Transmission and Distribution
DHANALAKSHMI COLLEGE OF ENGINEERING, CHENNAI – 301.
Sub: Transmission and Distribution
Branch: EEE
UNIT I
Code: EE2303
Sem: V
INTRODUCTION
PART A
1. What are the components of a power system?
The components of power systems are
1. generating stations
2. step up transformer stations
3. transmission lines
4. switching stations
5. step down transformer stations
6. primary distribution lines
7. service transformer banks
8. secondary distribution lines
2. What is meant by transmission and distribution system?
A large network which is used to deliver bulk power from power stations to the
load centers and large industrial consumers is called distribution system.
3. What are the transmission level voltages we have in India?
Primary transmission level voltage is 132 KV, 220KV or 400KV and secondary
transmission level voltage is 33KV or 66KV.
4. What are the various levels of generation in India?
3.3KV, 6.6KV, 11KV or 33KV.
5. What are the various levels of primary distribution in India?
11KV, 6.6KV or 3.3KV.
6. What is the usable voltage for secondary distribution?
415V & 240 V (415 volts for 3-phase loads and 240 volts for 1-phase loads)
7. What is a one-line diagram?
Schematic representation of the elements of electric power system is called as one
line diagram.
8. What is meant by primary and secondary transmission?
Transmission of electric power at 132KV by 3 phase 3 wire overhead system is
known as secondary transmission.
Transmission of electric power at 33KV by 3 phase 3 wire overhead system is
known as secondary transmission.
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9. What is meant by primary and secondary distributions?
The secondary transmission lines terminates at the substations where voltage is
reduced from 333KV to 11KV lines which run along the road sides of the city forms the
primary distribution.
A primary distribution line terminates at the distributing substations where voltage
is reduced from 11KV to 400 volts. Thus 3 phase 4 wire system which connect the
distributing substation and the consumer point forms the secondary distribution.
10. Distinguish between a feeder and a distributor.
SI.NO
1. a)
b)
Feeder
Distributor
Feeders are conductors or
transmission lines which carry
current from the stations to the
feeding points.
Feeders terminate into distributors
No tapping is taken from the
feeders.
2.
Current carrying capacity plays a
major role in designing a feeder.
So distributor is also a conductor from
which current is tapped off for the
supply to the consumer.
Whereas voltage drop plays a major role
in designing a distributor.
3.
Current loading remains the same
along its length.
Current loading factor varies along its
length.
11. Define the term distributor?
Distributor is a conductor from which current is tapped off for the supply to the
consumers. Feeders terminate into distributor.
12. What is a feeder? (Nov 2012)
Feeder is a conductor or transmission line which transmits current from the
generating stations to different distributing substations.
13. Why is electrical power preferably to be transmitted at a high voltage?
Electrical power is transmitted at high voltage because,
1.) It reduces the volume of conductor material used.
2.) It increases transmission efficiency.
3.) It decreases line drop.
14. What are the advantages of the HVDC transmission system over HVAC
transmission?
Advantages of HVDC transmission are
1.) It requires only two conductors as compared to three for ac transmission.
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2.) There is no inductance, capacitance, phase displacement and surge problems
in DC transmission.
3.) Due to the absence of inductance, the voltage drop in a DC transmission line
is less than AC line for the same load and sending end voltage. Hence DC
transmission has a better voltage regulation.
4.) There is no skin effect in a DC system. Therefore entire cress section of
conductor is utilized.
5.) For the same working voltage the potential stress on the insulation is less in
case of DC system than that in AC system. Therefore DC line requires less
insulation.
6.) DC line has less corona loss and reduced interference with communication
circuits.
7.) HVDC transmission line is free from dielectric loss, particularly in the case of
cables.
8.) No stabilizer is required for HVDC transmission over long distances.
15. What are the demerits of HVDC transmission?
1.) Electric power cannot be generated at high DC voltages
2.) The DC voltages cannot be stepped up for transmission of power at high
voltages.
3.) The DC switches and circuit breakers have their own limitations.
16. What are the disadvantages high voltage AC transmissions?
1.) An AC line requires more copper than a DC line
2.) The construction of an AC line is more complicated than a DC transmission
line.
3.) Due to skin effect in the ac system the effective resistance of the line is
increased.
4.) An AC line has capacitance. Therefore there is a continuous loss of power
due to charging current even when the line is open.
17. What are terminal equipments necessary in HVDC system?
The terminal equipments necessary in HVDC system are converters, inverters
mercury are valves, thyristors etc.
18. 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.
19. State 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.
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20. Mention the limitations of using very high transmission voltage.
The increased cost of insulation of conductors. The increased cost of transformers
switches gears and other terminal apparatus.
21. Mention the equipments that supply reactive power in HVDC converter stations?
AC filters
Static shunt capacitors
Synchronous condensers
StaticVAR compensators.
22. Why DC transmission is economical and preferable over AC transmission for large
distances only?
Because with larger distances, the saving in cost of DC overhead lines become
greater than the additional expenditure on terminal equipment.
23. Why is voltage regulation better in case of DC transmission?
Because of absence of inductance in DC systems.
24. What are the advantages of adopting EHV/UHV for transmission of AC electric
power?
-Reduced line losses
-High transmission efficiency
-Improved voltage regulation
-Reduced conductor material requirement
-Flexibility for future system growth
-increase in transmission capacity of the line
-increase of SIL.
25. Mention the problems associated with an EHV transmission?
The problems associated with EHV transmission are corona loss and radio
interference, requirements of heavy supporting structures erection difficulties and
insulation requirements.
26. What for series and shunt compensation provided in EHV lines?
Series compensation is provided to reduce the series reactance of the line so as
to improve stability, voltage regulation and transmission efficiency. Shunt
compensation is provided to reduce the line susceptance so as to improve the voltage
regulation under light load condition.
27. What is the voltage that has been selected for HVDC transmission?
± 500 KV.
28. What is FACTS?
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Flexible Alternating Current Transmission System is alternating current
transmission systems incorporating power electronic based and other static controllers to
enhance controllability and increase power transfer capability
29. State the IEEE definition for TCSC
A capacitive reactance compensator which consists of a series capacitor bank
shunted by a Thyristor-controlled reactor in order to provide smoothly variable series
capacitive reactance.
30. State the IEEE definition for SVC
A shunt connected static var generator or absorber whose output is adjusted to
exchange capacitive or inductive current so as to maintain or control specific
parameters of the electric power system (bus voltages)
31. State the IEEE definition for STATCOM
A static synchronous compensator is a static synchronous generator operated
as a shunt connected static var compensator whose capacitive or inductive output
current can be controlled independent of the AC system voltage.
32. State the IEEE definition for UPFC
Unified Power Flow Controller is a combination of static synchronous
compensator (STATCOM) and a static series compensator (SSSC) which are coupled via
a common DC link, to allow bidirectional flow of real power between the series output
terminals of the SSSC and the shunt output terminals of the STATCOM, and are
controlled to provide concurrent real and reactive series line compensation without an
external electric energy source
33. What is meant by sag?
The difference in level between points of supports and the lowest point on the
conductor is called sag.
34. Give any two factors that affect sag in an overhead line. (Nov 2012)
Wind and Ice loading
35. What are the advantages of high voltage AC transmission? (Nov’ 2011)
1.) The power can be generated at high voltages.
2.) The maintenance of ac substations is easy and cheaper.
3.) The ac voltage can be stepped up or stepped down by transformer with ease
and efficiency. This permits to transmit power at high voltages and distribute
it at safe potentials.
36. What is meant by string chart? (Nov’2011)
The curves of tension and sag vs temperature is called string chart.
37. What is a feeder?
(Nov 2012)
Feeder is a conductor or transmission line which transmits current from the generating stations to
different distributing substations
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PART B
1. Draw the layout of modern system and explain. What is the highest voltage level
available in India for EHV transmission?
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38. Give the advantages, disadvantages and applications of HVDC transmission
system.
(Nov 2012)
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APPLICATIONS OF HVDC SYSTEM:
The controllability of current-flow through HVDC rectifiers and inverters, their
application in connecting unsynchronized networks, and their applications in efficient
submarine cables mean that HVDC cables are often used at national boundaries for
the exchange of power (in North America, HVDC connections divide much of
Canada and the United States into several electrical regions that cross national
borders, although the purpose of these connections is still to connect unsynchronized
AC grids to each other). Offshore windfarms also require undersea cables, and their
turbines are unsynchronized. In very long-distance connections between just two
points, for example power transmission from a large hydroelectric power plant at a
remote site to an urban area, it is of great interest and several schemes of these kind
were built. For interconnections to Siberia, Canada, and the Scandinavian North, it
may be as result of decreased line-costs of HVDC make also of interest, but however
no such interconnection was realized as inverters are expensive, see List of HVDC
projects.
2. Explain how choice of voltage becomes a major factor in the line design.
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3. Compare EHVAC & HVDC
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Sl.
No
1.
HVDC / EHVDC
2.
Power transmitted per conductor is
more ( same amount of power is
transmitted with lesser no. of
conductors )
There is no inductance, and surge
problems, thus voltage drop is less
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Line construction is simple and cheaper
HVAC / EHVAC
Line construction is complex, cost of conductors and towers
are more
To transmit same amount of power, this system requires more
number of conductors.
Due to the presence of inductance, the voltage drop is more
for the same load and sending end voltage.
Entire cross section of the conductor is Due to skin effect, the current tends to concentrate on the
utilised.
surface of the conductor
Corona loss is less
Corona loss is proportional to the frequency. Hence AC this
loss is more
Require less insulation
The potential stress on the insulation is more for the same
working voltage, hence more insulation is required.
Radio interference is lless
There is no stability problems and
synchronizing difficulties
Electric power cannot be generated at
high DC voltage
The DC voltage cannot be stepped up or
stepped down
Contribution of DC link to the short
circuit current is less
Converters required at both end of the
line, they are very expensive
Converters absorb the reactive power
which must be supplied locally
Power control is fast and accurate as
there is less inertia due to the absent of
rotating synchronous machines
DC line is restricted to point to point
transmission
Unlimited
power
can
be
tranmitted
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Radio interference is more
Stability problem exist
Electric power can be generated at high DC voltage
can be stepped up or stepped down by using transformer
In AC system short circuit current is more
There is no investment on converters
No such problems in Ac transmission
Power control is slow
No such restriction with AC
Loading of the line is limited by transient stability limit. Thus
there is always under loading in the lines & the conductors are
not fully utilized.
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4. Deduce an approximate expression for sag in overhead lines when
(i)
supports are at equal levels. (Nov 2012)
(ii)
Supports are at unequal levels
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5. What is sag in overhead lines? Discuss the disadvantages of providing too small
or too large sag on a line
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6. An overhead transmission line conductor having parabolic configuration weighs
1.925 kg per metre of length. The area of X-section of the conductor is 2.2 cm2
and the ultimate strength is 8000 kg/cm2. The supports are 600m apart having
15m difference of levels. Calculate the sag from the taller of the two supports
which must be allowed so that the factor of safety shall be 5. Assume that ice
load is 1 kg per metre run and there is no wind pressure.
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39. What are the various types of HVDC links? Explain them in detail. (Nov’2011)
& (Nov 2012)
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7. Explain the principle of operation of compensators used for voltage control.
(Nov’2011)
(i)
Using shunt reactors
(ii)
shunt capacitors (iii)
series
capacitors (iv) static shunt compensation (synchronous condenser)
(v) thyristor controlled shunt compensation
Principle of operation
VR = Vs= I ( XL-XC)
XL=2πfL
XC=1/2πfC
8. A transmission line conductor at a river crossing is supported from two towers at a
height of 50 and 80 metres above water level. The horizontal distance between the
towers is 300 metres. If the tension in the conductor is 2000 kg, find the clearance
between the conductor and water at a point midway between the towers. Weight of
conductor per metre=0.844 kg. Derive the formula. (Nov’2012)
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l ch

