ByMd. Irfan Ahmed

Upkar Prakashan, Agra-2

(For BHEL, NTPC, CIL, DVC etc.)

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© Author

PublishersUPKAR PRAKASHAN2/11A, Swadeshi Bima Nagar, AGRA–282 002Phone : 4053333, 2530966, 2531101Fax : (0562) 4053330E-mail : care@upkar.in, Website : www.upkar.inBranch Offices4845, Ansari Road, Daryaganj, 16-11-23/37, Moosarambagh, Teegan Guda Paras Bhawan (First Floor),New Delhi–110 002 Opp. RTA Office Main Road, Beside– Khazanchi Road,Phone : 011–23251844/66 Andhra Bank, Hyderabad–500 036 (TS) Patna–800 004

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Code No. 1930

Printed at : UPKAR PRAKASHAN (Printing Unit) Bye-pass, AGRA

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Dedicated to My ParentsMr. Nisar Ahmed

&Mrs. Shabana Khatoon

Md. Irfan AhmedAssistant Professor

Career Point UniversityKota, Rajasthan

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Electrical Engineering is one of the core branches of engineering which has driven the world of

technology to excellence. The hard work and sincerity are strong pillars of success. I often face the questions

from the students they don’t know about the basics of the electrical subjects and the interviewer always

asks basics questions of the relevant subjects. So, I strive to impress our students to inculcate these values

and support. The book presents the Knowledge about all the electrical subjects. The chapters have been

developed using the basic principles of learning and motivational. Easy Explanation of all the chapters. The

contents of the book covers almost all the subject of electrical engineering. The book serves as a regular

book for electrical engineering to prepare the students for interview and make the careers in Electrical

sectors. It is mainly designed for prepare the students for how to face the technical interview.

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Contents

Chapter–1 Power System .................................................................................................. 3–15

Chapter–2 Electrical Machine ........................................................................................... 16–25

Chapter–3 Power Electronics ............................................................................................ 26–30

Chapter–4 Network Theory .............................................................................................. 31–34

Chapter–5 Measurement & Instrumentation .................................................................... 35–39

Chapter–6 Control System ................................................................................................ 40–50

Chapter–7 Analog & Digital Circuits ................................................................................ 51–57

Interview Questions with Solutions for

Electrical Engineers

Chapter-1

Power SystemQuestion 1. Define Electric Power System.Ans : An electric power system is a network of electrical components used to supply, transmit and use electric

power.Question 2. Define Power.Ans : Power is the time rate at which work is done or energy is transferred. In calculus terms, power is the

derivative of work with respect to time.Question 3. Define Electrical energy.Ans : The loss of electrical potential energy in maintaining current in a circuit is called electrical energy consumed

in the circuit.Question 4. Define Active, Reactive Power.Ans : Active Power (Actual Power, True Power, Watt-full Power, Useful Power, and Real Power) :In a DC Circuit, power supply to the DC load is simply the product of Voltage across the load and Current flowing

through it i.e., P = V I. because in DC Circuits, there is no concept of phase angle between current and voltage. In other words, there is no Power factor in DC Circuits.

But the situation is Sinusoidal or AC Circuits is more complex because of phase difference between Current and Voltage. Therefore average value of power (Real Power) is P = VI cosθ is in fact supplied to the load.

In AC circuits, When circuit is pure resistive, then the same formula used for power as used in DC as P = VI. Real Power formulas : P = VI (In DC Circuits) P = VI cosθ (in Single phase AC Circuits) P = 3 V I cosL L θ or (in Three Phase AC Circuits)

P = ( )S Q2 2− or

P = ( )VA VAR2 2− or

Real or True power = ( )Apparent Power Reactive Power2 2− or kW = ( )kVA kVAR2 2−

Reactive Power (Use-less Power, Watt less Power) : The powers that continuously bounce back and forth between source and load is known as reactive Power (Q).

Power merely absorbed and returned in load due to its reactive properties is referred to as reactive power. The unit of Active or Real power is Watt where 1W = 1V × 1 A.

