EE53-Transmission & Distribution
UNIT-1
INTRODUCTION
1. Mention the location of the HVDC transmission.
The locations of the HVDC transmission are Chandigarh, Delhi, Patiala and
Faridabad.
2. What is primary transmission?
The electric power at generating station is stepped up to 132kV(to reduce the
transmission loss) and transmitted by three phase, three wire overhead system to the
outskirts of the city. This forms the primary transmission.
3. What is secondary transmission?
The primary transmission line continues via transmission towers till the receiving
stations. At the receiving stations, the voltage level is reduced to 22kV or 33kV using the
step down transformer. There can be more than one receiving stations. Then at reduced
voltage level of 22kV or 33kV, the power is transmitted to various substations using
overhead three phase three wire system. This is secondary transmission. The conductors
used for the secondary transmission are called feeders.
4. Write the advantages of A.C transmission.
The advantages of A.C transmission are:
i. It can be generate power at very high voltages.
ii. The maintenance of A.C. substation is easy and cheaper.
iii. A.C. voltage can be stepped up or stepped down with the help of transformer.
iv. Construction is simple.
v. Fault can be detected very easily.
vi. There is no converter device required.
5. Write the disadvantages of A.C transmission.
The disadvantages of A.C transmission are:
i. More copper is required than D.C.
ii. Construction is complicated than D.C.
iii. Presence of skin effect to increase the effective resistance of the line.
iv. In A.C system due to the presence of capacitance there is continuous loss of power
due to charging current even when the line is open.
6. Mention the limitations of high transmission voltage.
The limitations of high transmission voltage are:
The cost of insulating the conductor is very high.
Cost of transformer, switchgear and other terminal apparatus are very high.
7. List the various systems of power transmission in D.C. system.
The various systems of power transmission in D.C. system are:
i. D.C. two wire system
ii. D.C. two wire with mid point earthed.
iii. D.C. three wire.
Single phase A.C. system
i. Single phase two wire
ii. Single phase two wire with mid point earthed
iii. Single phase three wire.
Two phase A.C. system
i. Two phase four wire
ii. Two phase three wire
Three phase A.C. system
i. Three phase three wire(Transmission system)
ii. Three phase four wire(Distribution system)
8. What is a feeder?
Feeder is defined as lines, which connect the distribution station and distributor.
9. What is a distributor?
Distributor is defined as a common bus bar, which connect the service main and feeder.
10. Distinguish between over head system and underground system.
Over head system Under ground system Construction cost is less. Construction cost is
high. To occupy the more spaces in metropolitan area. To enhance the city beauty It is operated
above 66KV. It is limits upto 66KV. Fault can be easily detected and rectified.Fault cannot be
easily detected.Maintenance cost is high. Maintenance cost is very less.
11. List the components of a power system.
The components of a power system are:
i. Generating station
ii. Step up transformer
iii. Step down transformer
iv. Transmission line
v. Switching station
vi. Primary distribution line
vii. Secondary distribution line
viii. Feeder
ix. Distributor and
x. Consumer
12. How distribution systems are classified?
Distribution systems are classified by,
i. Nature of current
ii. Type of construction
iii. Scheme of connection
iv. D.C distribution.
13. What is a service main?
Service main is defined as a small cable or conductor which connects the distributor to
the domestic terminal.
14. What is a distribution substation?
Distribution substation is a primary distribution system that delivers power to various
substations.
15. Why D.C. system is not used for transmission?
16. Give the types of loading in distribution system.
The types of loading in distribution system are:
Uniform loading
Concentrated loading
Combination of uniform loading and concentrated loading.
17. Define one line diagram.
One line diagram is defined as the way in which all electrical apparatus represented by
single line in symbolic form.
18. Which system is adopted for transmission of electric power?
A.C system is suitable for transmission of electric power due to the technical
difficulties of D.C system. There is possibility of saving the copper conductor.