 150  237  87m
2 l
l ch
b 
 387 m
2 l
D  11.85m
D2  31.6m
a
D2  D  19.75m
clearance  60.25m above water
x2
2c
a2
cd 
 1
2c
b2
cd h 
  2
2c
  l  2a 
1   2   h 
2c
l ch
l ch
a   &b  
2 l
2 l
y=c+
9.
A transmission line conductor at a river crossing is supported from two towers at a height of 50
and 80 metres above water level. The horizontal distance between the towers is 300 metres. If the
tension in the conductor is 2000 kg, find the clearance between the conductor and water at a point
midway between the towers. Weight of conductor per metre=0.844 kg. Derive the formula.
(Nov’2012)
=150 -237= -87m,
, D=11.85m, clearance =60.25m above water
level
,
,
,
,
, b
10.
Give the advantages, disadvantages and applications of HVDC transmission system.
(Nov 2012) (8)
Advantages
(i) Economical for bulk transmission of power. (ii) Cost of conductor reduces (iii) Lower
transmission losses (iv) No skin effect (v) Easy reversibility and controllability (vi)
Intermediate substations are not required (vii) Shunt compensation is not required
Disadvantages
(4)
(i) Costly terminal equipment (ii) more maintenance (iii) circuit breaking in multi terminal dc
system (iv) voltage transformation
Applications
(4)
(i) Easy to interconnect two different frequency system
11.
Deduce an approximate expression for sag in overhead lines when (Nov 2012)
(i)
supports are at equal levels.
(8)
When supports are at equal level
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(ii) When the supports are at unequal level
(8)
12. i) Briefly discuss about static VAR compensation ii) Explain break even distance (May 2012) (12)
Break even distance
(4)
The break-even distance is much smaller for submarine cables (typically about 50 km)
than for an overhead line transmission. The distance depends on several factors, as transmission
medium, different local aspects (permits, cost of local labour, etc)
UNIT II
TRANSMISSION LINE PARAMETERS
PART A
1. What are the primary constants of transmission lines?
(or)
What are line parameters?
Resistance, inductance, capacitance and conductance distributed uniformly
along the length of the line are called constants or parameters of transmission line.
2. Define resistance of transmission line?
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Resistance of transmission line in a single phase is defined as the loop
resistance per unit length of line. (Loop resistance is nothing but the sum of
resistances of both the wires for unit line length)
In a 3 phase it is defined as the resistance per phase. (ie) resistance of
one conductor
3. Define inductance of transmission line. Give its unit.
Inductance is defined as loop inductance per unit length of line (loop
inductance is the sum of inductances of both the wires for unit line length).
Its unit is henry per meter.
4. Define capacitance of transmission line.
Capacitance is defined as shunt capacitance between the two wires per unit
line length. (or) The capacitance between the conductors in a transmission line
is the charge per unit potential difference.
Its unit is farad per meter.
5. What is skin effect? Is it applicable to DC current also? (Nov 2012)
An alternating 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.
It is not applicable to DC current.
6. What is the effect of skin effect on the resistance of transmission line?
Due to skin effect the effective area of cross section of the conductor through
which current flows is reduced. Consequently the resistance of line is
increased when carrying an alternating current.
7. What is the cause of skin effect?
A solid conductor may consist of large number of strands, each carrying a
small portion of the total current. The inductance of the individual strands will
vary according to their positions. Thus the strands near the centre are
surrounded by a greater magnetic flux and hence have a larger inductance than
that near the surface. The presence of high reactance near the centre causes the
alternating current to flow near the surface resulting in skin effect.
8. On what factors does the skin effect depend?
The skin effect depends upon the following factors:1.) nature of material
2.) diameter o wire
3.) frequency and
4.) shape of wire
9. Give an expression for the loop inductance of a single phase, two wire system.
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Loop inductance
d

