Electrical Engineering Qs.

1. Electrical Q&A Part-1
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 currentwill have to return neutral so resultantcurrentis zero.
 Once there is a ground faultin the load side,current from phase will directlypass through earth and it will not
return through neutral through ELCB. That means once side currentis going and notreturning and hence
because ofthis difference in current ELCB will trip and it will safe guard the other circuits from faulty loads.If the
neutral is not grounded faultcurrent will definitely high and that full fault current will come back throu gh 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 circuitbreaker which is thermal operated and use for shortcircuit protection in small current
rating circuit.
 Normallyit is used where normal currentis less than 100A.
 MCCB moulded case circuitbreaker and is thermal operated for over load current and magnetic operation for
instanttrip in shortcircuit condition.Under voltage and under frequency may be inbuilt.
 Normallyit is used where normal currentis 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/Awhere area (A) is inversely proportional to resistance (R),so if area (A) increases,R
decreases & ifR is less the leakage currentwill take low resistance path so the earth pin should be thicker. It is
longer because the The First to make the connection and lastto disconnectshould be earth Pin.This assures
Safety for the person who uses the electrical instrument.
4) Why Delta Star Transformers are used for Lighting Loads?
 For lighting loads,neutral conductor is mustand hence the secondarymustbe 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 primarySo 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 currentis reduced , the
voltage drops during the starting of motor in systems are reduced.
6) What is meant by regenerative braking?
 When the supplyis 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 justmeasuring the currentonly because the current increases notonly
for over voltages but also for under voltage (As mostof the loads are non-linear in nature).So,the over voltage
protection & over current protection are completelydifferent.
 Over voltage relay meantfor sensing over voltages & protect the system from insulation break down and firing.
Over current relay meantfor sensing anyinternal shortcircuit, over load condition,earth fault thereby reducing

2. 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 betweentwo 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 ofstarting an induction motor
 DOL:direct online starter
 Star delta starter
 Auto transformer starter
 Resistance starter
 Series reactor starter
11) What is the difference betweenearth resistance and earthelectrode resistance?
 Only one of the terminals is evidentin the earth resistance.In order to find the second terminal we should
recourse to its definition:
 Earth Resistance is the resistance existing between the electricallyaccessible partof a buried electrode and
another pointof the earth, which is far away.
 The resistance ofthe electrode has the following components:
(A) the resistance ofthe metal and that of the connection to it.
(B) The contact resistance ofthe 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 that4ma is chosen
instead of0 mA is for fail safe operation.
 For example:A pressure instrumentgives output4mA 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 theywill get equal amountof electrical currentbut as the supplyvoltage is constant
across the Bulb (P=V^2/R).So the resistance of40Wbulb is greater and voltage across 40Wis 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. Normallythey
have low resistance.When DC supply is applied,due to low resistance,the currentthrough 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 Currenttransformers and is very importantfactor 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.

3.  A generating station is supposed to feed power to the grid and in case generating units are off, there is no
generation in the plantthen plantmay 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 supplythere is no inductance,only
resistance will actin the electrical circuit. So high electrical current will flow through primaryside of the
transformer.So for this reason coil and insulation will burn out
 When AC currentflow to primary winding itinduced alternating flux which also link to secondarywinding so
secondarycurrentflow in secondarywinding according to primarycurrent.Secondarycurrent also induced emf
(Back emf) in secondarywinding which oppose induced emfofprimarywinding and thus control primarycurrent
also.
 If DC currentapply to Primarywinding than alternating flux is not produced so no secondaryemfinduced in
secondarywinding so primarycurrentmay goes high and burn transformer winding.
18) Different betweenmegger and contact resistance meter?
 Megger used to measure cable resistance,conductor continuity,phase identification where as contactresistance
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 ofthe 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 electricalinstallations?
 Diversity factor is the ratio of the sum ofthe individual maximum demands ofthe various subdivisions ofa
system,or part of a system,to the maximum demand ofthe whole system,or part of the system,under
consideration.Diversityfactor is usuallymore than one.
21) Why humming sound occurred in HT transmission line?
 This sound is coming due to ionization (breakdown ofair into charged particles) ofair 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 mustalso make our own motors,transformers
or any other equipmentwe wantto use.
 We maintain the frequency at 50 Hz or 60hz because the world maintains a standard at50 /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 currentflow in circuit.
25) How can we start-up the 40w tube light with 230v AC/DC without using any choke/Coil?

4.  It is possible bymeans ofElectronic choke. Otherwise it’s notpossible to ionize the particles in tube. Light,with
normal voltage.
26) What is “pu” in electricalengineering?
 Pu stands for per unitand this will be used in power system single line diagram there itis like a huge electrical
circuit with no of components (generators,transformers,loads) with differentratings (in MVA and KV). To bring all
the ratings into common platform we use pu conceptin which, in general largestMVA and KV ratings of the
componentis considered as base values,then all other componentratings will getback into this basis.Those
values are called as pu values. (p.u=actual value/base value).
27) Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?
 Link is provided at a Neutral common pointin the circuitfrom which various connections are taken for the
individual control circuitand so it is given in a link form to withstand high Amps.
 But in the case of Fuse in the Phase of AC circuitit is designed such thatthe 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 blowoutand at the same time Supplyis on than due to open or break
Neutral Voltage is increase and equipmentmaybe 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 independentlyworking 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 ofeach bulb filamentis (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,currentflowing in all bulbs will be same.The energyreleased will be I2
R
 Thus,light outputwill be highestwhere resistance is highest.Thus, 60 watt bulb will be brightest.
 The 60W lamp as it has highestresistance & minimum currentrequirement.
 Highestvoltage drop across itX I [which is common for all lamps]=s highestpower.
 Note to remember:
 Lowestpower-lamp has highestelementresistance.
 And highestresistance will drop highestvoltage drop across itin a Series circuit
 And highestresistance in a parallel circuitwill pass minimum currentthrough it.So minimum power dissipated
across itas min currentX 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 clearlyshow very low resistance.A open
Capacitor will not show any movementon ohmmeter.
 A good capacitor will show low resistance initially,and resistance graduallyincreases.This shows thatCapacitor
is not bad. By shorting the two ends of Capacitor (charged by ohmmeter) momentarilycan give a weak spark.To
know the value and other parameters,you need better instruments
30) What is the difference betweenElectronic regulator and ordinary rheostat regulator for fans?

