UNIT I1. What are the components of a power system? The components of power systems are

1. generating stations 2. step up transformer stations3. transmission lines4. switching stations5. 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. In India generation voltages are usually ___________. 3.3KV,6.6KV,11KV or 33KV.

5. The voltages for primary distribution are 11KV,6.6KV or 3.3KV.

6. The voltage for secondary distribution in our country is _________. 400 volts.

7. What is an 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.

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. Primary distribution lines 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)

Feeder 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. What is a feeder? Feeder is a conductor or transmission line which transmits current from the generating stations to different distributing substations.

12. What are service mains? Conductors which connect consumer’s premises with the distributor are called service mains.

13. Define the term distributor? Distributor is a conductor from which current is tapped off for the supply to the consumers. Feeders terminate into distributor.

14. 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.

15. What are the advantages of the HVDC transmission system over HVAC transmission?

Advantages of HVDC transmission are1.) It requires only two conductors as compared to three for ac transmission.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.

16. What are the demerits of HVDC transmission?1.) Electric power cannot be generated at high dc voltages2.) 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.

17. What are the advantages of high voltage AC transmission?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.

18. What are the disadvantages high voltage AC transmission?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.

19. What are terminal equipments necessary in HVDC system? The terminal equipments necessary in HVDC system are converters, inverters mercury are valves, thyristors etc.

20. What are the elements of a distributor. A distribution system consists of feeders distributions and service mains.

UNIT II1. 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?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?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 consists 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 material2.) diameter o wire 3.) frequency and4.) shape of wire

9. Give an expression for the loop inductance of a single phase, two wire system.Loop inductance

=

= 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 causes 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. What is transposition? Why is it done? (or) Why transmission lines are transposed?

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.

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. 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

=

Where d1,d2,d3 are the distances between the conductors r- radius of the conductors

14. 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 GMD2.) 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 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)

15. Write an expression for electric potential at a charged single conductor?

Electric potential at a charged single conductor “A” is

Where QA=charge per meter length €o° = permittivity of free space r = radius of the conductor x = distance

16. 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.

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.

17. Explain proximity effect on conductors.The alternating magnetic flux in a conductor caused by the current flowing in a neighbouring conductor gives rise to circulating currents which cause an apparent increase in the resistance of a conductor. This phenomenon is called proximity effect.

18. What is the effect of proximity effect?Proximity effect results in 1.) the non uniform distribution of current in the cross section2.) the increase of resistance

19. 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.

20. 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.

21. 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.

22. What are the advantages of using bundled conductors? 1.) reduced reactance 2.) reduced voltage gradient3.) reduced corona loss4.) reduced radio interference5.) reduced surge impedance

unit IIIcharacteristics and performance of transmission lines

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.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

Where ’- 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.

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 refulation 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)

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 reactancecosR - 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 method3.) nominal method

8. Draw the T equivalent circuit of a medium transmission line.

Where - load current per phase- resistance per phase - inductive reactance per phase - capacitance per phase - receiving end power factor(lagging)- sending end voltage- voltage across capacitor C.

9. Draw the equivalent circuit of a medium line.

10. What is Ferranti effect?The phenomenon of rise in voltage at the receiving end of the lightly loaded or unloaded line is called as Ferranti’s effect.

11. 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

12.) What are the limitations of nominal T and methods in transmission lines problems?

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.

13. 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. 14. 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.

15. 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.

Where -line voltage at the receiving end Zo-surge impedance in ohms PSIL-surge impedance loading.

16. 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.

17. 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.

18. What are the methods use for voltage control of lines?The methods used for voltage control of lines are

1.) by using over compound generator 2.) by excitation control3.) by use of tap changing transformers4.) auto-transformer tap changing 5.) booster transformer6.) induction regulator and7.) by improvement of power factor.

19. What are the voltage regulating equipments used in transmission system?Synchronous motors, tap changing transformers, series shut capacitors, booster transformers, compound generators, induction regulator.

20. 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.

21. 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.

22. 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.

23. 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.

