Most important interview answer and question for EEE Part-3

51. 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
other wise it trips the circuit by its own tripping
mechanism.
52. What is SF6 Circuit Breaker?
SF6 is Sulpher hexa Flouride gas.. if this gas is used
as arc quenching medium in a Circuitbreaker means
SF6 CB.
53. What is frantic effect?
Output voltage is greater than the input voltage or
receiving end voltage is greater than the sending end
voltage.
54. What is meant by insulation voltage in cables?
explain it?
It is the property of a cable by virtue of it can
withstand the applied voltage without rupturing it is
known as insulation level of the cable.
55. Why we do 2 types of earthing on transformer
(ie:)body earthing & neutral earthing , what is
function. i am going to install a oo kva
transformer & 380 kva DG set what should the
earthing value?
The two types of earthing are Familiar as Equipment
earthing and system earthing. In Equipment
earthing:
body ( non conducting part)of the equipment
shouldd be earthed to safegaurd the human
beings.system Earthing : In this neutral of the supply
source ( Transformer or Generator) should be
grounded. With this,in case of unbalanced loading
neutral will not be shifted.so that unbalanced
voltages will not arise. We can protect the equipment
also. With size of the equipment( transformer or
alternator)and selection of relying system earthing
will be further classified into directly
earthed,Impedance earthing, resistive (NGRs)
earthing.
56. 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. 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.
57. Where should the lighting arrestor be placed
in distribution lines?
Near distribution transformers and out going feeders
of 11kv and incomming feeder of 33kv and near
power transformers in sub-stations.
58. Define IDMT relay?
It is an inverse definite minimum time relay.In IDMT
relay its operating is inversely proportional and also
a characteristic of minimum time after which this
relay operates.It is inverse in the sense ,the tripping
time will decrease as the magnitude of fault current
increase.
59. What are the transformer losses?
TRANSFORMER LOSSES – Transformer losses have
two sources-copper loss and magnetic loss. Copper
losses are caused by the resistance of the wire (I2R).
Magnetic losses are caused by eddy currents and
hysteresis in the core. Copper loss is a constant after
the coil has been wound and therefore a measurable
loss.
Hysteresis loss is constant for a particular voltage
and current. Eddy-current loss, however, is different
for each frequency passed through the transformer.
60. What is the count of hvdc transmission lines in
India?
Resolution:At present there are three hvdc
transmission lines in india
1)chandrapur to padghe(mumbai)–(100 MW at ±00
kV DC)
2)rehand to delhi (100 MW at ±00 kV DC)
3)talchal to kolar (200 MW)
61. What is meant by regenerative braking?
Resolution:When the supply is cutt off for a running
motor, it still continue running due to inertia. In
order to stop it quickly we place a load(resitor)
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”.
62. Why is the starting current high in a DC
motor?
Resolution:In DC motors, Voltage equation is V=Eb-
IaRa (V = Terminal voltage,Eb = Back emf in Motor,Ia
= Armature current,Ra = Aramture resistance).At
starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra
,where Ra is very less like 0.01ohm.i.e, Ia will become
enormously increased.
63. What are the advantages of star-delta starter
with induction motor?
Resolution:
(1). 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.
(2). Hence the starting current is reduced , the
voltage drops during the starting of motor in systems
are reduced.
64. Why Delta Star Transformers are used for
Lighting Loads?
Resolution: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.
65. Why in a three pin plug the earth pin is thicker
and longer than the other pins?
Resolution:It depends upon R=rho l/a where area(a)
is inversely proportional to resistance (R), so if (a)
increases, R decreases & if R is less the leakage
current will take low resistance path so the earth pin
should be thicker. It is longer because the The First to
make the connection and Last to disconnnect should
be earth Pin. This assures Safety for the person who
uses the electrical instrument.
66. Why series motor cannot be started on noload?
Resolution:Series motor cannot be started without
load because of high starting torque. Series motor
are used in Trains, Crane etc.
67. Why ELCB can’t work if N input of ELCB do not
connect to ground?
Resolution: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
much 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 wil 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.
68. How electrical power is generated by an A.C
Generator?
For the generation of elect power we need a prime
mover which supplies mechanical power input to the
alternator, can be steam turbines,or hydro turbines
.When poles of the rotor moves under the armature
conductors which are placed on the stator ,field flux
cut the armature conductor ,therefore voltage is
generated
and is of sinusoidal in nature…due to polarity change
of rotor poles(i,e) N-S-N-S.
69. Why an ac solenoid valve attract the plunger
even though we interchanges the terminal? Will
the poles changes?