= 10 7  r  2 ln 
r

r = relatively permeability of the material
d =Distance between two conductors
r =radius of the first conductor.
10. How inductance and capacitance of a transmission line are affected by the spacing
between the conductors?
If the conductors of a 3 phase transmission line are not equidistant from each
other the flux linkages, inductances and capacitances of various phases are not
different. This cause’s unequal voltage drops in the three phases and transfer
of power between phases due to mutual inductance even if the currents in the
conductors are balanced. Thus spacing between the conductors play a major
role in overhead transmission.
11. Write an expression for the inductance of each conductor for a 3 phase overhead
transmission line in which the conductors are unsymmetrical spaced but transposed.
If the current carrying conductors A,B,C are spaced asymmetrically and are
transposed to avoid the unbalancing effect then the inductance of each conductor
for a 3 phase overhead transmission line is

3 d d d
= 0.5  2 ln  1 2 3

r



 10 7 H / m


Where d1,d2,d3 are the distances between the conductors
r- radius of the conductors
12. What is the necessity for a double circuit line?
The necessity for a double circuit line in overhead transmission system is to
reduce the inductance perhaps.
13. Distinguish between GMD and GMR.
S.NO GMD(Dm)
1.)
GMD is also called as mutual GMD
2.)
GMD is defined as the geometrical
mean of the distances from one end of
the conductor to the other end. (i.e.
between the largest and smallest)
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GMR(Ds)
GMR is also called as self GMD
GMR is defined as the limit of
geometric mean of distances
between all the pairs of elements
in that area as the number of
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3.)
4.)
5.)
6.)
Mutual GMD depends only upon the
spacing and is independent of the exact
size, shape, orientation of the
conductor.
For a single phase line Dm=spacing
between conductors=D.
For a single circuit 3phi line
Dm=(d1*d2*d3)1/3
For a double circuit 3phi line Dm=(DAB*DBC*DCA)
elements increase without limit
Self GMD of a conductor depends
upon the size and shape of the
conductor and is independent of
spacing between the conductors.
For a single phase line Ds=0.7788r
For a single circuit 3 phi line
Ds=0.7788r
For a double circuit 3phi line
Ds=(DS1*DS2*DS3)
14. Write an expression for electric potential at a charged single conductor?
Electric potential at a charged single conductor “A” is