5.  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 ordinaryrheostattype 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 butin
rheostatcontrol resistance is decreased bysteps 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 heatwill be reduced,
 Cable becomes less bulkyand easy to carry, and very cheap to afford.
 It also reduces over heating of transformers.
32) What the main difference betweenUPS & inverter?
 Uninterrupted power supplyis mainlyuse for shorttime. Means according to ups VA it gives backup. Ups is also
two types: on line and offline. Online ups having high volt and amp for long time backup with high dc voltage. But
ups startwith 12v dc with 7 amps.but inverter is start with 12v,24,dc to 36v dc and 120amp to 180amp battery
with long time backup
33) Which type of A.C motor is used in the fan?
 It is Single Phase induction motor which mostlysquirrel cage rotor and are capacitor startcapacitor run.
34) What is the difference betweensynchronous generator and asynchronous generator?
 In simple,synchronous generator supplies’ both active and reactive power but asynchronous generator (induction
generator) supply’s onlyactive 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 & normallyfor 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 importantpartin Synchronous Generators;it controls the output voltage of the generator by controlling its
excitation current. Thus it can control the outputReactive Power of the Generator.
37) Difference betweena four point starter and three point starters?
 The shuntconnection in four pointstarter is provided separatelyfrom the line where as in three pointstarter it is
connected with line which is the drawback in three pointstarter
38) What happens if we connect a capacitor to a generator load?
 Connecting a capacitor across a generator always improves power factor,but it will help depends up on the
engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consu med
due to the improvementon pf.
 Don’t connecta capacitor across an alternator while it is picking up or withoutany other load
39) Why the capacitors work on ac only?
 Generally capacitor gives infinite resistance to dc components (i.e.,block the dc components).It allows the ac
components to pass through.
40) Why the up to dia 70mm² live conductor, the earthcable must be same size but above dia 70mm² live
conductor the earth conductor need to be only dia 70mm²?
 The current carrying capacity of a cable refers to it carrying a continuous load.

6.  An earth cable normallycarries no load,and under fault conditions will carry a significant instantaneouscurrent
but only for a short time mostRegulations define 0.1 to 5 sec before the fuse or breaker trips. Its size therefore
is defined by differentcalculating parameters.
 The magnitude ofearth fault currentdepends on:
 (a) the external earth loop impedance ofthe installation (i.e.beyond the supplyterminals)
 (b) the impedance ofthe active conductor in fault
 (c) the impedance ofthe 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 significantlybelow thatof the external earth
loop impedance (a);when It is quite large its impedance can be ignored.At this pointthere is no meritin
increasing the earth cable size
 i.e. Fault current = voltage / a + c
 (c) is also very small so the faultcurrent peaks out.
 The neutral conductor is a separate issue.It is defined as an active conductor and therefore mustbe 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 necessaryto provide single phase
voltages).However the increasing use ofnon-linear loads which produce harmonics has made this practice
dangerous,so for example the currentin some standard require full size neutrals.Indeed,in big UPS installations
I install double neutrals and earths for this reason.

7. Electrical Q&A Part-2
1) Why We use of Stones/Gravel in electricalSwitch Yard
 Reducing Step and Touch potentials during ShortCircuitFaults
 Eliminates the growth ofweeds 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.
2) 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 successfullydeliver 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
mustbe 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
continuouslyat12.5 hp, its insulation life could be decreased byas much as two-thirds of normal.If you need a
12.5-hp motor,buy one; service factor should onlybe used for short-term overload conditions
3) Why transmission line 11KV OR 33KV, 66KV not in 10KV 20KV?
 The miss conceptis Line voltage is in multiple of11 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 ofall 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 thatin the transmission line there would be a voltage loss ofaround 10%. So in order to get 100 at
the load pointthey started sending 110 from supplyside.This is the reason.It has nothing to do with form factor
(1.11).
 Nowadays that thoughthas changed and we are using 400 V instead of440 V, or 230 V instead of220 V.
 Also alternators are now available with terminal voltages from 10.5 kV to 15.5 kV so generation in multiples of11
does notarise. Now a days when, we have voltage correction systems,power factor improving capacitors,which
can boost/correctvoltage to desired level,we are using the exact voltages like 400KV in spite of 444KV
4) What is electricalcorona?
 Corona is the ionization of the nitrogen in the air, caused by an intense electrical field.
 Electrical corona can be distinguished from arcing in thatcorona starts and stops at essentiallythe same voltage
and is invisible during the day and requires darkness to see atnight.
 Arcing starts at a voltage and stops ata voltage about 50% lower and is visible to the naked eye day or nightif
the gap is large enough (about5/8″ at 3500 volts).
5) What are the indications of electricalcorona?
 A sizzling audible sound,ozone,nitric acid (in the presence ofmoisture in the air) that accumulates as a white or
dirty powder,light(strongestemission in ultravioletand weaker into visible and near infrared) that can be seen
with the naked eye in darkness,ultravioletcameras,and daylightcorona cameras us ing the solar-blind
wavelengths on earth created by the shielding ozone layer surrounding the earth.
6) What damage does corona do?
 The accumulation ofthe nitric acid and micro-arcing within itcreate carbon tracks across insulating materials.
Corona can also contribute to the chemical soup destruction ofinsulating cements on insulators resulting in
internal flash-over.

8.  The corona is the only indication.Defects in insulating materials thatcreate an intense electrical field can over
time resultin corona that creates punctures,carbon tracks and obvious discoloration ofNCIinsulators.
7) How long does corona require creating visible damage?
 In a specific substation the corona ring was mistakenlyinstalled backwards on a temporary500kV NCI insulato r,
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.
8) What voltage are corona rings typically installed at?
 It varies depending upon the configuration ofthe insulators and the type of insulator,NCInormallystartat 160kV,
pin and cap can vary starting at 220kV or 345kV depending upon your engineering tolerances and insulators in
the strings.
9) How do we select transformers?
 Determine primaryvoltage and frequency.
 Determine secondaryvoltage required.
 Determine the capacity required in volt-amperes.This is done by multiplying the load current (amperes) bythe
load voltage (volts) for single phase.
 For example:if the load is 40 amperes,such as a motor,and the secondaryvoltage is 240 volts, then 240 x 40
equals 9600 VA. A 10 KVA (10,000 volt-amperes) transformer is required.
 Always selectTransformer Larger than Actual Load. This is done for safety purposes and allows for expansion,in
case more loads is added ata later date. For 3 phase KVA, multiplyrated volts x load amps x 1.73 (square rootof
3) then divide by 1000.
 Determine whether taps are required.Taps are usuallyspecified on larger transformers.
10) Why Small Distribution Transformers not used for Industrial Applications?
 Industrial control equipmentdemands a momentaryoverload capacity of three to eighttimes’ normal capacity.
This is mostprevalentin solenoid or magnetic contactor applications where inrush currents can be three to eight
times as high as normal sealed or holding currents butstill maintain normal voltage atthis momentaryoverloaded
condition.
 Distribution transformers are designed for good regulation up to 100 percentloading,but their output voltage will
drop rapidly on momentaryoverloads ofthis type making them unsuitable for high inrush applications.
 Industrial control transformers are designed especiallyfor maintaining a high degree ofregulation even at eight
time’s normal load.This results in a larger and generallymore expensive transformer.
11) Can 60 Hz transformers be used at higher frequencies?
 Transformers can be used atfrequencies 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 at400 Hz than 60 Hz.
12) 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
usuallyexpressed in terms ofpercentage.
 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 generallyhave regulation from 2% to 4%, depending on the
size and the application for which they are used.
13) Why is impedance important?
 It is used for determining the interrupting capacityof a circuit breaker or fuse employed to protect the primaryof a
transformer.

