[Q:-01]
What do you mean by transmission and distribution of electrical energy ?
Ans
:
To transmit electrical energy in bulk
quantity from power station to the load centre at higher potential is known as
transmission.
To distribute electrical energy to the
consumer is known as distribution.
[Q:-02]
What are the different types of transmission and distribution of electrical
energy ?
Ans
:
(i) Primary
transmission and secondary transmission.
(ii) Primary
distribution and secondary distribution.
[Q:-03]
What are the methods of transmission and distribution of electrical energy ?
Ans
:
Overhead
system and underground system.
[Q:-04]
What are the standard voltages for (i) Generation, (ii)Transmission & (iii)
Distribution ?
Ans
:
(i) 6.6 kV, 11
kV, 33 kV,
(ii) 33 kV, 66
kV, 132 kV, 220 kV & 400 kV,
(iii) (a)
Primary distribution 6.6 kV, 11 kV, 33 kV, (b) Secondary distribution
A.C.-415/240 V & D.C.-450/225 V.
[Q:-05]
What is the basis of selection of voltage for transmission line ?
Ans
:
The basis of selecting the most economical
transmission voltage is to use 650 volt per km. of transmission line.
[Q:-06]
What are the advantages of high voltage transmission ?
Ans
:
The advantages
of high voltage transmission are as follows :—
(a) With the
increase in voltage the size of the conductor is reduced which further reduces
the cost of the supporting materials.
(b) With the
increase in voltage the line current is reduced resulting reduction in line
drop & line losses which causes higher efficiency.
(c) Due to low
voltage drop better voltage regulation is achieved.
[Q:-07]
What are the limitations of high voltage for transmission ?
Ans
:
In actual practice it is not possible to
increase the transmission voltage beyond a certain limit because of the
following facts :-
(a) The
insulation required between the conductor & earthed tower increases which
increases the cost of the tower.
(b) More
clearance is required between conductors and conductor and earthed members.
[Q:-08]
Why is d.c. not used for transmission ?
Ans
:
The d.c. is not used for transmission of
power as it is very difficult to get sufficiently high voltage in d.c.
generation and it is not easy to step up and step down the d.c. voltage.
[Q:-09]
What are the advantages of a.c. transmission over d.c. ?
Ans
:
(a) The
electrical power can be generated at high voltage easily.
(b) The
voltage can be stepped up at generating end by means of step up transformer to
the required value for transmission and then stepped down at distributing end
by means of step down transformer for distribution.
(c) The
maintenance of a.c. substation is easier and cheaper.
[Q:-10]
What are the disadvantages of AC transmission system?
Ans
:
(1) The volume
of conductor needed is much more than in D.C. system.
(2) Due to the
reactance of the transmission line the regulation of the line is increased.
(3) The
effective resistance of the line is increased due to skin effect
(4) The effect
of corona is more prominent in A.C. transmission lines.
(5) Sheath
loss occurs in the cable lines due to alternating current.
(6) The
construction of transmission lines in A.C. is difficult as compared to D.C.
(7) Synchronization
is required for the alternators running in parallel with other alternators.
[Q:-11]
What are the merits and demerits of d.c. transmission over a.c. transmission ?
Ans
:
Merits :—
(i) D.C.
system requires only two conductors' and hence much copper is saved.
(ii) It has no
inductance, capacitance, phase displacement and surge problem.
(iii) As there
is no skin effect in d.c. full cross section of conductor is utilized.
(iv) Less
insulation is required in d.c. for the same working voltage.
(v) Voltage
regulation is better in d.c. transmission.
Demerits :—
(i) It is very
difficult for high voltage d.c. generation due to commutating problem.
(ii) It is not
possible to step up the d.c. voltage.
[Q:-12]
Why is 3 phase, 3-wire, system generally used in transmission of A.C. power ?
Ans
:
Three phase three wire system is generally
used for its better efficiency of operation and easy convenience.
[Q:-13]
Why is underground system of transmission not used beyond 66 kV ?