19. Why all transmission and distribution systems are 3 phase systems?
A 3 phase a.c circuit using the same size conductors as the single phase circuit can carry
three times the power which can be carried by a 1 phase circuit and uses 3 conductors for
the 2 phases and one conductor for the neutral. Thus a 3 phase circuit is more economical
than a 1 phase circuit in terms of initial cost as well as the losses. Therefore all
transmission and distribution systems are 3 phase systems.
20. Why the transmission systems are mostly overhead systems?
Because of the cost consideration, the transmission systems are mostly overhead systems.
21. Why all overhead lines use ACSR conductors?
ACSR conductors comprises of hard drawn aluminium wires stranded around a core of
single or multiple strand galvanized steel wire. They provides the necessary conductivity
while the steel provides the necessary mechanical strength. Has less corona loss. The
breaking load is high and has less weight.
22. Why transmission lines are 3 phase 3 wire circuits while distribution lines are 3 phase 4 wire
circuits?
A Balanced 3 phase circuit does not require the neutral conductor, as the instantaneous sum of
the 3 line currents are zero. Therefore the transmission lines and feeders are 3 phase 3
wire circuits. The distributors are 3 phase 4 wire circuits because a neutral wire is
necessary to supply the 1 phase loads of domestic and commercial consumers.
23.Why overhead line conductors are invariably stranded?
They are stranded to make them flexible during erection and while in service.
24 . Stat the advantages of interconnected systems
Any area fed from one generating station during overload hours can be fed from another
power station and thus reserved capacity required is reduced, reliability of supply is
increased and efficiency is increased.
25. What is a ring distributor?
A ring distributor is a distributor which is arranged to form a closed circuit and is fed at
one or more than one point.
UNIT-2
TRANSMISSION LINE PARAMETERS
1. What are line parameters?
The line parameters are resistance, inductance, capacitance and shunt combination of
conductance.
2. What is skin effect? Is it applicable to direct current also?
The tendency of alternating current to concentrate near the surface of the conductor is
known as skin effect. Skin effect is not applicable to D.C. because it is absent in the D.C. system.
D.C.current when flowing through the conductor is uniformly distributed over the whole cross
section of the conductor.
3. What is the effect of skin effect on the resistance of transmission line?
Skin effect increases the resistance of transmission line and decreases the current flowing
through the line.
4. Mention the factors that depend on the skin effect.
The factors that depend on the skin effect are:
i. Nature of material
ii. Diameter of wire
iii. Frequency
iv. Shape of wire
Skin effect is negligible when the frequency is less than 50Hz and conductor diameter is
less than 1 cm.
5. What is unsymmetrical spacing?
When three phase line conductors are not equidistant from each other the conductor
spacing is said to be unsymmetrical spacing.
6. What is symmetrical spacing?
When three phase line conductors are equidistant from each other the conductor spacing
is said to be symmetrical spacing.
7. Define mutual GMD.
Mutual GMD is defined as the distance from one end of the conductor to the other end.
8. Define self GMD.
Self GMD is defined as the limit of geometrical mean of the distance between all
the pairs of elements in that area as the number of elements increase without limit.
9. What is the necessity for a double circuit line?
The necessity for a double circuit line in over head transmission system is to reduce the
inductance per phase.
10. What is Bundle conductor?
A bundle conductor is a conductor made up of two or more Conductors.
11. Write a short note on proximity effect on conductors.
The alternating magnetic flux in a conductor caused by the current flowing in a
neighboring conductor gives rise to circulating current in the former which cause an apparent
increase in the resistance of the conductor this phenomenon is called proximity effect.
12. What is ACSR conductor?
ACSR stand for Aluminium Core Steel Reinforced. It combine the lightness, electrical
conductivity and restlessness of aluminium with the high tensile strength of steel are now
employed as over heat conductors of very long distance transmission lines.
13. What is resistance of a transmission line?
In a single phase, or two wires d.c. line the total resistance is known as loop resistance i.e.
double the resistance of either conductor. Unit ohm (_). In a three phase, resistance per phase is
the resistance of the conductor.