9.  Example: Determine a minimum circuitbreaker 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 of520 amps at480 volts.
 Example: Determine the interrupting capacity, in amperes,ofa circuit breaker or fuse required for a 75 KVA,
three phase transformer,with a primary of 480 volts delta and secondaryof 208Y/120 volts. The transformer
impedance (Z) = 5%. If the secondaryis shortcircuited (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 of1,800 amps at480 volts.
 Note: The secondaryvoltage is not used in the calculation.The reason is the primary circuitof the transformer is
the only winding being interrupted.
14) What causes flash-over?
 Flash-over causes are notalways easilyexplained,can be cumulative or stepping stone like,and usuallyresultin
an outage and destruction.The firstflash-over components are available voltage and the configuration ofthe
energized parts,corona may be presentin many areas where the flash-over occurs,and flash-over can be
excited by stepping stone defects in the insulating path.
15) 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 secondaryterminals.Taps are generallyset at two and a half and
five percentabove and below the rated primary voltage.
16) Can Transformers be reverse connected?
 Dry type distribution transformers can be reverse connected withouta loss ofKVA rating, but there are certain
limitations.Transformers rated 1 KVA and larger single phase,3 KVA and larger three phases can be reverse
connected withoutany 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 inputto the transformer,then the original 480 volt primarywinding 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 atno load.
 For example,a small single phase transformer having a nameplate voltage of 480 volts primaryand 240 volts
secondary,would actually have a no load voltage of approximately250 volts, and a full load voltage of 240 volts.
If the 240 volt winding were connected to a 240 volt source,then the outputvoltage would consequentlybe
approximately460 volts at no load and approximately442 volts at full load. As the KVA becomes smaller,the
compensation is greater—resulting in lower outputvoltages.
 When one attempts to use these transformers in reverse,the transformer will notbe harmed;however,the output
voltage will be lower than is indicated by the nameplate.
17) What is the difference between“Insulating”, “Isolating”, and “Shielded Winding” transformers?

10.  Insulating and isolating transformers are identical.These terms are used to describe the separation ofthe primary
and secondarywindings.A shielded transformer includes a metallic shield between the primaryand secondary
windings to attenuate (lessen) transientnoise.
18) How many BTU’s of heat does a transformer generate?
 The heat a transformer generates is dependentupon the transformer losses.To determine air conditioning
requirements multiplythe 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 of2000 watts will generate 6820 BTUs/hour.
19) What is a transformer and how does it work?
 A transformer is an electrical apparatus designed to convert alternating currentfrom 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 completelystatic solid state device, which insures,under normal
operating conditions,a long and trouble-free life.It consists,in its simplestform,of two or more coils ofinsulated
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 secondaryor outputcoil. The change of voltage (or voltage ratio) between the primary
and secondarydepends on the turns ratio of the two coils.
20) Factors Affecting Corona Discharge Effect:
 Corona Discharge Effectoccurs because ofionization if the atmospheric air surrounding the voltage conductors,
so Corona Discharge Effectis affected by the physical state of the atmosphere as well as bythe condition ofthe
lines.
 (1) Conductor: Corona Discharge Effectis considerablyaffected by the shape,size and surface conditions ofthe
conductor .Corona Discharge Effect decreases with increases in the size (diameter) ofthe conductor, this effect is
less for the conductors having round conductors compared to flatconductors and Corona Discharge Effectis
concentrated on that places more where the conductor surface is notsmooth.
 (2) Line Voltage: Corona Discharge effectis not presentwhen 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 presentin between
the power conductors.In a stormyweather the ions presentaround the conductor is higher than normal weather
condition So Corona Breakdown voltage occurs at low voltages in the stormyweather condition compared to
normal conditions
 (4)Spacing betweenthe Conductors: Electro static stresses are reduced with increase in the spacing between
the conductors.Corona Discharge Effecttakes place at much higher voltage when the distance between the
power conductors increases.
21) Will a transformer change Three Phases to Single Phase?
 A transformer will notact 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 outwhile at the same time
presenting a balanced load to the three phase supplysystem.
 There are, however, circuits available to change three phase to two phase or vice versa using standard dual
wound transformers.Please contactthe factory for two phase applications.
22) Can 60 Hz transformers be operated at 50 Hz?
 Transformers rated below 1 KVA can be used on 50 Hz service.

11.  Transformers 1 KVA and larger, rated at 60 Hz, should notbe used on 50 Hz service, due to the higher losses
and resultantheatrise.Special designs are required for this service.However, any 50 Hz transformer will operate
on a 60 Hz service.
23) Can transformers be used in parallel?
 Single phase transformers can be used in parallel onlywhen 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 resultin a shorter life of the transformer.In addition,impedance values ofeach
transformer mustbe within 7.5% of each other.
 For example:Transformer A has an impedance of4%,transformer B which is to be parallel to A musthave
impedance between the limits of3.7% and 4.3%. When paralleling three phase transformers,the same
precautions mustbe observed as listed above,plus the angular displacementand phasing between the two
transformers mustbe identical.
24) 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 ultravioletlight to change the properties ofNCI 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 ofstate to a gas and rapid
expansion causing fracture or rupture of the material.
25) Causes of Corona
 Corona is causes bythe following reasons:
 The natural electric field caused by the flow of electrons in the conductor.Interaction with surrounding air.
Poor or no insulation is nota major cause butincreases corona.
 The use of D.C (Direct Current) for transmission.(Reason whymosttransmission is done in form ofAC)
26) Effects of Corona
1) Line Loss – Loss ofenergy because some energyis used up to cause vibration of the air particles.
2) Long term exposure to these radiations maynotbe good to health (yet to be proven).
3) Audible Noise
4) Electromagnetic Interference to telecommunication systems
5) Ozone Gas production
6) Damage to insulation ofconductor.
27) What is polarity, when associatedwith a transformer?
 Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondarywinding.
 Transformers 600 volts and below are normallyconnected 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 resultantvoltage 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.Polarityis a term
used only with single phase transformers.
28) What is exciting current?
 Exciting current, when used in connection with transformers,is the currentor amperes required for excitation. The
exciting current on mostlighting and power transformers varies from approximately10% on small sizes ofabout1

