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, 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?

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
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

• 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

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
• 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).
that thought has changed and we are using 400 V instead of 440 V, or 230 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
• 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
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
• 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
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
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

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
• 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
• 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
• 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
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

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

118) Why we use inductors

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

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

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

120) How many types of Neutral grounding system?

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