator its
voltage magnitude is less as compare to transmission.
• Primary Transmission:
It is a 3-phase, 3-wire transmission line connected in
between substation at generating station and
transmission substation or receiving substation e.g.
Primary Transmission voltage: - 220 KV, 400KV, 765 KV
• Secondary Transmission:
It is a 3-phase, 3-wire transmission line connected in
between transmission substation to sub transmission
substation or receiving substation. e.g. Secondary
Transmission voltage :- 220 KV, 132 KV, 110 KV

9. • Primary Distribution:
It is a 3-phase, 3-wire transmission
line connected in between receiving substation to
Distribution substation. e.g. primary distribution
voltage 33KV, 22KV, 11 KV for long distance line it
may be 66 KV
• Secondary Distribution:
It is a 3-phase, 4-wire Distribution line
in between Distribution substation to consumer
line. e.g. secondary distribution voltage 3-phase
400 Volt, for single phase 230 Volt

10. Standard transmission and distribution voltages
• Generation Voltages : 6.6kV, 11kV, 13.2kV or 33KV
• Transmission voltages :
• Primary Transmission – 110kV, 220kV or 400kV
• Secondary Transmission – 66kV or 33kV.
• Distribution voltages :
• Primary Distribution – 33kV , 22kV , 11kV or 6.6kV.
• Secondary Distribution – It is 3-Ph Four-Wire System
(R-Y-B-N) and Voltage level 3-Ph 400 Volt,
for single phase (Phase-Neutral) supply voltage is
230 volt

11. Advantages of transmission at high voltage.
1. With increase in the transmission voltage size of
the conductors is reduced (Cross section of the
conductors reduce as current required to carry
reduces).
2. As the reduction in current carrying requirement
losses reduces results in better efficiency
3. Due to low current voltage drop will be less so
voltage regulation improves.
4. Reduces the Power losses in the transmission line.

12. Limitations /disadvantages of transmission at high voltage.
1. With the increase in the voltage of transmission, the
insulation required between the conductors and the
earthed tower increases.
2. The increased cost of transformers , Switchgear and
other terminal apparatus.
3. This increase the cost of line support With increase
in the voltage of transmission, more clearance is
required between conductors and ground. Hence
higher towers are required.
4. With increase in the voltage transmission, more
distance is required between the conductors.
Therefore cross arms should be long.

13. Classifications of power transmission and distribution systems
1.According to the arrangement of conductor
a. Overhead Transmission system.
b. Underground Transmission system.
2. According to nature of Current.
i. D.C System
a. D.C two-wire.
b. D.C two-wire with mid-point earthed.
c. D.C Three-wire.
ii. Single Phase A.C System
a. Single Phase two wire.
b. Single Phase two-wire with mid-point earthed.
c. Single Phase three-wire.
iii. Two-Phase A.C System
a. Two-Phase four-wire.
b. Two-Phase Three wire.
iv. Three-Phase A.C System
a. Three-Phase Three-wire.
b. Three phase four-wire

14. • Two Wire D.C. System
It consists of two wires, one positive and other negative. The positive is
outgoing while the negative is return wire. The two wire d.c. system
where d.c. generators are used at the generating station is shown in
the Fig
Applications:
operation of d.c. motors, batteries, charging where d.c. supply is
must. It can be obtained by using rectifiers or by d.c. generators at
substations.

15. A.C Single Phase two wire.
This system may be used for very short distances. Single
Phase power is a two wire Alternating Current (AC) power
circuit.
Applications:
Most people use it every day because it’s the most common
household power circuit and powers their lights, TV, etc.
Typically there’s one power wire and one neutral wire and
power flows between the power wire (through the load)
and the neutral wire.

16. Three Phase Three Wire System
The three phase three wire system may be star or delta connected. If it
is star connected, then its neutral is grounded. The Fig. shows the
scheme of three phase three wire system for the primary distribution.
Applications :
The large consumers like factories which need bulk supply are directly
supplied from the substations. The power is also distributed to other
substations and distribution centers.