VA 
QA dx
2 o r x
Where QA=charge per meter length
€o° = permittivity of free space
r = radius of the conductor
x = distance
15. Write an expression for electric potential at a conductor in a group of charged
conductors?
Let A,B,C etc be the group of conductors operating at potentials such that
charges QA;Qn;Qc etc coulomb per metre length.
VA 
1
2 o


1
1
1
QA ln  QB ln  Qc ln  .....
r
d1
d2


Where r-radius of the conductor A
d1,d2……… -distance between the conductor A and other conductor B,C
etc.
o- Permittivity of free space.
16. Explain proximity effect on conductors.
The alternating magnetic flux in a conductor caused by the current flowing in
a neighboring conductor gives rise to circulating currents which cause an
apparent increase in the resistance of a conductor. This phenomenon is called
proximity effect.
17. What is the effect of proximity effect?
Proximity effect results in
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5.) the non uniform distribution of current in the cross section
6.) the increase of resistance
18. What is ACSR conductor?
ACSR conductor is an aluminum conductor with a steel core reinforced. It
consists of central core of galvanized steel strand surrounded by a number of
aluminum strands.
ACSR is a composite conductor which combines the lightness, electrical
conductivity and rustle ness of aluminum with the high tensil strength and has
a larger diameter. So to minimize the conona losses they are now used as
overhead conductors in the long distance transmission lines.
19. What is a composite conductor?
A conductor which operates at high voltages and composes of two or more
elements or strands, electrically in parallel is called as a composite conductor.
20. What is bundle conductor?
A bundle conductor is a conductor made up of two or more sub conductors
and is used as one phase conductors.
21. What are the advantages of using bundled conductors?
The advantages of using bundled conductors are
7.) reduced reactance
8.) reduced voltage gradient
9.) reduced corona loss
10.) reduced radio interference
11.) reduced surge impedance
22. Define symmetrical spacing.
In 3 phase system when the line conductors are equidistant from each other
then it is called symmetrical spacing.
23. Mention the sources of audible noise generation in EHV transmission systems?
Corona
Humming of transformers
Cooking Systems
Mechanical and Electrical auxiliaries
24. What is the need of transposition? (Nov’2011)
When three phase line conductors have unsymmetrical spacing the flux linkages and
inductances of each phase are not the same. This results in the unequal voltage drops in
the three phases even if the currents in the conductors are balanced. Therefore the voltage
at the receiving end will not be the same for all phases. To avoid the unbalancing effect
the positions of the line conductors are interchanged at regular intervals along the line so
that each conductor occupies the original position of every other conductor over an equal
distance. This exchanging of positions of conductors is called transposition.
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25. Define the term critical disruptive voltage? (Nov’2011)
The potential difference between conductors, at which the electric field intensity at
the surface of the conductor exceeds the critical value and corona occurs is known as
critical disruptive voltage.
26. What is skin effect? Is it applicable to DC current also?
(Nov 2012)
An alternating 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.It is not applicable to DC current.
27. Write the expression for a capacitance of a single-phase transmission line. (Nov 2012)
Capacitance per unit length between the conductors C AB 
Capacitance between line and neutral conductors C AN 
0
D
ln  
r
20
D
ln  
r
F /m
F /m
PART B
1. Derive an expression for the capacitance of a 3-phase line with equilateral
spacing
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2. Derive the capacitance of single phase two wire line taking earth’s effect into
account.
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3. Derive from first principles, the capacitance of a single phase overhead line.
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4. Derive the expression for the inductance of each line when the conductors are
unsymmetrical placed.
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5. Derive an expression to find the loop inductance of single phase overhead
transmission line.
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6. Explain in detail the theory of corona formation. Give its advantages and
disadvantages. And the expression of power loss. (Nov 2012)
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7. Derive from the fundamentals, an expression for critical disruptive voltage.
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8. Write short notes on the following: (i) STATCOM (ii) UPFC
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9. Explain the various factors affecting the corona loss. (Nov’2011)
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10. Find the inductance per phase per km of double circuit 3 phase line shown in fig.
The line is completely transposed and operated at a frequency of 50 Hz. r =6mm.
(Nov’2011)
5m
A
C’
3m
B
B’
6m
3m
C
5m
A’
Solution:
7. Derive an expression for capacitance of 3-phase un symmetrically spaced transmission line. (Nov
2012)
8. Explain the following with respect to corona (i) Corona (ii) effect (iii) disruptive critical voltage
(iv)visual critical voltage (v) corona power loss
(Nov2012)
(i) Corona effect
(4)
The Phenomenon of violet glow; hissing noise and production of ozone gas in an over head
transmission line is known as corona effect
r '  6  0.7788  4.67mm
Ds  3 Ds1 Ds 2 Ds 3
Ds1  4 Daa Daa ' Da ' a ' Da ' a  Ds 3
Ds  0.1828m
Dm  3 DAB DBC DCA  3 4.36  4.36  5.48  4.71m
Inductance per phase=0.2
Dm
 0.65mH / km
Ds
9. Find the inductance per Km of a three phase three wire transmission system consisting of 2 cm
diameter conductors spaced 4 m apart in horizontal plane. The conductors are regularly transposed.
(May 2012)
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5.03m
Inductance /phase/km
= 12.9X10-3 H
10. Find the capacitance between the conductors of a single phase 10 Km long line. The diameter of each
conductor is 1.213 cm. The spacing between the conductor is 1.25 m.
(May 2012)
-11
Capacitance of the line =
=0.5217X10 F
UNIT III
MODELLING AND PERFORMANCE OF TRANSMISSION LINES
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PART A
1. Give the lengthwise classification of transmission lines.
Transmission lines are classified as
1.) short transmission lines (length <80 km)
2.) medium transmission lines (80km<length < 250km)
3.) long transmission lines (length > 250 km)
2. Define regulation of a transmission line. (Nov 2012)
Regulation of a transmission line is defined as the change in voltage at the
receiving end when full load is thrown off the sending end voltage remaining
the same.
It is usually expressed as a percentage of receiving end voltage
%regulation 
VR'  VR
100
VR
Where VR' ’- no load voltage at the receiving end
VR- receiving end voltage
3. Define efficiency of a transmission line.
Efficiency of a transmission line is defined as the ratio of power received to
the power sent.

