ELECTRICAL ENGINEERING MOSTLY ASKED INTERVIEW QUESTIONS AND ANSWER PDF

Here we are providing Electrical Engineering mostly asked interview questions and answers PDF,We have created it based on interview review of various candidate of various organizations.We think it will be very helpful for upcoming various interview like UPPCL AE,CIL MT,VIZAG MT,ESE and others PSU.

1)     Why ELCB cannot work if Neutral input
of ELCB does not connect to ground?

  • ELCB
    is used to detect earth leakage fault. Once the phase and neutral are
    connected in an ELCB, the current will flow through phase and that same
    current will have to return neutral so resultant current is zero.
  • Once
    there is a ground fault in the load side, current from phase will directly
    pass through earth and it will not return through neutral through ELCB.
    That means once side current is going and not returning and hence because
    of this difference in current ELCB will trip and it will safe guard the
    other circuits from faulty loads. If the neutral is not grounded fault
    current will definitely high and that full fault current will come back
    through ELCB, and there will be no difference in current.

2)     What is the difference between MCB
& MCCB, Where it can be used?

  • MCB
    is miniature circuit breaker which is thermal operated and use for short
    circuit protection in small current rating circuit.
  • Normally
    it is used where normal current is less than 100A.
  • MCCB
    moulded case circuit breaker and is thermal operated for over load current
    and magnetic operation for instant trip in short circuit condition. Under
    voltage and under frequency may be inbuilt.
  • Normally
    it is used where normal current is more than 100A.

3)     Why in a three pin plug the earth pin
is thicker and longer than the other pins?

  • It
    depends upon R=ρL/A where area (A) is inversely proportional to resistance
    (R), so if  area (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 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.

4)     Why Delta Star Transformers are used
for Lighting Loads?

  • For
    lighting 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.

5)      What are the advantages of
star-delta starter with induction motor?

  • The
    main advantage of using the star delta starter is reduction of current
    during the starting of the motor. Starting current is reduced to 3-4 times
    of current of Direct online starting  Hence the starting current is
    reduced , the voltage drops during the starting of motor in systems are
    reduced.

6)     What is meant by regenerative braking?

  • When
    the supply is cut off for a running motor, it still continue running due
    to inertia. In order to stop it quickly we place a load (resistor) across
    the armature winding and the motor should have maintained continuous field
    supply so that back e.m.f voltage is made to apply across the resistor and
    due to load the motor stops quickly. This type of breaking is called as
    “Regenerative Breaking”.

7)     When voltage increases then current
also increases then why we need of over voltage relay and over current relay?
Can we measure over voltage and over current by measuring current only?

  • No.
    We cannot sense the over voltage by just measuring the current only
    because the current increases not only for over voltages but also for
    under voltage (As most of the loads are non-linear in nature).So, the over
    voltage protection & over current protection are completely different.
  • Over
    voltage relay meant for sensing over voltages & protect the system
    from insulation break down and firing. Over current relay meant for
    sensing any internal short circuit, over load condition, earth fault
    thereby reducing the system failure & risk of fire. So, for a better
    protection of the system. It should have both over voltage & over
    current relay.

8)     If one lamp connects between two phases
it will glow or not?

  • If
    the voltage between the two phases is equal to the lamp voltage 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.

9)     What are HRC fuses and where it is
used?

  • HRC
    stand for “high rupturing capacity” fuse and it is used in distribution
    system for electrical transformers

10)  Mention the methods for starting an induction motor?

  • The
    different methods of starting an induction motor
  • DOL:direct
    online starter
  • Star
    delta starter
  • Auto
    transformer starter
  • Resistance
    starter
  • Series
    reactor starter

11)  What is the difference between earth resistance and
earth electrode resistance?

  • 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.

12)  Why most of analog o/p devices having o/p range 4 to 20
mA and not 0 to 20 mA?

  • 4-20
    mA is a standard range used to indicate measured values for any process.
    The reason that 4ma is chosen instead of 0 mA is for fail safe operation.
  • For
    example: A pressure instrument gives output 4mA to indicate 0 psi  up
    to 20 mA to indicate 100 psi or full scale. Due to any problem in
    instrument (i.e) broken wire, its output reduces to 0 mA. So if range is
    0-20 mA then we can differentiate whether it is due to broken wire or due
    to 0 psi.

13)  Two bulbs of 100w and 40w respectively connected in
series across a 230v supply which bulb will glow bright and why?

  • Since
    two bulbs are in series they will get equal amount of electrical
    current but as the supply voltage is constant across the Bulb (P=V^2/R).So
    the resistance of 40W bulb is greater and voltage across 40W is more
    (V=IR) so 40W bulb will glow brighter.

14)  What happen if we give 220 volts dc supply to bulb
or tube light?

  • Bulbs
    or devices for AC are designed to operate such that it offers high
    impedance to AC supply. Normally they have low resistance. When DC supply
    is applied, due to low resistance, the current through lamp would be so
    high that it may damage the bulb element

15)  What is meant by knee point voltage?

  • Knee
    point voltage is calculated for electrical Current transformers and is
    very important factor to choose a CT. It is the voltage at which a CT gets
    saturated.

16)  What is reverse power relay?

  • Reverse
    Power flow relay are used in generating stations’ protection.
  • A
    generating station is supposed to feed 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.

17)  What will happen if DC supply is given on the primary of
a transformer?

  • Mainly
    transformer has high inductance and low resistance. In case of DC supply
    there is no inductance, only resistance will act in the electrical
    circuit. So high electrical current will flow through primary side of
    the transformer. So for this reason coil and insulation will burn out
  • When
    AC current flow to primary winding it induced alternating flux which also
    link to secondary winding so secondary current flow in secondary winding
    according to primary current.Secondary current also induced emf (Back emf)
    in secondary winding which oppose induced emf of primary winding and thus
    control primary current also.
  • If
    DC current apply to Primary winding than alternating flux is not produced
    so no secondary emf induced in secondary winding  so primary current
    may goes high and burn transformer winding.

18)  Different between megger and contact resistance meter?

  • Megger
    used to measure cable resistance, conductor continuity, phase
    identification where as contact resistance meter used to measure low
    resistance like relays, contactors.

19)  When we connect the capacitor bank in series?

  • 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

20)  What is Diversity factor in electrical installations?

  • Diversity
    factor is the ratio of the sum of the individual maximum demands of the
    various subdivisions of a system, or part of a system, to the maximum
    demand of the whole system, or part of the system, under consideration.
    Diversity factor is usually more than one.

21)  Why humming sound occurred in HT transmission line?

  • 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.

22)  Why frequency is 50 hz only & why should we maintain
the frequency constant?

  • We
    can have the frequency at any frequency we like, but then we must also
    make our own motors, transformers or any other equipment we want to use.
  • We
    maintain the frequency at 50 Hz or 60hz because the world maintains a
    standard at 50 /60hz and the equipments are made to operate at these
    frequency.

23)  If we give 2334 A, 540V on Primary side of 1.125 MVA
step up transformer, then what will be the Secondary Current, If Secondary
Voltage=11 KV?

  • As
    we know the Voltage & current relation for transformer-V1/V2 = I2/I1
    We Know, VI= 540 V; V2=11KV or 11000 V; I1= 2334
    Amps.
    By putting these value on Relation-
    540/11000= I2/2334
    So,I2 = 114.5 Amps

24)  What are the points to be considered for MCB (miniature
circuit breaker selection)?

  • I(L)x1.25=I(MAX)
    maximum current. Mcb specification is done on maximum current flow in
    circuit.

25)  How can we start-up the 40w tube light with 230v AC/DC
without using any choke/Coil?

  • It
    is possible by means of Electronic choke. Otherwise it’s not possible to
    ionize the particles in tube. Light, with normal voltage.

26)  What is “pu” in electrical engineering?

  • Pu
    stands for per unit and this will be used in power system single line
    diagram there it is like a huge electrical circuit with no of components
    (generators, transformers, loads) with different ratings (in MVA and KV).
    To bring all the ratings into common platform we use pu concept in which,
    in general largest MVA and KV ratings of the component is considered as
    base values, then all other component ratings will get back into this
    basis. Those values are called as pu values. (p.u=actual value/base
    value).

27)  Why link is provided in neutral of an ac circuit and
fuse in phase of ac circuit?

  • Link
    is provided at a Neutral common point in the circuit from which various
    connections are taken for the individual control circuit and so it is
    given in a link form to withstand high Amps.
  • But
    in the case of Fuse in the Phase of AC circuit it is designed such that
    the fuse rating is calculated for the particular circuit (i.e load) only.
    So if any malfunction happens the fuse connected in the particular control
    circuit alone will blow off.
  • If
    Fuse is provided in Neutral and if it is blowout and at the same time
    Supply is on than due to open or break Neutral Voltage is increase and
    equipment may be damage.

28)  If 200w, 100 w and 60 w lamps connected in series with
230V AC , which lamp glow brighter? Each lamp voltage rating is 230V.

  • Each
    bulb when independently working will have currents (W/V= I)
  • For
    200 Watt Bulb current (I200) =200/230=0.8696 A
  • For
    100 Watt Bulb current (I100) =100/230=0.4348 A
  • For
    60 Watt Bulb current (I60) =60/230=0.2609 A
  • Resistance
    of each bulb filament is (V/I = R)
  • For
    200 Watt Bulb R200= 230/0.8696= 264.5 ohms
  • For
    100 Watt Bulb R100= 230/0.4348 = 528.98 ohms and
  • For
    60 Watt Bulb R60= 230/0.2609=881.6 ohms respectively
  • Now,
    when in series, current flowing in all bulbs will be same. The energy
    released will be I2R
  • Thus,
    light output will be highest where resistance is highest. Thus, 60 watt
    bulb will be brightest.
  • The
    60W lamp as it has highest resistance & minimum current requirement.
  • Highest
    voltage drop across it X I [which is common for all lamps] =s highest
    power.
  • Note
    to remember:
  • Lowest
    power-lamp has highest element resistance.
  • And
    highest resistance will drop highest voltage drop across it in a Series
    circuit
  • And
    highest resistance in a parallel circuit will pass minimum current through
    it. So minimum power dissipated across it as min current X equal Voltage
    across =s min power dissipation

29)  How to check Capacitor with use of Multi meter.

  • Most
    troubles with Capacitors either open or short.
  • An
    ohmmeter (multi meter) is good enough. A shorted Capacitor will clearly
    show very low resistance. A open Capacitor will not show any movement on
    ohmmeter.
  • A
    good capacitor will show low resistance initially, and resistance gradually
    increases. This shows that Capacitor is not bad. By shorting the two ends
    of Capacitor (charged by ohmmeter) momentarily can give a weak spark. To
    know the value and other parameters, you need better instruments

30)  What is the difference between Electronic regulator and
ordinary rheostat regulator for fans?

  • The
    difference between the electronic and ordinary regulator is that in
    electronic regulator power losses are less because as we decrease the
    speed the electronic regulator give the power needed for that particular
    speed .But in case of ordinary rheostat type regulator the power wastage
    is same for every speed and no power is saved. In electronic regulator
    triac is employed for speed control. by varying the firing angle speed is
    controlled but in rheostat control resistance is decreased by steps to
    achieve speed control.

31)  What will happen when power factor is leading in
distribution of power?

  • If
    there is high power factor, i.e if the power factor is close to one:
  • Losses
    in form of heat will be reduced,
  • Cable
    becomes less bulky and easy to carry, and very cheap to afford.
  • It
    also reduces over heating of transformers.

32)  What the main difference between UPS & inverter?

  • Uninterrupted
    power supply is mainly use for short time. Means according to ups VA it
    gives backup. Ups is also two types: on line and offline. Online ups
    having high volt and amp for long time backup with high dc voltage. But
    ups start with 12v dc with 7 amps. but inverter is start with 12v,24,dc to
    36v dc and 120amp to 180amp battery with long time backup

33)  Which type of A.C motor is used in the fan?

  • It
    is Single Phase induction motor which mostly squirrel cage rotor and are
    capacitor start capacitor run.

34)  What is the difference between synchronous generator and
asynchronous generator?