Reactive power represent that the energy is first stored and then released in the form of magnetic field or electrostatic field in case of inductor and capacitor respectively. Reactive power is given by Q = VI sinθ which can be positive (+ ve) for inductive, negative (– ve) for capacitive load. The unit of reactive power is Volt-Ampere reactive. i.e., VAR where 1 VAR = 1V × 1A.

In more simple words, in Inductor or Capacitor, how much magnetic or electric field made by 1A × 1V is called the unit of reactive power.

Reactive power formulas : Q = V I sinθ Reactive Power = ( )Apparent Power True power2 2−

VAR = ( )VA P2 2−

kVAR = ( )kVA kW2 2−

Question 5. Define Apparent Power.Ans : Apparent Power : The product of voltage and current if and only if the phase angle differences between

current and voltage are ignored. Total power in an AC circuit, both dissipated and absorbed/returned is referred to as apparent power. The combination of reactive power and true power is called apparent power. In an AC circuit, the product of the r.m.s voltage and the r.m.s current is called apparent power.It is the product of Voltage and Current without phase angle. The unit of Apparent power (S) VA i.e., 1VA = 1V × 1A.When the circuit is pure resistive, then apparent power is equal to real or true power, but in inductive or capacitive circuit, (when Reactance exist) then apparent power is greater than real or true power.

Apparent power formulas : S = V I

Apparent Power = True Power + Reactive Power2 2

kVA = kW + kVAR2 2

Question 6. Define Power factor.Ans : (1) The cosine of angle between Current and Voltage is called Power Factor. P = VI cosθ Or

cosθ = P

V I or

cosθ = kWkVA

cosθ = True Power

Apparent Power(2) The ratio between resistance and Impedance is Called Power Factor.

cosθ = RZ

(3) The ratio between Actual Power and Apparent power is called power factor.

cosθ = kWkVA

Question 7. Why is power factor not more than one ?Ans : Power factor = cosθ, Where θ is the phase angle between supply voltage and the line current. Since maximum

value of cos θ is 1, the power factor of a load can never be more than 1.

Question 8. What is the difference between the lightning arrester and surge absorber ?Ans : Lightning arrester limits the duration and amplitude of the follow current while a surge absorber reduces

the steepness of the wave front for a particular surge

Question 9. What is meant by Lightning arrester or surge diverter ?Ans : A Surge Diverter is a device that is connected between line and earth, i.e., in parallel with the equipment

under protection at the substation. It limits the duration and amplitude of the follow current.

Question 10. Why Earth Wire is provided in the overhead transmission lines ?Answer : Earth wire is provided above the overhead transmission lines for protection against lightning strokes.

The ground wire or earth wire protects the phase conductors or line conductors by attracting itself the lightning strokes which in its absence would strike the phase conductors. Also the ground wire reduces the voltage electrostatically or electromagnetically induced in the conductors by the discharge of a neighboring cloud. It also provides the additional protective effect by causing the attenuation of traveling waves set in the lines by acting a short circuit secondary of the line current.

4 Interview (Electrical Engg.)

Question 11. What are the advantages of using bundled conductors ?Ans : 1. Reduced corona loss2. Reduced radio interference3. Reduced voltage gradient4. Reduced reactance and surge impedanceQuestion 12. What are ACSR conductors ? What are their advantages ?Ans : ACSR conductor is Aluminium conductor steel Reinforced. It has following advantages:(a) They are cheaper than copper Conductors.(b) They have superior mechanical strength and less weight.(c) Due to superior mechanical strength less no. of support towers and required so less cost of erection of line.(d) Corona losses are less due to larger radius.(e) These conductors are corrosion resistant.Question 13. Why transposition of conductors is done in power lines ?Ans : If the spacing is unsymmetrical than inductive reactance of each phase will be different which will cause

voltage drop of different magnitudes. Also due to unsymmetrical spacing magnetic field external to conductor will not be zero and transmission lines will interference with communication lines. Due to this transposition of conductors is done which means exchanging the position of conductors at regular interval.

