SRI KRISHNAENGINEERINGCOLLEGEPanapakkam, Chennai – 601 301.

Dept. of Electrical and Electronics EngineeringEC 2201-ELECTRICAL ENGINEERING

Prepared by: P.VIVEKUNIT- 1 D.C. MACHINES 1. State Faraday’s law of Electromagnetic Induction. Whenever there is a change in the magnetic flux linked with a circuit, an emf is induced in the circuit. The magnitude of the induced emf is proportional to the rate of change of flux linkages. 2. State Fleming’s left hand thumb rule. Stretch the fore finger, middle finger and the thumb of the left hand mutually perpendicular to one another. If the fore finger points in the direction of the magnetic field and the middle finger points the direction of current in the conductor, then the thumb will point the direction of motion of the conductor. 3.What are the different methods of Speed Control in a d.c Motor ? Speed can be varied by three Methods. 1. Field Control Methods 2. Armature Control Method 3.Variable voltage Method 4. What is cumulative & differential compound d.c Motors. If the flux set up by the series field aids the shunt field flux then the compound motor is called cumulative compound motors.If the flux setup by the series field opposes the shunt field flux then the compound motor is called differential compound motors. 5.State Fleming’s right hand thumb rule. Stretch the thumb, fore finger and the middle finger of the right hand mutually perpendicular to each other . If the fore finger points in the direction of field, thumb in the direction of motion of the conductor , the middle finger will point in the direction of the induced emf. 6. What is Commutation? The process of converting alternating emf into unidirectional emf is called commutation . The purpose of using commutator is to * Conduct electricity between rotating armature and fixed brushes. * Convert alternating emf into unidirectional emf & vice versa. 7. What are the various losses in DC Machines? 1. Copper loss (i) Armature copper loss

(ii) Shunt field copper loss (iii) Series field copper loss

2. Iron loss (i) Hysteresis loss

(ii) Eddy current loss 3. Mechanical loss

(i) Friction loss (ii) Windage loss

8. Define critical field resistance in DC shunt generator. Critical field resistance is defined as the resistance of the field circuit which will cause the shunt generator just to build up its emf at specified speed. 9. Define critical speed in DC shunt generator. Critical speed is defined as the speed at which the generator is to be driven to cause self excitedgenerator to build-up its emf for the given field circuit resistance. 10. How do you reverse the direction of a d.c Motor? Either the direction of the main field or the direction of current through the armature, conductors is to be reversed to reverse the direction of a d.c motor. 11. List any two conditions for voltage build-up of a dc shunt generator.

* The generator should have residual flux. * The field circuit resistance should be lesser than the critical field resistance. 12. Define the term armature reaction. The interaction between the flux setup by the current carrying armature conductors with the main field flux is defined as armature reaction. 13. What are the effects of armature reaction? The two unwanted effects of armature reaction are * Cross-magnetising effect. * Demagnetising effect. 14. What is the function of interpoles? Interpole winding is connected in series with the armature circuit and it is excited by the armature current .Their function is to provide additional emf in the coil undergoing commutation to neutralizes the reactance emf. 15. What are the reasons which causes the terminal voltage of DC shunt generator to get decreased as load current is increased?

* Armature circuit resistance drop * Demangneting effect of armature reaction * Weaking of field current.

16. State the basic parts of D.C Machine. Stator: Frame, Main pole, Field Coils, Inter poles, Inter pole winding, Brushes. Rotor : Armature core, Armature winding, Commutator, shaft. 17. Write down the emf equation for d.c.generator? p- -->noof poles