  • In
    simple, synchronous generator supplies’ both active and reactive power but
    asynchronous generator (induction generator) supply’s only active power
    and observe reactive power for magnetizing. This type of generators is
    used in windmills.

35)  What is the Polarization index value?

  • Its
    ratio between insulation resistance (IR)i.e meager value for 10min to
    insulation resistance for 1 min. It ranges from 5-7 for new motors &
    normally for motor to be in good condition it should be Greater than 2.5 .

36)  What is Automatic Voltage regulator (AVR)?

  • AVR
    is an abbreviation for Automatic Voltage Regulator.
  • It
    is important part in Synchronous Generators; it controls the output
    voltage of the generator by controlling its excitation current. Thus it
    can control the output Reactive Power of the Generator.

37)  Difference between a four point starter and three point
starters?

  • The
    shunt connection in four point starter is provided separately from the
    line where as in three point starter it is connected with line which is
    the drawback in three point starter

38)  What happens if we connect a capacitor to a generator
load?

  • Connecting
    a capacitor across a generator always improves power factor, but it will
    help depends up on the engine capacity of the alternator, otherwise the
    alternator will be over loaded due to the extra watts consumed due to the
    improvement on pf.
  •  Don’t
    connect a capacitor across an alternator while it is picking up or without
    any other load

39)  Why the capacitors work on ac only?

  • Generally
    capacitor gives infinite resistance to dc components (i.e., block the dc
    components). It allows the ac components to pass through.

40)  Why the up to dia 70mm² live conductor, the earth cable
must be same size but above dia 70mm² live conductor the earth conductor need
to be only dia 70mm²?

  • The
    current carrying capacity of a cable refers to it carrying a continuous
    load.
  • An
    earth cable normally carries no load, and under fault conditions will
    carry a significant instantaneous current but only for a short
    time
    most Regulations define 0.1 to 5 sec before the fuse or breaker
    trips. Its size therefore is defined by different calculating parameters.
  • The
    magnitude of earth fault current depends on:
  • (a)
    the external earth loop impedance of the installation (i.e. beyond the
    supply terminals)
  • (b)
    the impedance of the active conductor in fault
  • (c)
    the impedance of the earth cable.
  • i.e.
    Fault current = voltage / a + b + c
  • Now
    when the active conductor (b) is small, its impedance is much more than
    (a), so the earth (c) cable is sized to match. As the active conductor
    gets bigger, its impedance drops significantly below that of the external
    earth loop impedance (a); when It is quite large its impedance can be
    ignored. At this point there is no merit in increasing the earth cable
    size
  • i.e.
    Fault current = voltage / a + c
  • (c)
    is also very small so the fault current peaks out.
  • The
    neutral conductor is a separate issue. It is defined as an active
    conductor and therefore must be sized for continuous full load. In a
    3-phase system,
  • If
    balanced, no neutral current flows. It used to be common practice to
    install reduced neutral supplies, and cables are available with say
    half-size neutrals (remember a neutral is always necessary to provide
    single phase voltages). However the increasing use of non-linear loads
    which produce harmonics has made this practice dangerous, so for example
    the current in some standard require full size neutrals. Indeed, in big
    UPS installations I install double neutrals and earths for this reason.

41)    Why We use of Stones/Gravel in electrical
Switch Yard

  • Reducing
    Step and Touch potentials during Short Circuit Faults
  • Eliminates
    the growth of weeds and small plants in the yard
  • Improves
    yard working condition
  • Protects
    from fire which cause due to oil spillage from transformer and also
    protects from wild habitat.

42)    What is service factor?

  • Service
    factor is the load that may be applied to a motor without exceeding
    allowed ratings.
  • For
    example, if a 10-hp motor has a 1.25 service factor, it will successfully
    deliver 12.5 hp (10 x 1.25) without exceeding specified temperature rise.
    Note that when being driven above its rated load in this manner, the motor
    must be supplied with rated voltage and frequency.
  • However
    a 10-hp motor with a 1.25 service factor is not a 12.5-hp motor. If the
    10-hp motor is operated continuously at 12.5 hp, its insulation life could
    be decreased by as much as two-thirds of normal. If you need a 12.5-hp
    motor, buy one; service factor should only be used for short-term overload
    conditions

43)     Why transmission line
11KV OR 33KV, 66KV not in 10KV 20KV?

  • The
    miss concept is Line voltage is in multiple of 11 due to Form
    Factor.  The form factor of an alternating current waveform (signal)
    is the ratio of the RMS (Root Mean Square) value to the average value
    (mathematical mean of absolute values of all points on the waveform). In
    case of a sinusoidal wave, the form factor is 1.11.
  • The
    Main reason is something historical. In olden days when the electricity
    becomes popular, the people had a misconception that in the transmission
    line there would be a voltage loss of around 10%. So in order to get 100
    at the load point they started sending 110 from supply side. This is the
    reason. It has nothing to do with form factor (1.11).
  • Nowadays
    that thought has changed and we are using 400 V instead of 440 V, or 230 V
    instead of 220 V.
  • Also
    alternators are now available with terminal voltages from 10.5 kV to 15.5
    kV so generation in multiples of 11 does not arise.  Now a days when,
    we have voltage correction systems, power factor improving capacitors,
    which can boost/correct voltage to desired level, we are using the exact
    voltages like 400KV in spite of 444KV

44)    What is electrical corona?

  • Corona
    is the ionization of the nitrogen in the air, caused by an intense
    electrical field.
  • Electrical
    corona can be distinguished from arcing in that corona starts and stops at
    essentially the same voltage and is invisible during the day and requires
    darkness to see at night.
  • Arcing
    starts at a voltage and stops at a voltage about 50% lower and is visible
    to the naked eye day or night if the gap is large enough (about 5/8″ at
    3500 volts).

45)    What are the indications of electrical
corona?

  • A
    sizzling audible sound, ozone, nitric acid (in the presence of moisture in
    the air) that accumulates as a white or dirty powder, light (strongest
    emission in ultraviolet and weaker into visible and near infrared) that
    can be seen with the naked eye in darkness, ultraviolet cameras, and
    daylight corona cameras using the solar-blind wavelengths on earth created
    by the shielding ozone layer surrounding the earth.

46)    What damage does corona do?

  • The
    accumulation of the nitric acid and micro-arcing within it create carbon
    tracks across insulating materials. Corona can also contribute to the
    chemical soup destruction of insulating cements on insulators resulting in
    internal flash-over.
  • The
    corona is the only indication. Defects in insulating materials that create
    an intense electrical field can over time result in corona that creates
    punctures, carbon tracks and obvious discoloration of NCI insulators.

47)    How long does corona require creating
visible damage?

  • In
    a specific substation the corona ring was mistakenly installed backwards
    on a temporary 500kV NCI insulator, at the end of two years the NCI
    insulator was replaced because 1/3 of the insulator was white and the
    remaining 2/3 was grey.

48)    What voltage are corona rings typically
installed at?

  • It
    varies depending upon the configuration of the insulators and the type of
    insulator, NCI normally start at 160kV, pin and cap can vary starting at
    220kV or 345kV depending upon your engineering tolerances and insulators
    in the strings.

49)    How do we select transformers?

  • Determine
    primary voltage and frequency.
  • Determine
    secondary voltage required.
  • Determine
    the capacity required in volt-amperes. This is done by multiplying the
    load current (amperes) by the load voltage (volts) for single phase.
  • For
    example: if the load is 40 amperes, such as a motor, and the secondary
    voltage is 240 volts, then 240 x 40 equals 9600 VA. A 10 KVA (10,000
    volt-amperes) transformer is required.
  • Always
    select Transformer Larger than Actual Load. This is done for safety
    purposes and allows for expansion, in case more loads is added at a later
    date. For 3 phase KVA, multiply rated volts x load amps x 1.73 (square
    root of 3) then divide by 1000.
  • Determine
    whether taps are required. Taps are usually specified on larger
    transformers.

50)   Why Small Distribution Transformers not used for
Industrial Applications?

  • Industrial
    control equipment demands a momentary overload capacity of three to eight
    times’ normal capacity. This is most prevalent in solenoid or magnetic
    contactor applications where inrush currents can be three to eight times
    as high as normal sealed or holding currents but still maintain normal
    voltage at this momentary overloaded condition.
  • Distribution
    transformers are designed for good regulation up to 100 percent loading,
    but their output voltage will drop rapidly on momentary overloads of this
    type making them unsuitable for high inrush applications.
  • Industrial
    control transformers are designed especially for maintaining a high degree
    of regulation even at eight time’s normal load. This results in a larger
    and generally more expensive transformer.

51) Can 60 Hz transformers be used at higher frequencies?

  • Transformers
    can be used at frequencies above 60 Hz up through 400 Hz with no
    limitations provided nameplate voltages are not exceeded.
  •  However,
    60 Hz transformers will have less voltage regulation at 400 Hz than 60 Hz.

52) What is meant by regulation in a transformer?

  • Voltage
    regulation in transformers is the difference between the no load voltage
    and the full load voltage. This is usually expressed in terms of
    percentage.
  • For
    example: A transformer delivers 100 volts at no load and the voltage drops
    to 95 volts at full load, the regulation would be 5%. Distribution
    transformers generally have regulation from 2% to 4%, depending on the size
    and the application for which they are used.

53) Why is impedance important?

  • It
    is used for determining the interrupting capacity of a circuit breaker or
    fuse employed to protect the primary of a transformer.
  • Example:
    Determine a minimum circuit breaker trip rating and
    interrupting capacity for a 10 KVA single phase transformer with 4%
    impedance, to be operated from a 480 volt 60 Hz source.
  • Calculate:
  • Normal
    Full Load Current = Nameplate Volt Amps / Line Volts = 10,000 VA / 480 V =
    20.8 Amperes
  • Maximum
    Short Circuit Amps = Full Load Amps / 4% =20.8 Amps / 4%= 520 Amp
  • The
    breaker or fuse would have a minimum interrupting rating of 520 amps at
    480 volts.
  • Example:
    Determine the interrupting capacity, in amperes, of a circuit
    breaker or fuse required for a 75 KVA, three phase transformer, with a
    primary of 480 volts delta and secondary of 208Y/120 volts. The
    transformer impedance (Z) = 5%. If the secondary is short circuited
    (faulted), the following capacities are required:
  • Normal
    Full Load Current =Volt Amps / √ 3 x Line Volts= 75,000 VA / √ 3 x Line
    Volts √ 3 x 480 V =90 Amps
  • Maximum
    Short Circuit Line Current = Full Load Amps / 5%=  90 Amps /  5%
    =1,800 Amps
  • The
    breaker or fuse would have a minimum interrupting rating of 1,800 amps at
    480 volts.
  • Note:
    The secondary voltage is not used in the calculation. The
    reason is the primary circuit of the transformer is the only winding being
    interrupted.

54) What causes flash-over?

  • Flash-over
    causes are not always easily explained, can be cumulative or stepping
    stone like, and usually result in an outage and destruction. The first
    flash-over components are available voltage and the configuration of the
    energized parts, corona may be present in many areas where the flash-over
    occurs, and flash-over can be excited by stepping stone defects in the
    insulating path.

55) What are taps and when are they used?

  • Taps
    are provided on some transformers on the high voltage winding to correct
    for high or low voltage conditions, and still deliver full rated output
    voltages at the secondary terminals. Taps are generally set at two and a
    half and five percent above and below the rated primary voltage.

56) Can Transformers be reverse connected?