17. Three Phase Four Wire System

18. The fourth wire in this system is neutral and hence the
transformer secondary in such system is always star
connected. This system is generally preferred for the
secondary distribution. The single phase loads are
connected between one of the three lines and a neutral
while the three phase loads can be given three phase
supply directly, along with the provision of neutral for the
internal distribution.
The voltage between any of the lines and a neutral is 230
V while the voltage between any two lines is 400 V.
• Applications :
The four-wire system is used when a mixture of single-
phase and three-phase loads are to be served, such as
mixed lighting, Heating and motor loads.

19. Transmission through overhead transmission lines.
• An overhead power line is a structure used in electric power
transmission and distribution to transmit electrical energy
along large distances.
• It consists of one or more conductors (i.e., A.C.S.R)
suspended by towers or poles. Since most of the insulation is
provided by air, the porcelain insulators are used to insulate
current carrying conductor from other accessories.
• overhead power lines are generally the lowest-cost method of
power transmission for large quantities of electric energy.

20. Main components of overhead transmission lines
1. Supporting structure
(pole)
2. Line insulator
3. Overhead conductor
4. ‘V’ Cross arm
5. Top pin support
6. Two Pin Cross arm
7. Four pin cross arm
8. Stay set
(Stay wire of 7/8 or 7/10 SWG)
9. Lighting arrestors
10. Guarding wires
11. Continuous earth wire
12. Cables
13. Fuses and Isolating
switches
14. Different types of Clamp
(A-type, B-Type)
15. Bird guards
16. Vibration damper
17. Jumpers

21. Types of line insulators
1. Pin type insulator
2. Suspension or Disc type insulator.
3. Strain type insulator.
4. Shackle type insulator.
5. Stay or Guy or Egg type insulator.
1. Pin type insulator 2.Disc type insulator
3. Strain type insulator
4. Shackle type insulator.
5. Stay or Guy e insulator

22. ‘V’ Cross arm
Supporting structure (pole)
Line insulator
Top pin support & Two Pin Cross arm Four pin cross arm

23. Stay set
Lighting arrestors

24. Continuous earth wire Bird guards

25. Guarding wires

26. Vibration damper

27. Jumpers

28. Steps involved in erection of transmission tower.
1. No tower shall be erected on foundation before 10 days after concreting.
2. Check the correctness of diagonal and level of the stub of the foundation.
3. Towers are to be erected as per erection drawing
4. Assembly of tower parts shall be made as per mark number in the
erection drawing.
5. Special care shall be taken in selection of mark number for transverse and
longitudinal face of the square base tower.
6. Any buckling, damage to steel member, damage to galvanizing shall be
avoided.
7. No member shall be subjected to undue stress.
8. Collect material from store tallying with BOM(Bill of materials).
9. Preferably transport complete tower or complete section.
10. The tower member at location shall be kept on ground serially according
to sequence.

29. Transmission through UG (Underground) transmission system
An underground cable essentially consists of one or more
conductors covered with suitable insulation and surrounded by a
protecting cover. As cable is underground, there are less chances of
faults. As cable is laid underground, there are no chances of accidents
Advantages of UG Transmission system:
• No radio-interference.
• Short cut roots are possible.
• Less possibility of theft of energy.
• Voltage drop is less.
• More reliability to maintain supply as chances of faults is less.
• Availability of space is less. so no space is consumed in underground system as against
overhead system.
• Life of insulation is more.
• Its appearance is good, so it will not spoil the beauty of city due to overhead structure.

30. Comparisons between overhead and UG system
Sl.No Particular Overhead line Underground cable
1 Capital cost Less More
2 Erecting cost Less More
3 Time require for Less More
completion of work
4 Flexibility More flexibility No flexibility
5 Future expansion in
System voltage can
be System voltage cannot be
voltage level increased easily increased
6 Overload capacity More Less
7 Fault finding Easy Difficult
8 Charging Current Less More
9 Chances of fault More Less

31. 11 Safety Less More
12 Radio interference
Produces radio
interferences Not produces radio
interferences
13 Short cut route Difficult possible
14 Theft Of energy More possibility Less possibility
15 Voltage drop More less
16 Power factor Less More
17 Reliability Less More
18 Life Less More
19 Space consumed Space consumed No space consumed
20 Appearance Not good Very good

