Transmission line parameters1. What are the different types of conductors?

a) Solid conductors

b) Stranded conductors

c) Composite stranded conductors (upto 200KV)

d) Bundle conductors ( above 400KV )

2. Why all overhead lines use ACSR (Aluminium Conductor Steel Reinforced) conductors?

ACSR conductors comprises of hard drawn aluminium wires stranded around a core of single or multiple strand galvanized steel wire. They provide the necessary conductivity while the steel provides the necessary mechanical strength and it has less corona loss.

3. What is a bundle conductor and mention the advantages of using bundled conductors?

It is a conductor made up of 2 or more sub conductors and is used as one phase conductors.

Advantages of bundle conductors:

a) Reduced reactance

b) Reduced voltage gradient

c) Reduced corona loss

d) Reduced Interference.

4. What is the necessity of Double circuit lines and List out the advantages of double Circuit lines? Necessity of Double circuit lines is to enhance the power handling capacity

Advantages of double circuit lines:

a) Half of the load is shared by the other line

b) The increase in load current can be easily handled

c) In case of power failure in one circuit, the power will be supplied by the other lines.

5. What is skin effect?

The steady 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

6. Why Skin effect is absent in dc system?

The steady current when flowing through a conductor distributes itself uniformly over the whole cross section of the conductor .That is why skin effect is absent in DC system.

7. On what factors the skin effect depend?

Nature of the material,Diameter of the wire, Frequency and shape of the wire.

8. Define proximity effect?

The alternating magnetic flux in a conductor caused by the current flowing in a neighboring conductor gives rise to a circulating current which cause an apparent increase in the resistance of the conductor. This phenomenon is called as proximity effect.

9. What is the effect of proximity effect?

It results in the non uniform distribution of current in the cross section, and the increase of resistance.

10. What is meant by transposition of line conductors and advantages of transposition?

Transposition of conductors is defined as the interchanging the positions of the line conductors at regular intervals along the line so that each conductor occupies the original position of every other conductor at an equal distance.

Advantages of transposition:

a) Elimination of interference by using Transposition of linesb) Average Inductance is same in each section of the line. i.e G.M.D’s are equal for each section

11. Define Self GMD and Mutual GMD?

SelfGMD(GMR) :The Self-GMD means self geometrical mean distance .It reduces the

complexity of Inductance calculation

GMR = 0.7788r = 1r

Mutual GMD: The mutual GMD is the geometrical mean of the distances from one

Conductor to the other.

12. What is meant by Inductive Interference?

Usually, common lines are run along the power lines, in such cases, these lines are run on the same support and the same route. Under such conditions the power lines are likely to interfere with the communication lines. The interference is mainly due to electromagnetic and electrostatic fields. This can be eliminated by using transposition of lines.

13. Define Symmetrical spacing and Unsymmetrical spacing?

In 3-phase system when the line conductors are equidistant from each other then it is called

Symmetrical spacing.

Ex:

When 3-phase line conductors are not equidistant from each other, the conductor spacing is said to be Unsymmetrical spacing.

Ex:

14. Write the expressions for the inductance and capacitance of a 3-ph,3-wire lines?

Inductance /ph = H/m ln102 7

GMRGMD

C, L ,r C, L, d

IfIf

Capacitance/Ph = F/m

rGMD ln

2 0

If it is Symmetrical configauration, GMD =D (distance between the conductors) and Unsymmetrical configuration, GMD = 3

321 ..... ndddd

15. What is the effect of capacitance on earth?

mF

hd

Cab /

41r

d ln

2

2

0

ab C, hIf i.e habC 1

Transmission line theory and Mechanical design

1. Define voltage regulation and transmission efficiency?

Voltage regulation is defined as the difference in voltage at the receiving end of transmission line between conditions of no load and full load is called Voltage regulation and is expressed as a percentage of the receiving end voltage.