  • Dry
    type distribution transformers can be reverse connected without a loss of
    KVA rating, but there are certain limitations. Transformers rated 1 KVA
    and larger single phase, 3 KVA and larger three phases can be reverse
    connected without any adverse effects or loss in KVA capacity.
  • The
    reason for this limitation in KVA size is, the turns ratio is the same as
    the voltage ratio.
  • Example:
    A transformer with a 480 volt input, 240 volt output— can have the output
    connected to a 240 volt source and thereby become the primary or input to
    the transformer, then the original 480 volt primary winding will become
    the output or 480 volt secondary.
  • On
    transformers rated below 1 KVA single phase, there is a turn’s ratio
    compensation on the low voltage winding. This means the low voltage
    winding has a greater voltage than the nameplate voltage indicates at no
    load.
  • For
    example, a small single phase transformer having a nameplate voltage of
    480 volts primary and 240 volts secondary, would actually have a no load
    voltage of approximately 250 volts, and a full load voltage of 240 volts.
    If the 240 volt winding were connected to a 240 volt source, then the
    output voltage would consequently be approximately 460 volts at no load
    and approximately 442 volts at full load. As the KVA becomes smaller, the
    compensation is greater—resulting in lower output voltages.
  • When
    one attempts to use these transformers in reverse, the transformer will
    not be harmed; however, the output voltage will be lower than is indicated
    by the nameplate.

57) What is the difference between “Insulating”, “Isolating”, and
“Shielded Winding” transformers?

  • Insulating
    and isolating transformers are identical. These terms are used to describe
    the separation of the primary and secondary windings. A shielded
    transformer includes a metallic shield between the primary and secondary
    windings to attenuate (lessen) transient noise.

58) How many BTU’s of heat does a transformer generate?

  • The
    heat a transformer generates is dependent upon the transformer losses. To
    determine air conditioning requirements multiply the sum of the full load
    losses (obtained from factory or test report) of all transformers in the
    room by 3.41 to obtain the BTUs/hour.
    For example: A transformer with losses of 2000 watts will generate
    6820 BTUs/hour.

59) What is a transformer and how does it work?

  • A
    transformer is an electrical apparatus designed to convert alternating
    current from one voltage to another. It can be designed to “step up” or
    “step down” voltages and works on the magnetic induction principle.
  • A
    transformer has no moving parts and is a completely static solid state
    device, which insures, under normal operating conditions, a long and
    trouble-free life. It consists, in its simplest form, of two or more coils
    of insulated wire wound on a laminated steel core.
  • When
    voltage is introduced to one coil, called the primary, it magnetizes the
    iron core. A voltage is then induced in the other coil, called the
    secondary or output coil. The change of voltage (or voltage ratio) between
    the primary and secondary depends on the turns ratio of the two coils.

60) Factors Affecting Corona Discharge Effect:

  • Corona
    Discharge Effect occurs because of ionization if the atmospheric air
    surrounding the voltage conductors, so Corona Discharge Effect is affected
    by the physical state of the atmosphere as well as by the condition of the
    lines.
  • (1) Conductor:
    Corona Discharge Effect is considerably affected by the shape, size
    and surface conditions of the conductor .Corona Discharge Effect decreases
    with increases in the size (diameter) of the conductor, this effect is
    less for the conductors having round conductors compared to flat
    conductors and Corona Discharge Effect is concentrated on that places more
    where the conductor surface is not smooth.
  • (2)
    Line Voltage:
    Corona Discharge effect is not present when the applied line
    voltages are less. When the Voltage of the system increases (In EHV
    system) corona Effect will be more.
  • (3)
    Atmosphere:
    Breakdown voltage directly proportional to the density of the
    atmosphere present in between the power conductors. In a stormy weather
    the ions present around the conductor is higher than normal weather
    condition So Corona Breakdown voltage occurs at low voltages in the stormy
    weather condition compared to normal conditions
  • (4)Spacing
    between the Conductors:
    Electro static stresses
    are reduced with increase in the spacing between the conductors. Corona
    Discharge Effect takes place at much higher voltage when the distance
    between the power conductors increases.

61) Will a transformer change Three Phases to Single Phase?

  • A
    transformer will not act as a phase changing device when attempting to
    change three phase to single phase.
  • There
    is no way that a transformer will take three phase in and deliver single
    phase out while at the same time presenting a balanced load to the three
    phase supply system.
  • There
    are, however, circuits available to change three phase to two phase or
    vice versa using standard dual wound transformers. Please contact the
    factory for two phase applications.

62) Can 60 Hz transformers be operated at 50 Hz?

  • Transformers
    rated below 1 KVA can be used on 50 Hz service.
  • Transformers
    1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service, due
    to the higher losses and resultant heat rise. Special designs are required
    for this service. However, any 50 Hz transformer will operate on a 60 Hz
    service.

63) Can transformers be used in parallel?

  • Single
    phase transformers can be used in parallel only when their impedances and
    voltages are equal. If unequal voltages are used, a circulating current
    exists in the closed network between the two transformers, which will
    cause excess heating and result in a shorter life of the transformer. In
    addition, impedance values of each transformer must be within 7.5% of each
    other.
  • For
    example: Transformer A has an impedance of 4%, transformer B which is to
    be parallel to A must have impedance between the limits of 3.7% and 4.3%.
    When paralleling three phase transformers, the same precautions must be
    observed as listed above, plus the angular displacement and phasing
    between the two transformers must be identical.

64) What are causes of insulator failure?

  • Electrical
    field intensity producing corona on contaminated areas, water droplets,
    icicles, corona rings, … This corona activity then contributes nitric acid
    to form a chemical soup to change the bonding cements and to create carbon
    tracks, along with ozone and ultraviolet light to change the properties of
    NCI insulator coverings. Other detrimental effects include water on the
    surface or sub-surface freezing and expanding when thawing, as a liquid
    penetrating into a material and then a sudden temperature change causes
    change of state to a gas and rapid expansion causing fracture or rupture
    of the material.

65)  Causes of Corona

  • Corona
    is causes by the following reasons:
  • The
    natural electric field caused by the flow of electrons in the conductor.
    Interaction with surrounding air.
    Poor or no insulation is not a major cause but
    increases corona.
  • The
    use of D.C (Direct Current) for transmission.(Reason why most transmission
    is done in form of AC)

66) Effects of Corona
1)    
Line Loss – Loss of energy because some energy is used up to cause vibration of
the air particles.
2)    
Long term exposure to these radiations may not be good to health (yet to be
proven).
3)    
Audible Noise
4)    
Electromagnetic Interference to telecommunication systems
5)    
Ozone Gas production
6)    
Damage to insulation of conductor.
67) What is polarity, when associated with a transformer?

  • Polarity
    is the instantaneous voltage obtained from the primary winding in relation
    to the secondary winding.
  • Transformers
    600 volts and below are normally connected in additive polarity — that is,
    when tested the terminals of the high voltage and low voltage windings on
    the left hand side are connected together, refer to diagram below. This
    leaves one high voltage and one low voltage terminal unconnected.
  • When
    the transformer is excited, the resultant voltage appearing across a
    voltmeter will be the sum of the high and low voltage windings.
  • This
    is useful when connecting single phase transformers in parallel for three
    phase operations. Polarity is a term used only with single phase
    transformers.

68) What is exciting current?

  • Exciting
    current, when used in connection with transformers, is the current or
    amperes required for excitation. The exciting current on most lighting and
    power transformers varies from approximately 10% on small sizes of about 1
    KVA and smaller to approximately .5% to 4% on larger sizes of 750 KVA. The
    exciting current is made up of two components, one of which is a real
    component and is in the form of losses or referred to as no load watts;
    the other is in the form of reactive power and is referred to as KVAR.

69) What is Boucholz relay and the significance of it in to the
transformer?

  • Boucholz
    relay is a device which is used for the protection of transformer from its
    internal faults,
  • it
    is a gas based relay. whenever any internal fault occurs in a transformer,
    the boucholz relay at once gives a horn for some time, if the transformer
    is isolated from the circuit then it stop its sound itself otherwise it
    trips the circuit by its own tripping mechanism.

70) Why we do two types of earthing on transformer (Body earthing
& neutral earthing)

  • The
    two types of earthing are Familiar as Equipment earthing and system
    earthing.
  • In
    Equipment earthing: body (non conducting part) of the equipment should be
    earthed to safeguard the human beings.
  • The
    System Earthing : In this neutral of the supply source ( Transformer or
    Generator) should be grounded. With this, in case of unbalanced loading
    neutral will not be shifted. So that unbalanced voltages will not arise.
    We can protect the equipment also. With size of the equipment (
    transformer or alternator)and selection of relying system earthing will be
    further classified into directly earthed, Impedance earthing, resistive
    (NGRs) earthing.

71) Conductor corona is caused by?

  • Corona
    on a conductor can be due to conductor configuration (design) such as
    diameter too small for the applied voltage will have corona year-around
    and extreme losses during wet weather, the opposite occurs during dry
    weather as the corona produces nitric acid which accumulates and destroys
    the steel reinforcing cable (ACSR) resulting in the line dropping. Road
    salts and contaminants can also contribute to starting this deterioration.

72) What is flash-over and arcing?

  • Flash-over
    is an instantaneous event where the voltage exceeds the breakdown
    potential of the air but does not have the current available to sustain an
    arc, an arc can have the grid fault current behind it and sustain until
    the voltage decreases below 50% or until a protective device opens.
  • Flash-over
    can also occur due to induced voltages in unbounded (loose bolts, washers,
    etc) power pole or substation hardware, this can create RFI/TVI or
    radio/TV interference. Arcing can begin at 5 volts on a printed circuit
    board or as the insulation increases it may require 80kVAC to create
    flash-over on a good cap and pin insulator.

73) How to Minimizing Corona Effects

  • Installing
    corona rings at the end of transmission lines.
  • A
    corona ring, also called anti-corona ring, is a toroid of (typically)
    conductive material located in the vicinity of a terminal of a high
    voltage device. It is electrically insulated.
  • Stacks
    of more spaced rings are often used. The role of the corona ring is to distribute
    the electric field gradient and lower its maximum values below the corona
    threshold, preventing the corona discharge.

74) What is BIL and how does it apply to transformers?

  • BIL
    is an abbreviation for Basic Impulse Level. Impulse tests are dielectric
    tests that consist of the application of a high frequency steep wave front
    voltage between windings, and between windings and ground. The Basic
    Impulse Level of a transformer is a method of expressing the voltage surge
    (lightning, switching surges, etc.) that a transformer will tolerate
    without breakdown.
  • All
    transformers manufactured in this catalog, 600 volts and below, will
    withstand the NEMA standard BIL rating, which is 10 KV.
  • This
    assures the user that he will not experience breakdowns when his system is
    properly protected with lightning arrestors or similar surge protection
    devices.

75) The difference between Ground and Neutral?

  • NEUTRAL
    is the origin of all current flow. In a poly-phase system, as its phase
    relationship with all the three phases is the same, (i.e.) as it is not
    biased towards any one phase, thus remaining neutral, that’s why it is
    called neutral.
  • Whereas,
    GROUND is the EARTH on which we stand. It was perceived to utilize this
    vast, omnipresent conductor of electricity, in case of fault, so that the
    fault current returns to the source neutral through this conductor given
    by nature which is available free of cost. If earth is not used for this
    purpose, then one has to lay a long. long metallic conductor for the
    purpose, thus increasing the cost.
  • Ground
    should never be used as neutral. The protection devices (eg
    ELCB, RCD etc) work basically on principle that the phase currects are
    balanced with neutral current. In case you use ground wire as the neutral,
    these are bound to trip if they are there – and they must be there. at
    least at substations. And these are kept very sensitive i.e. even minute
    currents are supposed to trip these.
  • One
    aspect is safety – when someone touches a neutral, you don’t want him to
    be electrocuted – do you? Usually if you see the switches at home are on
    the phase and not neutral (except at the MCB stage). Any one assumes the
    once the switch is off, it is safe (the safety is taken care of in 3 wire
    system, but again most of the fixtures are on 2 wire) – he will be shocked
    at the accidental touching of wire in case the floating neutral is
    floating too much.

76) What is impedance of a transformer?

  • If
    you mean the percentage impedance of the transformed it means the ratio of
    the voltage( that if you applied it to one side of the transformer while
    the other side of the transformer is short cuitcuted, a full load current
    shall flow in the short circuits side), to the full load current.
  • More
    the %Z of transformer, more Copper used for winding, increasing cost of the
    unit. But short circuit levels will reduce, mechanical damages to windings
    during short circuit shall also reduce. However, cost increases
    significantly with increase in %Z.
  • Lower
    %Z means economical designs. But short circuit fault levels shall increase
    tremendously, damaging the winding & core.
  • The
    high value of %Z helps to reduce short circuit current but it causes more
    voltage dip for motor starting and more voltage regulation (% change of
    voltage variation) from no load to full load.