14. What is the need for transpositions of line conductors in a transmission system?
When the three phase line conductors have unsymmetrical spacing the flux linkages and
inductances of each phase are not the same which creates unequal voltage drops in three phases
even if the current in the conductor are same. To avoid the above action transposition is required.
15. What is transposition of conductors?
The transposition is defined as interchanging of position of the line conductors at regular
intervals along the line so that each conductor occupies the original position of every other
conductor over at equal distance. Such an exchange of positions is known as transposition.
16.State kelvin’s law.
The annual expenditure on the variable part of the transmission system should be
equal to the annual cost of energy wasted in the conductor used in that system.
17.State any two limitations of kelvin’s law.
It is difficult to estimate accurately the annual charge on the capital outlay. It does not give the
exact economical size of the conductor.
18.Define resistance of the transmission line.
It is defined as the loop resistance per unit length of the line in a single phase system. In 3 phase
system it is defined as the resistance per phase.
19.Define inductance of a line.
It is defined as the loop inductance per unit length of the line.Its unit is henrys per meter.
20.Define capacitance of a line.
It is defined as shunt capacitance between the two wires per unit line length. Its unit is farads
per meter.
21.What is skin effect?
The steady current when flowing through the conductor ,does not distribute uniformly, rather it
has the tendency to concentrate near the surface of the conductor. This phenomenon is
called skin effect.
22.Why skin effect is absent in dc system?
The steady current when flowing through a conductor distributes itself uniformly over the
whole cross section of the conductor.That is why skin effect is absent in dc system.
23.What is the effect of skin effect on the resistance of the line?
Due to skin effect the effective area of cross section of the conductor through which current
flow is reduced. Hence the resistance of the line is increased when ac current is flowing.
24.On what factors the skin effect depend?
Nature of the material, Diameter of the wire , Frequency and shape of the wire.
25.Define symmetrical spacing.
In 3 phase system when the line conductors are equidistant from each other then it is
called symmetrical spacing.
26.What is the necessity for a double circuit line?
To reduce the inductance per phase and to increase the efficiency.
UNIT-III
MODELLING AND PERFORMANCE OF TRANSMISSION LINES
1. Define regulation of a transmission line.
The difference in voltage at the receiving end of a transmission line from no load
to full load is called voltage regulation and is expressed as a percentage of the receiving
end voltage. % voltage regulation = (VS-VR)/VR*100 Where, VS - sending end voltage
VR - receiving end voltage
2. What are the units for the ABCD constants?
The limits of ABCD constants are:
A and D are dimension less.
B is expressed in ohm and C is expressed in mho.
3. Define ferrantic effect.
This phenomenon of rise in voltage at the receiving end of the open circuited or lightly
loaded line is called ferranti effect. VR>VS Where, VR – receiving end voltage VS – sending
end voltage
4. What is attenuation in a transmission line?
Attenuation is the power loss in transmission line. It is also called as transmission loss.
The difference between the sending end power and receiving end power.
5. Give the classification of transmission lines in terms of distances.
The classification of transmission lines in terms of distances is given by,
i. Short transmission lines <20kV upto 50km
ii. Medium transmission lines >20kV<100kV>50 upto150km
iii. Long transmission lines >100kV>150km
6. What is the cause of ferrantic effect?
A long transmission line has considerable shunt capacitance effect when such a line is
unloaded at the receiving it still draws a current from the generating end to charge the distributed
capacitance. This causes ferrantic effect.
7. Mention the methods used to control the voltage of transmission lines.
The methods used to control the voltage of transmission lines are:
i. By using over compound generator
ii. By excitation control
iii. By use of tap changing transformers
iv. Autotransformer tap changing
v. Booster transformer
vi. Induction regulator and by improvement of power factor
8. What is surge impedance loading?
Surge impedance loading of a power transmission line is the power delivered by a line to
a pure resistive load equal to its surge impedance.
