AGNI COLLEGE OF TECHNOLOGY

Transmission and Distribution (EE 6402)

PART-A
1. Why is power transmitted at high voltage?
The power can be generated at high voltages
. The maintenance of ac substation is easy and cheaper
Refer Notes

2. Distinguish between feeder and distributor?

Refer Notes

3.Define Proximity effect on conductors.

The alternating magnetic flux in a conductor caused by the current flowing in a neighboring
conductor gives rise to the circulating currents which causes non-uniformity of current and
an apparent increase in the resistance of the conductor. This phenomenon is known as
proximity effect.

4.What is skin effect?

The tendency of alternating current to concentrate near the surface of a conductor is known
as skin effect.

5. What is the importance of voltage control? (1)

Refer Notes

6.What is Ferranti effect?

 In a medium or long transmission line when open circuited or lightly loaded, receiving end
voltage is found to be more than the sending end voltage. This phenomenon of rise in
voltage at the receiving end of the open circuited or lightly loaded line is called Ferranti
effect. It is due to voltage drop across the inductance (due to charging current) being phase
with the sending end voltage. Thus both capacitance and inductance are necessary to produce
this phenomenon. It occurs only in medium and long transmission line.

7.What is the purpose of insulator?

i) The overhead line conductors should be supported on the poles or towers in such a way
that currents from conductors do not flow to earth through supports i.e., line conductors
must be properly insulated from supports.
ii) This is achieved by securing line conductors to supports with the help of insulators.
iii) The insulators provide necessary insulation between line conductors and supports and
thus prevent any leakage current from conductors to earth

8.What is the main purpose of armouring?

Refer Notes

9. What are the materials used in bus bars?

Refer Notes

10. What are the classifications of substation according to service?

According to service requirement :

i) Transformer substations
ii) Switching substations
iii) Power factor correction substations
iv) Converting substations
v) Industrial substations
According to Constructional Features :
i) Indoor sub-station
ii) Outdoor sub-station
iii) Underground sub-station
iv) Pole-mounted sub-station
v)
11. Define voltage regulation

Voltage regulation is defined as the change in voltage at the receiving (or load) end when the
full-load is thrown off, the sending-end (or supply) voltage and supply frequency remaining
www.vidyarthiplus.com www.vidyarthiplus.com unchanged.. % voltage regulation= ((Vs-
Vr)/Vr)*100 where Vs is the voltage at the sending end Vr is the receiving end voltage
12.What is corona? Mention the factors affecting corona.

The phenomenon of violet glow, hissing noise and production of ozone gas in an overhead
transmission line is known as corona.
Factors affecting corona
i)Atmosphere.
ii)Conductor size.
iii)Spacing between conductors,
iv) Line voltage

13.Why are transmission line are three phase three wire while the distribution line are three
phase four wire circuits
i)In transmission, the loads are always balanced, hence the fourth wire is not needed.
ii)In distribution, no loads are balanced. The fourth wire is needed to supply the power to the
single phase residential load.

14. State kelvin’s law

The most economical cross-sectional area of the conductor is the one which makes the annual
interest and depreciation on capital cost of transmission line equal to the annual cost of energy
lost in the conductor.This is called as kelvin’s law.
State the limitations of kelvin’s law
i) It is not easy to estimate the energy loss in the line without actual load curves, which are
not available at the time of estimation.
ii) Kelvin’s law did not consider many physical factors like voltage regulation, corona loss,
temperature rise.
iii) The diameter of the conductor may be so small as to cause high corona loss.
iv) The conductor may be too weak to stamp from mechanical point of view.

15. Distinguish between self & mutual GMD.

Refer Notes

16.Mention the advantages of transposition of conductors.

To avoid the unbalancing effect due to unsymmetrical spacing, we generally interchange the
positions of the conductors at regular intervals along the line so that each conductor occupies
the original position of every other conductor over an equal distance. Such an exchange of
positions is known as transposition.
i)The transposing of line conductors are required to make the voltage drop equal for all the
phases.
ii)Reduces the disturbances to the nearby communication circuits.
iii)Effect of unbalanced current is neutralized.

17.Define transmission efficiency.

The ratio of receiving end power to the sending end power of a transmission line is known as the
transmission efficiency of the line i.e.

18.Write the formula for finding surge impedance of transmission line.

19.State the properties of insulating materials

i) High mechanical strength in order to withstand conductor load, wind load etc.
ii) High electrical resistance of insulator material in order to avoid leakage currents to earth.
iii) High relative permittivity of insulator material in order that dielectric strength is high.
iv)The insulator material should be non-porous, free from impurities and cracks otherwise the
permittivity will be lowered.
v) High ratio of puncture strength to flashover

20.Define string efficiency.