12. 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 componentand 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.
29) 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 boucholzrelay at once gives a
horn for some time,ifthe transformer is isolated from the circuitthen it stop its sound itselfotherwise ittrips the
circuit by its own tripping mechanism.
30) Why we do two types of earthing on transformer (Body earthing & neutral earthing)
 The two types of earthing are Familiar as Equipmentearthing and system earthing.
 In Equipmentearthing:body (non conducting part) of the equipmentshould be earthed to safeguard the human
beings.
 The System Earthing : In this neutral of the supplysource ( Transformer or Generator) should be grounded.With
this,in case of unbalanced loading neutral will notbe shifted.So that unbalanced voltages will notarise.We can
protect the equipmentalso.With size of the equipment( transformer or alternator)and selection ofrelying system
earthing will be further classified into directlyearthed, Impedance earthing,resistive (NGRs) earthing.
31) 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 wetweather,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.
32) What is flash-over and arcing?
 Flash-over is an instantaneous eventwhere the voltage exceeds the breakdown potential ofthe air but does not
have the current available to sustain an arc, an arc can have the grid fault currentbehind 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 itmayrequire 80kVAC to create flash-over on a good cap and pin
insulator.
33) How to Minimizing Corona Effects
 Installing corona rings atthe 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 ofa high voltage device. It is electricallyinsulated.
 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.
34) What is BIL and how does it apply to transformers?
 BIL is an abbreviation for Basic Impulse Level.Impulse tests are dielectric tests thatconsistofthe application ofa
high frequency steep wave front voltage between windings,and between windings and ground.The Basic
Impulse Level of a transformer is a method ofexpressing the voltage surge (lightning,switching surges,etc.) that
a transformer will tolerate withoutbreakdown.
 All transformers manufactured in this catalog,600 volts and below,will withstand the NEMA standard BIL rating,
which is 10 KV.

13.  This assures the user thathe will not experience breakdowns when his system is properlyprotected with lightning
arrestors or similar surge protection devices.
35) The difference betweenGround 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 anyone phas e,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, omnipresentconductor
of electricity, in case of fault, so that the fault currentreturns 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 basicallyon 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 mustbe there. at leastat substations.And these are kept very sensitive
i.e. even minute currents are supposed to trip these.
 One aspectis safety – when someone touches a neutral,you don’t wanthim to be electrocuted – do you? Usually
if you see the switches athome 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,butagain mostof the fixtures are
on 2 wire) – he will be shocked at the accidental touching ofwire in case the floating neutral is floating too much.
36) What is impedance of a transformer?
 If you mean the percentage impedance ofthe transformed itmeans the ratio of the voltage( that if you applied itto
one side of the transformer while the other side of the transformer is shortcuitcuted,a full load current shall flow
in the shortcircuits side),to the full load current.
 More the %Z of transformer,more Copper used for winding,increasing costofthe unit. But shortcircuit levels will
reduce,mechanical damages to windings during shortcircuitshall also reduce.However,costincreases
significantlywith increase in %Z.
 Lower %Z means economical designs.Butshortcircuit fault levels shall increase tremendously,damaging the
winding & core.
 The high value of %Z helps to reduce shortcircuit current but it causes more voltage dip for motor starting and
more voltage regulation (% change of voltage variation) from no load to full load.
37) How are transformers sizedto 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
approximately5 to 7 times the running load amperage.This severe starting overload will resultin a drop of the
transformer outputvoltage.
 When the voltage is low the torque and the horsepower ofthe motor will drop proportionatelyto the square ofthe
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 approximately50% of normal running
speed The underlying problem is low voltage at the motor terminals.If the ampere rating ofthe motor and

14. transformer over current device falls within the motor’s 50% RPM draw requirements,a problem is likelyto
develop. The over current device may not open under intermediate motor ampere loading conditions.
 Overheating of the motor and/or transformer would occur,possiblycausing failure ofeither 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 ofthe motor should notexceed 65%
of the transformer’s full load ampere rating.
 (2) If several motors are being operated from one transformer,avoid having all motors startat the same time.If
this is impractical,then size the transformer so that the total running currentdoes notexceed 65% of the
transformer’s full load ampere rating.
38) 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 faultcurrent, w.r.t winding, core burning.
 Relay co-ordination and faultclearing time.
 Standard practice of limiting earth faultcurrent. In case no data or calculation is possible,go for limiting E/F
current to 300A or 500A, depending on sensivity of relay.
39) Why a neutral grounding contactor is needed in diesel generator?
 There would notbe any current flow in neutral if DG is loaded equallyin 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 byCT and trips the DG. so neutral in grounded to give low resistance path to fault
current.
 An electrical system consisting ofmore than two low voltage Diesel Generator sets intended for paralle l operation
shall meetthe following conditions:
 (i) Neutral of only one generator needs to be earthed to avoid the flow of zero sequence current.
 (ii) During independentoperation,neutrals ofboth 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 whilstin operation.
 Solution:
 Considering the requirementofearthing 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 independentoperation ofany generator its neutral contactor shall be closed.
 Operation of neutral contactors shall be preferably made automatic using breaker auxiliarycontacts.
40) 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 presentbetween the faulted body & the neutral,the magnitude ofthis voltage
being determined bythe fault current magnitude and the impedance value.
 This voltage mightcirculate enough currentin a person accidentallycoming in contactwith 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 atthe instantof an earth fault, the faulted phase goes to neutral

15. potential and the high fault current would invariably cause the Over current or shortcircuit protection device to
operate in sufficientlyquick time before any harm could be done

16. Electrical Q&A Part-3
1) What is the reason of grounding or earthing of equipment?
 with a ground path,in case of shortcircuit the shortcircuit currentgoes to the body of the equipment& then to the
ground through the ground wire.Hence if at the momentoffault if a person touches the equipmentbodyhe will
not get a shock cause his bodyresistance (in thousands ofohms) 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.
2) 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 between5In 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 instantaneouslywhen the
current is between these limits.10kA is the shortcircuit withstands capacityof the MCB.
 Under normal condition,a current limiting type MCB will trip on shortcircuit (magnetic trip) and the current during
circuit interruption will be much less than the prospective current. However, the MCBs have to have a shortcircui t
capacity more than or equal to the fault level at the location where it is installed.
3) 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 usuallyconsists ofline inductance,line to earth capacitance and resistance.Resistance can be
neglected with respectto the line inductance .When the load on the system falls the energy stored in the
capacitance gets discharged.The charging currentcauses 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 significantamountofcharging 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 shortlines,butsignificantin medium and long tra nsmission
line.Hence, this phenomenon is applicable for medium and long transmission line.
The main impactofthis phenomenon is on over voltage protection system,surge protection system,insulation
level etc.
4) Can single phase transformers be used for three phase applications?
 Yes. Three phase transformers are sometimes notreadilyavailable whereas single phase transformers can
generallybe found in stock.Three single phase transformers can be used in delta connected primaryand wye or
delta connected secondary.They should never be connected wye primary to wye secondary,since this will result
in unstable secondaryvoltage. The equivalentthree phase capacity when properlyconnected of three single
phase transformers is three times the nameplate rating ofeach single phase transformer.
5) What is BIL and how does it apply to transformers?