9. What is 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. Transmission efficiency = Receiving end power
Sending end power
10. List any six voltage regulating equipments used in transmission system.
The six voltage regulating equipments used in transmission system are:
i. Synchronous motors.
ii. Tap changing transformer
iii. Series shunt capacitance
iv. Booster transformer
v. Compound generators
vi. Induction regulator
11. What is a power circle diagram?
A power circle diagram is a diagram drawn for the transmission line network involving
the generalized circuit constants, the sending end voltage VS and the receiving end voltage VR.
12. State the need for compensation in a transmission line.
The need for compensation in a transmission line are:
i. Improvement in system power factor which is an important aspect in achieving better
ii. economy in power distribution.
iii. Reduction in supply side reactive burden.
iv. To improve the power transfer capability
v. Reduction in investment per kW of a load supplied.
13. Give formula for surge impedance.
The formula for surge impedance is given by
Surge impedance, ZC = √L/C
Where, ZC – Surge impedance
L - Inductance
C – Capacitance.
14. Define corona.
Corona is defined as a self sustained electric discharge in which the field intensified
ionization is localized only over a portion of the distance between the electrodes.
Indications of corona are:
i. Hissing noise
ii. Ozone gas
iii. Bluish white glow
15. How is corona loss related with the diameter of the conductor?
Corona loss related with the diameter of the conductor by the following Way,
Corona loss α √r/d
Where, d - Diameter of the conductor.
Hence lower the diameter of the conductor, higher is the loss.
16. What is local corona?
The corona does not start simultaneously on the whole surface, but it takes place at
different points of the conductor which are pointed is known as local corona,
17.Define critical disruptive voltage.
It is defined as the minimum phase voltage at which corona occurs.
18.Define visual critical voltage.
It is the minimum phase voltage at which corona appears all along the line
conductors.
19.State any two merits of corona.
Reduces the effects of transients produced by surges.System performance is improved.
20.Give the two demerits of corona.
The transmission efficiency is affected. Corrosion occurs.
21.Mention the methods of reducing corona effect.
By increasing the conductor size and conductor spacing
22.Why ACSR conductors are used in lines?
If the size of the conductor is larger corona effects are reduced and reduces the
proximity effect.Hence they are used in lines.
23.Define medium lines.
Lines having length between 60 and 150 km and line voltages between 20 and 100kv are
called medium lines.
24.Define short lines.
Lines having lengths below 60km and voltages below 20kv are called short lines.
25.Mention the limitations of end condenser method.
This over estimates the effects of line capacitance.It is assumed to be lumped or
concentrated.
UNIT- 4
INSULATORS AND CABLES
1. What is the purpose of insulator?
The insulator provides necessary clearances against the highest voltage in worst
atmospheric conditions to which the line is likely to be subjected. The insulators also
prevent short circuiting between the different phases of the conductor and provide
necessary mechanical support for the line conductors.
2. What are the important properties that an overhead line insulator must posses?
The important properties that an overhead line insulator must posses are:
i. High mechanical strength
ii. High relative permittivity
iii. High insulation resistance
3. What is factor of safety?
The ratio of puncture strength to flash over voltage is termed as factor of safety.
Factor of safety = Puncture strength
Flash over voltage
4. Mention the main causes for failure of insulators.
The main causes for failure of insulators are:
i. Cracking of insulators
ii. Porosity of material
iii. Improper verification
iv. Flash-over
v. Mechanical stress
vi. Short circuits
vii. Deposition of dust
5. What is an Insulator?
Insulator is the element which provides necessary insulation between line conductor and
support. It prevents any leakage current from conductor to earth.
6. List the characteristics of insulators.
The characteristics of insulators used for overhead transmission are:
i. They should have high mechanical strength in order to withstand the load due to
weight of conductors
ii. They should have high relative permittivity in order to provide high dielectric strength.
7. Enumerate the different types of insulators used for overhead
transmission lines?