E=(ФNZ/60)(P/A) V

Z--->Total noof conductor Ф--->flux per pole , N--->speed in rpm.18. Why the armature core in d.c machines is constructed with laminated steel sheets instead of solid steel sheets? Lamination highly reduces the eddy current loss and steel sheets provide low reluctance path to magnetic field. 19. Why commutator is employed ind.c.machines? Conduct electricity between rotating armature and fixed brushes, convert alternating emfinto unidirectional emf(mechanical rectifier). 20. Distinguish between shunt and series field coil construction? Shunt field coils are wound with wires of small section and have more noof turns. Series field coils are wound with wires of larger cross section and have less noof turns. 21. How does d.c. motor differ from d.c. generator in construction? Generators are normally placed in closed room and accessed by skilled operators only. Therefore on ventilation point of view they may be constructed with large opening in the frame. Motors have to be installed right in the place of use which may have dust, dampness, inflammable gases, chemicals….etc. to protect the motors against these elements, the motor frames are made either partially closed or totally closed or flame proof. 22.How will you change the direction of rotation of d.c.motor? Either the field direction or direction of current through armature conductor is reversed. 23. What is back emf in d.c. motor? As the motor armature rotates, the system of conductor come across alternate north and south pole magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the conductor is in opposite to current. As this emf always opposes the flow of current in motor operation it is called as back emf. 24. What is the function of no-voltage release coil in d.c. motor starter? As long as the supply voltage is on healthy condition the current through the NVR coil produce enough magnetic force of attraction and retain the starter handle in ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value then electromagnet may not have enough force to retain so handle will come back to OFF position due to spring force automatically. 25. Enumerate the factors on which speed of a d.c.motor depends? N= (V-IaRa)/Ф so speed depends on air gap flux, resistance of armature, voltage applied to armature. 26. Under What circumstances does a dc shunt generator fails to generate? Absence of residual flux, initial flux setup by field may be opposite in direction to residual flux, shunt field circuit resistance may be higher than its critical field resistance, load circuit resistance may be less than its critical load resistance. 27. Define critical field resistance of dc shunt generator? Critical field resistance is defined as the resistance of the field circuit which will cause the shunt generator just to build up its emf at a specified field. 28. Why is the emf not zero when the field current is reduced to zero in dc generator?

Even after the field current is reduced to zero, the machine is left out with some flux as residue so emf is available due to residual flux. 29. On what occasion dc generator may not have residual flux? The generator may be put for its operation after its construction, in previous operation, the generator would have been fully demagnetized. 30. What are the conditions to be fulfilled by for a dc shunt generator to build back emf? The generator should have residual flux, the field winding should be connected in such a manner that the flux setup by field in same direction as residual flux, the field resistance should be less than critical field resistance, load circuit resistance should be above critical resistance. 31. Define armature reaction in dc machines? The interaction between the main flux and armature flux cause disturbance called as armature reaction. 32. What are two unwanted effects of armature reactions? Cross magnetizing effect & demagnetizing effect. 33. What is the function of carbon brush used in dc generators? The function of the carbon brush is to collect current from commutator and supply to external load circuit and to load.

UNIT- II Transformers

1.What are the two types of transformer cores used? (1)core type (2)shell type. 2.What is a transformer ? Define an ideal transformer? Transformer is a static electrical device, which transfers electrical energy one circuit to another circuit without any change in frequency . It decreases or increases the input ac voltage. An Ideal transformer is one wich does not involve any power loss and doesnot have any change in frequency . It decreases or increases the input ac voltage. 3.What are the test to be connected in a transformer to obtain its Equivalent circuit? (a)Open circuit test (b)Short circuit test 4.Why transformer are rated in KVA? Copper loss of a transformer depends on current and iron loss depends on voltage. Hence total transformer loss depends on volt-ampere(VA) and not on phase angle between voltage and current ie, it is indepen dent of load power factor . That is why rating of transformers is in KVA & not in KW. 5.What is the necessity of using stepped core ? In transformers of medium and large capacity. Only circular coils are used which are mechanically stronger. It is more economical to place circular shape coils around Stepped core. With increase in steps , the diameter gets reduced & ultimately reduce the volume of the copper conductor and also the resistance of the coil.

6.What are Instrument transformers? To measure high currents in the order of several hundred amperes and voltages of several kilo-volts, instrument transformers are used along with ammeters & voltmeters of lower range. Type: Current transformer - to measure high current. Potential transformer - to measure high voltage. 7. What is the purpose of constructing transformer core by silicon content steel laminations? Steel and steel alloys offers less reluctance to the path of magnetic field. Addition of 3 to 5 percent silicon with steel reduces Hysteresis loss in the core . Laminating the core reduces eddy current loss occurring in the core. 8. Name two important electrical performances of transformers . The two important electrical performances of transformers are (i)Voltage regulation (ii)Efficiency.9. Mention the difference between core and shell type transformers? In core type, the windings surrounded the core considerably and in shell type the coresurround the windings i.e winding is placed inside the core. 10. What is the purpose of laminating the core in a transformer? To reduce the eddy current loss in the core of the transformer.11. Give the emf equation of a transformer and define each term? Emf induced in primary coil E1= 4.44fФmN1 volt emf induced in secondary coil E2 =4.44 fФmN2. f--->freq of AC input Фm---->maximum value of flux in the core N1,N2--->Number of primary & secondary turns. 12. Does transformer draw any current when secondary is open? Why? yes, it(primary) will draw the current from the main supply in order to magnetize the core and to supply for iron and copper losses on no load. There will not be any current in the secondary since secondary is open. 13. Define voltage regulation of a transformer? When a transformer is loaded with a constant primary voltage, the secondary voltage decreases for lagging PF load, and increases for leading PF load because of its internal resistance and leakage reactance. The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full load voltage is termed as regulation. %regulation down=(V2noload-V2F.L)*100/V2noload