The ratio of voltage across the whole string to the product of number of discs and the voltage
across the disc nearest to the conductor is known as string efficiency i.e.,

where n = number of discs in the string

21.What is meant by tower spotting?

Refer Notes

22.What is meant by sag?

The difference in level between points of supports and the lowest point on the
conductor is called sag.
 Factors that affecting sag :
i)Weight of the conductor, ii) Length of the span, iii) Working tensile strength, iv)
Temperature.
What is sag template ?
For normal or average spans and for standard towers, the sag and the nature of the
curve that the
conductor will occupy under expected loading conditions is calculated and plotted as
a template

23.What is power circle diagram?

It is a diagram drawm for the transmission lines network involving the generalized circuit
constants and the sending end and receiving end voltage.

24.Name the various types of grounding

The methods commonly used for grounding the neutral point of a 3-phase system
are :
i)Solid or effective grounding
ii)Resistance grounding
iii)Reactance grounding
iv)Peterson-coil grounding

PART-B

1.Discuss in detail about HVDC links the advantages, disadvantages and applications of
HVDC transmission

iv)Back-Back HVDC link

i) Monopolar link
ii) Bipolar link :
iv) Back-Back HVDC link :
D.C. transmission. For some years past, the transmission of electric power by d.c. has been
receiving the active consideration of engineers due to its numerous advantages.
Advantages. The high voltage d.c. transmission has the following advantages over high
voltage
a.c. transmission :
(i) It requires only two conductors as compared to three for a.c. transmission.
(ii) There is no inductance, capacitance, phase displacement and surge problems in d.c.
transmission.
(iii) Due to the absence of inductance, the voltage drop in a d.c. transmission line is less than
the
a.c. line for the same load and sending end voltage. For this reason, a d.c. transmission line
has better voltage regulation.
(iv) There is no skin effect in a d.c. system. Therefore, entire cross-section of the line
conductor
is utilised.
(v) For the same working voltage, the potential stress on the insulation is less in case of d.c.
system than that in a.c. system. Therefore, a d.c. line requires less insulation.
(vi) A d.c. line has less corona loss and reduced interference with communication circuits.
(vii) The high voltage d.c. transmission is free from the dielectric losses, particularly in the
case
 of cables.
(viii) In d.c. transmission, there are no stability problems and synchronising difficulties.
Disadvantages
(i) Electric power cannot be generated at high d.c. voltage due to commutation problems.
(ii) The d.c. voltage cannot be stepped up for transmission of power at high voltages.
(iii) The d.c. switches and circuit breakers have their own limitations.
2.
3. Explain the real and reactive power flow in lines. Also explain the methods of voltage control.
 4.
What are the various properties of insulators? Also briefly explain about suspension and pin type
insulators. Draw the schematic diagram.

Types of Insulators
There are several types of insulators but the most commonly used are pin type, suspension
type, strain insulator and shackle insulator.
1. Pin type Insulators

The pin type insulator is secured to the cross-arm on the pole. There is a groove on the upper
end of the insulator for housing the conductor. The conductor passes through this groove and
is bound by the annealed wire of the same material as the conductor.Pin type insulators are
used for transmission and distribution of electric power at voltages upto 33 kV. Beyond
 operating voltage of 33 kV, the pin type insulators become too bulky and hence
uneconomical

2.  2. Suspension Type insulator

For high voltages (>33 kV), it is a usual practice to use suspension type insulators shown in
Figure. consist of a number of porcelain discs connected in series by metal links in the form
of a string. The conductor is suspended at the bottom end of this string while the other end of
the string is secured to the cross-arm of the tower. Each unit or disc is designed for low
voltage, say 11 kV. The number of discs in series would obviously depend upon the working
voltage. For instance, if the working voltage is 66 kV, then six discs in series will be
provided on the string.

5. Explain the neutral and resistance grounding

Neutral Grounding
The process of connecting neutral point of 3-phase system to earth (i.e. soil) either
directly or through some circuit element is called neutral grounding. Neutral grounding
provides protection to personal and equipment. It is because during earth fault, the current
path
is completed through the earthed neutral and the protective devices (e.g. a fuse etc.) operate to

isolate the faulty conductor from the rest of the system. This point is illustrated in Fig.
Fig. shows a 3-phase, star-connected system with neutral earthed. Suppose a single line to
ground fault occurs in line R at point F. This will cause the current to flow through ground path
as shown in Fig.1. Note that current flows from R phase to earth, then to neutral point N
and back to R-phase. Since the impedance of the current path is low, a large current flows
through this path. This large current will blow the fuse in R-phase and isolate the faulty line R.
This will protect the system from the harmful effects of the fault. One important feature of
grounded neutral is that the potential difference between the live conductor and ground will not
exceed the phase voltage of the system i.e. it will remain nearly constant.