Power delivered
Power sent out
100 
VR I RCosR
100
Vs I sCoss
Where VR, IR, CosR are the receiving end voltage, current and power factor
respectively.
Vs, Is, Coss are the sending end voltage, current and power factor
respectively.
4. Explain the influence of power factor on the regulation of a transmission line.
1.) when the load PF ( cosR ) is lagging or unity or leading that IR cosR >
IXL sin R then voltage regulation is positive (receiving end voltage is lesser
than the sending end voltage) and increases with the decrease in power factor
for lagging loads (for a given VR and I.
2.) when the load PF is leading to this extent that IR cosR < IXL sin R
the voltage regulation is negative and decreases with the decrease in PF for
leading loads (for a given VR and I)
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5. Under what circumstances, the receiving end voltage may be higher than that of
the sending end?
When load PF cosR
is leading, IXL sin R >IR cosR
then
regulation is negative (i.e.). the receiving end voltage may be higher than that
of the sending end.
Where I – load current
XL -loop reactance
cosR - receiving end power factor(leading)
6. Explain how capacitance effects are taken into account in medium
transmission lines.
Medium transmission lines have sufficient length (80-250km) and operate at
voltages greater than 20kv. In such lines the capacitive current is appreciable
and hence cannot be neglected. So to obtain reasonable accuracy the effects of
capacitance must be taken into account.
7. What are the methods that are used for obtaining the performance
calculations of medium lines?
The methods that are used for obtaining the performance calculation of
medium lines are
1.) end condenser method
2.) nominal T method
3.) nominal  method
8. What is the difference between nominal T and nominal  configuration?
S.NO
Nominal T
1.)
In this the whole line capacitance is
assumed to be concentrated at the
middle point of the line and half the
line resistance and reactance are
lumped on its either side
Full charging current flows over half
2.)
the line
3.)
T-equivalent circuit
Nominal 
In this the whole line capacitance is
assumed to be divided into two
halves, one half being connected at
the receiving end and other half at
the receiving end.
Capacitance at the receiving end
has no effect on the line drop. But
the charging current of the second
half capacitance is added to obtain
the total sending current
-equivalent circuit
9. What are the limitations of nominal T and  methods in transmission lines
problems?
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Generally the capacitance is uniformly distributed over the entire length of the
line. But for easy calculations in nominal T and  the capacitance is concentrated
at one or two points also in nominal  method the capacitance connected in the
load side has no effect on voltage drop. Due to all these there may be considerable
error in calculation.
10. How the capacitance effects are taken into account in a long transmission
line?
Long transmission lines have sufficient length and operate at voltage higher
than 100kv the effects of capacitance cannot be neglected. Therefore in order
to obtain reasonable accuracy in long transmission lines calculations, the
capacitance effects must be taken into account.
11. What is surge impedance?
The square root of the ration of line impedance (Z) and shunt admittance(Y) is
called the surge impedance (Z) of the line.
12. Define surge impedance loading or natural power of the line?
Surge impedance loading is defined as the load of unity PF that can be
delivered by the line of negligible resistance.
V2
PSIL  RL
Zo
Where
2
-line voltage at the receiving end
VRL
Zo-surge impedance in ohms
PSIL-surge impedance loading.
13. What are the ABCD constants?
ABCD constants are generalized circuit constants of a transmission line. They
are usually complex numbers. Input voltage and current are expressed in terms
of output voltage and current. The constants A and D are dimensionless B and
C are ohms and mhos respectively.
14. What is a power circle diagram?
A power circle diagram is a diagram drawn for the transmission line network
involving the generalized circuit constants and the sending end voltage Vs and
receiving end voltage VR.
15. What is the use of power circle diagram?
Power circle diagram is used to determine the maximum power that can be
transmitted over the line both at the receiving end and sending end.
16. Define attenuation in a transmission lines?
Attenuation is defined as the power loss in line. It is nothing but the
transmission loss (i.e.). the difference between the sending end power and
receiving end power.
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17. What is steady state stability limit?
Steady state stability limit is the maximum flow of power through a particular
point of power system without loss of stability when the power is increased
very gradually.
18. Define critical disruptive voltage.
It is the minimum phase to neutral voltage at which corona occurs
19. Define visual critical voltages
Visual critical voltage is defined as the min. phase neutral voltage at which
corona glow appears all along the line conductors
20. Write an expression for the power loss due to corona.
 f  25  r

V  V c  2 X 10-5 KW/km/ph
P  242.2

   d
where f - supply frequency Hz
V – phase to neutral r.m.s voltage in kV
Vc – critical disruptive voltage (r.m.s) per phase
21. What are the units for A,B,C and D in the ABCD parameters?
A and D are dimensionless B and C are ohms and mhos respectively.
22. What are the voltages regulating equipments used in transmission system?
Synchronous motors, tap changing transformers, series shut capacitors,
booster transformers, compound generators, induction regulator.
23. Differentiate between voltage stability and rotor angle stability.
Voltage stability:
-It means load stability.
-It is mainly related to reactive power transfer.
-Here problems arise mainly in the event of faults.
Rotor angle stability:
-It means basically generator stability.
-It is mainly interlinked to real power transfer.
-Here problems arise during and after faults.
24. What are the methods used for voltage control of lines?
The methods used for voltage control of lines are
 by using over compound generator
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