17.  BIL is an abbreviation for Basic Impulse Level.Impulse tests are dielectric tests thatconsistofthe application ofa
high frequency steep wave front voltage between windings,and between windings and ground.The BIL of a
transformer is a method ofexpressing the voltage surge that a transformer will tolerate withoutbreakdown.
6) Where Auto-recloser is used?
 For Generator protection / Transformer Protection / Transmission Line /Bus bar protection.
 Many faults on overhead transmission lines are transientin 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 ofCB on the both ends of the lines.
 Open-0.3 second-Close-3minute-Close this is the sequence ofAR. i.e.-OPEN,C-CLOSED
 whenever faults occurs CB opens,then after 0.3 sec it closes automatically,if faults persists then itwill open after
3 min it closes and ifstill fault persists.It remains in open condition.
 Auto reclosure is generallyused for Transmission lines where the general types of faults are transientin nature.
 It can be three phase auto-reclosure or single pole auto-reclosure.
 The single pole auto reclosures are generallyfor 400kV line below this three pole auto- reclosures are used.
 The reason for a line the single pole reclosures provides a better stabilityof the system since some partofpower
is still transferred through the healthy phases.
 Also 400kV breaker till date has a independentdrive/trip/ close coils for the three poles,below that all breakers
have common drive/trip / closing coils for the three poles.
7) What is difference betweenpower transformers & distribution transformers?
 Distribution Transformers are designed for a maximum efficiencyat 50% of load.Whereas power transformers
are designed to deliver max efficiency ay 90% and above loads.
 The distributions transformers have low impedance so as to have a better regulation power transformers have
higher so as to limitthe SC current.
 Power transformers are used to step up voltages from 11 KV which is the generating voltage to 132 or whatever
will be the transmission voltage levels.Power transformers are having Star-Delta connection.It will be located at
power generating stations.
 Distribution transformers are used to step down voltages from transformer levels to 11 KV/415 V. Will be having
Delta-Star. It will be located in substations near load centers.
 The main basic difference lies in the Design stage itselfas power transformer are to operate at near full load so
there sensing is such thatthey achieve equal.of copper losses & iron losses atfull loads whereas this is achieved
in the design itselfatabout50% loading in disttransformer butfriends there is a dilemma as our dist.transformer
are almostfullyloaded & beyond so they never go operate at their full eff. & also poor voltage regulation.
 The difference between power and distribution transformers refers to size & inputvoltage. Distribution
transformers vary between 25 kVA and 10 MVA, with inputvoltage between 1 and 36 kV. Power transformers are
typically units from 5 to 500 MVA, with input voltage above 36 kV. Distribution transformer design to have a max
efficiency at a load lower than full load. Power transformer design to have a max efficiency at full load
8) 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 keptclose.In case it is kept open,under balanced
load conditions the currentthrough neutral will not flow & nothing harmful will take place but in case an earth fault
takes place then there will be no earth fault currentflowing through the system & the generator will run as a
ungrounded generator.Thus the earth fault will not be cleared.

18.  If more number ofgenerators are connected parallel.We will have a close loop and hence negative sequence
current will flow. This will increase the rotor temperature.Hence ifmore number ofgenerators are connected then
only one is earthed and others are open.
 In case of Two or more generators connected to a common bus withouta transformer in between,basicallyi n
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 notbe valid for such systems.
 Sometime we maywant 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 justa small tes tlike checking insulation with a
meggar etc. for such things we need a NIS.
 Neutral isolator is required ifwe have delta transmission system and atthe 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 startflowing through the neutral, which we
can sense thru a CT & relay & hence can immediatelyidentify & clear the fault in about 100 ms by opening the
associated breaker/prime mover/excitation.Quicker the fault clearance less is the damage.
9) Why shorting type terminal required for CT?
 During maintenance or secondaryinjection you will need to bypass the CT & for the same you need shorting link.
During sec.injection you will shortcircuit the main CT & bypass it. Open circuiting the CT will saturate it &
damage it.
10) 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.
11) 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
toit.LA connection is securelymade 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.Butthen proper earthing has to be assured.
12) Can 60 Hz transformers be operated at 50 Hz?
 Transformers 1 KVA and larger, rated at 60 Hz, should notbe used on 50 Hz service due to higher losses and
resultantheatrise.However, any 50 Hz transformer will operate on 60 Hz service.
13) Can transformers be used in parallel?
 Single phase transformers can be used in parallel onlywhen their voltages are equal.If unequal voltages are
used,a circulating currentexists in the closed network between the two transformers which will cause excess
heating and resultin a shorter life of the transformer. In addition impedance values ofeach transformer mustbe
within 7.5% of each other.
14) Can Transformers be reverse connected?
 Dry type distribution transformers can be reverse connected withouta loss ofKVA rating, but there are certain
limitations.Transformers rated 1 KVA and larger single phase,3 KVA and larger three phases can be reverse
connected withoutany adverse effects or loss in KVA capacity.
15) Why short circuit do not take place when electrode is touched to ground.

19.  Basicallyduring welding we force a short-circuitat the electrode tip. The fault condition produces large magnitude
currents.Greater the Currentvalue 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.Butwelding is a very complex& detailed
phenomenon.Besides there are manyprinciples on which welding operates.Some maybe a welding,dc welding,
arc, constantvoltage, constantcurrent etc
16) What’s the difference between generator breaker and simple breaker?
 Breaker is one which disconnects the circuitin fault condition and It is similar for all equipment.Based on the
equipmentvoltage and maximum shortcircuitcurrentthe ratings will be decided.For better understanding we call
generator or transformer or line etc breakers.
17) What is accuracyClass of the instrument?
 Generally the class indicates the accuracy with which the meter will indicate or equipmentwill measure with
respectto its input.
 The accuracy of differentequipmentwill depend on number offactors.
 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 ofaccuracy 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
Similarlyindicating instruments shall have accuracies & accordinglyapplication as depicted below for testing the
following values are generallyused:
 for routine tests : accuracy class 1
 for type tests : accuracy class 0.5 or better.
 indicating meters generallywill have accuracy of 1.
18) First pole to clear factor-Circuit breakers
 The first pole to clear factor (kpp) is depending on the earthing system ofthe network. The firstpole to clear factor
is used to calculating the transientrecovery 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 resonantearthed system.
3. kpp = 1.0 corresponds to special cases e.g.railwaysystems.
 A special case is when there is a three phase faultwithout involving earth in a system with earthed neutral. This
case responds to kpp = 1.5 . This special case is however notnormallyconsidered in the standards.
19) 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 limitthe
value of fault current. Hence we inserta resistance in the neutral circuitto limitthis faultcurrent. 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 itto the protection relays & hence the relays can then isolate the system from
the fault; so that the system is isolated from the faultbefore the harm is done by the fault current. That is the
reason thatall the equipmentwill be designed for faultKA values for 1 sec so that the total operation(CTsensing-
relay functioning-circuitbreaker 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 equipmentare designed to carry the fault current. Thus all your
objectives of:

20.  preventing the arcing.
 limiting the fault current.
 isolating the faulted system are achieved
20) Why are NGR’s rated for 10sec?
 NGR are placed in the neutral circuit & hence will be energized only in the fault conditions thus their continuous
loading is notexpected. Hence they are selected for intermittentrating.Similarlywhen we place a transformer in
the neutral grounding circuitthe KVA rating obtained after the calculation is multiplied bya diversity factor to
obtain smaller rating cause the therefore It will not be continuouslyrated.
 NIS is also provided to cut the circulating negative sequence currentin 2 more generators connected in parallel.
in some grid conditions theyask to keep neutral isolated after being connected to grid.
21) 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 of10% in the flux density
(voltage) causes an increase of50% in the magnetizing force (current). Its significance lies mainlyin PS class
core of CTs used for diff protection
22) Design method for neutral grounding resistor?
 NGR design basics:
 Capacitive coupling of generator,equipmentand the ground
-Generator to ground capacitance.
-Generator cable to ground capacitance (or bus duct as the case maybe)
-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.
23) 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 ofdisc 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 suitabilitywith respectto EM strength.
 After this we need to consider the force that the stack has to bear. If we have a strain type of fitting i.e. the stack
has to bear horizontal conductor tension,weightload of the conductor,wind load,ice load etc then the number of
insulator discs required maybe more.
 But for a suspension type system which has to bear only the weightthen number ofdiscs required maybe less
than whatwe get by dividing by 11. That is the reason we have seen only23/24 discs in 400 kv line cause in that
case the creep age obtained musthave been enough & also the strain requirement.
 33kv insulators are generallyused in a vertical installation & are not stacked together because thatwill make the
suspension very rigid
24) Do taps work the same when a transformer is reverse fed?

21.  Taps are normallyin the primarywinding to adjustfor varying incoming voltage.If the transformer is reverse fed,
the taps are on the output side and can be used to adjustthe output voltage.
25) 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 thatH
terminals are primaryand X terminals secondary.This is true for step down transformers,butin a step up
transformer the connections should be reversed. Low voltage primary would connectto X terminals while high
voltage secondarywould connecton the H terminals.
26) 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 primaryleads to any
two wires of a three phase system,regardless ofwhether the source is three phase 3-wire or three phase 4-wire.
The transformer outputwill be single phase.
27) Why in Double circuit wire are transposed (R – B, Y – Y, B – R)
 This is done to avoid
1. Proximity effect
2. Skin effect
3. Radio interference
4. Reduction in noise in communication Signals
28) 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.
29) Which is more dangerous AC or DC
 Low frequency (50 – 60 Hz) AC currents can be more dangerous than similar levels ofDC current since the
alternating fluctuations can cause the heartto lose coordination,inducing ventricular fibrillation,which then rapidly
leads to death.
 However any practical distribution system will use voltage levels quite sufficientto ensure a dangerous amountof
current will flow, whether it uses alternating or directcurrent. Since the precautions againstelectrocution are
similar,ultimately,the advantages of AC power transmission outweighed this theoretical risk,and it was
eventually adopted as the standard.
30) 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 currentwill be flowing though both the generator taking earth as path. This leads to in crease
in loss and increase in temperature (This mayleads 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.
31) 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 emfis maximum atthe centre because ofmaximum lines offield there. Hence the

22. maximum opposition ofcurrentat inner side of conductor and minimum opposition atthe 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 accountthe inductance effect, its simple consider the DC current. Since its constant& not varying
hence no back emf but if we graduallystart increasing the frequencythen 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.
32) 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 currentsurrounded by another conductor
(sheath).Hence the Alternative current in the conductor induces voltages in the sheath or the armour.Hence
grounding these cables atboth ends will cause the potential ofthe 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 onlywhen one wants to avoid the voltage development
because can either go with the de rated cable or if one updates the cable in advance.
 When only one end of the cable sheath is grounded then there is no path for the circulating currentto 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 limitthe length of the cable used.
This method is called single pointbonding.This is thus used onlyfor shortlengths.
 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 existbetween sheath
& ground,the same being maximum atthe link boxes where bonding is done.This method provides maximum
possible currentcarrying capacity with the maximum possible lengths.
33) 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 manuallyby hand.so EDO means
Electrically spring charged Draw Out breaker
 In MDO breaker the spring charging is also done byhand manuallyand draw out aboutalso by hand only. so
MDO means Manual spring charge Draw Out breaker
34) 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) ofthe transformer depends on currentand voltage purely, not on load i.e,
phase angle between the currentand voltage i.e. why transformer rated in kVA
 Transformer is nota load and having no effect on P.F (that’s why no change in its power factor) and it only
transfer the constantpower from one voltage level to another voltage level withoutchanging frequency. since
both the losses viz copper loss(depends on current) and iron loss(depends on voltage) are independentofpower
factor, that is why a Transformers rating is noton kW, but on KVA
35) Why the secondary of CT never open when burden is connected on the CT.?
 secondaryof CT is never opened as because CTis always connected to the line so opening the secondarywill
mean there will be no counter mmfto balance the primary currentas a resultof which a very high induced emf