24. Define critical disruptive voltage.

It is the minimum phase to neutral voltage at which corona occurs

25. 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

26. Write an expression for the power loss due to corona.

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

UNIT IV

1. What are the advantages of underground cables over overhead lines?1.) underground cables are less liable to any interruptions caused

by lightning or storms, birds and other severe weather conditions.

2.) They reduce accidents caused by the breaking of the conductors3.) They provide better general appearance4.) They have small voltage drop across them.

2. What are the usual insulating materials for cables?1.) rubber2.) polyethylene3.) polyvinyl chloride4.) fibrous material such as paper or jute etc5.) enamel6.) gutta-percha7.) vulcanized Indian rubber8.) varnished cam brie etc.

3. What is vulcanization?A compound of pure rubber with mineral matter such as sulphur, zinc oxide etc is rolled into thin sheets and cut into strips, that rubber compound is then applied to the conductor and is heated to a temperature of about 150°C. this process is called vulcanization.

4. What is “empire tape”?Empire tape is a cotton cloth impregnated and coated with varnish. It is nothing but varnished cam brie.

5. How are cables classified based on an operating voltage?

According to operating voltage cables are classified as1.) low tension cables (LT) –up to 1000 volts2.) high tension cables (HT) – up to 11,000 volts3.) super tension cables (ST) – from 22,000 volts to 33,000volts4.) extra high tension cables (EHT) – from 33 to 66K volts 5.) extra super voltage cable – beyond 132 KV

6. What is the purpose of a metallic sheath in a cable?The purpose of a metallic sheath in a cable is to prevent the entry of moisture or gases or other damaging liquids into the insulating material.

7. What do you understand by the term “bedding” in a cableOver the metallic sheath there is a layer of bedding which consists of paper tape compounded with fibrous material like jute or Hessian tape etc. its purpose is to protect the metallic sheath from corrosion and from mechanical injury due to armouring.

8. What is “serving” in a cable?Like bedding, a layer of fibrous material is provided over the armouring in order to protect the armouring from atmospheric conditions. This is called “serving”.

9. What are the different types of cables that are generally used for 3 phase service?

The different types of cables that are used for 3 phase service are1.) belted cable – up to 11KV2.) screened cables from 22KV to 66KV3.) pressure cables – Beyond 66KV

10. Up to what voltage range are belted cables used?Belted cables are used for voltages up to 11KV in some extra ordinary cases they are used up to even 22KV.

11. How will you overcome the limitations of solid types cables in pressure coils?

Solid types cables are limited to a voltage of 66KV. For voltages beyond 66KV solid type cables are unreliable because there is danger of break down of insulation due to the presence of voids. These voids are eliminated by increasing the pressure of the compound in pressure coils. So pressure coils can be used for voltages greater than 66KV.

12. What are the methods of laying of underground cables?

Laying of underground cables can be done in 3 methods.1.) direct laying 2.) draw in system 3.) solid system

13. Define insulation resistance?The opposition offered by insulation to leakage current is called insulation resistance.

14. Write the expression for the insulation resistance of a single core cable?Insulation resistance of a single core cable is given by

Where R - insulation resistance -resistivity of the insulationl-length of the cabler1-conductor radius in a single core cable r2-internal sheath radius

15. Write the expression for the capacitance of a single core cable?Capacitance of a single core cable is

farad

Where - capacitance of the cable in faradl- length of the cable in metres

D-inner sheath diameter in metresd-conductor diameter in metres

relative permittivity of the insulation

16. What is dielectric stress in a cable?Under operating conditions, electrostatic forces act along the layer of insulation. This is known as dielectric stress. The dielectric stress in the cable is equal to the potential gradient in a cable.

17. What is inter sheath grading?The process of achieving uniformity in dielectric stress by using homogenous dielectric is known as inter sheath grading. But the dielectric divided into various layers, by suitably placing the metallic inter sheaths.

18. List the characteristics of insulators?Insulators used for over head transmission

1.) should have high mechanical strength in order to withstand the load due to weight of conductors

2.) should have high relative permittivity in order to provide high dielectric strength

3.) should have high insulation resistance to prevent leakage of currents to strength

4.) should have high ratio of puncture strength to flashover voltage5.) should be non porous free from impurities and cracks otherwise

the permittivity will be lowered.