32. Compare HVDC and HVAC system
Sl.No Particular HVDC HVAC
1
Number of conductor
required for single
circuit
One conductor.& Ground is
used as a return path
Three conductors (R.Y.B)
2 For double circuit
Two conductors.& Ground
is used as a return path
Six conductors (R,Y,B &
R,Y,B)
3 Design of Tower Light Heavy
4
Intermediate
substation
Not required Required at every 250 Km
5 Capital cost of S/S More Less
6
Transmission line
cost/km
Less More
7 Ground return Possible Not possible
8 Frequency Absent Present
9 Skin effect Absent Present
10 Proximity effect Absent Present

33. 11 Ferranti effect Absent Present
12 Corona losses Less More
13 Radio interference Absent Present
14 Effect of L &C Absent Present
15 Value of resistance Less More 1.6 times than DC
16 Copper loss Less More
17
Transmission
Efficiency
More Less
18
Voltage drop in
transmission line
Less More
19 % Regulation Better Good
20
Limitation on length
of cable
Charging current is
absent
so no limitation on
length of
cable
Due to charging current
there is limitation on length
of cable

34. 21 String efficiency 100 % Less than 100 %
22 Losses in S/s More Less
23 Maintenance cost of S/S More Less
24 Asynchronous tie Possible Not possible
25 Reliability & availability
One bipolar line is
sufficient
Two AC circuit are
necessary
26 Control system Difficult, costly Simpler cheaper
27
Power handling
capacity
No limit
There is limit due to power
angle & inductance
28 Voltage control
Easier as L&C are not
effective
Difficult for long distance
lines due to presence of
L & C
29 Stability limit
No limit due to absent of
power angle & inductance
EHVAC limits due to power
angle & inductance
30 Power flow control
Power can be quickly(fast)
controlled,
Power flow cannot be easily
controlled, (slow)

35. 31 Power transfer ability High Lower
32 Transient performance Excellent Poor
33
Back to Back
conversion stations
Possible Not Possible
34 Short circuit current level Less More
35 Reliable circuit Not available Available

36. Classification of UG cables
The underground cables are classified in two ways; by the voltage capacity, or by
the construction.
• By Voltage
1. LT cables: Low-tension cables with a maximum capacity of 1000 V or 1kV
2. HT Cables: High-tension cables with a maximum of 11KV
3. ST cables: Super-tension cables with a rating of between 22 KV and 33 KV
4. EHT cables: Extra high-tension cables with a rating of between 33 KV and 66 KV
5. Extra super voltage cables: with maximum voltage ratings beyond 132 KV
• By Construction
1. Belted cables: Maximum voltage of 11KV
2. Screened cables: Maximum voltage of 66 KV
3. Pressure cables: Maximum voltage of more than 66KV

37. Essential properties of insulating material used in for UG cables
1. High insulation resistance to avoid leakage current.
2. High dielectric strength to avoid electrical breakdown of the cable.
3. High mechanical strength to withstand the mechanical handling of cables.
4. Non-hygroscopic i.e., it should not absorb moisture from air or soil. The
moisture tends to decrease the insulation resistance and hastens the
breakdown of the cable. In case the insulating material is hygroscopic, it
must be enclosed in a waterproof covering like lead sheath.
5. Non-inflammable.
6. Low cost so as to make the underground system a viable proposition.
7. Unaffected by acids and alkalies to avoid any chemical action.

38. Types U.G Cables Based On Construction.
1. Belted Cable (Upto 11KV)
2. Screened Cable (From 22KV to 66KV)
I. H-Type Cables (It was first designed by M. Hochstadter)
II. S.L Type Cables (Separate Lead type)
III. H.S.L. Type Cables (cable is combination of H type and S.L. type
cable)
3. Pressure Cables (upto 66KV)
I. Oil Filled Cables:
II. Gas Filled Cables:

39. Belted Cable (Up to 11KV)
Screened Cable (From 22KV to 66KV)
Pressure Cables (up to 66KV)

40. General construction of a single core UG cable.

41. • Core or conductor :
It function is carry current. Cross section of conductor
is directly proportional to current. Cable may have 1 or
more than 1 core conductor.
• Insulation:
Each core of conductor is provided with suitable
thickness of insulation to avoid short circuit between
two conductors.
• Metallic (Lead) Sheath:
It is provided over insulation to provide the protection
of core from entry of moisture , gases or other damaging
liquids (acids & alkaline) in the soil & atmospheric , The
metallic sheath is of lead or lead alloys is provided on
the insulation recently aluminum is also being used as a
metallic sheath.