%regulation up=(V2noload-V2F.L)*100/V2F.L 14. Full load copper loss in a transformer is 1600W. what will be the loss at half load? If x is the ratio of actual load to full load then copper loss = x2(F.L copper loss) here Wc = (0.5)2 – 1600=400W. 15. Define all day efficiency of a transformer? It is computed on the basis of energy consumed during a certain period, usually a day of 24 hrs. All day efficiency=output in kWh/input in kWh tor 24 hrs.

16. Why transformers are rated in kVA? Copper loss of a transformer depends on current & iron loss on voltage. Hence total losses depends on Volt-Ampere and not on PF. That is why the rating of transformers are in kVA and not in kW. 17. What are the typical uses of auto transformer? 1. To give small boost to a distribution cable to correct for the voltage drop. 2. As induction motor starter. 18. What are the application of step-up & step-down transformer? Step-up transformers are used in generating stations. Normally the generated voltage will be either 11kV. This voltage(11kV) is stepped up to 110kV or 220kV or 400kV snd transmitted through transmission lines(simply called as sending end voltage). Step-down transformers are used in receiving stations. The voltage are stepped down to 11kV or 22kV are stepped down to 3phase 400V by means of a distribution transformer and made available. 19. Mention the difference between core and shell type transformers? In core type, the windings surrounded the core considerably and in shell type the core surround the windingsi.e winding is placed inside the core. 20. What is the purpose of laminating the core in a transformer? To reduce the eddy current loss in the core of the transformer.21. How transformers are classified according to their construction? 1. core type 2.shell type. In core type, the winding (primary and secondary) surround the core and in shell type, the core surround the winding. 22. Explain on the material used for core construction? The core is constructed by sheet steel laminations assembled to provide a continuous magnetic path with minimum of air gap included. The steel used is of high silicon content sometimes heat treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities. The eddy current loss is minimized by laminating the core, the laminations being used from each other by light coat of core-plate vanish or by oxide layer on the surface. The thickness of lamination varies from 0.35mm for a frequency of 50Hz and 0.5mm for a frequency of 25Hz. When will a Bucholtz relay operates and give an alarm. 23.How does change in frequency affect the operation of a given transformer? With a change in frequency, iron and copper loss, regulation, efficiency & heating varies so the operation of transformer is highly affected. 24. What is the angle by which no-load current will lag the ideal applied voltage? In an ideal transformer, there are no copper & core loss i.e loss free core. The no load current is only magnetizing current therefore the no load current lags behind by angle 900. However the winding possess resistance and leakage reactance and therefore the no load current lags the applied voltage slightly less than 900. 25. List the arrangement of stepped core arrangement in a transformer? 1. To reduce the space effectively 2. To obtain reduce length of mean turn of the winding

3. To reduce I2R loss. Why are breathers used in transformers? Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil. Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases. 26. What is the function of transformer oil in a transformer? 1. It provides good insulation 2. Cooling. 27. Can the voltage regulation go –ive? If so under what condition?Yes, if the load has leading PF. 28. Distinguish power transformers & distribution transformers? Power transformers have very high rating in the order of MVA. They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will be very high. Distribution transformers are used in receiving side. Voltage levels will be medium. Power ranging will be small in order of kVA. Complicated controls are not needed. 29. What is the purpose of providing Taps in transformer and where these are provided? In order to attain the required voltage, taps are provided, normally at high voltages side(low current).

UNIT- IIIInduction Motors 1. What are the advantages of phase Induction motor?

* Simple and extremely rugged Construction * Low cost and reliability * Require minimum maintenance * High Efficiency and good power factor * Simple starting arrangement

2. How can the direction of rotation of a 3phase Induction motor be reversed? If the direction of rotation is to be changed then the direction of the rotation magnetic field is to be reversed which can be achieved by reversing the phase sequence of the 3- phase supply given to the motor . 3. What are the effects of in increasing rotor resistance on starting current and starting torque? * Reduces the rotor current and hence the current drawn from the supply. * Improves the starting torque developed by improving the motor circuit power factor in high proportion to the decrease in motor current 4 . Why it is not possible for the rotor speed of an Induction motor to be equal to the speed of its rotating magnetic field? If the rotor speed equals synchronous speed, it will lead to absence of relative speed and hence the rotor emf and current will become zero and the machine will not be able to develop any mechanical torque to run as a motor. 5. What are the advantages of slip ring Induction motor over cage motors? * Rotor circuit is accessible for external connection. * By adding suitable external resistance to the rotor circuit the starting current is reduced with the added advantage of improving starting torque developed.