Question 14. What are the suppliers of reactive VARs and what are the absorbers of reactive VARs in power system ?

Ans : Reactive VAR Suppliers :(a) Transmission line at no load(b) Cables(c) Synchronous condensers(d) Shunt Capacitors(e) Alternator at lagging power factorReactive VAR Absorber :(a) Transformer(b) Loaded transmission line(c) Inductors(d) Alternator at leading power factorQuestion 15. If Spacing between the conductors is increased what will be its effect on inductance and

capacitance of the line.

Ans : L = 2 × 107 In dr

C = 2 0∏∈

In dr

These two equations show that if spacing between the conductors is increased, inductance will increase and capacitance will decreased. Due to this, power cables have more capacitive effect than inductive effect.

Question 16. What is the limit of loading till receiving end voltage will be more than sending end voltage ?Ans : If the loading is less than surge impedance loading then reactive power generated is more than absorbed,

therefore the receiving end voltage is greater than sending end voltage. So Surge impedance loading (SIL) is limit of loading. If loading is more than SIL then receiving end voltage will become less than sending end voltage.

Question 17. What are skin and proximity effect ?Ans : Skin effect : Flow of alternating current is non-uniform across the cross section of the conductor, with outer

filaments of the conductor carrying more current than filaments closer to the center. This is known as skin effect.Proximity effect : When two conductors are lying close to each other non-uniform current takes place closer the

portion higher is the current density.Question 18. What is Ferranti effect ?Ans : Because of the shunt parameter or (capacitive effect) there is tendency that the receiving end voltage under

light load or no load condition becomes more than the sending end voltage this phenomena is known as Ferranti effect.

Ferranti effect is significant only on medium and long lines.

Interview (Electrical Engg.) 5

Question 19. What are the problems of HVAC transmission which are not present in HVDC ?Ans : (a) Because of large charging currents, the use of HVAC for underground transmission over long distance

is prohibited.(b) HVAC link is a synchronous link which poses stability problem. Further for long line, line inductance increases

which reduces steady state stability limit.Question 20. What are the advantages of HVDC transmission ?Ans : (a) No charging current(b) No stability problem(c) Less corona loss and less radio interference(d) No reactive power compensation required(e) Low short circuit currentQuestion 21. What is the economic distance for HVDC transmission ?Ans : Terminal cost of HVDC equipment is high but cost per unit length of HVDC line is less as compared to cost

per unit length of HVAC line. So at a particular distance cost of HVAC line is equal to HVDC line for same power transferred and reliability. Beyond the distance HVDC is more economical than HVAC line.

Question 22. What is the problem associated with HVDC circuit breaking ?Ans : Because of natural current zero doesn’t exist in dc current due to this low resistance switching is not

possible. Although transient faults can be cleared by grid control but permanent faults require combination of grid control, faults locators and isolating switches. The lack of dc breakers has inhibited the networking of dc.

Question 23. What functions filters perform on ac and dc side of HVDC line ?Ans : Filters are provided on the ac side of the converters for HVDC transmission to reduce harmonic currents

and voltages on the ac side which would otherwise produce interference with other equipments or with communication lines. Filters on dc side are used to smoothen the dc output and thus these are effective in reducing interference with communication lines. Filters also provide reactive VARs required for converter and inverter operation.

Question 24. What is corona ?Ans : Corona phenomena is the ionization of air surrounding the power conductor or the phenomenon of violet

glow, hissing noise and production of ozone gas in an overhead transmission line is known as corona.Question 25. What are the factors which increase corona loss ?Ans : (a) Electric field around the conductor.(b) Atmospheric factors: if pressure is high and temperature is low then corona loss is low.(c) Dust, rain and snow increase corona loss.(d) Lower the diameter of conductor higher the corona loss.(e) Corona loss increases with frequency.(f) Smooth surface of conductor reduce corona loss.Question 26. Define stability.Ans : The stability of a physical system is referred as its capability to return to the original equilibrium position

on the occurrence of a disturbance or to another equilibrium state which is generally in the proximity of the initial equilibrium point.

