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.

• 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.

• 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.

• 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)

• 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.

• 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.

• 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.

• 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.

• 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).

• 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,

• 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.

• 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.

• 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.

• 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.)

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.

• No.
  Phase converters or phase shifting devices such as reactors
  and capacitors are required to convert single phase power to three phases.

• 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

• 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.

• 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.

• 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)

• 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.

• 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.

• Near
  distribution transformers and out going feeders of 11kv and incoming
  feeder of 33kv and near power transformers in sub-stations.

• 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.

• 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.

• 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.

• 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

• 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.

• 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.

• 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.