by excitation control
by use of tap changing transformers
auto-transformer tap changing
booster transformer
induction regulator and
by improvement of power factor.
25. What is Ferranti effect? (Nov’2011)
The phenomenon of rise in voltage at the receiving end of the lightly loaded or
unloaded line is called as Ferranti’s effect.
26. Distinguish between attenuation and phase constant. (Nov’2011)
In long transmission line,
Z
Characteri stics impedance Z c 
Y
Propagatio n constant   ZY    j
  Attenuatio n constant
  Phase constant
PART B
1. Deduce the expression for (a) %regulation (b) ABCD parameters of a
short transmission line.
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2. Two transmission line with generalized constants A1B1C1D1 and A2B2C2D2 are in
parallel. Obtain the overall constants.
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3. Two transmission line with generalized constants A1B1C1D1 and A2B2C2D2 are in
series. Obtain the overall constants.
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4. Deduce the expression for (a) %regulation (b) ABCD parameters of a medium
transmission line represented in nominal  configuration.
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5. Deduce the expression for (a) %regulation (b) ABCD parameters of a medium
transmission line represented in nominal T configuration.
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6. A balanced 3-phase load of 30MW is supplied at 132kV, 50Hz and 0.85pf.
lagging by means of a transmission line. The series impedance of a single
conductor is (20+j52) ohms and the total phase-neutral admittance is 315X10-6
siemen. Using nominal T method, determinei) the A, B, C and D constants of
the line (ii) sending end voltage (iii) regulation of the line..
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7. A132kV, 50Hz, 3-phase transmission line delivers a load of 50MW at 0.8p.f.
lagging at the receiving end. The generalized constants of the transmission line
are: A=D=0.951..4deg ; B=9678deg; C=0.001590deg.Find the regulation of
the line and charging current. Use nominal T method.
8. A 3 phase , 50 Hz, 150 km line has a resistance, inductive reactance and capacitive
shunt admittance of 0.1 ohm and 3 x 10-6S per km per phase. If the line delivers 50
MW at 110 KV and 0.8 pf lagging, determine the sending end voltage and current.
Assume a nominal π circuit for the line.
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9. A balanced 3 phase load of 30 MW is supplied at 132 KV, 50 Hz and 0.85 pf lagging
by means of transmission line. The series impedance of a single conductor is 20 +j 52
ohms and the total phase neutral admittance is 315 x 10-6 mho. Using nominal T
method. Determine (a) The A,B, C and D constants of the line. (b) sending end
voltage (c) Regulation of the line.
(Nov’2011)
zy
 0.992 0.18o
2
zy 

B  z  1    55.5 690
4 

C  Y  0.000315 900
(ii) Vs  AVR  BI R
(i ) A  D  1 
IR  131  j81.62
Vs  82428  j 5413
Vs (line )  143kV ; Vs(ph)  82.6kV
 Vs


 VR  / VR  X 100  9.25%

 A

 iii  percentage regulation= 
10. A 3 phase line having an impedance of 5 + j20 ohms per phase delivers a load of 30
MW at a power factor of 0.8 lag and voltage of 33 KV. Determine the capacity of the
phase modifier to be installed at the receiving end if the voltage at the sending end is
to be maintained at 33 KV. Assume the shunt admittance is neglected. (Nov’ 2012)
Z  R  jX  5  j 20  20.61 75.960
Assume Y=0 (neglected)
 1 
A= 1+ YZ   1 00  D
 2 
B  Z  20.61 75.960
    75.96
Vr ( ph )  33 / 3  19.05kV  Vs
VsVr
 17.61MW
B
AVr2
 17.61MW
B
MM '  15.2MVA( perphase)
capacity of the phase modifier for 3phase=15.2  3 =45.6 MVA
11. A 15km long 3-phase overhead line delivers 5MW at 11kV at 0.8 lagging p.f line loss
is 12% of power delivered. Line inductance is 1.1mH per km per phase. Find
sending end voltage and voltage regulation. (Nov 2012)
12. (i) Perform the analysis of long transmission lines using RIGOROUS method.
(ii) Explain the concept of surge impedance loading. (Nov 2012)
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UNIT IV
INSULATORS AND CABLES
PART A
1. Where polythene cables are used?
 Non-hygroscopic used in cables for submarines and damp soil.
 Lighter used as aerial cables for vertical installations.
2. State the advantages of polythene insulators.

Non-hygroscopic

Light in weight

Low dielectric constant

Low loss factor and

Low thermal resistance.
3. By what materials cable sheaths are made?
Lead sheaths and Aluminium sheaths.
4. In what way Aluminium sheaths are superior to lead sheaths?
Aluminium sheaths are smaller in weight, high mechanical strength, greater
conductivity, cheap, easy to manufacture and install, withstand the required gas pressure
without reinforcement.
5. Where corrugated seamless aluminium sheath is used in cables?
Corrugated seamless aluminium (CSA) sheath is used in high voltage oil filled cables
and telephone lines.
6. Why corrugated seamless aluminium sheath is used in cables?
It is used because it is very flexible and easily by repeated bending the sheath is
not distorted and it is not damaged. It has lesser weight and reduced thickness.
7. Why protective covering is done in cables?
To protect the cables from mechanical damage, corrosion and electrolytic action
when laid direct in the ground the protective covering is made.
8. By what material protective covering is made in cables?