Yes because the poles changes for every half-cycle of
ac voltage so the polarity of AC voltage is
continuously changing for every half cycle. so,
interchanging of terminals in ac system does not
show any difference. That’s why the ac solenoid
attract the plunger even though it’s terminals are
interchanged.
70. What is derating?, why it is necessary, it is
same for all means for drives, motors,and cables.
The current currying of cables will change
depending upon the site temperature (location of
site), type of run (it will run through duct, trench,
buried etc.), number of tray, depth of trench,
distance between cables. Considering this condition
actual current currying capacity of cable reduce than
current currying capacity (which given to cable
Catalogue) this is called derating.
71. Why temperature rise is conducted in bus bars
and isolators?
Bus bars and isolators are rated for continuous
power flow, that means they carry heavy currents
which rises their temperature. so it is necessary to
test this devices for temperature rise.
72. When voltage increases then current also
increases then what is the need of over voltage
relay and over current relay? Can we measure
over voltage and over current by measuring
current only?
No.We can’t 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.
73. If one lamp connects between two phases it
will glow or not?
If the voltage between the two phase 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.
74. How do you select a cable size (Cu & Al) for a
particular load?
At first calculate the electrical current of the load,
after that derate the electrical current considering
derating factor(depending on site condition and
laying of cable) after choose the cable size from cable
catalog considering derating electrical current.After
that measure the length of cable required from
supply point of load to load poin. Calculate the
voltage drop which will max 3% (resistance and
reactance of cable found from
cable catalog of selecting cable) if voltage drop>3%
then choose next higher size of cable.
75. 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.
76. Which power plant has high load factor?
All base load power plants have a high load factor. If
we use high efficiency power plants to supply the
base load,we can reduce the cost of generation.Hydel
power plants have a higher efficiency than thermal
& nuclear power plants.
77. 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
78. 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.
79. What is use of lockout relay in ht voltage?
A lock-out relay is generally placed in line before or
after the e-stop switch so the power can be shut off at
one central location. This relay is powered by the
same electrical source as the control power and is
operated by a key lock switch. The relay itself may
have up to 24 contact points within the unit itself.
This allows the control power for multiple machines
to be locked out by the turn of a single key switch.
80. What is the power factor of an alternator at no
load?
At no load Synchronous Impedance of the alternator
is responsible for creating angle difference. So it
should be zero lagging like inductor.
81. How to determine capacitor tolerance codes?
Resolution:In electronic circuits, the capacitor
tolerance can be determined by a code that appears
on the casing. The code is a letter that often follows a
three-digit number (such as 130Z).The first two are
the 1st and 2nd significant digits and the third is a
multiplier code. Most of the time the last digit tells
you how many zeros to write after the first two digits
and these are read as Pico-Farads.
82. Why most of analog o/p devices having o/p
range 4 to 20 mA and not 0 to 20 mA?
Resolution: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.
83. Two bulbs of 100w and 40w respectively
connected in series across a 230v supply which
bulb will glow bright and why?
Resolution: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.
84. What is meant by knee point voltage?
Resolution: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.(CT-current transformer).
85. What is reverse power relay?
Resolution:Reverse Power flow relay are used in
generating stations’s protection. A generating
stations is supposed to fed power to the grid and in
case generating units are off,there is no generation
in the plant then plant may take power from grid. To
stop the flow of power from grid to generator we use
reverse power relay.
86. What will happen if DC supply is given on the
primary of a transformer?
Resolution: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.
87. What is the difference between isolators and –
>electrical circuit breakers? What is bus-bar?
Resolution:Isolators are mainly for switching
purpose under normal conditions but they cannot
operate in fault conditions .Actually they used for
isolating the CBs for maintenance. Whereas CB gets
activated under fault conditions according to the
fault detected.Bus bar is nothing but a junction
where the power is getting distributed for
independent loads.
88. What are the advantage of free wheeling diode
in a Full Wave rectifier?
Resolution:It reduces the harmonics and it also
reduces sparking and arching across the mechanical
switch so that it reduces the voltage spike seen in a
inductive load
89. What is the function of interposing current
transformer?
Resolution:The main function of an interposing
current transformer is to balance the currents
supplied to the relay where there would otherwise
be an imbalance due to the ratios of the main
current transformers.
Interposing current transformer are equipped with a
wide range of taps that can be selected by the user to
achieve the balance required.
90. What are Motor Generator Sets and explain
the different ways the motor generator set can be
used ?