* Additional speed control methods can be employed in the rotor circuit. 6.State the different modes of operation of 3 phase Induction machines? (i)Motor Mode (ii)Braking by plugging mode (iii)Generator Mode. 7. What are the methods of speed contol in Induction motors? 1. Control from stator side (a) By changing the applied voltage (b) By changing the applied frequency C By Changing the number of stator poles 2. Control from rotor side (d) rotor rheostat control (e) by operating two motors in concatenation or cascade (f) by injection an emf in rotor circuit 8. In what respect does the cage induction motor differ from slip ring induction motor? The rotor circuit is a permanently closed circuit in the cage rotor. The rotor consist of thick copper bars & the ends of on both sides of the bar are welded or soldered. The rotor circuit in slip ring Induction motor is not a closed one. It consist of 3 phase windings. Here addition of resistance is possible. 9. Name the two different theories with which principle of 1 phase Induction Motors are explained. The two different theories are

* Double revolving field theory * Cross field theory.

10. Why are centrifugal switches provided on many 1 phase Induction motors? Centrifugal switches are provided on many 1 phase Induction motors to disconnect the starting/auxillary winding from the supply when the motor reaches about 70% of its synchronous speed. 11.Distinguish between capacitor start and run induction motors ? The Second winding in Capacitor start motor is called starting winding which will be connected across the supply along with starting capacitace at the time of starting. The starting winding will be disconnected from the supply when the speed reaches about 70% of the synchronous motor. In the capacitor start and run motor the second winding is called start and run motor second winding which along with the capacitor remains connected to the supply all the time. 12. How is the direction of a capacitor Induction motor be reversed? The direction rotation can be reversed by Interchanging the terminals of either the main or starting winding. 13. What will be the direction of rotation of a shaded pole 1 phase Induction motor? The shaded pole Induction motor rotates in the direction specified by the unshaded to shaded region in the pole face. 14. What are the inherent characteristics of plain 1 phase Induction motor? A plain 1 phase Induction motor is not used in practice due to the following inherent characteristics. * A plain 1 phase Induction motor does not have any starting torque.

* However, if the rotor is initially given a starting torque , by some means, the motor can pick up its speed in a direction at which the initial torque is given and deliver the required output. 15. Which type of 1 phase Induction motor would you use for the following application? (i)Celling fan (ii)Wet-grinder Celling fan- Capactiorstart and run motor ; Wet grinder-Capacitor start motor. 16.What could be the reasons if split phase motor fails to start and hums loudly? It could be due to the starting winding being open for grounded or burntout. 17. What could be the reasons if a split phase motor runs too slow? * Wrong supply voltage and frequency * Overload * Grounded starting and running windings . * Short circuited or open windings in field circuit. 18. What are the 2 types of 3phase induction motor? Squirrel cage and slip ring induction motor. 19. Why an induction motor is called as rotating transformer? The rotor receives same electrical power in exactly the same way as the secondary of a two winding transformer receiving its power from primary. That is why induction motor is called as rotating transformer. 20. Why an induction motor never runs at its synchronous speed? If it runs at sy.speed then there would be no relative speed between the two, hence no rotor emf, so no rotor current, then no rotor torque to maintain rotation. 21. What are slip rings? The slip rings are made of copper alloys and are fixed aroud the shaft insulating it. Through these slip rings and brushes rotor winding can be connected to external circuit. 22. What are the advantage of cage motor? Since the rotor have low resistance, the copper loss is low and efficiency is very high. On account of simple construction of rotor it is mechanically robust, initial cost is less, maintenance cost is less, simple starting arrangement. 23. Give the condition for maximum torque for 3phase induction motor, when it is running? The rotor resistance and reactance should be same for max.torque i.e. R2=Sx2. 24. List out the method for speed control of 3phase cage type induction motor?

By changing supply frequency By changing no of poles By operating the two motors in cascade.