23. may appear in the secondaryas flux is very high and no counter mmfand this will be dangerous for the personnel
in the secondaryside and for pt if it is shorted then with full voltage applied to the primary.
 If we shortthe secondarythen much high current will circulate in the secondarydue to high induced emfmuch
higher than the actual full load currentas a resultof which the transformer’s secondarywinding mayburn out.
36) 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 secondaryvalues.
 Step 2 :
Based upon the calculated value divide into various zones
 Zone 1 (Forward) means 80% ofyour protected line length.
 Zone 2 (Forward) means 100% ofprotected line length + 20% Adjacent Shortestline
 Zone 3 (Forward) means 100% ofprotected line length + 50% Adjacent Longestline.
 Zone 4 (Reverse) means 10% ofprotected line.
37) Difference betweenCT class 0.2 and 0.2S?
 0.2S & 0.5S: Special type of measurementCTs they guarantee the declared accuracy, even with
20% loading.And some definite error can be defined even with a load as low as 1%. Thus they are suitable for
industries where loads are commissioned in steps or stages.Also for tariff metering purposes.
 0.2S: Special class for metering.It is more accurate than 0.2 classes.Generallyif we use 0.2s class CTthan VA
burden of core is also come down.
 In 0.2 classes CT,ratio & phase angle errors mustbe within the specified limits at5%, 20%,100% & 120% of
rated secondarycurrent.Whereas in 0.2s class CT,ratio & phase angle errors mustbe within the specified limits
at 1%, 5%, 20%, 100% & 120% of rated secondarycurrent.Also in 0.2s class,Ratio & Phase angle errors limits
are lower than 0.2 classes.
38) Why we use inductors
 Inductors have the property to oppose sudden changes in Current. When connected to the primary side of
transformer,ifany sudden shortcircuit(very high) currentflows due to some faultin the system,the inductor will
oppose the flow of that current saving the transformer.
 Secondly, for the problem oflagging 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 oflagging current.
39) Why do we need a bigger breaker when reverse feeding a transformer?
 Typically the output winding is wound firstand is therefore closestto the core. When used as exciting winding a
higher inrush currentresults.In mostcases the inrush currentis 10 to 12 times the full load current for 1/10 of a
second.When the transformer is reverse fed the inrush currentcan be up to 16 times greater.In this case a
bigger breaker with a higher AIC rating mustbe used to keep the transformer online.
40) How many types of Neutral grounding system?
 There are primarilythree types of grounding system which are:
 (1)Solid grounding – The neutral pointof the system is grounded withoutanyresistance.If the ground fault
occurs,high ground currentpasses 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 currentto 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 faultcurrent is limited to in the rage from 25A to 600A.

24.  (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 thatvery small currentflows 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’ttrip.This system musthave a ground faultmonitoring system.The use of line to
neutral (single phase) is prohibited (NEC,250.36(3)) in HRGsystem,however, phase to neutral is used with
using the additional transformer having its neutral grounded.When ground faultoccurs in HRG system,the
monitoring systems gives alarm and the plantoperators startthe standbymotor and stop the faulty one for the
maintenance.This way,the process plantis notinterrupted.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 butthey are not
commonlyused.

25. Electrical Q&A Part-4
1) 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 about10Hz you
will see the meter reading fluctuating instead.
2) 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 againstthe conductor above when they siton the
wire below.
 Designed to maintain the mechanical requirementto prevent arching between conductors while maintaining a
tower heightthat 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 dropletof water/ice to fall on bottom conductor.
3) What is the difference betweenSurge Arrester & Lightning Arrestor
 LA is installed outside and the effect of lightning is grounded,where as surge arrestor installed inside panels
comprising ofresistors which consumes the energyand nullify the effect of surge.
 Transmission Line Lightning Protection:
 The transmission line towers would normallybe 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 least1kM out from all substation structures.
Those earth wires to be properly electricallyto each supporting transmission tower,and bonded back to the
substation earth system.
 It is importantto have the catenaries’ earth conductors above the power conductor lines,at a sufficientdistance
and position thata lightning strike will not hit the power conductors.
 In some cases itis 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 itis often necessaryto have catenary earthing along the full distance of the transmission
line.
 Without specifics,(and you could not presentlygive tower pictures in a Postbecause ofa CR4 Server graphics
upload problem),specifics would include:
 Structure Lightning Protection:
 At the Substation,itis normal to have vertical electrodes bonded to the structure,and projecting up from the
highestpoints ofthe structure,with the location and number ofthose electrodes to be sufficientthat 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 pointin the
structure,and in those cases small structure extension towers are installed,with electrodes atthe tapered peak of
those extension towers.

26.  The extension towers are normally600mm square approximatelyuntil the extension tower changes shape atthe
tapered peak, and in some cases projectupwards 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 normallyhas 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)
4) 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 presentbetween the phases ofthe 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 presentaround 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 ofthe
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 potenti al between the
power conductors makes the insulating medium presentbetween the power conductors to startconducting and
reaches a voltage (Critical Breakdown Voltage) where the insulating air medium acts as conducting medium
results in breakdown ofthe insulating medium and flash over is observed.All this above said phenomenon
constitutes CORONA DISCHARGE EFFECT in electrical Transmission lines.
5) Methods to reduce Corona Discharge Effect:
 Critical Breakdown voltage can be increased byfollowing factors
 By increasing the spacing betweenthe conductors:
 Corona Discharge Effectcan be reduced by increasing the clearance spacing between the phases ofthe
transmission lines.However increase in the phases results in heavier metal supports.Costand Space
requirementincreases.
 By increasing the diameter of the conductor:
 Diameter ofthe conductor can be increased to reduce the corona discharge effect.By using hollow conductors
corona discharge effectcan be improved.
 By using Bundled Conductors:
 By using Bundled Conductors also corona effectcan 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 buti presented here to understand the Corona Ring in the Power system.
Corona Rings or Grading Rings are presenton the surge arresters to equallydistribute the potential along the
Surge Arresters or Lightning Arresters which are presentnear the Substation and in the Transmission lines.
6) 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 thatmay 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

27. test setfor mostpin and cap type insulators.A recentarticle 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 presentat normal operating voltage.If the RFI is present,de-energize the insulator (line) and if the
RFI goes away, suspectthe insulator (line).Then there can be insulators thathave 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 thatwill immediatelyarc-over tripping the testset current
overload.
7) 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 applysmall
voltage across itand measure the current.
 Then again connectalternate windings ofsame two windings and applysmall amountof 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.
8) How to measure Transformer Impedance?
 Follow the steps below:
 (1) Short the secondaryside of the transformer with currentmeasuring devices (Ammeter)
 (2) Apply low voltage in primary side and increase the voltage so that the secondarycurrentis the rated
secondarycurrentof the transformer.Measure the primaryvoltage (V1).
 (3) Divide the V1 by the rated primary voltage of the transformer and multiplyby 100. This value is the percentage
impedance ofthe transformer.
 When we divide the primaryvoltage V1 with the full load voltage we will get the shortcircuitimpedance ofthe
transformer with refereed to primaryor Z01. For getting the percentage impedance we need to use the formula =
Z01*Transformer MVA /(Square of Primary line voltage).
9) Why Bus Couplers are normally 4-Pole. Or When Neutral Isolation is required?
 Neutral Isolation is mandatorywhen you have a Mains Supply Source and a Stand-by Power Supply Source. This
is necessarybecause ifyou do not have neutral isolation and the neutrals ofboth 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 respectto earth will increase,which mightharm anymaintenance personnel working on the
OFF source.It is for this reason that PCC Incomers & Bus Couplers are normally4-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 independantsources ,where the neutral of one source and
the neutral of another source should notmix the examples are electricityboard power supply and standalone
generator supplyetc. the neutral return current from one source should notmixwith or return to another source.
as a mandatorypointthe neutral of any transformer etc are to be earthed, similarlythe neutral of a generator also
has to be earthed.While paralling (under uncontrolled condition) the neutral currentbetween the 2 sources will
crises cross and create tripping ofanyone source breakers.
 also as per IEC standard the neutral of a distribution system shall notbe earthed more than once. means earthing
the neutral further downstream is notcorrect,