77) How are transformers sized to operate Three Phase induction
type squirrel cage motors?

  • The
    minimum transformer KVA rating required to operate a motor is calculated
    as follows:
  • Minimum
    Transformer KVA =Running Load Amperes x 1.73x Motor Operating Voltage /
    1000
  • NOTE:
    If motor is to be started more than once per hour add 20%
    additional KVA. Care should be exercised in sizing a transformer for an
    induction type squirrel cage motor as when it is started, the lock rotor
    amperage is approximately 5 to 7 times the running load amperage. This
    severe starting overload will result in a drop of the transformer output
    voltage.
  • When
    the voltage is low the torque and the horsepower of the motor will drop
    proportionately to the square of the voltage.
  • For
    example: If the voltage  were to drop to 70% of nominal, then motor
    horsepower and torque would drop to 70 % squared or 49% of the motor
    nameplate rating.
  • If
    the motor is used for starting a high torque load, the motor may stay at
    approximately 50% of normal running speed The underlying problem is low
    voltage at the motor terminals. If the ampere rating of the motor and
    transformer over current device falls within the motor’s 50% RPM draw
    requirements, a problem is likely to develop. The over current device may
    not open under intermediate motor ampere loading conditions.
  • Overheating
    of the motor and/or transformer would occur, possibly causing failure of
    either component.
  • This
    condition is more pronounced when one transformer is used to power one
    motor and the running amperes of the motor is in the vicinity of the full
    load ampere rating of the transformer. The following precautions should be
    followed:
  • (1)When
    one transformer is used to operate one motor, the running amperes of the
    motor should not exceed 65% of the transformer’s full load ampere rating.
  • (2)
    If several motors are being operated from one transformer,
    avoid having all motors start at the same time. If this is impractical,
    then size the transformer so that the total running current does not
    exceed 65% of the transformer’s full load ampere rating.

78) Which Point need to be consider while Neutral Earthing of
Transformer?

  • The
    following points need to check before going for Neutral Grounding
    Resistance.
  • Fault
    current passing through ground, step and touch potential.
  • Capacity
    of transformer to sustain ground fault current, w.r.t winding, core
    burning.
  • Relay
    co-ordination and fault clearing time.
  • Standard
    practice of limiting earth fault current. In case no data or calculation
    is possible, go for limiting E/F current to 300A or 500A, depending on
    sensivity of relay.

79) Why a neutral grounding contactor is needed in diesel
generator?

  • There
    would not be any current flow in neutral if DG is loaded equally in 3
    phases , if there any fault(earth fault or over load) in any one of the phase
    ,then there will be un balanced load in DG . at that time heavy current
    flow through the neutral ,it is sensed by CT and trips the DG. so neutral
    in grounded to give low resistance path to fault current.
  • An
    electrical system consisting of more than two low voltage Diesel Generator
    sets intended for parallel operation shall meet the following conditions:
  • (i)
    Neutral of only one generator needs to be earthed to avoid the flow of
    zero sequence current.
  • (ii)
    During independent operation, neutrals of both generators are required in
    low voltage switchboard to obtain three phases, 4 wire system including
    phase to neutral voltage.
  • (iii)
    required to achieve restricted earth fault protection (REF) for both the
    generators whilst in operation.
  • Solution:
  • Considering
    the requirement of earthing neutral of only one generator, a contactor of
    suitable rating shall be provided in neutral to earth circuit of each
    generator. This contactor can be termed as “neutral contactor”.
  • Neutral
    contactors shall be interlocked in such a way that only one contactor
    shall remain closed during parallel operation of generators. During
    independent operation of any generator its neutral contactor shall be
    closed.
  • Operation
    of neutral contactors shall be preferably made automatic using breaker
    auxiliary contacts.

80) Neutral grounded system vs solidly grounded system

  • In
    India, at low voltage level (433V) we MUST do only Solid Earthing
    of the system neutral.
  • This
    is by IE Rules 1956, Rule No. 61 (1) (a). Because, if we option for
    impedance earthing, during an earth fault, there will be appreciable
    voltage present between the faulted body & the neutral, the magnitude
    of this voltage being determined by the fault current magnitude and the
    impedance value.
  • This
    voltage might circulate enough current in a person accidentally coming in
    contact with the faulted equipment, as to harm his even causing death.
    Note that, LV systems can be handled by non-technical persons too. In
    solid earthing, you do not have this problem, as at the instant of an earth
    fault, the faulted phase goes to neutral potential and the high fault
    current would invariably cause the Over current or short circuit
    protection device to operate in sufficiently quick time before any harm
    could be done

81)    What is the reason of grounding or earthing of
equipment?

  • with
    a ground path, in case of short circuit the short circuit current goes to
    the body of the equipment & then to the ground through the ground
    wire. Hence if at the moment of fault if a person touches the equipment
    body he will not get a shock cause his body resistance (in thousands of
    ohms) will offer a high resistance path in comparison to the ground wire.
    Hence the fault current will flow thru the ground wire & not thru
    human body.
  • Providing
    a ground path helps in clearing the fault. A CT in the ground connection
    detects the high value fault current hence the relay connected to the CT
    gives breaker a trip command.
  • Grounding
    helps in avoiding arcing faults. IF there would have been no ground then a
    fault with the outer body can cause a arcing to the ground by breaking the
    air. This is dangerous both for the equipment & the human beings.

82)    A type-C MCB has thermo magnetic capability
5In to 10In that means a short circuit current will be interrupted as the value
will reach between 5In to 10In but the MCB breaking capacity is (for example)
define as 10kA.

  • 5In
    to 10In is the pickup threshold for the magnetic trip element. The MCB
    will trip instantaneously when the current is between these limits. 10kA
    is the short circuit withstands capacity of the MCB.
  • Under
    normal condition, a current limiting type MCB will trip on short circuit
    (magnetic trip) and the current during circuit interruption will be much
    less than the prospective current. However, the MCBs have to have a short
    circuit capacity more than or equal to the fault level at the location
    where it is installed.

83)    What is Ferrari Effect?

  • Ferranti
    Effect is due to the rise in voltage at the receiving end than that of the
    sending end. This occurs when the load on the system reduces suddenly.
  • Transmission
    line usually consists of line inductance, line to earth capacitance and
    resistance. Resistance can be neglected with respect to the line
    inductance .When the load on the system falls the energy stored in the
    capacitance gets discharged. The charging current causes inductive
    reactance voltage drop. This gets added vector ally to the sending end
    voltage and hence causes the voltage at the receiving end to raise
  • A
    Long transmission line draws significant amount of charging current. If
    such line is open circuited or very lightly loaded at the receiving end,
    the voltage at the receiving end may become greater than sending end
    voltage. This effect is known Ferranti effect and is due to the voltage
    drop across the line inductance (due to charging current) being in phase
    with the sending end voltages. Therefore both capacitance and inductance
    is responsible to produce this phenomenon.
  • The
    capacitance (charging current) is negligible in short lines, but significant
    in medium and long transmission line. Hence, this phenomenon is applicable
    for medium and long transmission line.
    The main impact of this phenomenon is on over
    voltage protection system, surge protection system, insulation level etc.

84)    Can single phase transformers be used for
three phase applications?

  • Yes.
    Three phase transformers are sometimes not readily available whereas
    single phase transformers can generally be found in stock. Three single
    phase transformers can be used in delta connected primary and wye or delta
    connected secondary. They should never be connected wye primary to wye
    secondary, since this will result in unstable secondary voltage. The
    equivalent three phase capacity when properly connected of three single
    phase transformers is three times the nameplate rating of each single
    phase transformer.

85)    What is BIL and how does it apply to
transformers?

  • BIL
    is an abbreviation for Basic Impulse Level. Impulse tests are dielectric
    tests that consist of the application of a high frequency steep wave front
    voltage between windings, and between windings and ground. The BIL of a
    transformer is a method of expressing the voltage surge that a transformer
    will tolerate without breakdown.

86)    Where Auto-recloser is used?

  • For
    Generator protection / Transformer Protection / Transmission Line / Bus
    bar protection.
  • Many
    faults on overhead transmission lines are transient in nature 90% of
    faults are used by birds, tree branches. These condition results in
    arching faults and the arc in the fault can be extinguished by
    de-energizing the lines by opening of CB on the both ends of the lines.
  • Open-0.3
    second-Close-3minute-Close this is the sequence of AR. i.e.-OPEN,C-CLOSED
  • whenever
    faults occurs CB opens, then after 0.3 sec it closes automatically, if
    faults persists then it will open after 3 min it closes and if still fault
    persists. It remains in open condition.
  • Auto
    reclosure is generally used for Transmission lines where the general types
    of faults are transient in nature.
  • It
    can be three phase auto-reclosure or single pole auto-reclosure.
  • The
    single pole auto reclosures are generally for 400kV line below this three
    pole auto- reclosures are used.
  • The
    reason for a line the single pole reclosures provides a better stability
    of the system since some part of power is still transferred through the
    healthy phases.
  • Also
    400kV breaker till date has a independent drive/ trip/ close coils for the
    three poles, below that all breakers have common drive/ trip / closing
    coils for the three poles.

87)    What is difference between power transformers
& distribution transformers?

  • Distribution
    Transformers are designed for a maximum efficiency at 50% of load. Whereas
    power transformers are designed to deliver max efficiency ay 90% and above
    loads.
  • The
    distributions transformers have low impedance so as to have a better
    regulation power transformers have higher so as to limit the SC current.
  • Power
    transformers are used to step up voltages from 11 KV which is the
    generating voltage to 132 or whatever will be the transmission voltage
    levels. Power transformers are having Star-Delta connection. It will be
    located at power generating stations.
  • Distribution
    transformers are used to step down voltages from transformer levels to 11
    KV/415 V. Will be having Delta-Star. It will be located in substations
    near load centers.
  • The
    main basic difference lies in the Design stage itself as power transformer
    are to operate at near full load so there sensing is such that they
    achieve equal. of copper losses & iron losses at full loads whereas
    this is achieved in the design itself at about 50% loading in dist
    transformer but friends there is a dilemma as our dist. transformer are
    almost fully loaded & beyond so they never go operate at their full
    eff. & also poor voltage regulation.
  • The
    difference between power and distribution transformers refers to size
    & input voltage. Distribution transformers vary between 25 kVA and 10
    MVA, with input voltage between 1 and 36 kV. Power transformers are
    typically units from 5 to 500 MVA, with input voltage above 36 kV.
    Distribution transformer design to have a max efficiency at a load lower
    than full load. Power transformer design to have a max efficiency at full
    load

88) What will be happen if the neutral isolator will be open or
close during the running condition of power?

  • During
    normal condition the neutral isolating switch should be kept close. In
    case it is kept open, under balanced load conditions the current through
    neutral will not flow & nothing harmful will take place but in case an
    earth fault takes place then there will be no earth fault current flowing
    through the system & the generator will run as a ungrounded generator.
    Thus the earth fault will not be cleared.
  • If
    more number of generators are connected parallel. We will have a close
    loop and hence negative sequence current will flow. This will increase the
    rotor temperature. Hence if more number of generators are connected then
    only one is earthed and others are open.
  • In
    case of Two or more generators connected to a common bus without a transformer
    in between, basically in hydro stations, one of the Neutral Isolation
    Switch(NIS) is kept closed & rest are opened to prevent circulating
    currents to flow between generators. Hence the above explanation will not
    be valid for such systems.
  • Sometime
    we may want to test generator and may want to isolate the neutral from
    ground. like for example meggaring etc. In such case we would like to open
    ground connection cable in case we want to remove the NIS? we will
    certainly not like to open all the bolted connections for just a small
    test like checking insulation with a meggar etc. for such things we need a
    NIS.
  • Neutral
    isolator is required if we have delta transmission system and at the time
    to connection with the Grid Neutral isolation is required.
  • If
    we ungrounded the neutral then the generator is connected to the ground
    via Phase to earth capacitances. Hence during faults arcing grounds can
    take place. Which are dangerous both to human & equipment.
  • When
    we provide earthed neutral, for a fault, earth fault current will start
    flowing through the neutral, which we can sense thru a CT & relay
    & hence can immediately identify & clear the fault in about 100 ms
    by opening the associated breaker/prime mover/excitation. Quicker the
    fault clearance less is the damage.