100Re% 0

Vr

VrVrg

Where 0Vr Receiving end voltage at no is load and Vr is Receiving end voltage at load

Transmission Efficiency is defined as the ratio of power delivered at the receiving end to the power sent from the sending end.

100 rP %

sP

2. Classify the lines as per line length?Short lines (80Km < L)

Medium lines (80Km -160Km)

Long lines (160Km >L)

3. What are the values of A, B, C, D constants in ‘T’, ‘П’ and short lines?

If AD-BC =1, Then network is called symmetrical network If A=D, Then network is called Reciprocal network

A B C D

Short line 1 Z 0 1

Nominal-T 1+ Z(1+ ) Y 1+

Nominal- П 1+ Z Y(1+ ) 1+

4. What are the units of ABCD (Generalized) constants of a transmission line?

A and D dimensionlessB is in Ohms andC is in Siemens (mhos)

5. Mention the limitation of nominal-T and nominal- П methods in the line problems?

Generally the capacitance is uniformly distributed over the entire length of the line .But for easy calculations the capacitance is concentrated at one or two points. Due to these effects there are error in the calculations.

6. What is end condenser method?It is a method used for obtaining the performance calculations of medium lines .Here the capacitance of the line is lumped or concentrated at the receiving end.

7. Define Ferranti effect and what is the reason for Ferranti effect?

The phenomenon of rise in voltage at the receiving end of the open circuited or lightly loaded line is called Ferranti effect. Reason for Ferranti effect is capacitance associated with the line generates more reactive power than the reactive power which is absorbed

8. Define surge impedance?

The characteristic impedance of a lossless line is called as surge impedance

CL

YZCZ

9. What is surge impedance loading and Mention the significance of surge impedance loading?

The unity power factor load that can be delivered by a line of negligible resistance is called the surge impedance loading.

Significance:

No reactive power is generated or absorbed at the line ends. The total reactive power absorbed by the line inductance is equal to the total reactive power generated by line capacitance.

10. What is meant by Infinite line?

If the receiving end voltage and current are numerically equal to the corresponding sending end values , i.e |VS|=|VR| and | IS |=| IR | ,then such a line is called Infinite line or flat line.

11. What is corona?

The ionization of insulating material(air),near the surface of the conductor at certain Points is called corona (Or) The complete disruption of dielectric strength of Insulating material(air) near the surface of the conductor at certain points.

12. Define Critical disruptive voltage and Visual critical voltage?

Critical disruptive voltage is minimum phase voltage at which corona occurs.

Visual critical voltage is minimum voltage at which corona appears along the line conductors

13. What are the various factors which affect corona?

i) Atmosphere

ii) Conductor size and shape

iii) Spacing between conductors

iv) Line voltage.

14. What are the various factors which affect corona loss?

voltagedisruptive Critical is dV

factordensity Air is δ Where //2 25 510241 KmphKWdVpVdrfP

The factors affecting corona loss are system frequency, system voltage, air density, conductor diameter, profile of the conductor, conductor surface condition, atmospheric conditions and load current .

15. What are the adverse effects of corona? i) Power loss but not very important except under abnormal weather conditions

ii) Corrosion due to production of Ozone gas

iii) Interference with neighbouring communication lines.

16. What are the identifications of corona?

i) A violet glow is observed around the conductor

ii) It produces a hissing noise

iii) It is accompanied by a power loss-the wattmeter connected in the electric circuit will show a reading.

iv) Occurance of beds and tufts.

17. What are the advantages and disadvantages of corona?

Advantages:Corona acts as a safety valve to the transmission line conductor against direct lightening strokes.

Disadvantages:

a) There is a some real power loss apart from the ohmic loss

b) Corona causes an interference with the neighboring communication lines (even though

lines are in symmetrical configuration)

18. How can the Corona effect be minimized?

Corona effect can be minimized in a transmission line by using

a) Larger diameter conductors

b) Hollow conductors

c) ACSR conductors or bundled conductors

19. Define Sag of a line and mention the factors that affect sag in the transmission line ?

The difference in level between the points of supports and the lowest point of the conductor is called as sag.