Question 27. Define Steady and Transient state stability.Ans : Steady state stability : The steady state stability is the stability of the system under conditions of gradual

or relatively slow change in load.Transient State Stability : The Transient State Stability refers to the maximum flow of power possible through

a point without losing the stability with sudden and large changes in the network conditions.Question 28. Why is radio interference ?Ans : Radio interference is the adverse effect introduced by corona on wireless broadcasting. The corona discharge

emit radiation which may introduce noise signals in the communication lines, radio and television revievers. The amplitude of radio interference level varies inversely as the frequency at which the interference is measured. Thus the services in higher frequency band are less affected.

Question 29. What is diversity factor and load factor ?Ans : Diversity factor : It is the ratio of sum of individual maximum demand to the maximum demand on power

station.Diversity factor should be always greater than 1.Load factor : It is the ratio of Average load to the Maximum demand.

6 Interview (Electrical Engg.)

Question 30. Define base load and Peak load.Ans : Base Load : The unvarying load which occurs almost the whole day on the station is known as base load.Peak Load : The various peak demands of load over and above the base load of the station is known as peak

load.Question 31. Define load and load duration curve.Ans : Load Curve : The curve which shows the variation of load on the power station with respect to time is

known as a load curve.Load Duration Curve : The load elements of a load curve are arranged in the order of descending magnitudes

thus the curve is called load duration curve.Question 32. What is the difference between Grounding and earthing ?Ans : Grounding means connecting the neutral point of the circuit or network to an reference potential commonly

used is zero. Earthing means connecting the body of a particular thing to zero potential in order to protect the material from any fault current.

Question 33. Difference between Grid and Sub-stations.Ans : Grid : The transmission system of an area (or state) is known as a grid.Substation : A place where electricity is distributed and transformed.Question 34. Why one pin in three plug is long in comparison to other pin ?

Ans : It depends upon R = ρ la

where area(a) is inversely proportional to resistance (R), so if (a) increases, R

decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

Question 34. Difference between Bus and node ?Ans : Bus : The element from which any point of real power or the reactive power is being withdrawn or injected

there is no change in the voltage.Node : “Node” is refers to any point on a circuit where two or more circuit elements meet. For two nodes to be

different, their voltages must be different.Question 35. Explain primary and back up protection.Ans : (i) Primary Protection : The primary protection is the first line of defense and is responsible to protect all

the power system elements from all the types of fault. The protection provided by each zone to its element is known as primary protection.

(ii) Back-up Protection : In case of failure of Primary protection a backup protection should be provided to remove the faulted part from the system.

The back-up protection is provided as the main protection can fail due to many reasons like(a) Failure in circuit breakers(b) Failure in Protective Relay(c) Failure in tripping Circuit(d) Failure in dc tripping voltage(e) Loss of voltage or current supply to the relayQuestion 36. What is the difference between feeder and distributor ?Ans : Feeder : A feeder is a conductor which connects the substation (or localized generating station) to the

area where power is to be distributed. No tappings are taken from the feeder so that current in it remains the same throughout.

Distributors : A distributor is a conductor from which tappings are taken for supply to the consumers. The current through a distributor is not constant because tappings are taken at various places along its length.

Question 37. Define Fault and types of fault.Ans : Fault : A fault in its electrical equipment is defined as defect in its electrical circuit due to which the flow

of current is diverted from the intended paths.Types of faults are :(i) Symmetrical fault : â LLL Fault â LLLG Fault

Interview (Electrical Engg.) 7

(ii) Unsymmetrical fault â LG Fault â LLG Fault â LL Fault â Single Phase open circuit â Two Phase open circuitQuestion 38. Define Radial and Ring main system.Ans : Radial System :â If distributer is connected to the supply system from one end only, it is called radial system of distribution. It

is also true for a feeder that is if a feeder is connected to the supply system from one end only that feeder is called radial feeder.

â The radial system is employed only when power is generated at low voltage and the substation is located at the Centre of the load.