Resolution:Motor Generator Sets are a combination
of an electrical generator and an engine mounted
together to form a single piece of equipment. Motor
generator set is also referred to as a genset, or more
commonly, a generator.
The motor generator set can used in the following
different ways:
1.Alternating current (AC) to direct current (DC)
2.DC to AC
3.DC at one voltage to DC at another voltage
4.AC at one frequency to AC at another harmonicallyrelated
frequency
91. What is power quality meter ?
Power Quality meters are common in many
industrial environment. Small units are now
available for home use as well. They give operators
the ability to monitor the both perturbations on the
power supply, as well as power used within a
building, or by a single machine or appliance. In
some situations, equipment function and operation
is monitored and controlled from a remote location
where communication is via modem, or highspeed
communication lines.So we can understand the
importance of power measurement through power
quality meters.
92. What is the different between digital phase
converter and ordinary phase converter?
Digital phase converter are a recent development in
phase converter technology that utilizes proprietary
software in a powerful microprocessor to control
solid state power switching components. This
microprocessor, called a digital signal processor
(DSP), monitors the phase conversion process,
continually adjusting the input and output modules
of the converter to maintain perfectly balanced
three-phase power under all load conditions.
93. Explain the operation of variable frequency
transformer?
A variable frequency transformer is used to transmit
electricity between two asynchronous alternating
current domains. A variable frequency transformer
is a doubly-fed electric machine resembling a
vertical shaft hydroelectric generator with a threephase
wound rotor, connected by slip rings to one
external ac power circuit.
A direct-current torque motor is mounted on the
same shaft. Changing the direction of torque applied
to the shaft changes the direction of power flow;
with no applied torque, the shaft rotates due to the
difference in frequency between the networks
connected to the rotor and stator.The variable
frequency transformer behaves as a continuously
adjustable phase-shifting transformer. It allows
control of the power flow between two
networks .
94. What is the main use of rotary phase
converter ?
Rotary phase converter will be converting single
phase power into true balanced 3 phase power,so it
is often called as single phase to three phase
converter . Often the advantages of 3 phase motors,
and other 3 phase equipment, make it worthwhile to
convert single phase to 3 phase so that small and
large consumers need not want to pay for the extra
cost of a 3 phase service but may still wish to use 3
phase equipment.
95. Use of switch mode power converter in realtime
basis?
Switch mode power converter can be used in the
following 5 different ways
1) step down an unregulated dc input voltage to
produce a regulated dc output voltage using a circuit
known as Buck Converter or Step-Down SMPS,
2)step up an unregulated dc input voltage to produce
a regulated dc output voltage using a circuit known
as Boost Converter or Step-Up SMPS,
3)step up or step down an unregulated dc input
voltage to produce a regulated dc output voltage,
4)invert the input dc voltage using usually a circuit
such as the Cuk converter, and
5)produce multiple dc outputs using a circuit such as
the fly-back converter.
96. Which type of oil is used as a transformer oil?
Transformer oil, or insulating oil, is usually a highlyrefined
mineral oil that is stable at high
temperatures and has excellent electrical insulating
properties. It is used in oil filled transformers, some
types of high voltage capacitors, fluorescent lamp
ballasts, and some types of high voltage switches and
circuit breakers. Its functions are to insulate,
suppress corona and arcing, and to serve as a
coolant.
Well into the 170s, polychlorinated biphenyls (PCB)s
were often used as a dielectric fluid since they are
not flammable. They are toxic, and under incomplete
combustion, can form highly toxic products such as
furan.
Starting in the early 170s, concerns about the toxicity
of PCBs have led to their banning in many countries.
Today, non-toxic, stable silicon-based or fluoridated
hydrocarbons are used, where the added expense of
a fireresistant liquid offsets additional building cost
for a transformer vault. Combustion-resistant
vegetable oil-based dielectric coolants and synthetic
pentaerythritol tetra fatty acid (C7, C8) esters are also
becoming increasingly common as alternatives to
naphthenic mineral oil. Esters are non-toxic to
aquatic life, readily biodegradable, and have a lower
volatility and higher flash points than mineral oil.
97. 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
98. what are the points to be consider for
MCB(miniature circuit breaker selection?
I(L)*1.25=I(MAX) maximum current. Mcb
specification are done on maximum current flow in
circuit.
99. what is the full form of KVAR?
We know there are three types of power in Electrical
as Active, apparent & reactive. So KVAR is stand for
“Kilo
Volt Amps with Reactive component.
100. What is excitation?
Excitation is applying an external voltage to DC
shunt coil in DC motors.