89)    Why shorting type terminal required for CT?

  • During
    maintenance or secondary injection you will need to bypass the CT &
    for the same you need shorting link. During sec. injection you will short
    circuit the main CT & bypass it. Open circuiting the CT will saturate
    it & damage it.

90) Why fuse is given for only PT and not CT?

  • Fuse
    if given for CT blows off due to a fault then rather than protecting the
    CT it will make it open circuited hence it will be saturated &
    damaged. For PT it gives overload & SC protection.

91) Why is insulating base required for LA?

  • The
    LA is provided with a dedicated Prper earthing which may be in the form of
    a buried treated electrode next to it.LA connection is securely made with
    the electrode via a surge counter. If we directly earth the LA through
    structure then the surge counter will not be able to measure the no of
    surges. For lesser rating the counter is not provided, hence we can bypass
    the insulated base. But then proper earthing has to be assured.

92) Can 60 Hz transformers be operated at 50 Hz?

  • Transformers
    1 KVA and larger, rated at 60 Hz, should not be used on 50 Hz service due
    to higher losses and resultant heat rise. However, any 50 Hz transformer
    will operate on 60 Hz service.

93) Can transformers be used in parallel?

  • Single
    phase transformers can be used in parallel only when their voltages are
    equal. If unequal voltages are used, a circulating current exists in the
    closed network between the two transformers which will cause excess
    heating and result in a shorter life of the transformer. In addition
    impedance values of each transformer must be within 7.5% of each other.

94) Can Transformers be reverse connected?

  • Dry
    type distribution transformers can be reverse connected without a loss of
    KVA rating, but there are certain limitations. Transformers rated 1 KVA
    and larger single phase, 3 KVA and larger three phases can be reverse
    connected without any adverse effects or loss in KVA capacity.

95) Why short circuit do not take place when electrode is touched to
ground.

  • Basically
    during welding we force a short-circuit at the electrode tip. The fault
    condition produces large magnitude currents. Greater the Current value
    have greater I2R heat produced. The arcing energy elevates the temperature
    & hence melts the electrode material over the joint.
  • The
    transformer is designed to withstand such high currents. But welding is a
    very complex & detailed phenomenon. Besides there are many principles
    on which welding operates. Some may be a welding, dc welding, arc, constant
    voltage, constant current etc

96) What’s the difference between generator breaker and simple
breaker?

  • Breaker
    is one which disconnects the circuit in fault condition and It is similar
    for all equipment. Based on the equipment voltage and maximum short circuit
    current the ratings will be decided. For better understanding we call
    generator or transformer or line etc breakers.

97) What is accuracy Class of the instrument?

  • Generally
    the class indicates the accuracy with which the meter will indicate or equipment
    will measure with respect to its input.
  • The
    accuracy of different equipment will depend on number of factors.
  •  For
    example for a PT accuracy will depend on its leakage reactance &
    winding resistance. For a PT accuracy gives the voltage & phase error
    & it varies with the VA burden of secondary. Also better core material
    will give better heat dissipation & reduce error. class of accuracy
    will give the voltage error for a PT
  • different
    type of PTs available are:0.1, 0.2, 0.5, 1, 5 & error values will be:
    class% voltage error(+/_) phase displacement
    Similarly indicating instruments shall have
    accuracies & accordingly application as depicted below for testing the
    following values are generally used:
  • for
    routine tests : accuracy class 1
  • for
    type tests : accuracy class 0.5 or better.
  • indicating
    meters generally will have accuracy of 1.

98) First pole to clear factor-Circuit breakers

  • The
    first pole to clear factor (kpp) is depending on the earthing system of
    the network. The first pole to clear factor is used to calculating the
    transient recovery voltage for three phase faults. In general following
    cases apply:-
    1. kpp = 1.3 corresponds to three phase faults
    in system with an earthed neutral.
    2. kpp = 1.5 corresponds to three phase faults
    in isolated or resonant earthed system.
    3. kpp = 1.0 corresponds to special cases e.g.
    railway systems.
  • A
    special case is when there is a three phase fault without involving earth
    in a system with earthed neutral. This case responds to kpp = 1.5 . This
    special case is however not normally considered in the standards.

99) Why we use a resistance to ground the neutral when we need
always low resistivity for the grounding?

  • If
    we ground the generator directly then whenever a fault will take place at
    any phase with ground the fault current flowing throw the faulted phase-to
    ground-to neutral will be very high cause there will be no resistance to
    limit the value of fault current. Hence we insert a resistance in the
    neutral circuit to limit this fault current. Also we need to reduce the
    fault current to such a value that the protection CTs are able to identify
    the fault current without saturating the CTs. Communicate it to the
    protection relays & hence the relays can then isolate the system from
    the fault; so that the system is isolated from the fault before the harm
    is done by the fault current. That is the reason that all the equipment
    will be designed for fault KA values for 1 sec so that the total
    operation(CT sensing-relay functioning-circuit breaker operation ) time
    will be less than 1 sec. hence the Breakers will isolate the fault before
    1 sec i.e. within the time period the equipment are designed to carry the
    fault current. Thus all your objectives of:
  • preventing
    the arcing.
  • limiting
    the fault current.
  • isolating
    the faulted system are achieved

100) Why are NGR’s rated for 10sec?

  • NGR
    are placed in the neutral circuit & hence will be energized only in
    the fault conditions thus their continuous loading is not expected. Hence
    they are selected for intermittent rating. Similarly when we place a
    transformer in the neutral grounding circuit the KVA rating obtained after
    the calculation is multiplied by a diversity factor to obtain smaller
    rating cause the therefore It will not be continuously rated.
  • NIS
    is also provided to cut the circulating negative sequence current in 2
    more generators connected in parallel.
    in some grid conditions they ask to keep neutral
    isolated after being connected to grid.

101) How to calculate knee point voltage and significance of knee
point voltage?

  • Knee
    point voltage: That point on the magnetizing curve (BH curve) where an
    increase of 10% in the flux density (voltage) causes an increase of 50% in
    the magnetizing force (current). Its significance lies mainly in PS class
    core of CTs used for diff protection

102) Design method for neutral grounding resistor?

  • NGR
    design basics:
  • Capacitive
    coupling of generator, equipment and the ground
    -Generator to ground capacitance.
    -Generator cable to ground capacitance (or bus
    duct as the case may be)
    -Low voltage winding of trafo & ground
    capacitance.
    -Surge arrestor capacitance.
  • The
    total capacitance is then obtained from the above values & then we
    calculate from that the capacitive reactance. The capacitive current then
    produced is calculated from the generator voltage & the capacitive
    reactance obtained above. Once the current is obtained we can then
    calculate the electrostatic KVA from the current multiplied with voltage.

103) Criterion is there for selection of Insulation Disc in
Transmission and Distribution Line.

  • 11kV
    is the phase to earth voltage for 220kV =220/ (sqrt(3)*11)=12 No’s of disc
    are suitable.The number can be increased to increase the creep age
    distance.
  • While
    selecting the disc insulators one has to keep in mind the following
    things:
    1. EM-strength of the string. All the forces
    coming on to the string & the ability of the string to withstand them.
    2. Sufficient Cree page distance so as not to
    cause a flashover .
    3. Interface with the type of conductor used
    (moose, tarantula, zebra etc)
  • So
    we will get the value of no of discs by dividing the phase to earth
    voltage with 1.732. Once that is done then we need to see its suitability
    with respect to EM strength.
  • After
    this we need to consider the force that the stack has to bear. If we have
    a strain type of fitting i.e. the stack has to bear horizontal conductor
    tension, weight load of the conductor, wind load, ice load etc then the
    number of insulator discs required may be more.
  • But
    for a suspension type system which has to bear only the weight then number
    of discs required may be less than what we get by dividing by 11. That is
    the reason we have seen only 23/24 discs in 400 kv line cause in that case
    the creep age obtained must have been enough & also the strain
    requirement.
  • 33kv
    insulators are generally used in a vertical installation & are not
    stacked together because that will make the suspension very rigid

104) Do taps work the same when a transformer is reverse fed?

  • Taps
    are normally in the primary winding to adjust for varying incoming
    voltage. If the transformer is reverse fed, the taps are on the output
    side and can be used to adjust the output voltage.

105) Why may I get the wrong output voltage when installing a step
up transformer?

  • Transformer
    terminals are marked according to high and low voltage connections. An H
    terminal signifies a high voltage connection while an X terminal signifies
    a lower voltage connection. A common misconception is that H terminals are
    primary and X terminals secondary. This is true for step down
    transformers, but in a step up transformer the connections should be
    reversed. Low voltage primary would connect to X terminals while high
    voltage secondary would connect on the H terminals.

106) Can a single phase transformer be used on a three phase source?

  • Yes.
    Any single phase transformer can be used on a three phase source by
    connecting the primary leads to any two wires of a three phase system,
    regardless of whether the source is three phase 3-wire or three phase
    4-wire. The transformer output will be single phase.

107) Why in Double circuit wire are transposed (R – B, Y – Y, B – R)

  • This
    is done to avoid
    1. Proximity effect
    2. Skin effect
    3. Radio interference
    4. Reduction in noise in communication Signals

108) Selection of LA

  • The
    voltage rating of LA is selected as: Line voltage x sqrt(2)/ sqrt(3) so
    for 11kV line its 9kV
  • In
    that case also the values would not differ much if We takes the TOV factor
    as 1.4. However, we can take the value of 1.56 as TOV to be more precise.

109) Which is more dangerous AC or DC

  • Low
    frequency (50 – 60 Hz) AC currents can be more dangerous than similar
    levels of DC current since the alternating fluctuations can cause the
    heart to lose coordination, inducing ventricular fibrillation, which then
    rapidly leads to death.
  • However
    any practical distribution system will use voltage levels quite sufficient
    to ensure a dangerous amount of current will flow, whether it uses
    alternating or direct current. Since the precautions against electrocution
    are similar, ultimately, the advantages of AC power transmission outweighed
    this theoretical risk, and it was eventually adopted as the standard.

110) What all are the applications where high speed grounding
switches are used.

  • Generator
    neutral is earthed directly or through distribution transformer. This
    neutral earthing is through done through a switch. This is general
    practice for only one generator.
  • For
    two generators in parallel to a bus the neutral earthing is different. If
    both the neutral earthing is closed the negative sequence current will be
    flowing though both the generator taking earth as path. This leads to
    increase in loss and increase in temperature (This may leads to false
    tripping also). Hence once the second generator is synchronized with the
    bus or grid the neutral is isolated.
  • Neutral
    grounding switch we do not need a high speed grounding switch. A normal
    switch with the correct rating capacity would also work.

111) What is Skin Effect and how does it happen??

  • According
    to faradays law of electromagnetic induction, a conductor placed in a
    changing magnetic field induces an emf. The effect of back emf is maximum
    at the centre because of maximum lines of field there. Hence the maximum
    opposition of current at inner side of conductor and minimum opposition at
    the surface. Hence the current tries to follow at the surface. It is due
    to this reason that we take hollow tube conductors in bus duct.
  • Taking
    into account the inductance effect, its simple consider the DC current.
    Since its constant & not varying hence no back emf but if we gradually
    start increasing the frequency then the flux cutting the conductor goes on
    increasing, hence greater the frequency greater the alternating flux
    cutting the conductor & hence greater the back emf & therefore
    greater the skin effect.

112) Why we ground the sheath of single core power cables and to
avoid grounded at both the ends?