The different types of insulators used for overhead transmission lines are:
i. Pin type insulators
ii. Suspension type insulator
iii. Strain insulators
iv. Shackle insulators
8. What is a strain insulator and where is it used?
When there is a dead end or a corner or a sharper curve, the transmission lines is
subjected to a greater tension. The insulators which are used to release the excessive tension of
such transmission lines is called as strain insulators. For high voltage transmission lines the
strain insulators consisting of an assembly of suspension type insulators are used.
9. What is a shackle insulator?
When the low voltage transmission line meets a dead end or a corner or a sharper curve it
is subjected to a greater tension. The insulators which are used to relieve the low voltage line of
excessive tension are called shackle insulators.
10. Define string efficiency.
String efficiency is defined as the voltage across the whole string to the product of
number of discs and the voltage across the unit nearest to the conductor. String efficiency =
Voltage across the whole string Number of discs * voltage across the unit nearest to the
conductor
11. List the various methods for improving string efficiency.
The various methods for improving string efficiency are:
i. Longer cross - arms method
ii. Capacitance grading method
iii. Static shielding method or guard ring method
12. What is guard ring?
A guard ring is a metal ring electrically connected to the conductor and surrounding the
bottom insulator. It is used to equalize the potential distribution across each unit in a string.
13. How will you classify testing of insulators?
Testing of insulators can be classified into three types. They are,
i. Design tests
ii. Sample tests and iii. Routine tests
14. Distinguish between design and routine tests.
The difference between design and routine tests is that the design test is performed
ascertain the electrical and mechanical performance of a particular type of insulators. Whereas
the routine test is performed to weed out the defective insulators.
15. Mention the advantages of suspension type insulators over pin type insulators.
The advantages of suspension type insulators over pin type insulators are:
i. They are cheaper than pin type insulators beyond 11kV
ii. In the event of failure of an insulator one unit can be replaced instead of whole string.
iii. Each unit of suspension type insulators is designed for low voltage usually 11kV.
Depending upon the working voltage, the desired number of units can be connected in series.
16. What is a cable?
One or more conductors covered with insulating materials and surrounded by a protective
cover is called a cable.
17. Mention the advantages of underground cables over overhead lines.
The advantages of underground cables over overhead lines are
i. They reduce accidents caused by the breaking of the conductors
ii. They provide better general appearance
iii. They have small voltage drop across them.
18. What types of insulating materials are used in cables?
Some of the insulating materials used in the manufacture of cables are
i. Rubber
ii. Polyethylene
iii. Polyvinyl chloride
iv. Enamel
v. Gutta-percha
vi. Vulcanized Indian rubber
vii. Varnished cambric etc.
19.Why cables are not used for long distance transmission?
Cables are not used for long distance transmissions due to their large charging currents.
20.Mention the 3 main parts of the cable?
Conductor, dielectric, sheath.
21.What is the function of conductor?
Conductor provides the conducting path for the current.
22. What is the purpose of insulation in a cable?
The insulation or dielectric withstands the service voltage and isolates the conductor with
other objects.
23. What is the function of sheath in a cable?
The sheath does not allow the moisture to enter and protects the cable from all external
influences like chemical or electrochemical attack, fire etc.
24. Mention the conductor materials in cables?
Copper, Aluminum Al occupies greater space than copper for a given conductance. Both
are high electrical conductivity.
25. What is the purpose of stranding of conductors?
Stranding increases the resistance of the cable. It has flexibility.
26.Define the segmental conductors.
The stranded wires which are compacted by the rollers to minimize the air spaces between the
individual wires are called segmented conductors. Here the conductor size is reduced for
a given conductance.
27.State the properties of insulating materials.
It should have high insulation resistance, high dielectric strength, good mechanical properties,
non-hygroscopic, capable of being operated at high temperatures, low thermal resistance
and low power factor.
UNIT- 5
SUBSTATION, GROUNDING SYSTEM AND DISTRIBUTION SYSTEM
1. What is substation?
Substation is the assembly of apparatus used to change some characteristics of electric
supply.
2. Give the conditions of laying out a substation.
The conditions for laying out a substation are:
It should be located at a proper site.