Weight of the conductor, length of the span, working tensile strength and the temperature.

20. What is meant by receiving-end power circle diagram?

The circle diagram drawn with receiving-end true and reactive power components as the horizontal and vertical ordinates is called the receiving-end power circle diagram.

21. What is stringing chart?

These are graphical curves, which will provide the variation of Sag w.r.to temp and Tension of the conductor .Stringing chart gives the data for sag and tension to be allowed at a particular temperature .It is very useful while erecting the transmission line conductors for adjusting the sag and tension properly.

22. What is the shape of Sag-Span curve?

The shape of the sag-span curve is a catenary (Y=k coshx ).But if the span is long and the sag is small ,the sag-span curve approaches parabola (Y=kx2 )

23. What is the use of Sag templates?

Main purpose of Sag templates is to locate the center point of the Tower w.r.to other Tower

Distribution and Mechanical design1. Define Kelvin’s law and Mention any two limitations of 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.

i.e aP

aP 32

Limitations: i) It is difficult to estimate accurately the annual charge on the capital outlay ii) It does not give the exact economical size of the conductor

2. What are the different types of distributors?

i) Based on nature of current a) DC distributorb) AC distributor

ii) Based on constructiona) Overhead distribution systemb) Underground distribution system

iii) Based on schemea) Radial distributorb) Ring main distributorc) Interconnected distributor

3. Why transmission lines are 3 phase 3 wire circuits while distribution lines are 3 phase 4-wire circuits ?A balanced 3 phase circuits 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 load.

4. What is a ring main distributor and advantages of ring main distributor? A ring main distributor is a distributor which is arranged to form a closed circuit and is fed at one or more than one circuit.

Advantages:

1. Less voltage fluctuations at consumer’s terminals 2. Less copper is required as each part of the ring carries less current than in radial system 3. Better flexibility and voltage regulation.

5. What are the factors on which choice of transmission voltage depends? The choice of transmission line voltage mainly depends on the following factors

1. Length of transmission line

2. Amount of power to be transmitted

3. Percentage Voltage regulation required

4. Cost of Switchgear, terminal equipment and conductor material

6. 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.

7. Mention the differences between 3-phase, 3-wire and 3 phase,4-wire distribution system?i) 3 phase, 3 wire is employed for balance 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

8. What is the difference between Distributor ,feeder and Service mains ?Feeder: It is one which the magnitude of current is same throughout it’s length

Distributor: In this magnitude of current is varies throughout it’s length.

Servicemain: These are the conductors, which connects the distributor to the consumer’s mains.

9. What are the main parts of a distribution system?

The main parts of a distribution system are

a) Secondary substation …………………… ….…33/11KV

b) Primary distribution line (or feeder) ………… 11KV

c) Distribution substation ………………………11KV/400V

d) Secondary distribution line (or distributor) …...400V and 230V

10. What are the apparatus in a substation?

A substation usually contains the following major apparatus : 1.Transformers 2.Circuit breakers 3.Horn-gap switches 4.Disconnect switches 5.Grounding switches 6.Surge arresters 7.Current limiting reactors 8.Instrument transformers

11. 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: Space required and clearances between conductors are more. Time required for erection and possibility of faults are less.

12. State the various types of substations according to its service requirements?

Transformer substation

Switching substation

Power factor correction substation

Frequency changer substation

Converting substation

Industrial substation

13. What is the fundamental basis for the selection of conductor size for a feeder?

The fundamental basis for the selection of conductor size for a feeder is the current carrying capacity .Voltage drop consideration is not important in a feeder .It is because there are no consumers tapped off from a feeder and the receiving end voltage can be raised to the desired value.

14. What is the fundamental basis for the selection of conductor size for a distributor?

The fundamental basis for the selection of conductor size for a distributor is the voltage drop. It is because according to Indian Electricity Rules ,the consumer’s voltage must be kept within ± 6% of the declared voltage.