  • A
    single core cable with a sheath is nothing but a conductor carrying
    current surrounded by another conductor (sheath). Hence the Alternative
    current in the conductor induces voltages in the sheath or the armour.
    Hence grounding these cables at both ends will cause the potential of the
    armour to be same as ground potential & hence shall become safe for
    the personnel.
  • But
    grounding the cables at both the end will cause a problem. In that case
    the circulating currents will start flowing with the armor, the ground
    & with the two ends of the grounding completing the circuit. This will
    also provide path for the fault currents to flow. Hence this whole thing
    will cause the cable to produce some I2R losses, hence heating & hence
    the current carrying capacity will be de rated. This system of cable
    earthing is called both-end bonding. This system is suggested only when
    one wants to avoid the voltage development because can either go with the
    de rated cable or if one updates the cable in advance.
  • When
    only one end of the cable sheath is grounded then there is no path for the
    circulating current to flow. Hence the current carrying capacity of the
    cable will be good. But in this case potential will be induced between
    sheath & ground. This potential is proportional to the length of the
    cable & hence this will limit the length of the cable used. This
    method is called single point bonding. This is thus used only for short
    lengths.
  • There
    is another system called the cross bonding system in which the sheath are
    sectionaliosed & cross connected so that the circulating currents are
    minimized. Although some potential will also exist between sheath &
    ground, the same being maximum at the link boxes where bonding is done.
    This method provides maximum possible current carrying capacity with the
    maximum possible lengths.

113) What is EDO & MDO type breaker?

  • In
    the Breakers for the operation spring charging is must.
  • In
    EDO breaker the spring charging is done with a motor and draw out manually
    by hand. so EDO means Electrically spring charged Draw Out breaker
  • In
    MDO breaker the spring charging is also done by hand manually and draw out
    about also by hand only. so MDO means Manual spring charge Draw Out
    breaker

114) Why transformer rating is in KVA or KW?

  • Because
    power factor of the load is not defined in case of transformer that’s why
    it is not possible to rate transformer in KW.
  • The
    losses (cu loss and iron loss) of the transformer depends on current and
    voltage purely, not on load i.e, phase angle between the current and
    voltage i.e. why transformer rated in kVA
  • Transformer
    is not a load and having no effect on P.F (that’s why no change in its
    power factor) and it only transfer the constant power from one voltage
    level to another voltage level without changing frequency. since both the
    losses viz copper loss(depends on current) and iron loss(depends on
    voltage) are independent of power factor, that is why a Transformers
    rating is not on kW, but on KVA

115) Why the secondary of CT never open when burden is connected on
the CT.?

  • secondary
    of CT is never opened as because CT is always connected to the line so
    opening the secondary will mean there will be no counter mmf to balance
    the primary current as a result of which a very high induced emf may appear
    in the secondary as flux is very high and no counter mmf and this will be
    dangerous for the personnel in the secondary side and for pt if it is
    shorted then with full voltage applied to the primary.
  • If
    we short the secondary then much high current will circulate in the
    secondary due to high induced emf much higher than the actual full load
    current as a result of which the transformer’s secondary winding may burn
    out.

116) Distance relay setting

  • Step1:

    Get the conductor Details (i.e Positive Sequence
    Impedance (Z), Zero Sequence Impedance(Z0)) which is in Primary value.
    Convert in terms of secondary values.
  • Step
    2 :

    Based upon the calculated value divide into
    various zones
  • Zone
    1 (Forward) means 80% of your protected line length.
  • Zone
    2 (Forward) means 100% of protected line length + 20% Adjacent Shortest
    line
  • Zone
    3 (Forward) means 100% of protected line length + 50% Adjacent Longest
    line.
  • Zone
    4 (Reverse) means 10% of protected line.

117) Difference between CT class 0.2 and 0.2S?

  • 0.2S
    & 0.5S:
    Special type of measurement CTs they guarantee the declared
    accuracy, even with
    20% loading. And some definite error can be
    defined even with a load as low as 1%. Thus they are suitable for
    industries where loads are commissioned in steps or stages. Also for
    tariff metering purposes.
  • 0.2S:
    Special class for metering. It is more accurate than 0.2 classes.
    Generally if we use 0.2s class CT than VA burden of core is also come
    down.
  • In
    0.2 classes CT, ratio & phase angle errors must be within the
    specified limits at 5%, 20%, 100% & 120% of rated secondary current.
    Whereas in 0.2s class CT, ratio & phase angle errors must be within
    the specified limits at 1%, 5%, 20%, 100% & 120% of rated secondary
    current. Also in 0.2s class, Ratio & Phase angle errors limits are
    lower than 0.2 classes.

118) Why we use inductors

  • Inductors
    have the property to oppose sudden changes in Current. When
    connected to the primary side of transformer, if any sudden short circuit
    (very high) current flows due to some fault in the system, the inductor
    will oppose the flow of that current saving the transformer.
  • Secondly,
    for the problem of lagging current. Capacitors are connected across the
    inductor to improve the lagging current. So Mainly Inductor is used to (i)
    protected the transformer, (ii) solved the problem of lagging current.

119) Why do we need a bigger breaker when reverse feeding a
transformer?

  • Typically
    the output winding is wound first and is therefore closest to the core.
    When used as exciting winding a higher inrush current results. In most
    cases the inrush current is 10 to 12 times the full load current for 1/10
    of a second. When the transformer is reverse fed the inrush current can be
    up to 16 times greater. In this case a bigger breaker with a higher AIC
    rating must be used to keep the transformer online.

120) How many types of Neutral grounding system?