42. • Bedding:
Over the metallic sheath there is layer of bedding. The
purpose of bedding is protecting the metallic sheath against
corrosion & from the mechanical injury due to armouring. It
is made from fibrous material such as jute, hessian tape.
• Armouring:
This layer is over a bedding only underground cable and not
for over head cable . It covers the bedding, which consists
of 1 or 2 layers of galvanized steel wire or steel tapes. Its
purpose is to protect the cable from mechanical injury,
while rough handling & at the time of maintenance.
• Serving:
This layer is last layer which comes over armouring. Its
function is to protect armouring or to cover armouring
against rusting and it also helps for easy handling of cables.
It is similar to bedding & consists of fibrous material such as
jute.

43. Construction of 3 core XLPE(Cross-Linked Polyethylene) cable

44. • Conductors
Usually stranded copper (Cu) or Aluminum (Al) is used.
Copper is denser and heavier, but more conductive than
aluminum. Electrically equivalent aluminum conductors
have a cross-sectional area approximately 1.6 times
larger than copper, but half the weight. The size of the
copper / Aluminum conductor forming one of the cores of
a cable is expressed in square millimeters (mm2), and the
current rating of the cable is dependent upon the cross-
sectional area of each core. Multi-core Aluminum or
copper conductor are produced by two shapes.

45. • Conductor Screen
• This screen consists of a lapped copper tape or metallic
foil usually less than 1.0 mm in thickness, which is the
interface between the conductor and the insulation.
The main purpose of conductor screen is to maintain a
uniformly divergent electric field, and to contain the
electric field within the cable core.
• Conductor screen is semi-conducting material because
semi-conducting materials do not conduct electricity well
enough to be a conductor but will not hold back voltage.
It smoothes out the surface irregularities of the
conductor.

46. Filler & Binding Tap (Laying-Up):
• In case of three core cables, the three cores are laid up with polymer
compound or non-hygroscopic fillers like polypropylene (PP) fillers and a
binder tape is applied with an overlap to provide a circular shape to the
cable.
•These binder tapes can be of PVC or foamed Polyethylene.
•Inner Sheath (Bedding) for Armored Cables. Extruded layer of PVC or PE is
applied over the laid up cores for armored cables.

47. Insulation
• There are different Type of Insulation Material used for cable but widely
used are-Cross-linked polyethylene: (XLPE), They are known as PEX or
XLPE Cable.
• It is form of polyethylene with cross links.
XLPE creates by direct links or bonds between the carbon backbones of
individual polyethylene chains forms the cross linked polyethylene
structure.
• The result of this linkage is to restrict movement of the polyethylene
chains relative to each other, so that when heat or other forms of energy
are applied to the basic network structure cannot deform and the
excellent properties that polyethylene has at room temperature are
retained at higher temperatures. The cross linking of the molecules also
has the effect of enhancing room temperature properties.
• The useful properties of XLPE are temperature resistance, pressure
resistance (stress rupture resistance), environmental stress crack
resistance (esc), and resistance to UV light, chemical resistance, oxidation
resistance, room temperature and low temperature properties.
• XLPE cables work for the working voltage of 240 V to 500 KV.

48. Insulation Screen
• An extruded layer of semi conducting is applied over the insulation layer
to insure that the electric stress is homogenous around the insulated core.
The semi conducting layer shall be firmly bonded to the outer layer of the
insulation layer.
• The purpose of insulation screen is same as conductor screen.
• The purpose of insulation screen is to reduce voltage stress at the
interface between the conducting and insulating component.
• A cylindrical, smooth surface between the insulation and metallic shield.
• Insulation screen is a layer of black cross linked semi conductive
compound of approx 1mm thickness and is either fully bonded to the
insulation layer, or can be “cold strippable” by hand.
• When terminating or jointing the cables, it is necessary to remove a part
of the insulation screen.