It should provide safe and reliable arrangement.
It should be easily operated and maintained.
It should involve minimum capital cost.
3. Mention the classification of substation according to service.
According to service, substations are classified as,
Transformer substations
Switching substations
Power factor correction substations
Frequency changer substations
Converting substations
Industrial substations
4. List the classification of transformer substations.
Transformer substations are classified as,
Step-up substation
Primary grid substation
Secondary substation
Distribution substation
5. Define busbar.
Busbar is defined as a conductor to which a number of circuits are connected.
6. Give the materials mainly used in busbar?
The materials that are mainly used in busbar are:
Copper
Aluminium
7. What are the factors to be considered for busbar design?
The factors to be considered for busbar design are:
Material
Cross section of conductors
Temperature rise
Distance between phase conductors
Enclosure design
8. Which tests are necessary on station busbars?
The tests conducted on station busbars are:
Temperature rise test
Rated short time current test
Rated momentary current test
High voltage test
9. What is neutral grounding (or) neutral earthing?
The neutral point of star connected three phase winding of power transformers,
generators, motors, earthing transformers are connected to low resistance ground. Such a
connection is called neutral grounding (or) neutral earthing.
10. State the advantages of neutral grounding.
The advantages of neutral grounding are:
Arcing grounds are reduced or eliminated
The life of insulation is long
Reduced maintenance, repairs and breakdowns
Stable neutral point
Improved service reliability
Greater safety
11. Define earth resistance.
Earth resistance is defined as the resistance of the earthing electrode to the real earth and
is expressed in ohms. Earth resistance ER = V/I
Where,
V – Voltage between the electrode and the voltage spike
I – Injected current
12. What are the devices that are used for transferring D.C. power at the substation by using
converting machinery?
The devices that are used for transferring D.C. power at the substation by using
converting machinery are:
Mercury arc rectifier
Rotary converters and
Motor generator set
13. What is an interconnector?
The interconnector or inter connected network is a common development of simple ring
system to reduce the power loss and voltage drop.
14. What is an interconnected system?
Interconnected system is a system in which the feeder ring is energized by two or more
than two generating stations or substations.
15. Write the methods of earthing.
The methods of earthing are:
Pipe earthing
Plate earthing
16. Classify the connection schemes.
Classification of connection schemes are:
Radial system
Ring main system
Interconnected system
17. List the disadvantages of D.C. three wire distribution system.
Disadvantages of D.C. three wire distribution system are:
i. Three wires are required
ii. A balancer is required and as such the cost is increased
iii. The safety is partially reduced
18. Write the advantages of radial system.
The advantages of radial system are
i. The initial cost is low
ii. Useful when the generation is at low voltage.
iii. Preferred when the station is located at the centre of the load.
19. What is a ring distributor?
A ring distributor is a distributor which is arranged to form a closed circuit and is fed at
one or more than one point.
20. State any two advantages of ring main system.
Less voltage fluctuations at consumer’s terminals.Less copper is required as each part of
the ring carries less current than in radial system.
21.Mention the disadvantages of a 3 wire system
In 3 wire system a third wire is required.The safety is partially reduced.A balancer
is required and therefore cost is increased.
22.What are the advantages of a 3 wire dc distribution system over a 2 wire dc distribution
system?
If 3 wire system is used to transmit the same amount of power over the same
distance with same efficiency with same consumer voltage we require 0.3125 times
copper as required in 2 wire system.
23.Mention the differences between 3 wire and 3 phase 4 wire distribution system?
3 phase 3 wire is employed for balanced loads, but 3 phase 4 wire is
employed for unbalanced loads.3 phase 3 wire is used for transmission but 3 phase 4 wire is used
for distribution of power to consumers.
24.Mention the methods used for voltage control of lines
Tap changing auto- transformer, booster transformer , excitation control and induction
regulator.
25.What is sending end power circle diagram?
The circle drawn with sending end true and reactive power as the horizontal and
vertical co-ordinates are called sending end power circle diagram.