15. 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 in necessary to supply the 1-phase loads of domestic and commercial consumers.

16. What are the advantages of adopting EHV/UHV for transmission of ac electrical power?Reduced line losses, High transmission efficiency, Improved regulation and reduced conductor material requirement.

17. Mention the limitations of using very high transmission voltage?i) The increased cost of insulating the conductor ii) The increased cost of transformers, switch gears and other terminal apparatus

Insulators, Grading of Insulators and Underground cables

1. What are the different types of Insulators based on shape?

a) Pin type Insulatorsb) Shackle Insulatorsc) String Or Suspension type Insulatorsd) Strain Or Tension Insulators

2. What is the purpose of Arcing horn?

It fixed at the top end of the string serves the purpose of arcing shield and protects the insulator string from flashover whenever over-voltage appears between the tower structure and the line conductor.

3. What are the two functions that are performed by grading ring?

The grading ring performs two functions i.e equalizes the voltage distribution across the insulator units and when used in conjunction with arcing horn fixed at the top end of the string serves the purpose of arcing shield and protects the insulator string from flashover

4. Define String efficiency of the insulators?

The ratio of voltage across the whole string and the product of the number of units and units and voltage across the unit nearest to the line conductor is known as the string efficiency .

unitmost lower theacross Voltagenstring theacross Voltageefficiency String i.e

5. Why is string efficiency for a D.C system 100%?

If D.C Voltage is applied across a string of suspension insulators, then voltage across each unit will be the same .It is because insulator capacitances are ineffective for D.C. Therefore, String efficiency for a D.C system is 100%

6. What is the purpose of surge absorber?

The device, which reduces the steepness of the wave front of a particular surge and thus minimizes the danger due to overvoltages, is known as surge modifier or surge absorber.

7. Mention any two methods of testing of Insulators?

i) Power frequency Dry Flash over Test

ii) Power Frequency Wet Flash-over Test

iii)Impulse Frequency Flash over Test

8. Explain dielectric loss briefly?The energy losses occurring in the dielectric of cables are due to leakage and so called dielectric hysteresis .The former loss ,is due to passing of current by conduction through the resistance of dielectric ,and is independent of supply frequency and therefore it occurs both D.C and A.C .The leakage current is proportional to the applied voltage and ,therefore ,the loss is proportional to the square of the applied voltage.

9. How cables are classified?

i) Based on Voltage :

a) LV cables (1KV)

b) HV cables (upto 11KV)

c) Super voltage cables (22KV-33KV)

d) EHV cables (33KV-66KV)

e) Extra super high voltage cable (132 Kv and above)

ii) Based on no of cores

a) 1-core cables

b) 3-core cables

c) 3.5-core cables

10. Write an expression for insulation resistance?

ohmsrR

lRi ln

2

It is noted that where as resistance of the core of the cable is directly proportional to the

length of the cable but leakage resistance is inversely proportional to the length of the cable.

11. Why the capacitance of the cable is very high than the capacitance of the overhead lines?

The distance 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.

12. 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.

13. What are the methods, that are used for improving string efficiency?

Various methods used for improving string efficiency are by using insulators with larger discs or by providing each insulator unit with a metal cap, by using longer cross-arms ,by capacitance grading or by static shielding .

14. For what purpose metalic sheath is provided in underground cables?

(i) ARMOURING: (Galvonized steel material is used for armouring) To protect the cable against external stress(ii) BEDDING : (Jute material is used for bedding)

a) Prevent the corrosion on sheath b) Suppress mechanical stress

(iii) SHEATH : (aluminium foils are used for sheath) To prevent the entry of moisture into Insulation(iv) INSULATION: (vulcanized rubber, impregnated paper are used for insulation).

To prevent the leakage current from the surface of the core to the ground

Prepared by,

P.VENKATESH,

Lecturer,

V R Siddhartha Engineering College,Vijayawada.