49. Bedding (Inner Sheath)
• It could be also called inner sheath or inner jacket, which serves as a bedding
under cable armoring to protect the laid up cores and as a separation sheath.
• Inner sheath is over laid up of cores.
• It gives circular shape of the cable and it also provides Bedding for the
armoring.
• Inner sheath is provided by extrusion of thermoplastic over the laid up of
cores.
• Inner sheath is provided by wrapping at thermoplastic tape.
• All multi-core cables have either extruded PVC inner sheath or thermoplastic
wrapped inner sheath, which is compatible with insulation material and
removable without any damage to insulation. Single core cables have no inner
sheath.

50. Metallic Screen
Medium Voltage & High voltage cables
have an earthed metallic screen over the
insulation of each core. This screen consists
one or multi layers of a lapped conductive
copper wires, copper tape or metallic foil, lead,
aluminum helically with overlap over insulation
screen. The metallic shield needs to be
electrically continuous over a cable length to
adequately perform its functions of
electrostatic protection, electromagnetic
protection, and protection from transients,
such as lightning and surge or fault currents.

51. • Armoring:
The armor provides mechanical protection against
crushing forces. Armor also can serve as an Earth
Continuity Conductor (ECC). The armoring type
could be: Mechanical protection of the cable is
provided by a single layer of wire / strip strands
laid over the bedding. Steel wire or strip is used for
3-core or 4-core cables, but single-core cables have
aluminum wire armoring. When an electric current
passes through a cable, it produces a magnetic
field (the higher the voltage the bigger the field).
The magnetic field will induce an electric current in
steel armor (eddy currents), which can cause
overheating in AC systems. The non-magnetic
aluminum armor prevents this from happening.

52. • Over Sheath (Outer Jacket):
It is the outer protection part of the cable against
the surrounding environment. Protected against water ingress,
protection against termite, protection against UV and protection against
differing soil compositions.
It is applied over armoring in case of armored cable and
over inner sheath in case of unarmored cable called as ‘Outer
Sheath.’ The standard sheath color is Black other colors such as Red,
Light Blue can also be provided. High-voltage cables are identified by
outer sheaths colored red; a black sheath indicates a low-voltage cable.

53. • Advantages of XLPE Insulated Cable
1. XLPE has higher current carrying capacity.
2. It can withstand higher temperature compared to PVC cable.
3. XLPE have high overload capacity.
4. XLPE has lower dielectric and constant power factor.
5. XLPE cables are lighter in weight, has smaller bend radius, and hence lesser
installation cost.
6. XLPE has higher short circuit rating.
7. XLPE has a higher moisture & chemical resistance.
8. Cable installation job for XLPE is easier than PVC insulated cables because of less
weight, less diameter and less bending radius.
9. The volume resistivity (ohm-cm) for XLPE is way higher.

54. Methods of laying UG cable
1. Direct laying cable
2. Draw- in system
3. Solid System

55. 1.Direct laying

57. • This is the simple and economic method of cable laying. For
this, a trench of 0.5 meter width and 1.5 meter deep is dug in
the ground as per cable route. Soil is taken out.
• A layer of sand of 10cm is spread at the bottom of trench then
cable is laid on this sand, again a layer of sand over the cable
and over it a layer of bricks along the length of cable is provided.
• The remaining portion is covered by soil. By this the cable is
protected mechanically and possibility of moisture is avoided by
use of sand layers. If two or more cables are to be laid in same
trench, sufficient space should be provided between them to
avoid transfer of fault from one cable to other.

58. • Advantages
(i) It is a simple and less costly method.
(ii)It gives the best conditions for dissipating the heat generated
in the cables.
(iii) It is a clean and safe method as the cable is invisible and free
from external disturbances.
• Disadvantages
(i) The extension of load is possible only by a completely new
work which may cost as much as the original work.
(ii) The alterations in the cable network cannot be made easily.
(iii) The maintenance cost is very high.
(iv) Localization of fault is difficult.
(v) It cannot be used in congested areas.

