dhanalakshmi college of engineering,...

Transmission and Distribution

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DHANALAKSHMI COLLEGE OF ENGINEERING, CHENNAI – 301.

Sub: Transmission and Distribution Code: EE2303

Branch: EEE Sem: V

UNIT I 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 Feeder Distributor

1. a)

b)

Feeders are conductors or

transmission lines which carry

current from the stations to the

feeding points.

No tapping is taken from the

feeders.

Feeders terminate into distributors

So distributor is also a conductor from

which current is tapped off for the

supply to the consumer.

2. Current carrying capacity plays a

major role in designing a feeder.

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 HVDC / EHVDC HVAC / EHVAC

1. Line construction is simple and cheaper Line construction is complex, cost of conductors and towers

are more

2. Power transmitted per conductor is

more ( same amount of power is

transmitted with lesser no. of

conductors )

To transmit same amount of power, this system requires more

number of conductors.

3. There is no inductance, and surge

problems, thus voltage drop is less

Due to the presence of inductance, the voltage drop is more

for the same load and sending end voltage.

4. Entire cross section of the conductor is

utilised.

Due to skin effect, the current tends to concentrate on the

surface of the conductor

5. Corona loss is less Corona loss is proportional to the frequency. Hence AC this

loss is more

6. Require less insulation The potential stress on the insulation is more for the same

working voltage, hence more insulation is required.

7. Radio interference is lless Radio interference is more

8. There is no stability problems and

synchronizing difficulties

Stability problem exist

9. Electric power cannot be generated at

high DC voltage

Electric power can be generated at high DC voltage

10. The DC voltage cannot be stepped up or

stepped down

can be stepped up or stepped down by using transformer

11. Contribution of DC link to the short

circuit current is less

In AC system short circuit current is more

12. Converters required at both end of the

line, they are very expensive

There is no investment on converters

13. Converters absorb the reactive power

which must be supplied locally

No such problems in Ac transmission

14. Power control is fast and accurate as

there is less inertia due to the absent of

rotating synchronous machines

Power control is slow

15. DC line is restricted to point to point

transmission

No such restriction with AC

16. Unlimited power can be

tranmitted

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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2

2

2

2

2

150 237 872

3872

11.85

31.6

19.75

60.25 above water

xy=c+

2c

12

22

21 2

2

&2 2

l cha m

l

l chb m

l

D m

D m

D D m

clearance m

ac d

c

bc d h

c

l ah

c

l ch l cha b

l l

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

=

r

dr ln210 7

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

= mHr

ddd/10ln25.0 7

3321

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) GMR(Ds)

1.) GMD is also called as mutual GMD GMR is also called as self 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)

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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elements increase without limit

3.) Mutual GMD depends only upon the

spacing and is independent of the exact

size, shape, orientation of the

conductor.

Self GMD of a conductor depends

upon the size and shape of the

conductor and is independent of

spacing between the conductors.

4.) For a single phase line Dm=spacing

between conductors=D.

For a single phase line Ds=0.7788r

5.) For a single circuit 3phi line

Dm=(d1*d2*d3)1/3

For a single circuit 3 phi line

Ds=0.7788r

6.) For a double circuit 3phi line Dm-

=(DAB*DBC*DCA)

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

ro

AA

x

dxQV

2

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.

.....

1ln

1ln

1ln

2

1

21 dQ

dQ

rQV cBA

o

A

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 mF

r

DCAB /

ln

0

Capacitance between line and neutral conductors mF

r

DCAN /

ln

2 0

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)

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 '

31 2 3

41 ' ' ' ' 3

33

m

s

6 0.7788 4.67

0.1828

4.36 4.36 5.48 4.71

DInductance per phase=0.2 0.65 /

D

s s s s

s aa aa a a a a s

s

m AB BC CA

r mm

D D D D

D D D D D D

D m

D D D D m

mH km

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)

5m

5m

6 m

3m

3m

C‟ A

B

C

B‟

A‟


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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)

Capacitance of the line = =0.5217X10-11

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

100%'

R

RR

V

VVregulation

Where '

RV ‟- 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.

100100 sss

RRR

CosIV

CosIV

outsentPower

deliveredPower

Where VR, IR, CosR are the receiving end voltage, current and power factor

respectively.

Vs, Is, Coss 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 ( cosR ) is lagging or unity or leading that IR cosR >

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 cosR < 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 cosR is leading, IXL sin R >IR cosR 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

cosR - 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 Nominal

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

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.

2.) Full charging current flows over half

the line

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

3.) T-equivalent circuit -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.

o

RLSIL

Z

VP

2

Where 2

RLV -line voltage at the receiving end

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.

225

2.242 cVVd

rfP

X 10

-5 KW/km/ph

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,

constant Phase

constantn Attenuatio

ZYconstant n Propagatio

impedance sticsCharacteri

j

Y

ZZ c

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, determinei) 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=0.951..4deg ; B=9678deg; C=0.001590deg.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 10-6

S 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)

0

0

s

R

s

( ) s(ph)

s

( ) 1 0.992 0.182

1 55.5 694

0.000315 90

(ii) V

I 131 81.62

V 82428 5413

143 ; V 82.6

Vpercentage regulation=

A

o

R R

s line

R

zyi A D

zyB z

C Y

AV BI

j

j

V kV kV

iii V

/ 100 9.25%RV X

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) 0

0

0

( )

2

5 20 20.61 75.96

Y=0 (neglected)

1A= 1+ 1 0

2

20.61 75.96

75.96

33/ 3 19.05

17.61

17.61

' 15.2 ( )

capacity of the phase modifier for 3phas

r ph s

s r

r

Z R jX j

Assume

YZ D

B Z

V kV V

V VMW

B

AVMW

B

MM MVA perphase

e=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

45. What are the methods of grading of cables? (Nov‟2011)

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. Ex

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)

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

220V 5 A 10 A 12 A

220V 15A

5 A 15 A

0.2

0.2 0.2

0.4

0.2

0.2

0.3

0.2

0.3

0.3

0.25

0.2


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