25. Name the two winding of single phase induction motor? Running and starting winding.26. What are methods available for making single phase induction motor a self starting? By slitting the single phase, by providing shading coil in the poles.27. What is the function of capacitor in single phase induction motor? To make phase difference between starting and running winding, to improve PF and

to get more torque. 28. State any 4 use of single phase induction motor? Fans, wet grinders, vacuum cleaner, small pumps, compressors, drills. Explain why singlephase induction motor is not a self starting one? When motor fed supply from single phase, its stator winding produces an alternating flux, which doesn’t develops any torque. 29. What kind of motors used in ceiling fan and wet grinders? Ceiling fan - Capacitor start and capacitor run single phase induction motor, wet grinders- Capacitor start capacitor run single phase induction motor. 30. What is the application of shaded pole induction motor? Because of its small starting torque, it is generally used for small toys, instruments, hair driers, ventilators..etc. 31. In which direction a shaded pole motor runs? The rotor starts rotation in the direction from unshaded part to the shaded part. 32. Why single phase induction motor have low PF? The current through the running winding lags behind the supply voltage by large angle so only single phase induction motor have low PF. 33. Differentiate between “capacitor start” & “Capacitor start capacitor run” single phase induction motor? Capacitor start – capacitor is connected series with starting winding, but it will be disconnected from supply when motor pick up its speed. Capacitor start capacitor run- starting winding and capacitor will not be disconnected from supply even though motor pickup its speed. 34. What are the principal advantages of rotating field type construction? Relatively small amount of power required for field system can easily supplied to rotating system using slip rings and brushes, more space is available in the stator part of the machine to provide more insulation, it is easy to provide cooling system, stationary system of conductors can easily be braced to prevent deformation. 35. Write two extra features of slip ring induction motor? Rotor has 3phase winding. extra resistance can be added in rotor circuit for improving PF with the help of three slip rings.

UNIT- IV

Synchronous and special machines

1. What are the advantages of rotating field system? 1. It is easy to insulate stationary armature winding for high ac voltages; which may have as high as 30 KV or more. 2 . It is easy to provide cooling system for a stationary system of conductor. 3 . The output current can be taken directly from fixed terminals on the stator to the load circuit, with out having to pass it through brush contacts . 4 . The relatively small amount of power required for the field system can be easily supplied to the rotating field system through slip rings and brushes. 2. How are alternators classified? According to type of field system * Stationary field system * Rotating field system According to shape of field system * Salient pole type * Smooth cylindrical type. 3. What are the advantages of short winding? * They save copper of end connections. * It improves the wave form of generated emf. ie. the generated emf can be made to approximate to a sine wave easily. * Due to elimination of high frequency harmonics, eddy current & hysteresis losses are reduced there by increasing the efficiency. 4.Name the various methods of predetermining the voltage regulation of 3 phase Alternator. * Synchronous Impedance / EMF method * Ampere- turn/ MMF Method * Potier/ ZPF Method. 5. Why is the synchronous impedance method of estimating voltage regulation is considered as pessismistic method? Compared to other methods , the value of voltage regulation obtained by the synchronous impedance method is always higher than the actual value and therefore this method is called the pessimistic method. This is because the value of synchronous impedance obtained by this method is higher than the actual value. 6. What is meant by infinite bus-bars? The source or supply lines with non-variable voltage and frequency are called infinite bus bars. The source lines are said to have Zero source impedance. 7. What is necessity of predetermination of voltage regulation ? Most of the Alternators are manufactured with large power, voltage ratings. Conducting load testfor such high power, Voltage rating machines are not possible. Hence there is a necessity to predetermine the voltage regulation at any desired load currents and power factors. 8. Why is the MMF method of estimating the voltage regulation is considered as the optimistic