19. What is safety factor of insulation?The ratio of puncture strength to flash over voltage is known as safety factor

20. What is a strain insulator and where is it used?When there is a dad end or a corner or a sharper curve the transmission line is subjected to a greater tension. The insulator which are used to relieve the line of excessive tension are called strain insulators. For high voltage transmission lines the strain insulators consisting of an assembly of suspension type insulators are used.

21. Define string efficiency?String efficiency is defined as the ratio of voltage across the whole string to the product of number of discs and the voltage across the unit nearest to the conductor.

Where n= number of discs in the strings.

22. List the various methods by which voltage across the units can be equalized?

(or) List the various methods of improving string efficiency.

The various methods of improving string efficiency are1.) longer cross – arms method 2.) capacitance grading method 3.) static shielding method or guard ring method

23. What is a guard ring?A guard ring is a metal ring electrically connected to the conductor and surrounding the bottom insulator.

UNIT-V

1. How will you classify distribution systems?

Distribution systems can be classified as follows:1) Depending upon the supply system it can be classified as a) AC distribution system b) DC distribution system2) According to the type of construction it can be classified as a) Overhead distribution system b) Underground distribution system3) According to the scheme of connection it can be classified as a) Radial system b) Ring main system c) Interconnected system4)According to the no. of conductors used it can be classified as a) 2-wire system b) 3-wire system c) 4-wire system

2. Distinguish between a feeder and a distributor. Sl. No. Feeder Distributor1 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 form which current is tapped off for the supply to 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

3. What is a ring distributor?A ring distributor is a distributor which is arranged to form a closed circuit and is fed at one or more than one point

4. State the advantages of interconnected system?The advantages of interconnected system areAny area fed from one generation station during overload hours can be fed from another power station and thus reserved capacity required is reduced, reliability of supply is increased and load factor, efficiency in increased.

5. What is the purpose of interconnector in a ring main distributor?The purpose of interconnector in a ring main distributor is to reduce the voltage drops in the various sections of the distributor.

6. Explain ring main system.In ring main system of distribution the primaries of distribution transformers form a loop. The loop circuit starts from the substation bus bars, makes a loop through the

area to be served and returns to the substation. The distributors are tapped form different points of the feeders through distribution transformers.

7. State the advantages of ring main system?1) Less voltage fluctuation at consumer’s terminals2) Ring main system is more reliable. In the event of fault on

any section of the feeder, the continuity of supply can be maintained by isolating the faulty section

3) Less copper is required as each part of the ring carries less current than in radial system

8. Draw the diagram of ring main system.9. What is the difference between 3wire and 3phase 4

wire distribution system? Sl. No. 3 phase 3 wire distribution 3phase 4wire distribution1

2.

3phase 3 wire is employed for balanced loads

It is used for transmission

3 phase 4 wire system is employed for unbalanced loads

It is used for distribution of power to consumers

10. What will be the consequences of disconnecting the neutral in a 3phase 4 wire system?When the neutral is disconnected in a 3phase 4wire balanced system no change produced. But in case of unbalanced 3phase 4wire system if the neutral is disconnected the loads which are connected between any two line conductors and the neutral, are connected in series and potential difference across the combined load becomes equal to line voltage. The potential difference across each load is thus changed as per rating of the load.

11. Write the classification of substation.Depending upon the purpose

1) Generating or step up substation2) Grid sub station3) Secondary sub station4) Distribution sub station5) Special purpose sub station

Depending upon the physical feature 1) Indoor sub station 2) Outdoor substation 3)pole mounted substation 4) under ground sub station

12. Name the equipments used in a substation.

Bus bars, insulators, isolating switches, circuit breakers, load interrupting switches, power transformers, Instrument transformers, Current transformer, Potential transformer, indicating and measuring instruments, grounding.

13. List the types of bus bars?1)single bus bar system, 2) double bus with double breaker scheme 3) double bus with single breaker scheme 4) Breaker and a half scheme 5) main and transfer bus bar scheme 6) Ring bus 7) Double bus bar with bypass isolator

14. Define screening coefficient.The screening coefficient for n electrodes in parallel is defined as,