59. 2.Draw in system

61. • In this method, conduit or duct of glazed stone or cast iron or concrete are laid
in the ground with manholes at suitable positions along the cable route.
• The cables are then pulled into positions from manholes. Figure shows section
through four way underground duct line. Care must be taken that where the duct
line changes direction, depths, dips and offsets be made with a very long radius
or it will be difficult to pull a large cable between the manholes. The distance
between the manholes should not be too long so as to simplicity for the pulling
of the cables.
• The cables to be laid in this way must be provided with Serving of hessian and
jute in order to protect them when being pulled into the ducts

62. Advantages
(i) Repairs, alterations or additions to the cable network
can be made without opening the ground.
(ii) As the cables are not armoured, therefore, joints
become simpler and maintenance cost is reduced
considerably.
(iii) There are very less chances of fault occurrence due
to strong mechanical protection provided by the
system.
Disadvantages
(i) The initial cost is very high.
(ii)The current carrying capacity of the cables is reduced
due to the close grouping of cables and unfavorable
conditions for dissipation of heat.

63. 3. Solid System

64. • In this method of laying, the cable is laid in open pipes
or through dug out in earth along the cable route.
• The throughing is of cast iron, stoneware, asphalt
treated wood. After the cable is laid in position, the
throughing is filled with bituminous (ಡಾಮರು,ಪೆಟೆ್ರೋಲಿಯಂನ
ಒಂದು ಉತ್ಪನನ) or asphaltic compound and covered over.
• Cables laid in this manner are usually plain lead covered
because throughing affords good mechanical protection.

65. Disadvantages
(i) It is more expensive than direct laid system.
(ii) It requires skilled labour and favourable
weather conditions.
(iii) Due to poor heat dissipation facilities, the
current carrying capacity of the cable is
reduced

66. Faults in UG cables
• Open circuit fault : when there is a break in the
conductor of a cable, it is called open-circuit
fault.
• Short circuit fault: When two conductors of a
multi-core cable come in electrical contact with
each other due to installation failure, is called a
short-circuit fault.
• Earth fault : when the conductor of a cable
comes in contact with earth, it is called earth
fault or ground fault.

67. Classification of transmission lines based on distance:
• Short Distance Transmission Line - (up to 50 KM)
• Medium Distance Transmission Line - (up to 50 to 150 KM)
• Long Distance Transmission Line - (above 150 KM)
Classification of transmission lines based Voltage level:
• High voltage Transmission Line (HV) up to 33 KV
• Extra High Voltage Transmission Line (EHV) up to 400 KV
• Ultra High voltage Transmission Line (UHV) above 400 KV

68. 1. Short transmission lines.
• When the length of an overhead transmission line is
upto about 50 km and the line voltage is
comparatively low (< 20 kV), it is usually considered
as a short transmission line.
• Due to smaller length and lower voltage, the
capacitance effects are small and hence can be
neglected.
• Therefore, while studying the performance of a short
transmission line, only resistance and inductance of
the line are taken into account.

69. 2. Medium transmission lines.
• When the length of an overhead transmission line is
about 50-150 km and the line voltage is moderately
high (>20kV <100 kV), it is considered as a medium
transmission line.
• Due to sufficient length and voltage of the line, the
capacitance effects are taken into account.
• For purposes of calculations, the distributed
capacitance of the line is divided and lumped in the
form of condensers shunted across the line at one or
more points.

70. 3. Long transmission lines.
• When the length of an overhead transmission line
is more than 150 km and line voltage is very high
(> 100 kV), it is considered as a long transmission
line.
• For the treatment of such a line, the line
constants are considered uniformly distributed
over the whole length of the line.

71. Transmission line constants
A transmission line has
Resistance, Inductance and Capacitance
uniformly distributed along the whole
length of the line.

