Transmission and Distribution
1. 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.
2. Why the transmission systems are mostly overhead
systems?
Because
of the cost consideration, the transmission systems are mostly overhead
systems.
3. 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.
4. 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.
5. Why overhead line conductors are invariably
stranded?
They are
stranded to make them flexible during erection and while in service.
6. 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.
7. 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.
8. 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.
9. 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.
10.
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.
11.
Mention
the differences between 3 wire and 3 phase 4 wire distribution system?
i)3 phase
3 wire is employed for balanced loads, but 3 phase 4 wire is employed for
unbalanced loads.
ii)3
phase 3 wire is used for transmission but 3 phase 4 wire is used for
distribution of power to consumers.
12. 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.
13. 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.
14. 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.
15.
Mention
the demerits of HVDC transmission.
i. Electric
power cannot be generated at high dc voltages.
ii. The dc
voltages cannot be stepped up for transmission of power at high voltages.
iii. The dc
switches and circuit breakers have their own limitations.
16.
What are
the advantages of high voltage ac transmission.
The po
wer can be generated at high voltages. The maintenance of ac substation is easy
and cheaper.
17. Mention the disadvantages of high voltage ac
transmission.
An ac
line requires more copper than a dc line. The construction of an ac line is
more complicated than a dc transmission line. Due to skin effect in the ac
system the effective resistance of the line is increased.
18 Mention the limitations of using very high
transmission voltage.
The
increased cost of insulating the conductor. The increased cost of transformers ,switch
gears and other terminal apparatus.
19.Mention the terminal equipments necessary in
HVDC system.
Converters,
mercury arc valves and thyristors.
20.Why HVDC line do not require any reactive power
compensation ?
Due to
absence of charging currents .
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 efficency
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.Define an electric power supply system?
The
system of generation, transmission and distribution of electrical power is
called the electrical power system .
29.What are service mains?
Service
mains are conductors which connect the consumers terminals to the distributor.
30.What is the usable voltage for secondary
distribution ?
415 40 V
(415 volts for 3-phase loads and 240 volts for 1-phase loads)
31.Why galvanized steel wire is not suitable for
EHT lines for the purpose of transmitting large amounts of power over long
distance?
because
of Poor conductivity
High
internal reactance & eddy current & hysterisis
31.Mention the transmission voltages that are used
in India?
33/66/110/132/220/400/765
KV. 765KV is EHV.
32.
On what
basis the conductor size is determined for transmission lines upto 220 KV?
is
determined on the basis of its current carrying capacity.
33.
On what
basis the conductor size of EHV line is determined?
is
decided on the basis of corona
34.
Mention
the sources of audible noise generation in EHV transmission systems?
Corona
Humming
of transformers Cooking Systems
Mechanical
and Electrical auxillaries
35.
Mention
the material universally employed for overhead transmission line?
ASCR
(Aluminium Conductor Steel Reinforced )
36.
What is
the economical value of span for 400kv transmission line ?
350-400m.
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