  • There
    are primarily three types of grounding system which are:
  • (1)Solid
    grounding
    – The neutral point of the system is grounded without any
    resistance. If the ground fault occurs, high ground current passes through
    the fault. Its use is very common in low voltage system, where line to
    neutral voltage is used for single phase loads.
  • (2)
    Low Resistance grounding (LRG)
    – This is used for
    limiting the ground fault current to minimize the impact of the fault
    current to the system. In this case, the system trips for the ground
    fault. In this system, the use of line to neutral (single phase) is
    prohibited. The ground fault current is limited to in the rage from 25A to
    600A.
  • (3)
    High Resistance Grounding (HRG)
    – It is used where
    service continuity is vital, such as process plant motors. With HRG, the
    neutral is grounded through a high resistance so that very small current
    flows to the ground if ground fault occurs. In the case of ground fault of
    one phase, the faulty phase goes to the ground potential but the system
    doesn’t trip. This system must have a ground fault monitoring system. The
    use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in
    HRG system, however, phase to neutral is used with using the additional
    transformer having its neutral grounded. When ground fault occurs in HRG
    system, the monitoring systems gives alarm and the plant operators start
    the standby motor and stop the faulty one for the maintenance. This way, the
    process plant is not interrupted. The ground fault current is limited to
    10A or less.
  • There
    are other two types such as Corner Grounding (for Delta system) and
    ungrounded system but they are not commonly used.
121)    What value AC meters show, is it the RMS or
peak voltage?
  • AC
    voltmeters and ammeters show the RMS value of the voltage or
    current. DC meters also show the RMS value when connected to varying DC
    providing the DC is varying quickly, if the frequency is less than about
    10Hz you will see the meter reading fluctuating instead.
122)    Why in the transmission tower construction
Middle arm is longer than the upper and lower Arm.
  • Conductor
    of Upper Arm and Lower Arm will stay apart.
  • To
    prevent big birds (Ostriches etc) from bumping their heads against the
    conductor above when they sit on the wire below.
  • Designed
    to maintain the mechanical requirement to prevent arching between
    conductors while maintaining a tower height that is manageable, and of
    course preventing head injuries to birds
  • The
    arms are of different links to prevent a broken upper line from falling on
    one or more of the phase lines below.
  • The
    clearance from other phase.
  • Mutual
    inductance minimization.
  • Preventing
    droplet of water/ice to fall on bottom conductor.
123)    What is the difference between Surge
Arrester & Lightning Arrestor
  • LA
    is installed outside and the effect of lightning is grounded, where as
    surge arrestor installed inside panels comprising of resistors which
    consumes the energy and nullify the effect of surge.
  • Transmission
    Line Lightning Protection:
  • The
    transmission line towers would normally be higher than a substation
    structure, unless you have a multi-storey structure at your substation.
  • Earth
    Mats are essential in all substation areas, along with driven earth
    electrodes (unless in a dry sandy desert site).
  • It
    is likewise normal to run catenaries’ (aerial earth conductors) for at
    least 1kM out from all substation structures. Those earth wires to be
    properly electrically to each supporting transmission tower, and bonded
    back to the substation earth system.
  • It
    is important to have the catenaries’ earth conductors above the power
    conductor lines, at a sufficient distance and position that a lightning
    strike will not hit the power conductors.
  • In
    some cases it is thus an advantage to have two catenary earth conductors,
    one each side of the transmission tower as they protect the power lines
    below in a better manner.
  • In
    lightning-prone areas it is often necessary to have catenary earthing
    along the full distance of the transmission line.
  • Without
    specifics, (and you could not presently give tower pictures in a Post
    because of a CR4 Server graphics upload problem), specifics would include:
  • Structure
    Lightning Protection:
  • At
    the Substation, it is normal to have vertical electrodes bonded to the
    structure, and projecting up from the highest points of the structure,
    with the location and number of those electrodes to be sufficient that if
    a lightning strike arrived, it would always be a vertical earthed
    electrode which would be struck, rather than any electrical equipment.
  • In
    some older outdoor substation structures, air-break isolator switches are
    often at a very high point in the structure, and in those cases small
    structure extension towers are installed, with electrodes at the tapered
    peak of those extension towers.
  • The
    extension towers are normally 600mm square approximately until the
    extension tower changes shape at the tapered peak, and in some cases
    project upwards from the general structure 2 to 6 metres, with the
    electrode some 2 to 3 metres projecting upwards from the top of the
    extension tower.
  • The
    substation normally has a Lightning Counter – which registers a strike on
    the structure or connected  to earth conductors, and the gathering of
    that information (Lightning Days, number per Day/Month/Year, Amperage of
    each strike)
124)    How Corona Discharge Effect Occur in
Transmission Line?
  • In
    a power system transmission lines are used to carry the power. These
    transmission lines are separated by certain spacing which is large in
    comparison to their diameters.
  • In
    Extra High Voltage system (EHV system ) when potential difference is
    applied across the power conductors in transmission lines then air medium
    present between the phases of the power conductors acts as insulator
    medium however the air surrounding the conductor subjects to electro
    static stresses. When the potential increases still further then the atoms
    present around the conductor starts ionize. Then the ions produced in this
    process repel with each other and attracts towards the conductor at high
    velocity which intern produces other ions by collision.
  • The
    ionized air surrounding the conductor acts as a virtual conductor and
    increases the effective diameter of the power conductor. Further increase
    in the potential difference in the transmission lines then a faint
    luminous glow of violet color appears together along with hissing noise.
    This phenomenon is called virtual corona and followed by production of
    ozone gas which can be detected by the odor. Still further increase in the
    potential between the power conductors makes the insulating medium present
    between the power conductors to start conducting and reaches a voltage
    (Critical Breakdown Voltage) where the insulating air medium acts as
    conducting medium results in breakdown of the insulating medium and flash
    over is observed. All this above said phenomenon constitutes CORONA
    DISCHARGE EFFECT in electrical Transmission lines.
125)    Methods to reduce Corona Discharge Effect:
  • Critical
    Breakdown voltage can be increased by following factors
  • By
    increasing the spacing between the conductors:
  • Corona
    Discharge Effect can be reduced by increasing the clearance spacing
    between the phases of the transmission lines. However increase in the
    phases results in heavier metal supports. Cost and Space requirement
    increases.
  • By
    increasing the diameter of the conductor:
  • Diameter
    of the conductor can be increased to reduce the corona discharge effect.
    By using hollow conductors corona discharge effect can be improved.
  • By
    using Bundled Conductors:
  • By
    using Bundled Conductors also corona effect can be reduced this is because
    bundled conductors will have much higher effective diameter compared to
    the normal conductors.
  • By
    Using Corona Rings or Grading Rings:
  • This
    is of having no greater significance but i presented here to understand
    the Corona Ring in the Power system. Corona Rings or Grading Rings are
    present on the surge arresters to equally distribute the potential along
    the Surge Arresters or Lightning Arresters which are present near the
    Substation and in the Transmission lines.
126)    How to test insulators?
  • Always
    remember to practice safety procedures for the flash-over voltage distance
    and use a sturdy enclosure to contain an insulator that may shatter, due
    to steam build-up from moisture in a cavity, arcing produces intense heat,
    an AM radio is a good RFI/arcing detection device, a bucket truck AC
    dielectric test set (130KV) is a good test set for most pin and cap type
    insulators. A recent article said the DC voltage required to “search out
    defects can be 1.9 times the AC voltage.
  • Insulators
    have a normal operating voltage and a flash-over voltage. Insulators can
    have internal flash-over that are/are not present at normal operating
    voltage. If the RFI is present, de-energize the insulator (line) and if the
    RFI goes away, suspect the insulator (line). Then there can be insulators
    that have arcing start when capacitor or other transients happen, stop
    when the line is de-energized or dropped below 50% of arc ignition
    voltage. Using a meg-ohm-meter can eliminate defective insulators that
    will immediately arc-over tripping the test set current overload.
127) How to identify the starting and ending leads of winding
in a motor which is having 6 leads in the  terminal box
  • If
    it is a single speed motor then we have to identify 6 leads.
  • Use
    IR tester to identify 3 windings and their 6 leads. Then connect any two
    leads of two winding and apply small voltage across it and measure the
    current.
  • Then
    again connect alternate windings of same two windings and apply small
    amount of voltage (same as before) and measure current.
  • Check
    in which mode you get the max current and then mark it as a1-a2 &
    b1-b2. You get max current when a2-b1 will be connected and voltage
    applied between a1-b2.
  • Follow
    the same process to identify a1-a2, b1-b2, c1-c2.now we will be able to
    connect it in delta or star.
128)    How to measure Transformer Impedance?
  • Follow
    the steps below:
  • (1)
    Short the secondary side of the transformer with current measuring devices
    (Ammeter)
  • (2)
    Apply low voltage in primary side and increase the voltage so that the
    secondary current is the rated secondary current of the transformer.
    Measure the primary voltage (V1).
  • (3)
    Divide the V1 by the rated primary voltage of the transformer and multiply
    by 100. This value is the percentage impedance of the transformer.
  • When
    we divide the primary voltage V1 with the full load voltage we will get
    the short circuit impedance of the transformer with refereed to primary or
    Z01. For getting the percentage impedance we need to use the formula =
    Z01*Transformer MVA /(Square of Primary line voltage).
129)    Why Bus Couplers are normally 4-Pole. Or
When Neutral Isolation is required?
  • Neutral
    Isolation is mandatory when you have a Mains Supply Source and a Stand-by
    Power Supply Source. This is necessary because if you do not have neutral
    isolation and the neutrals of both the sources are linked, then when only
    one source is feeding and the other source is OFF, during an earth fault,
    the potential of the OFF Source’s Neutral with respect to earth will
    increase, which might harm any maintenance personnel working on the OFF
    source. It is for this reason that PCC Incomers & Bus Couplers are
    normally 4-Pole. (Note that only either the incomer or the bus coupler
    needs to be 4-pole and not both).
  • 3pole
    or 4pole switches are used in changing over two independant sources ,where
    the neutral of one source and the neutral of another source should not mix
    the examples are electricity board power supply and standalone generator
    supply etc. the neutral return current from one source should not mix with
    or return to another source. as a mandatory point the neutral of any
    transformer etc are to be earthed, similarly the neutral of a generator
    also has to be earthed. While paralling (under uncontrolled condition) the
    neutral current between the 2 sources will crises cross and create
    tripping of anyone source breakers.
  • also
    as per IEC standard the neutral of a distribution system shall not be
    earthed more than once. means earthing the neutral further downstream is
    not correct,
130) Why Three No’s of Current transformer in 3 phase Star
point is grounded.
  • For
    CT’s either you use for 3 phase or 2 phase or even if you use only 1 CT’s
    for the Over current Protection or for the Earth Faults Protection, their
    neutral point is always shorted to earth. This is NOT as what you explain
    as above but actually it is for the safety of the CT’s when the current is
    passing thru the CT’s.
  • In
    generally, tripping of Earth faults and Over current Protection has
    nothing to do with the earthing the neutral of the CT’s. Even these CT’s
    are not Grounded or Earthed, these Over current and the Earth Faults
    Protection Relay still can operated.
  • Operating
    of the Over current Protection and the Earth Faults Relays are by the
    Kirchhoff Law Principle where the total current flowing into the points is
    equal to the total of current flowing out from the point.
  • Therefore,
    for the earth faults protection relays operating, it is that, if the total
    current flowing in to the CT’s is NOT equal total current flowing back out
    of the CT’s then with the differences of the leakage current, the Earth
    Faults Relays will operated.
131) What is tertiary winding of Transformer?
  • Providing
    a tertiary winding for a transformer may be a costly affair. However,
    there are certain constraints in a system which calls for a tertiary
    transformer winding especially in the case of considerable harmonic levels
    in the distribution system. Following is an excerpt from the book “The
    J&P Transformer Book”.
  • Tertiary
    winding is may be used for any of the following purposes:
  • (A)To
    limit the fault level on the LV system by subdividing the infeed that is,
    double secondary transformers.
  • (B)The
    interconnection of several power systems operating at different supply
    voltages.
  • (C)
    The regulation of system voltage and of reactive power by means of a
    synchronous capacitor connected to the terminals of one winding.
  • It
    is desirable that a three-phase transformer should have one set of
    three-phase windings connected in delta thus providing a low-impedance
    path for third-harmonic currents. The presence of a delta connected
    winding also allows current to circulate around the delta in the event of
    unbalance in the loading between phases, so that this unbalance is reduced
    and not so greatly fed back through the system.
  • Since
    the third-order harmonic components in each phase of a three-phase system
    are in phase, there can be no third-order harmonic voltages between lines.
    The third-order harmonic component of the magnetising current must thus
    flow through the neutral of a star-connected winding, where the neutral of
    the supply and the star-connected winding are both earthed, or around any
    delta-connected winding. If there is no delta winding on a star/star
    transformer, or the neutral of the transformer and the supply are not both
    connected to earth, then line to earth capacitance currents in the supply
    system lines can supply the necessary harmonic component. If the harmonics
    cannot flow in any of these paths then the output voltage will contain the
    harmonic distortion.
  • Even
    if the neutral of the supply and the star-connected winding are both
    earthed, then although the transformer output waveform will be
    undistorted, the circulating third-order harmonic currents flowing in the
    neutral can cause interference with telecommunications circuits and other
    electronic equipment as well as unacceptable heating in any liquid neutral
    earthing resistors, so this provides an added reason for the use of a
    delta connected tertiary winding.
  • If
    the neutral of the star-connected winding is unearthed then, without the
    use of a delta tertiary, this neutral point can oscillate above and below
    earth at a voltage equal in magnitude to the third-order harmonic
    component. Because the use of a delta tertiary prevents this it is sometimes
    referred to as a stabilizing winding.
  • When
    specifying a transformer which is to have a tertiary the intending
    purchaser should ideally provide sufficient information to enable the
    transformer designer to determine the worst possible external fault
    currents that may flow in service. This information (which should include
    the system characteristics and details of the earthing arrangements)
    together with a knowledge of the impedance values between the various
    windings, will permit an accurate assessment to be made of the fault
    currents and of the magnitude of currents that will flow in the tertiary
    winding. This is far preferable to the purchaser arbitrarily specifying a
    rating of, say, 33.3%, of that of the main windings.
132) Why do transformers hum?
  • Transformer
    noise is caused by a phenomenon which causes a piece of magnetic sheet
    steel to extend itself when magnetized. When the magnetization is taken
    away, it goes back to its original condition. This phenomenon is