72. Resistance
• It is the opposition of line conductors to
current flow. The resistance is distributed
uniformly along the whole length of the line.
• The resistance of transmission line conductors
is the most important cause of power loss in a
transmission line.
• The resistance R of a line conductor having
resistivity ρ, length l and area of cross section a
is given by ;

73. Inductance.
• When an alternating current flows through a conductor, a
changing flux is set up which links the conductor.
• Due to these flux linkages, the conductor possesses
inductance. Mathematically, inductance is defined as the
flux linkages per ampere i.e.,
Inductance
where, ψ = flux linkages in Weber-turns
I = current in amperes

74. Capacitance.
• We know that any two conductors separated by an
insulating material constitute a capacitor. As any two
conductors of an overhead transmission line are
separated by air which acts as an insulation, therefore,
capacitance exists between any two overhead line
conductors.
• The capacitance between the conductors is the charge
per unit potential difference i.e., Capacitance,
where q = charge on the line in coulomb
v = p.d. between the conductors in volts

75. • Voltage regulation : The difference in voltage at
the receiving end of a transmission line
between conditions of no load and full load is
called voltage regulation and is expressed as a
percentage of the receiving end voltage.

76. • Transmission Efficiency :The ratio of receiving
end power to the sending end power of a
transmission line is known as the transmission
efficiency of the line i.e.
where VR, IR and cosφR are the receiving end voltage, current
and power factor while VS, IS and cosφS are the corresponding
values at the sending end.

77. Short transmission line
Equivalent circuit
The effects of line capacitance are neglected for a short transmission
line. Therefore, while studying the performance of such a line, only
resistance and inductance of the line are taken into account. The
equivalent circuit of a single phase short transmission line is shown in
Figure.

78. • vector diagram

79. • From the right angled triangle ODC, we get

81. Problems on performance of short transmission lines.
1.A 3-phase line delivers 3600 kW at a p.f. 0·8
lagging to a load. If the sending end voltage is 33
kV,
Determine,
(i)The receiving end voltage.
(ii)line current
(iii)Transmission efficiency.
The resistance and reactance of each conductor
are 5·31 Ω and 5·54 Ω respectively.

84. 2. A short 3-φ transmission line with an
impedance of (6 + j 8) Ω per phase has sending
and receiving end voltages of 120 kV and 110
kV respectively for some receiving end load at
a p.f. of 0·9 lagging.
Determine (i) power output and (ii) sending
end power factor

86. Meaning of Corona
• Corona is a phenomenon associated with
all transmission lines. Under certain
conditions, the localized electric field near
energized components and conductors can
produce a tiny electric discharge or corona,
that causes the surrounding air molecules to
ionize, or undergo a slight localized change of
electric charge.

87. Formation of corona
When AC Voltage given across two conductors
separated by distance ‘d’, When Voltage is increased
greater than breakdown voltage of air i.e. 30KV/cm,
then air around the conductor gets ionized and
ionized air is conducting under this condition corona
will takes place (form) .
• During corona following observations are noted:
Luminous violet glow occurs around the conductor.
Hissing sound will produce.
Ozone gas will produce.
This phenomenon is known as “corona” effect

89. Factors affecting corona
i) Magnitude of Voltage : If voltage across two
conductors is greater than 30 KV/cm, i.e. breakdown
voltage of air than corona formation starts. Corona
will not start if voltage is below 30 KV/cm
ii) Distance between two conductor: If spacing
between two conductors is very large as compare to
their diameter than there is no possibility of corona
formation. Because value of voltage at which corona
occurs increases.
iii) Size of conductor: If size (Cross section) of conductor
is more, than magnitude of voltage required to occur
the corona increases.

90. iv) Condition of conductor & Hardware: Rough and irregular
surface of conductor and hardware will give more corona
than solid, smooth body conductor & hardware.
v) Atmospheric Condition: As corona takes place due to
ionization of air so it depends on condition of air so for dry air
formation of corona occurs late than in wet air (damp
atmosphere condition/rainy season/thunderstorms/fog air
becomes more conductivity)
vi) Effect of supply Frequency: Corona loss varies directly as
the supply frequency.
vii) Effect of density of air: Corona loss increases with the
decrease in the density of air (The corona loss of
transmission line passing through hilly area is higher than that
of a similar line in plain due to reduced value of air density at
high level /altitude)

91. Advantages of corona:
1. Due to corona formation, the air
surrounding the conductor becomes
conducting and hence virtual diameter of
conductor is increased.
2. The increased diameter reduces the
electrostatic stress of the conductor.
3. Corona reduces the effects of transients
produced by surges.
4. It acts as safety valve against over voltage
due to lighting stroke.