method? The final result obtained by this method is a very close to the actual value. It takes in to account the saturation effect. Hence this method is called optimistic method. 9. What are the advantages of salient type pole construction used insy.machines? They allow better ventilation, the pole faces are so shaped radial air gap length increases from pole center to pole tips so flux distortion in air gap is sinusoidal so emf is also sinusoidal. 10. Which type of sy.generators are used in hydroelectric plants and why? As the speed of operation is low, for hydro turbines used in hydroelectric plants, salient pole type sy.generator is used because it allows better ventilation also better than smooth cylindrical type rotor. 11. Why are alternators rated in KVA and not in KW? As load increases I2R loss also increases, as the current is directly related to apparent power delivered by generator, the alternator has only their apparent power in VA/KVA/MVA as their power rating. 12. Why thesy.impedance method of estimating voltage regulation is is considered as pessimistic method? Compared to other method, the value of voltage regulation obtained by this method is always higher than the actual value so it is called as pessimistic method. 13. Why MMF method of estimating voltage regulation is considered as optiimistic method? Compared to EMF method, MMF method involves more noof complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes core saturation effect. As this method require more effort, final result is very close to actual value, hence this method is called as optiimistic method. 14.What are the advantages of salient type pole construction used in sy.machines? They allow better ventilation, the pole faces are so shaped radial air gap length increases from pole center to pole tips so flux distortion in air gap is sinusoidal so emf is also sinusoidal. 15.Why are alternators rated in KVA and not in KW? As load increases I2R loss also increases, as the current is directly related to apparent power delivered by generator, the alternator has only their apparent power in VA/KVA/MVA as their power rating. 16.Why the sy.impedance method of estimating voltage regulation is is considered as pessimistic method? Compared to other method, the value of voltage regulation obtained by this method is always higher than the actual value so it is called as pessimistic method. 17.Why MMF method of estimating voltage regulation is considered as optimistic method? Compared to EMF method, MMF method involves more no of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes core saturation effect. As this method require more effort, final result is very close to actual value, hence this method is called as optimistic method. 18.Which type of sy.generators are used in hydroelectric plants and why? As the speed of operation is low, for hydro turbines used in hydroelectric plants, salient pole type sy.generator is used because it allows better ventilation also better than smooth cylindrical type rotor. UNIT-V

Transmission and distribution system

1. What is string efficiency? The ratio of voltage across the whole string to the product of number of disc and the voltage across the disc nearest to the conductor is known as string efficiency. 2. Write the generation, transmission and distribution voltage in India. Generation voltage 11KV Transmission voltage 132KV, 220KV ,400KV Subtransmissionvoltage 66KV or 33KV Distribution voltage - Primary distribution 11KV - Secondary voltage 415V (2phase) 240 V(single phase) 3. What are the main constitutes of a power system. 1. Generation 2. Transmission 3. Distribution. 4. What are the different kinds of generating stations? 1. Hydroelectric 2. Thermal 3. Nuclear. 5. Give any two hydroelectric stations in India? 1. Kundha 2. Mettur. 6. Give any two thermal stations in India ? 1. Tuticorin 2. Neyveli Lignite 7. Give any two nuclear station in India? 1. Kalpakkam 2.Tarapur. 8. Write the classification of substation? 1. Step up substation 2. Primary substation 3. Secondary substation 4. Distribution substation. 9. What is feeder, distributor and service main? Feeder: Feeder is the line which supplies a point of distribution network without being tapped at any intermediate point. This are the conductors which connect the substation to the areas served by these stations. Distributor:

The distributors are characterized by numerous tappings which are taken from them for the supply to the various consumer. Service mains: The service mains are the connecting links between the distributors and the consumer terminals. 10. What is the difference between feeder and distributor? Current loading of a feeder is same along the whole of its length, whereas a distribution has a variable loading due to the large number of individual tappings. 11. What are the elements of distribution systems? 1. Feeder 2. Distributor 3. Service main. 12. Define Kelvins law. “The most economical area of conductors is that for which the total annual cost of transmission line is minimum”. (or) “The most economical area of conductor is that for which the variable part of annual charge is equal to the cost of energy losses per year. 13. Write any two limitation of Kelvins law . 1. It is not easy to estimate the energy loss in the line without actual load curves, which are not available at the time of estimination. 2. This law does not take in to account several physical factor like safe current density, mechanical strength, corona loss etc. 14. Write the classifications of HVDC links? (i)Monopolar link (ii)Bipolar link (iii)Homopolar link. 15. Give any desirable properties of insulators (1) High ratio of puncture strength to flash over. (2) High electrical resistance of insulators materials in order to avoid leakage currents to earth. (3) High mechanical strength in order to withstand conductor load , wind load etc. (4) High relative permittivity of insulator material in order that dielectric strength is high. 16. Name the types of insulator 1. Pin type insulators 2. Suspension type insulators 3. Strain insulators 4. Shackle insulators. 17. What are the methods of improving string efficiency? (1) By using longer cross- arms (2) By using a guard ring (3) By grading the insulators. 18. Write the classification of cables. According to voltage level, 1.Low tension cables- upto 1000V 2.High tension cables- upto 11 KV 3.Super tension cables –from 22 kV to 33 KV 4.Extra high tension cables-from 33 to 66 KV 5.Extra super voltage cables- beyond 132 KV