    scientifically referred to as magnetostriction.
  • A
    transformer is magnetically excited by an alternating voltage and current
    so that it becomes extended and contracted twice during a full cycle of
    magnetization. The magnetization of any given point on the sheet varies,
    so the extension and contraction is not uniform. A transformer core is
    made from many sheets of special steel to reduce losses and moderate the
    ensuing heating effect.
  • The
    extensions and contractions are taking place erratically all over a sheet
    and each sheet is behaving erratically with respect to its neighbour, so
    you can see what a moving, writhing construction it is when excited. These
    extensions are miniscule proportionally and therefore not normally visible
    to the naked eye. However, they are sufficient to cause a vibration, and
    consequently noise. Applying voltage to a transformer produces a magnetic
    flux, or magnetic lines of force in the core. The degree of flux
    determines the amount of magnetostriction and hence, the noise level Why
    not reduce the noise in the core by reducing the amount of flux?
    Transformer voltages are fixed by system requirements. The ratio of these
    voltages to the number of turns in the winding determines the amount of
    magnetization. This ratio of voltage to turns is determined mainly for
    economical soundness. Therefore the amount of flux at the normal voltage
    is fixed. This also fixes the level of noise and vibration. Also,
    increasing (or decreasing) magnetization does not affect the
    magnetostriction equivalently. In technical terms the relationship is not
    linear.
133) How can we reduce airborne noise?
  • Put
    the transformer in a room in which the walls and floor are massive enough
    to reduce the noise to a person listening on the other side. Noise is
    usually reduced (attenuated) as it tries to pass through a massive wall.
    Walls can be of brick, steel, concrete, lead, or most other dense building
    materials.
  • Put
    the object inside an enclosure which uses a limp wall technique. This is a
    method which uses two thin plates separated by viscous (rubbery) material.
    As the noise hits the inner sheet some of its energy is used up inside the
    viscous material. The outer sheet should not vibrate.
  • Build
    a screen wall around the unit. This is cheaper than a full room. It will
    reduce the noise to those near the wall, but the noise will get over the
    screen and fall elsewhere (at a lower level). Screens have been made from
    wood, concrete, brick and with dense bushes (although the latter becomes
    psychological)
  • Do
    not make any reflecting surface coincident with half the wave length of
    the frequency. What does this mean? Well, every frequency has a wave
    length. To find the wave length in air, for instance, you divide the speed
    of sound, in air (generally understood as 1130 feet per second) by the
    frequency. If a noise hits a reflecting surface at these dimensions it
    will produce what is called a standing wave. Standing waves will cause
    reverberations (echoes) and an increase in the sound level. If you hit
    these dimensions and get echoes you should apply absorbent materials to the
    offending walls (fibreglass, wool, etc.)
134) What is polarity, when associated with a transformer?
  • Polarity
    is the instantaneous voltage obtained from the primary winding in relation
    to the secondary winding. Transformers 600 volts and below are normally
    connected in additive polarity. This leaves one high voltage and one low
    voltage terminal unconnected. When the transformer is excited, the
    resultant voltage appearing across a voltmeter will be the sum of the high
    and low voltage windings. This is useful when connecting single phase
    transformers in parallel for three phase operations. Polarity is a term
    used only with single phase transformers.
135) What is exciting current?
  • Exciting
    current is the current or amperes required for excitation. The exciting
    current on most lighting and power transformers varies from approximately
    10% on small sizes of about 1 KVA and less to approximately 2% on larger
    sizes of 750 KVA.
136) Can a three phase transformer be loaded as a single phase
transformer?
  • Yes,
    but the load cannot exceed the rating per phase and the load must be
    balanced. (KVA/3 per phase)
  • For
    example:
    A 75 kVA 3 phase transformer can be loaded up to 25 kVA on
    each secondary. If you need a 30 kVA load, 10 kVA of load should be
    supplied from each secondary.
137) What are taps and when are they used?
  • Taps
    are provided on some transformers on the high voltage winding to correct
    for high or low voltage conditions, and still deliver full rated output
    voltages at the secondary terminals.
  • Standard
    tap arrangements are at two-and-one-half and five percent of the rated
    primary voltage for both high and low voltage conditions.
  • For
    example, if the transformer has a 480 volt primary and the available line
    voltage is running at 504 volts, the primary should be connected to the 5%
    tap above normal in order that the secondary voltage be maintained at the
    proper rating.
138) What is the difference between “Insulating,”
“Isolating,”and“Shielded Winding” transformers?
  • Insulating
    and isolating transformers are identical. These terms are used to describe
    the isolation of the primary and secondary windings, or insulation between
    the two.
  •  A
    shielded transformer is designed with a metallic shield between the
    primary and secondary windings to attenuate transient noise.
  • This
    is especially important in critical applications such as computers,
    process controllers and many other microprocessor controlled devices.
  •  All
    two, three and four winding transformers are of the insulating or
    isolating types. Only autotransformers, whose primary and secondary are
    connected to each other electrically, are not of the insulating or
    isolating variety.
139) Can transformers be operated at voltages other than nameplate
voltages?
  • In
    some cases, transformers can be operated at voltages below the nameplate rated
    voltage.
  •  In
    NO case should a transformer be operated at a voltage in excess of its
    nameplate rating, unless taps are provided for this purpose. When
    operating below the rated voltage, the KVA capacity is reduced
    correspondingly.
  • For
    example, if a 480 volt primary transformer with a 240 volt secondary is
    operated at 240 volts, the secondary voltage is reduced to 120 volts. If
    the transformer was originally rated 10 KVA, the reduced rating would be 5
    KVA, or in direct proportion to the applied voltage.
140) Can a Single Phase Transformer be used on a Three Phase
source?
  • Yes.
    Any single phase transformer can be used on a three phase
    source by connecting the primary leads to any two wires of a three phase
    system, regardless of whether the source is three phase 3-wire or three
    phase 4-wire. The transformer output will be single phase.
141) Can Transformers develop Three Phase power from a Single
Phase source?
  • No.
    Phase converters or phase shifting devices such as reactors
    and capacitors are required to convert single phase power to three phases.
142) Can Single Phase Transformers be used for Three Phase
applications?
  • Yes.
    Three phase transformers are sometimes not readily available
    whereas single phase transformers can generally be found in stock.
  • Three
    single phase transformers can be used in delta connected primary and wye
    or delta connected secondary. They should never be connected wye primary
    to wye secondary, since this will result in unstable secondary voltage.
    The equivalent three phase capacity when properly connected of three
    single phase transformers is three times the nameplate rating of each
    single phase transformer. For example: Three 10 KVA single phase
    transformers will accommodate a 30 KVA three phase load
143) Difference between Restricted Earth Fault & Unrestricted
Earth Fault protections?
  • Restricted
    earth fault is normally given to on star connected end of power equipment
    like generators, transformers etc. mostly on low voltage side. For REF
    protection 4 no’s CTs are using one each on phase and one in neutral. It
    is working on the principle of balanced currents between phases and
    neutral. Unrestricted E/F protection working on the principle of comparing
    the unbalance on the phases only. For REF protection PX class CT are using
    but for UREF 5P20 Cts using.
  • For
    Differential Protection CTs using on both side HT & LV side each
    phase, and comparing the unbalance current for this protection also PX
    class CTs are using.
144) Can transformers be operated at voltages other than nameplate
voltages?
  • In
    some cases, transformers can be operated at voltages below the nameplate
    rated voltage. In NO case should a transformer be operated in excess of
    its nameplate rating unless taps are provided for this purpose. When
    operating below the rated voltage the KVA capacity is reduced
    correspondingly.
145) How many types of cooling system it transformers?
  • ONAN
    (oil natural,air natural)
  • ONAF
    (oil natural,air forced)
  • OFAF
    (oil forced,air forced)
  • ODWF
    (oil direct,water forced)
  • OFAN
    (oil forced,air natural)
146) What is the function of anti-pumping in circuit breaker?
  • when
    breaker is close at one time by close push button, the anti pumping
    contactor prevent re close the breaker by close push button after if it
    already close.
147) There are a Transformer and an induction machine. Those two
have the same supply. For which device the load current will be maximum?
  • The
    motor has max load current compare to that of transformer because the
    motor consumes real power.. and the transformer is only producing the
    working flux and it’s not consuming. Hence the load current in the
    transformer is because of core loss so it is minimum.
148) Where the lighting arrestor should be placed in distribution
lines?
  • Near
    distribution transformers and out going feeders of 11kv and incoming
    feeder of 33kv and near power transformers in sub-stations.
149) Why Delta Star Transformers are used for Lighting Loads?
  • For
    lighting 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.
150) NGR grounded system vs. solidly grounded system
  • In
    India, at low voltage level (433V) we must do only Solid Earthing of the
    system neutral. This is by IE Rules 1956, Rule No. 61 (1) (a).Because, if
    we have opt for impedance earthing, during an earth fault, there will be
    appreciable voltage present between the faulted body & the neutral,
    the magnitude of this voltage being determined by the fault current
    magnitude and the impedance value.
  • This
    voltage might circulate enough current in a person accidentally coming in
    contact with the faulted equipment, as to harm his even causing death.
    Note that, LV systems can be handled by non-technical persons too.
  • In
    solid earthing, you do not have this problem, as at the instant of an
    earth fault, the faulted phase goes to neutral potential and the high
    fault current would invariably cause the Over current or short circuit
    protection device to operate in sufficiently quick time before any harm
    could be done.
151) Why Do not We Break Neutral in AC Circuits?
  • Neutral
    is connected to earth at some point, thus it has some value as a return
    path in the event of say and equipment earth being faulty. It’s a bit like
    asking ‘why don’t we break the Earth connection’
  • It
    was stupid and dangerous, as it was possible for the neutral fuse to blow;
    giving the appearance of ‘no power’ when in fact the equipment was still
    live.
152) What is Minimum Value of Insulation Resistance / Polarization
Index?
  • Motor
    Insulation Resistance:
  • The
    acceptable meg-ohm value = motor KV rating value + 1 (For LV and MV
    Motor).
  • Example,
    for a 5 KV motor, the minimum phase to ground (motor body) insulation is 5
    + 1 = 6 meg-ohm.
  • Panel
    Bus Insulation Resistance:
  • The
    acceptable meg-ohm value = 2 x KV rating of the panel.
  • Example,
    for a 5 KV panel, the minimum insulation is 2 x 5 = 10 meg-ohm
  • IEEE
    43 – INSULATION RESISTANCE AND POLARIZATION INDEX (
    min
    IR at 400C in MΩ)
Minimum
Insulation Resistance
TEST
SPECIMEN
R1
min = kV+1 R1 min = 100
For
most windings made before about 1970, all field windings, and others not
described below For most dc armature and ac windings built after about 1970
(form wound coils)
R1
min = 5
For
most machines with random -wound stator coils and form-wound coils rated
below 1kV
153) What is service factor?
  • Service
    factor is the load that may be applied to a motor without exceeding
    allowed ratings. For example, if a 10-hp motor has a 1.25 service factor;
    it will successfully deliver 12.5 hp (10 x 1.25) without exceeding
    specified temperature rise. Note that when being driven above its rated
    load in this manner, the motor must be supplied with rated voltage and
    frequency.
  • Keep
    in mind, however, that a 10-hp motor with a 1.25 service factor is not a
    12.5-hp motor. If the 10-hp motor is operated continuously at 12.5 hp, its
    insulation life could be decreased by as much as two-thirds of normal. If
    you need a 12.5-hp motor, buy one; service factor should only be used for
    short-term overload conditions.
154) Calculate the size the CT on the neutral point of the
secondary side of 11/0.415 kV Transformer
  • For
    high impedance relays (differential or restricted earth fault relays),
    ‘Class X’ current transformers are recommended to be used.
  • Please
    note that both CTs (neutral & phase) shall have the same
    characteristics. The following is an example to size the CT:
  • Input
    data:
    11/0.415 kV ,2500 KVA Power transformer ,Transformer
    impedance is 6% ,Length of cable from neutral CT to the relay is 200 m
    ,Cross section of CT cable to be used is 6 mm² -copper and resistance is
    0.0032 Ω/m
  • Step
     1: Calculation of CT Rated Primary Current
  • I
    = kVA/ (0.415×1.732) = 2500/ (0.415×1.732) = 3478.11 A, CT with primary
    current of 4000 A to be selected.
  • Select
    the secondary current of the CT 1 or 5 A. selecting 1 A secondary
    current, as the cross section and length of pilot wires can have a
    significant effect on the required knee voltage of the CT and therefore
    the size and cost of the CT. When the relay is located some distance from
    the CT, the burden is increased by the resistance of the pilot wires.
  • Step
    2: Calculation of maximum Fault Current
  • Ift
    = kVA/ (0.415×1.732x Z)
  • Ift
    = 2500/ (0.415×1.732×0.06) = 57968.59 A (say 58000 A)
  • Step
    3: Calculation of the Knee Voltage of the CT (Vkp)
  • Vkp
    = (2x Iftx (Rct+Rw)/CT transformation ratio)
  • Where:
    Rct  is the CT resistance (to be given by the manufacturer), Here Rct
    is1.02 Ω. 
  •  Rw:
    total CT cable resistance= 2x cable length (200 m) x wire resistance=
    2x200x0.0032= 1.28 Ω
  • CT
    transformation ratio = CT Primary Current/CT Secondary Current
  • CT
    transformation ratio = 4000/5= 800 A, for CT with 5 A secondary current;
    or,
  • CT
    transformation ratio = 4000/1= 4000 A, for CT with 1 A secondary current.
    We will use 1 A in this example.
  • Vkp
    = (2x58000x (1.02+1.28)/4000)= 66.7 V.
  • The
    Vkp of the CT should be higher than the setting of relay stability voltage
    (Vs), to ensure stability of the protection during maximum Through fault
    current.
  • To
    calculate the stability voltage,we should follow the related formula given
    by the relay manufacturer, as each relay manufacturer has its own formula.
  • we
    may calculate the Vkp as above using a CT with secondary current of 5 A,
    and you will notice the difference in the Vkp.
155) When should we use Molded Case Circuit Breakers and Mini
Circuit Breakers?
  • MCB
    is Miniature Circuit Breaker, since it is miniature it has limitation for
    Short Circuit Current and Amp Rating MCB:
  • MCB
    are available as Singe module and used for :-
  • Number
    of Pole :- 1,2,3,4 – 1+ N , & 3 + N
  • Usually
    Current range for A.C. 50-60 HZ, is from 0.5 Amp – 63 Amp. Also available
    80A, 100A, and 125 Amp.
  • SC
    are limited 10 KA
  • Applications
    are as: – Industrial, Commercial and Residential application.
  • Tripping
    Curve:
  • (1)
    B Resistive and lighting load,
  • (2)
    C Motor Load,
  • (3)
    D Highly inductive load.
  • MCCB:
  • MCCB:
    – Moulded Case Circuit Breaker.
  • MCCB
    are available as Singe module and used for:
  • Number
    of Pole :- 3 pole , & 4 Pole
  • Current
    range for A.C:
  • For
    3.2 /6.3/12.5/25/50/100/125/160 Amp and Short Circuit Capacity 25/35/65
    KA.
  • For
     200 250 Amp and Short Circuit Capacity 25/35/65 KA
  • For
    400 630/800 Amp and Short Circuit Capacity 50 KA
  • Protection
    release :
  • Static
    Trip :- Continuous adjustable overload protection range 50 to 100 % of the
    rated current Earth fault protection can be add on with adjustable earth
    fault pick up setting 15 to 80 % of the current.
  • Micro
    processor Based release:
  • Over
    load rated current 0.4 to1.0 in steps of o.1 of in trip time at 600 % Ir
    (sec) 0.2.0.5,1, 1.5 , 2 ,3
  • Short
    Circuit :-2 to10 in steps of 1 lr , short time delay (sec) 0.02.0.05,0.1,
    0.2 ,0.3
  • Instantaneous
    pick up :2 to10 in steps of 1 in Ground fault pick up Disable: 0.2 to 0.8
    in steps of 0.1 of in Ground fault delay (sec): 0.1 to 0.4 in steps of 0.1
  • MCB
    (Miniature Circuit Breaker) Trip characteristics normally not adjustable,
    factory set but in case of MCCB (Moulded Case Circuit Breaker) Trip
    current field adjustable.
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