92. Disadvantages of corona:
1.Corona is accompanied by loss of energy.
Due to more losses transmission efficiency
get reduced.
2.The current drawn by corona is non-
sinusoidal which causes non-sinusoidal
voltage drops in the line. This may cause
inductive interference with neighboring
communication lines.
3.Ozone is produced, which causes corrosion
of the conductor due to chemical action.
4. Harmonics are produced which will cause
radio interference

93. Methods to reduce corona.
1. By increasing distance between two conductor
i.e. by using longer cross arm.
2. By using larger size(diameter) of conductor e.g./
using ACSR, bundled conductor
3.By using smooth body conductor and hardware.

94. Meaning of skin effect
• When alternating current flows through conductor it has
tendency to flow away from center of conductor. i.e.
maximum current density is near skin of conductor and
goes on reducing towards center core is known as skin
effect. (Since the inductive reactance (XL) at the center of
the conductor is more than surface of conductor)
OR
• The tendency of alternating current to concentrate near
the surface of a conductor is known as skin effect.

95. Skin effect can be reduced by
1. Use stranded conductors instead of solid conductors.
2. Use hollow conductors instead of solid conductor.
3. Use A.C.S.R /A.A.A.C conductors for transmission purpose
4. Use D.C. supply whenever possible as Skin effect is absent
(Since frequency 0) instead of A.C. supply.
**All Aluminium Conductor (AAC),
**All Aluminium Alloy Conductor(AAAC) and
**Aluminium Conductor Steel Reinforced (ACSR).

96. Ferranti effect.
When receiving end voltage (VR) is found to be
greater than sending end voltage (VS).This
phenomenon is known as Ferranti effect.
• When these effects occur:
Suppose transmission line is subjected to following
Conditions:
1. When there is no load on transmission line (IL = 0)
2. When There is no load at receiving sub-station or Lightly
loaded .
3. When there is sudden load thrown OFF.
4. When there is sudden load shading.
5. When Transmission line is open circuited due to load
failure.

97. Transposition of conductors
• Transposition of line conductors means changing the
positions of 3- phases on the line supports twice over
the total length of the line.
• OR Exchanging the position of 3 phases (R-Y-B) at
regular interval.
• Each phase occupies 3 different positions consequently
on line support (Tower) as shown in fig

98. Necessity of transposition
• Due transposition of conductor inductance of
each line is same LA = LB = LC, So drop due to
inductive reactance in each line is same so
voltage at receiving end between any two line
become same.
• So to obtain same voltage in any two line at
receiving end (VRY = VYB = VRB) transposition is
necessary.
• Radio interferences are less due to transposition

99. MODEL QUESTIONS BANK
Cognitive Level: REMEMBER, UNDERSTAND
1) Explain the typical ac power supply scheme (single line
diagram of typical ac power supply scheme)
2) Compare DC and AC power transmission.
3) List the advantages and limitations of High transmission
voltage.
4) Classify the various types of for power transmission
system.
5) Explain briefly the different elements of transmission
line.
6) Explain voltage regulation and efficiency.
7) List the standard voltages used for Transmission systems.
8) Explain briefly the main components of overhead lines.
9) Explain briefly desirable properties of Insulators.
10) Define Corona and its formation.

100. 11) List the factors affecting corona.
12) List the advantages and disadvantages of Corona.
13) List the methods to reduce corona.
14) Explain briefly Constants of a transmission line.
15) Explain voltage regulation and transmission efficiency
16) Explain Short transmission lines with vector diagram.
17) Explain Skin effect and Ferranti effects.
18) Classify the UG cables based on construction.
19) Explain requirements of insulating materials used in UG
cables.
20) Explain construction of a 3 core UG cable.
21) Classify the UG cables based on voltage.
22) Explain with diagram the construction of XLPE cable.
23) List the types of cable faults