Bitumen &Bituminized materials

PVC and

Layers of fibrous materials.
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9. What is meant by serving of a cable?
Layers of fibrous material permitted with waterproof compound applied to
the exterior of the cable is called serving of a cable.
10. Why armouring is done in the cables?
To protect the sheath from mechanical damage.
11. Why armouring is not done in single core cables?
The presence of magnetic material within the alternating magnetic field of a
single core cable produces excessive losses. Hence single core cables are left
unarmoured with non-magnetic materials like tin-bronze or silicon-bronze tapes
or wires.
12. Why Aluminium is used as an armour material?
It has non-magnetic properties, high conductivity and mechanical strength.
13. What is meant by grading of cables?
The method of equalizing the stress in the dielectric of the cable is called the
grading of cables.
14.
Why the capacitance of the cable is very high than the capacitance of the overhead
lines?
The distances between the conductors are small. The distance between the
cores and the earthed sheath is also small. The permittivity of the cable insulation is
3 to 5 times greater than that of air insulation.
15. Write the expression of the capacitance of a single core cable.
C = (2πε0εr ) / ln ( R/r ) F/m
Where,
R = Resistance of a conductor
r = Radius of a conductor
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ε0 = 8.854 x 10-12 F/m
εr = Relative permittivity of the cable insulation
16. What is meant by charging current of a cable?
The capacitance of a cable determines the charging current. The charging current
restricts the use of cables on EHV lines. The current carrying capacity of an a.c.
cable is also reduced by the charging current.
17. Why power loss occurs in the dielectric of a cable?

Due to conductivity of insulation

Dielectric absorption.

Ionization or corona
18. Mention the methods of laying the cables.
Direct laying, draw in system and solid system
19. Mention the advantages of direct laying of cables.
It is simple and cheaper method. It gives the best conditions for
dissipating the heat generated in the cables.
20. State any two disadvantages of direct laying method.

Localisation of fault is difficult.

It cannot be used in congested areas where excavation is inconvenient.
21. Mention the disadvantages of pressure cables.
The cost of the pressure tube is high.
22. Mention the types of gas pressure cables/
External and internal pressure cables.
23. What are the types of oil filled cables?
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
Single core conductor channel cables

Sheath channel cables and

3 core filler space channel.
24. What are the types of pressure cables?

Oil filled cables and

Gas pressure cables.
25. What is the operating range of pressure cables?
It is greater than 66 kV.
26. What are the advantages of separate lead screened (SL) cables over H- type cables?

The possibility of core to core breakdown decreases to a large extent.

Bending of cables becomes easy owing to no overall lead sheath.
27. Mention the disadvantages of oil filled cables.

Expensive

Laying and maintenance of cables is quite complicated.
28. What are the types of screened cables?
H type and
Separate Lead screened (SL) type cables.
29. Why the working voltage level of belted cables is limited to 22 kV?
It is limited because beyond 22 kV tangential stresses acting along
the layers of paper insulation set up large current. These current causes local
heating resulting in the risk of breakdown insulation at any moment.
30. Up to what voltage range are belted cables used?
Upto 11 kV. In some extra ordinary cases they are used upto even 22 kV.
31. What are the different types of cables that are generally used for 3 phase service?
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
Belted cable

Screened cables and

Pressure cables.
32. Why cables are not used for long distance transmission?
Cables are not used for long distance transmissions due to their large charging
currents.
33. Mention the 3 main parts of the cable?
 Conductor
 Dielectric
 Sheath
34. What is the function of conductor?
Conductor provides the conducting path for the current.
35. What is the purpose of insulation in a cable?
The insulation or dielectric withstands the service voltage and isolates the
conductor with other objects.
36. 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.
37. Mention the conductor materials in cables?
 Copper
 Aluminium
38. What is the purpose of stranding of conductors?
Stranding increases the resistance of the cable .It has flexibility.
39. 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.
40 .State the properties of insulating materials.
It should have high insulation resistance, high dielectric strength, good
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mechanical properties, non-hygroscopic, capable of being operated at high
temperatures ,low thermal resistance and low power factor.
41. Mention the commonly used power cables.
 Impregnated paper
 Polyvinyl chloride
 Polyethene
42. Mention the advantages of PVC over paper insulated cables.
 Reduced cost and weight
 Insulation is resistant to water
 Simplified jointing
 Increased flexibility.
43. State the merits of paper insulated cables.
 High current carrying capacity
 Long life and
 Greater reliability
44.
What are the advantages of string insulators? (Nov’2011)
(i)
Number of units can be increased.
(ii)
Replacement of fault insulator unit is possible.
(iii)
Low tension due to its swinging
What are the methods of grading of cables? (Nov’2011)
45.

Capacitance grading and

Inter sheath grading.
PART B
1. With the neat diagram, show and explain the various parts of a oil filled cable .
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2. Derive the expression for the insulation resistance of a single core cable.
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3.
Deduce an expression for capacitance C for a single core cable.
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4. Discuss grading of underground cables
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5. Draw and explain pin and suspension type insulators.
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6. A single core cable has a conductor diameter of 1 cm and insulation thickness of 0.4
cm. If the specific resistance of insulation is 5X1014 ohm-cm. Calculate the insulation
resistance for a 2 Km length of the cable.
7. In a 3 phase overhead system each line is suspended by a string of 3 insulators. The
voltages across top unit and middle unit are 8Kv & 11Kv respectively. Calculate a)
the ratio of capacitance between pin and earth to the self capacitance of the each unit.
b) the line voltage c) string efficiency.
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8. A 3 phase overhead transmission line is being supported by three disc insulators. The
potentials across top unit and middle unit are 9 KV and 11 KV respectively. Calculate (i)
the ratio of capacitance between pin and earth to the self capacitance of each unit. (ii) the
line voltage and (iii) string efficiency.
(Nov’2011)
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9. With neat diagrams explain constructional features of various types of cables.
(Nov’2011)
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11. The insulation resistance of the single core cable is 495 Mohms per Km. If the core
diameter is 2.5 cm and the resistivity of insulation is 4.5 x 1014 ohm-cm. Find the
insulation thickness.
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UNIT V
SUBSTATION, GROUNDING SYSTEM AND DISTRIBUTION
SYSTEM
PART A
1. What is a substation?
The assembly of apparatus used to change some characteristic ( eg: voltage , A.C to
D.C frequency power factor etc) of electric supply is called a substation.
2. Mention any two layouts of laying out a substation.

Location should be at the center of the load

Should provide safe and reliable arrangement.
3. How substations are classified?

Service requirement

Constructional feature.
4. State the various types of substation according to its service requirements.

Transformer substation

Switching substation

Power factor correction substation

Frequency changer substation

Converting substation

Industrial substation
5. List the types of substations classified according to its construction.