16 marks Questions:
Unit I
1. (i) Discuss various types of HVDC links.
(ii) List out the main components of a HVDC system.
2. (i) Draw and explain the structure of modern power systems with typical voltage levels.
(ii)What is the highest voltage level available in India?
3. (i) Explain the effect of high voltage on volume of copper and on efficiency.
(ii)Explain why the transmission lines are 3 phase 3-wire circuits while distribution lines are 3
phase 4-wire circuits.
4. (i) Draw the model power system with single line representation. Show its essential
constituent sections.
(ii) What are the AC transmission and distribution level voltages we have in India?
(iii) What are the different kinds of DC links? Draw relevant diagrams.
5. (i) Explain why EHV transmission is preferred? What are the problems involved in EHV AC
transmission?
(ii) With neat schematic, explain the principle of HVDC system operation.
6. Explain about FACTS with neat diagram
7. Explain TCSC and SVS systems
8.
Explain with neat diagram about STATCOM and UPFC
9. Compare EHVAC and HVDC transmission
10. Explain the applications of HVDC transmission system
Unit II
1. Derive an expression for inductance of a single phase transmission system.
2. Derive an expression for capacitances of a single phase transmission system and discuss the effect
of earth on capacitance with suitable equation.
3.
(i) Derive an expression for inductance of a single-phase overhead line.
(ii) A conductor is composed of seven identical copper strands each having a radius r. Find the
self-GMD of the conductor.
4. i) Derive an expression for the capacitance between conductors of a 1Ф overhead line.
ii) Find the capacitance between the conductors of a single-phase 10 km long line. The diameter
of each conductor is 1.213cm. The spacing between conductors is 1.25m. Also find the
capacitance of each conductor neutral.
5.
i) Derive the expression for inductance of a two wire 1Ф transmission line
ii) Derive the expression for capacitance of a 1Ф transmission line
6.
i)What are the advantages of bundled conductors?
ii) Derive the expression for capacitance of a double circuit line for hexagonal spacing.
iii) Why is the concept of self GMD is not applicable for capacitance?
7.
i) Explain clearly the skin effect and the proximity effects when referred to overhead lines.
ii) Write a short note on the inductive interference b/w power and communication lines.
8. Derive the expression for the capacitance per phase of the 3Ф double circuit line flat vertical
spacing with transposition.
9. A 3Ф overhead transmission line has its conductors arranged at the corners of an equilateral
triangle of 2m side. Calculate the capacitance of each line conductor per km. Given the
diameter of each conductor is 1.25cm.
10. Find the capacitance per km per phase of a 3Ф line arrangement in a horizontal plane spaced 8
metres apart. The height of all conductors above the earth is 13 metres. The diameter of each
conductor is 2.6 cm. the line is completely transposed and takes the effect of ground into
account.
Unit III
1. Determine the efficiency and regulation of a 3phase, 100Km, 50 Hz transmission line delivering
20 MW at a power factor of 0.8 lagging and 66 kV to a balanced load. The conductors are of
copper, each having resistance 0.1 Ω/ Km, 1.5 cm outside dia, spaced equilaterally 2 metres
between centres. Use nominal T method.
2. A three phase 5 km long transmission line, having resistance of 0.5 Ω / km and inductance of
1.76mH/km is delivering power at 0.8 pf lagging. The receiving end voltage is 32kV. If the
supply end voltage is 33 kV, 50 Hz, find line current, regulation and efficiency of the
transmission line.
3. Derive the expressions for sending end voltage in nominal T method and end Condenser method.
4. What is an equivalent p circuit of long line? Derive expression for parameters of this circuit in
terms of line parameters.
5. A 50Hz transmission line 300 km long total series impedance of 40+j25 Ω and total shunt
admittance of 10-3 mho. The 220 KV with 0.8 lagging power factor. Find the sending end voltage,
current, power and power factor using nominal П method.
6. Define regulation of a transmission line and derive the approximate expression for the regulation
of a short transmission line. What is corona loss? How do you determine this loss?