Ans
:
Due to
insulation difficulties.
[Q:-14]
What medium is preferred for power transmission and why ?
Ans
:
Overhead line
is preferred because it is economical.
[Q:-15]
What Is the value of the dielectric strength of air under normal condition ?
Ans
:
It is about : 30
kV/cm.
[Q:-16]
Upto what limit of length the U.G. cables over recommended ?
Ans
:
100 km.
[Q:-17]
What are the merits and demerits of underground system over O.H. system ?
Ans
:
Merits :—
(i) It is more
safer.
(ii)
Maintenance cost is low.
(iii) Chances
of power failure or faults and accident are very few.
(iv) Voltage
drop is low.
(v) It is free
from interruption of supply due to thunder storms, severe weather condition or
such other causes.
Demerits :—
(i) It is more
expensive.
(ii) It is not
flexible like O.H. line.
(iii) For
E.H.V. transmission above 66 kV insulation difficulties will arise.
(iv) Fault
location and repairing are difficult and expensive.
(v) It draws
higher charging current due to high capacitance.
(vi) Tapping
and jointing are difficult.
[Q:-18]
What is skin effect ?
Ans
:
When an alternating current passes through
a conductor it does not flow uniformly all over the conductor. The
concentration of current is higher on the surface of the conductor than at the
centre, thus increasing the effective resistance. This effect is called the
skin effect which is prominent at higher frequency. The skin effect increases
with the increase of cross section and permeability of conductor and this
effect is reduced in stranded conductors.
[Q:-19]
What is proximity effect ?
Ans
:
The proximity effect is the distortions of
the current distribution in the cross section of a conductor caused by the
mutual induction between the currents in the go and return conductors arranged
closely parallel to one another. The concentration of currents in the
conductors is higher nearest each other thus increasing their effective
resistance. It is directly proportional to the magnitude of the current and
inversely proportional to the distance between conductors.
[Q:-20]
Why is the effective a.c. resistance more than d.c. resistance of a
transmission line ?
Ans
:
Due to skin
effect & proximity effect.
[Q:-21]
Why does a transmission line possess Inductance & capacitance ?
Ans
:
When a conductor carries current it is
surrounded by a magnetic field and in case of a.c. this magnetic field is not
constant but changing and links with the same conductor and also other
conductors. Due to these magnetic flux linkages the transmission line possesses
inductance.
When two conductors are separated by an
insulating medium it constitute a capacitor. In case of transmission line the
two conductors form the two plate of a capacitor separated by air medium from
one another thus possessing a capacitance.
[Q:-22]
What Is the reason for the existence of the capacitance In a transmission line
?
Ans
:
It is due to
the p.d. between the conductors.
[Q:-23]
Upto what length of transmission line the capacitance can be neglected ?
Ans
:
75 km.
[Q:-24]
What are the factors governing the inductance & capacitance of a
transmission line ?
Ans
:
(i) Distance
between the conductor (inductance increases but capacitance decreases with the
increase in distance).
(ii) Radius of
conductor (inductance decreases but capacitance increases with the increase in
radius).
(iii) Length
of the line (both inductance & capacitance increase with the increase in
length).
[Q:-25]
How are inductance and capacitance of a transmission line related with spacings
of conductors and radius of conductor ?
Ans
:
If D = spacing
of conductors , and
r = radius of
conductor both in same unit then the loop inductance of a single phase line
L = (1 + 4 loge
D/r) x 10-7 Henry/metre.
In case of
three phase line symmetrically spaced, the inductance per conductor
L = (0.5 + 2
loge D/ r) x 10-7 Henry/metre.
If Є0
= the permittivity of free space = 8.854 x 10-12.
The
capacitance to neutral per phase single or three phase
C = 10-9
/ 18 loge D / r Farads/metre
If the
spacings between conductors are not equilateral but is perfectly transposed
then,
D = 3√(D1
x D2 x D3)
where D1,
D2 and D3 are the distances between individual
conductors.
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