Ring main System :â The drawback of radial electrical power distribution system can be overcome by introducing a ring main

system.â A ring main is arranged in the form of closed loop. It may have one or more feeding points.â If one feeder is under fault or maintenance, the ring distributor is still energized by other feeders connected

to it. In this way the supply to the consumers is not affected even when any feeder becomes out of service.Question 39. Difference between Inductor and Inductance.Ans : Inductor : An Inductor is a device that stores energy in a magnetic field.Inductance : The Inductance is defined as the flux linkage per unit current. When an alternating current flows

through a conductor, a changing flux is set up which links the conductor. Due to these flux linkages the conductor possesses inductance.

Mathematically it can be written as

L = yI

Henry

S = V IWhere, y = Flux Linkage in weber-turns I = Current in amperesQuestion 40. Define Kelvin’s Law.Ans : The cost of conductor material is very considerable part of the total cost of a transmission line. Therefore the

determination of proper size of conductor for the line is of vital importance. The most economical area of conductor is that for which the total annual cost of transmission line is minimum. This is known as kelvin’s law.

Question 41. What are the disadvantages of low power factor ?Ans : (i) Voltage drop in the line is more.(ii) Reactive power requirement increases.(iii) Transmission loss increases.(iv) Real power transferred decreases.Question 42. If one lamp connects between two phases it will glow or not.Ans : If the voltage between the two phase is equal to the lamp volt age then the lamp will glow. When the voltage

difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

Question 43. Why Delta Star Transformers are used for Lighting Loads ?Ans : For lightning loads, neutral conductor is must and hence the secondary must be star winding and this

lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta star transformer is used for lighting loads.

Question 44. Why the capacitor works on ac only ?Ans : Generally capacitors gives infinite resistance to dc components (i.e., block the dc components). It allows

the ac components to pass through.Question 45. What is reverse power relay ?Ans : Reverse Power flow relay are used in generating stations protection. A generating station is supposed to fed

power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

8 Interview (Electrical Engg.)

Question 46. Why AC systems are preferred over DC systems ?Ans : Due to following reasons, AC systems are preferred over DC systems :(a) It is easy to maintain and change the voltage of AC electricity for transmission and distribution.(b) Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC

transmission.(c) From power stations, AC is produced so it is better to use AC then DC instead of converting it.(d) When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will

naturally tend to zero at some point making the current easier to interrupt.Question 47. How can you relate power engineering with electrical engineering ?Ans : Power engineering is a sub division of electrical engineering. It deals with generation, transmission and

distribution of energy in electrical form. Design of all power equipment’s also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e., called on grid systems and they might work on off grid systems that are not connected to the system.

Question 48. Why humming sound occurred in HT transmission line ?Ans : This sound is coming due to ionization (breakdown of air into charged particles) of air around transmission

conductor. This effect is called as Corona effect, and it is considered as power loss.Question 49. When we connect the capacitor bank in series ?Ans : we connect capacitor bank in series to improve the voltage profile at the load end in transmission line there

is considerable voltage drop along the transmission line due to impedance of the line. So in order to bring the voltage at the load terminals within its limits i.e., (+ or – 6%) of the rated terminal voltage the capacitor bank is used in series.

Question 50. Where should the lighting arrestor to be placed in distribution lines ?Ans : Near distribution transformers and outgoing feeders of 11kv and incoming feeder of 33kv and near power

transformers in sub-stations.Question 51. Why should be the frequency is 50 Hz only why not others like 45, 95 56 or anything, why

should we maintain the frequency constant if so why it is only 50 ?Ans : We can have the frequency at any frequency you like, but then you must also make your own motors,

transformers or any other equipment you want to use. We maintain the frequency at 50 Hz or 60 Hz because the world maintains a standard at 50 /60 Hz and the equipment are made to operate at these frequency.