7. A 220kV, 3Ф transmission line has an impedance per phase of (40+j200) Ω and an admittance of
(0+j0.0015) mho. Determine the sending end voltage and sending end current when the receiving
end current is 200 A at 0.95 pf lagging. Use nominal T method. Determine the efficiency and
regulation of a three phase 200 km, 50Hz transmission line delivering 100MW at a pf of 0.8
lagging and 33kV to a balanced load. The conductors are of copper, each having resistance
0.1Ω/km & 1.5cm outside dia, spaced equilaterally 2m b/w centres. Neglect leakage reactance.
(Use nominal T and П methods)
8. Explain the Ferranti effect with a phasor diagram and its causes.
9. Explain the classification of lines based on their length of transmission.
10. What are ABCD constants.
Unit IV
1. Discuss two methods to increase the value of string efficiency, with suitable sketches.
2. Explain any two methods of grading of cables with necessary diagrams.
3. (i)What are different methods to improve string efficiency of an insulator?
(ii) In a 3-unit insulator, the joint to tower capacitance is 20% of the capacitance of each unit. By
how much should the capacitance of the lowest unit be increased to get a string efficiency of
90%. The remaining two units are left unchanged. (i)Derive the expression for insulator
resistance, capacitance and electric stress in a single core cable. Where is the stress maximum and
minimum?
(ii) A single core 66kv cable working on 3-phase system has a conductor diameter of 2cm and
sheath of inside diameter 5.3cm. If two inner sheaths are introduced in such a way that the stress
varies between the same maximum and minimum in the three layers find:
a) position of inner sheaths
b) voltage on the linear sheaths
c) maximum and minimum stress
(i)Draw the schematic diagram of a pin type insulator and explain its function
(ii) A 3 phase overhead transmission line is being supported by three disc insulators. The
potential across top unit (i.e. near the tower) and the middle unit are 8kV and 11kV respectively.
Calculate,
a) The ratio of capacitance b/w pin and earth to the self capacitance of each unit
b) Line Voltage
c) String Efficiency
(i) Describe with the neat sketch, the construction of a 3 core belted type cable.
(ii) A conductor of 1cm diameter passes centrally through porcelain cylinder of internal diameter
2 cms and external diameter 7cms. The cylinder is surrounded by a tightly fitting metal sheath.
The permittivity of porcelain is 5and the peak voltage gradient in air must not exceed 34kV/cm.
Determine the maximum safe working voltage.
4. (i) What are the various properties of insulators? Also briefly explain about suspension type
insulators.
(ii) Calculate the most economical diameter of a single core cable to be used on 132kV, 3 phase
system. Find also the overall diameter of the insulation, if the peak permissible stress does not
exceed 60kV/cm. also derive the formula used here
5. (i) Briefly explain about various types of cables used in underground system.
6. (ii) A string of 4 insulator units has a self capacitance equal to 4 times the pin to earth
capacitance. Calculate,
a) Voltage distribution as a % of total voltage
b) String efficiency
7. (i) Give any six properties of a good insulator.
8.
(ii) With a neat diagram, explain the strain and stay insulators.
9.
(iii) A cable is graded with three dielectrics of permittivity 4, 3 and 2. The maximum
permissible potential gradient for all dielectrics is same and equal to 30 kV/cm. The core
diameter is 1.5cm and sheath diameter is 5.5cm.
10.
(i) Explain the constructional features of one LT and HT cable
(ii) Compare and contrast overhead lines and underground cables
Unit V
1.With a neat sketch explain double bus with double breaker and double bus with single breaker.
State their advantages and disadvantages.
2.Explain the following:
Neutral grounding
3. Resistance grounding.
4.Explain about the various types of substations
5.Write short notes on Sub mains
6. Stepped and tapered mains
7.Explain the substation bus schemes.
8.Write short notes on Busbar arrangement in substation
9. Grounding grids
10.(i) Explain the design principles of substation grounding system.
(ii) Explain the equipments in a transformer substation.