Indoor substation

Outdoor substation

Pole mounted substation
6. Mention any two comparisons between indoor and outdoor substations.
INDOOR:

Space required and clearances between the conductors are less.

Time required for erection and possibility of faults are more.
OUTDOOR:
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
Space required and clearances between conductors are more.

Time required for erection and possibility of faults are less.
7. List the various substation equipments.

Transformer

Busbars

Insulators

Isolators

Circuit breaker

Relays

Lightning arresters.
8. Define step potential.
It is the voltage between the feet of a person standing on the floor of the
substation with 0.5m spacing between two feet during the flow of earth fault
current through the earthing system.
9. Define touch potential.
It is the voltage between the fingers of raised hand touching the faulted
structure and the feet of the person standing on the substation floor.
The person should not get shocked even if the earth structure is carrying faulted
current .i.e touch potential should be very low.
10. Define sag of a line.
The difference in level between the points of supports and the lowest point of the
conductor is called as sag.
11. Mention the factors that affect sag in the transmission line.

Weight of the conductor

Length of the span

Working tensile

Strength and

Temperature.
12. What is the reason for the sag in the transmission line?
While erecting the line, if the conductors are stretched too much
between supports then there prevails an excessive tension on the line which
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may break the conductor. In order to have safe tension in the conductor a sag in the
line is allowed.
13. What is power circle diagram?
It is a diagram drawn for the transmission lines network involving the generalized circuit
constants and the sending end and receiving end voltage.
14. What are the voltages regulating equipments used in transmission system?

Synchronous motors

Tap changing transformers

Series and shunt capacitors

Booster transformers

Compound generators and

Induction regulator.
15. Mention the methods used for voltage control of lines.

Tap changing auto- transformer

Booster transformer

Excitation control and

Induction regulator.
16. 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.
17. What is receiving end power circle diagram?
The circle drawn with receiving end values are called receiving end power
circle diagram.
18. Mention two significance of neutral grounding

The system voltage during the earth fault depends on neutral earthing.

Protection against arcing grounds , unbalanced voltages with respect to earth,
protection from lightning.
19. What is neutral grounding?
Connecting the neutral or star point of any electrical equipment(generator
,transformer etc) to earth.
20. Define coefficient of earthing.
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It is defined as the ratio of highest rms voltage of healthy line to earth to line to line
rms voltage.
21. Define resonant frequency
It is defined as a reactance earthing with selected value of reactance to match with the
line to ground capacitance.
22. Mention the disadvantages of ungrounded neutral

Occurrence of insulation breakdown leading to the heavy phase to phase fault
condition.

Voltages due to lightning surges do not find path to earth.
23. Mention two advantages of neutral grounding.

Arcing grounds are eliminated.

The over voltages due to lightning and switching surges are discharged to ground.
24. Name the various types of grounding.

Solid grounding

Resistance grounding

Reactance grounding

Resonant grounding
25. Give the response of resistance for earth driven rods.
R= ρ /2πl *ln(2l/d)
Where,
l = length of the rod
d = diameter of the rod
ρ = resistivity of the rod
26. For the uniformly current carrying ground driven rod, give the resistance value.
R=ρ/(2πl) *(ln(8l/d)-1)
Where
ρ = Resistivity of the rod
l = length of the rod
d = diameter of the rod
27. Define screening coefficient.
Screening coefficient for ‘n’ electrodes in parallel is
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= (resistance of one electrode)/(resistance of n electrodes in parallel * n)
28.What are the various methods of earthing in substation? (Nov’2011)
Using grid mats with several earth electrodes and using grounding resistance.
29. Define the terms feeders and service mains? (Nov’2011)
Feeder is a conductor or transmission line which transmits current from the
generating stations to different distributing substations.
Conductors which connect consumer’s premises with the distributor are called
service mains.
PART B
1.
Explain in detail the methods of neutral grounding systems.
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27. Explain in detail the types of sub stations.
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28. A 800 meters 2 wire dc distributor AB fed from both ends is uniformly loaded
at the rate of 1.25 A/m run. Calculate the voltage at the feeding points A and B
if the minimum potential of 220 V occurs at point C at a distance of 450 m from
the end A. Resistance of each conductor is 0.05 ohm/km
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29. A 2 wires dc distributor cable AB is 2 km long and supplies loads of 100 A, 150
A, 200 A and 50 A situated 500 m, 1000 m, 1600 m and 2000 m from the
feeding point A. Each conductor has a resistance of 0.01 ohm per 1000 m.
Calculate the potential difference at each load point if a potential difference of
300 V is maintained at point A.
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30. A 2 wire dc distributor 200 m long is uniformly loaded with 2 A/m. Resistance
of single wire is 0.3 ohm/km. If the distributor is fed at one end, calculate (i) the
voltage drop upto a distance of 150 m from the feeding point. (ii) the maximum
voltage drop.
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31. Explain the classification of substation based on service requirement and
constructional feature.
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32. Exalin various equipments in transformer substation.
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33. E
x
p
l
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ain ring distributor and ring main distributor with interconnector.
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9. A 3 wire dc distributor is fed at one end at 220 V between wires and middle wire as
shown in fig. The numbers between section indicate the resistance of the respective
section. Calculate the voltage between middle wire and outer at each load point. (Nov’12)
0.2 
0.2 
0.2 
5A
220V
0.4 
0.2 
220V
10 A
0.2 
0.3 
0.2 
0.3 
15 A
5A
15A
0.3 
12 A
0.25 
0.2 
VAD=220 – 2.2 = 217.8V
VAB=4.4 V
VDH= -2.6 V
VHE=0.4 V
VBC= 24 V, VBF=2.4 V, VFL=0.3 V
VCL=215.9 V
VHI=203.7V
VFJ=196.7 V
VGK= 188.9 V
10. Draw the circuit arrangement and explain the various elements of the following bus-bar
arrangements.
(Nov’2011)
(i) Single bus scheme
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