Advantages of Neutral Grounding

The following are the advantages of neutral grounding
(i) Voltages of the healthy phases do not exceed line to ground voltages i.e. they remain
nearly
constant.
(ii) The high voltages due to arcing grounds are eliminated.

(iii) The protective relays can be used to provide protection against earth faults. In case earth
fault occurs on any line, the protective relay will operate to isolate the faulty line.
(iv) The over voltages due to lightning are discharged to earth.
(v) It provides greater safety to personnel and equipment.
( vi ) It provides improved service reliability.
(vii) Operating and maintenance expenditures are
reduced

5.5.3 Methods of Neutral Grounding

The methods commonly used for grounding the neutral point of a 3-phase system
are :
( i ) Solid or effective grounding
( ii ) Resistance
grounding
(iii) Reactance grounding
(iv) Peterson-coil grounding
6. Explain the structure of power system with its single line diagram
Electric Supply System: The conveyance of electric power from a power station to
consumers’ premises is known as electric supply system.
An electric supply system consists of three principal components viz., the power station, the
transmission lines and the distribution system. Electric power is produced at the power
stations which are located at favorable places, generally quite away from the consumers. It is
then transmit-ted over large distances to load centres with the help of conductors known as
transmission lines. Finally, it is distributed to a large number of small and big consumers
through a distribution network.
The electric supply system can be broadly classified into
(i)d.c. or a.c. system (ii) overhead or underground system.
Now-a-days, 3-phase, 3-wire a.c. system is universally adopted for generation and
transmission of electric power as an economical proposition.
However, distribution of electric power is done by3-phase, 4-wire a.c. system. The
underground system is more expensive than the overhead system. Therefore, in our country,
overhead system is mostly adopted for transmission and distribution of electric power.

The large network of conductors between the power station and the consumers can be
broadly divided into two parts-transmission system and distribution system, Each part can be
further sub-divided into two-primary transmission and secondary transmission and primary
distribution and secondary distribution.
Fig. shows the structure of electrical power system. It may be noted that it is not necessary
that all power schemes include all the stages shown in the figure. For example, in a certain
power scheme, there may be no secondary transmission and in another case, the scheme may
be so small that there is only distribution and no transmission.
(i) Generating station :
The generating station where electric power is produced by 3-phase alternators operating in
parallel. The usual generation voltage is 11 kV. For economy in the transmission of electric
power, the generation voltage (i.e11 kV) is stepped upto 132 kV (or more) at the generating
station with the help of 3-phase transformers. The transmission of electric power at high
voltages has several advantages including the saving of conductor material and high
transmission efficiency. It may appear advisable to use the highest possible voltage for
transmission of electric power to save conductor material and have other advantages. But
there is a limit to which this voltage can be increased. It is because in-crease in transmission
voltage introduces insulation problems as well as the cost of switchgear and transformer
equipment is increased. Therefore, the choice of proper transmission voltage is essentially a
question of economics. Generally the primary transmission is carried at 66 kV, 132kV, 220
kV or 400 kV.
(ii) Primary transmission :
The electric power at 132 kV is transmitted by 3-phase, 3-wire overhead system to the out-
skirts of the city. This forms the primary transmission.
(iii)Secondary transmission.
The primary transmission line terminates at the receiving station (RS) which usually lies at
the outskirts of the city. At the receiving station, the voltage is reduced to 33kV by step-down
transformers. From this station, electric power is transmitted at 33kV by 3-phase, 3-wire
over- head system to various sub-stations (SS) located at the strategic points in the city. This
forms the secondary transmission.
(iv)Primary distribution.
The secondary transmission line terminates at the sub-station (SS)where voltage is reduced
from 33 kV to 11kV, 3-phase, 3-wire. The 11 kV lines run along the important road sides of
the city. This forms the primary distribution. It may be noted that big consumers (having
demand more than 50 kW) are generally supplied power at
11 kV for further handling with their own sub-stations.
(v)Secondary distribution.
The electric power from primary distribution line (11 kV) is
delivered to distribution sub-stations (DS). These sub-
stations are located near the consumers’ localities and step
down the voltage to 400 V, 3-phase, 4-wire for secondary
distribution. The voltage between any two phases is 400 V
and between any phase and neutral is 230 V. The single-
phase residential lighting load is connected between any one
phase and neutral, whereas 3-phase, 400 V motor load is
connected across 3-phase lines directly. It may be worthwhile to mention here that secondary
distribution system consists of feeders, distributors and service mains.
Fig shows the elements of low voltage distribution system. Feeders (SC or SA)radiating from
the distribution sub-station (DS) supply power to the distributors (AB, BC, CD and AD). No
 consumer is given direct connection from the feeders. Instead, the consumers are connected
to the distributors through their service main.

7. Derive the expression for capacitance of a single phase and three phase transmission system
Subject In Charge Course Co-Ordinator HoD/EEE