Question 52. What are HRC fuses and where it is used ?Ans : HRC stand for “high rupturing capacity” fuse and it is used in distribution system for electrical transformers.Question 53. Which power plant has high load factor ?Ans : All base load power plants have a high load factor. If we use high efficiency power plants to supply the base

load, we can reduce the cost of generation. Hydro power plants have a higher efficiency than thermal & nuclear power plants.Question 54. What is use of lockout relay in HT voltage ?Ans : A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at

one central location. This relay is powered by the same electrical source as the control power which is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

Question 55. Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv ?Ans : Transformer Induced voltage equation contains 4.44 factor. E = 4.44*f*N*f E = Induced emf per phase N = number of turns f = frequency f = maximum flux per poleFrom the equation we see that E is proportional to 4.4 and it is in turn multiple of 11. So always transmission

voltage is multiple of 11 Question 56. Which type of motor is used in trains, what is the rating of supply used ?Ans : DC series is in the trains to get high starting torque while starting of the trains and operating voltage is

1500v dc.Question 57. Battery banks are in connected in series or parallel and why ?Ans : Battery banks are always connected in series in order to get a multiplied voltage where the AH or current

capacity remaining same. Example : 24 nos. 2V, 200AH batteries connected in series will give 48V, 200Ah output (AH = Ampere hours)

Interview (Electrical Engg.) 9

Question 58. Why Diversity Factor is always Greater than one ?Ans : Diversity factor is always greater than one is defined as. Greater the diversity factor, lesser the cost of

generation Since more diversity factor means less maximum demand which corresponds to lesser plant capacity that reduces cost of plant and hence that of generation.

Question 59. Which type of relay is preferred for Short medium and long transmission line ?Ans : Short Line → Reactance Relay Medium Line → OHM Relay Long Line → MHO RelayQuestion 60. Explain pilot wire, pilot protection and pilot wire protection.Ans : Pilot wire : An auxiliary conductor used in connection with remote measuring devices or for operating

apparatus at a distant point.Pilot Protection : A form of line protection that uses a communication channel as a means to compare electrical

conditions at the terminals of the line.Pilot wire protection : Pilot protection in which a metallic circuit is used for the communicating means between

relays at the circuit terminals.Question 61. Explain causes of faults.Ans : (i) Faults are causes by breaking of conductor or failure of insulation.(ii) Sudden change in load(iii) overloading(iv) Mechanical defect(v) Lack of maintenance(vi) overheating of stator and rotor(vii) Lightning stroke(viii) overvoltage(ix) Under frequency(x) Loss of excitationQuestion 62. Explain operating force or torque and restraining force or torque.Ans : Operating Force or Torque : A force or torque which tends to close the contacts of the relay.Restraining Force or Torque : A force or torque which opposes the operating force or torque.Question 63. Explain PSM and TMS.Ans : Plug setting multiplier (PSM) : It is the value of current taken as multiples of pick-up currents which is

called the PSM.

PSM = Secondary Current

Relay Current Setting

PSM = Fault Current

Relay Current Setting × CT RatioTime multiplier Setting (TMS) : The operating time of the relay can be set a desire value. In the induction disc

type relay the angular distance by which the moving part of the relay travels for closing the contacts can be adjusted to get different operating time. There are 10 steps in which time can be set that is known as TMS. The values of the TMS are 0.1, 0.2 ………….1

Question 64. Define Pick-up (level).Ans : The threshold value of the actuating quantity (current, voltage, etc.) above which the relay operates.Question 65. What is the difference between earth resistance and earth electrode resistance ?Ans : Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should

recourse to its definition : Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.

The resistance of the electrode has the following components :(A) The resistance of the metal and that of the connection to it.(B) The contact resistance of the surrounding earth to the electrode.Question 66. What is Overcurrent relay and under voltage relay ?Ans : Overcurrent Relay : A relay which operates when the actuating current exceeds a certain preset value or

its pick-up value.Under voltage Relay : A relay which operates when the system voltage falls below a certain preset value.

10 Interview (Electrical Engg.)

Interview Questions with Solutions ForElectrical Engineers

Publisher : Upkar Prakashan Author : Md. Irfan Ahmed

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