28. 10) 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 currentProtection or
for the Earth Faults Protection, their neutral point is always shorted to earth. This is NOT as whatyou explain as
above but actually it is for the safety of the CT’s when the currentis passing thru the CT’s.
 In generally, tripping of Earth faults and Over currentProtection 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 currentProtection 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 outfrom 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 ofthe leakage current, the Earth Faults
Relays will operated.
11) What is tertiary winding of Transformer?
 Providing a tertiary winding for a transformer maybe a costly affair. However, there are certain constraints in a
system which calls for a tertiary transformer winding especiallyin 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 maybe used for any of the following purposes:
 (A)To limitthe fault level on the LV system by subdividing the infeed that is,double secondarytransformers.
 (B)The interconnection ofseveral power systems operating atdifferentsupplyvoltages.
 (C) The regulation of system voltage and of reactive power by means ofa synchronous capacitor connected to
the terminals ofone winding.
 It is desirable thata three-phase transformer should have one setof three-phase windings connected in delta
thus providing a low-impedance path for third-harmonic currents.The presence ofa delta connected winding also
allows currentto circulate around the delta in the event of unbalance in the loading between phases,so thatthis
unbalance is reduced and notso greatly fed back through the system.
 Since the third-order harmonic components in each phase ofa three-phase system are in phase,there can be no
third-order harmonic voltages between lines.The third-order harmonic componentofthe magnetising current
mustthus flow through the neutral of a star-connected winding,where the neutral of the supplyand 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 s upplyare not both connected to earth, then line to
earth capacitance currents in the supplysystem lines can supplythe necessaryharmonic component.If the
harmonics cannotflow in any of these paths then the outputvoltage 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 equipmentas 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,withoutthe 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 ideallyprovide sufficient
information to enable the transformer designer to determine the worstpossible external faultcurrents that may
flow in service. This information (which should include the system characteristics and details ofthe earthing

29. arrangements) together with a knowledge of the impedance values between the various windings,will permitan
accurate assessmentto be made of the fault currents and of the magnitude ofcurrents that will flow in the tertiary
winding.This is far preferable to the purchaser arbitrarilyspecifying a rating of, say, 33.3%, of that of the main
windings.
12) Why do transformers hum?
 Transformer noise is caused bya phenomenon which causes a piece ofmagnetic sheetsteel to extend itself
when magnetized.When the magnetization is taken away, it goes back to its original condition.This phenomenon
is scientificallyreferred to as magnetostriction.
 A transformer is magneticallyexcited by an alternating voltage and current so that it becomes extended and
contracted twice during a full cycle of magnetization.The magnetization ofany given point on the sheetvaries, so
the extension and contraction is not uniform.A transformer core is made from manysheets ofspecial steel to
reduce losses and moderate the ensuing heating effect.
 The extensions and contractions are taking place erraticallyall over a sheetand each sheetis behaving
erratically with respectto its neighbour,so you can see what a moving,writhing construction itis when excited.
These extensions are miniscule proportionallyand therefore not normallyvisible to the naked eye. However, they
are sufficientto cause a vibration, and consequentlynoise.Applying voltage to a transformer produces a
magnetic flux, or magnetic lines offorce in the core. The degree of flux determines the amountof
magnetostriction and hence,the noise level Why not reduce the noise in the core by reducing the amountofflux?
Transformer voltages are fixed by system requirements.The ratio of these voltages to the number of turns in the
winding determines the amountofmagnetization.This ratio of voltage to turns is determined mainlyfor
economical soundness. Therefore the amountof flux at the normal voltage is fixed. This also fixes the level of
noise and vibration.Also, increasing (or decreasing) magnetization does notaffect the magnetostriction
equivalently. In technical terms the relationship is notlinear.
13) 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 usuallyreduced (attenuated) as ittries to pass through a massive wall.Walls
can be of brick, steel,concrete, lead,or mostother 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 sheetsome ofits energy is used up inside the
viscous material.The outer sheetshould notvibrate.
 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 getover the screen and fall elsewhere (ata 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 coincidentwith 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 (generallyunderstood as 1130 feetper second) by the frequency. If a noise hits a reflecting surface
at these dimensions itwill produce whatis called a standing wave.Standing waves will cause reverberations
(echoes) and an increase in the sound level.If you hit these dimensions and getechoes you should apply
absorbentmaterials to the offending walls (fibreglass,wool,etc.)
14) What is polarity, when associatedwith a transformer?
 Polarity is the instantaneous voltage obtained from the primary winding in relation to the secondarywinding.
Transformers 600 volts and below are normallyconnected in additive polarity. This leaves one high voltage and
one low voltage terminal unconnected.When the transformer is excited,the resultantvoltage appearing across a

30. 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.Polarityis a term used only with single phase transformers.
15) What is exciting current?
 Exciting current is the current or amperes required for excitation. The exciting current on mos tlighting and power
transformers varies from approximately10% on small sizes ofabout1 KVA and less to approximately2% on
larger sizes of 750 KVA.
16) 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 mustbe 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.
17) 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 secondaryterminals.
 Standard tap arrangements are at two-and-one-halfand five percent of the rated primaryvoltage for both high
and low voltage conditions.
 For example,if the transformer has a 480 volt primaryand the available line voltage is running at 504 volts, the
primaryshould be connected to the 5% tap above normal in order that the secondaryvoltage be maintained at
the proper rating.
18) 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 ofthe primary
and secondarywindings,or insulation between the two.
 A shielded transformer is designed with a metallic shield between the primaryand secondarywindings to
attenuate transientnoise.
 This is especiallyimportantin critical applications such as computers,process controllers and manyother
microprocessor controlled devices.
 All two, three and four winding transformers are ofthe insulating or isolating types.Only autotransformers,whose
primaryand secondaryare connected to each other electrically, are not of the insulating or isolating variety.
19) Can transformers be operated at voltages other than nameplate voltages?
 In some cases,transformers can be operated atvoltages below the nameplate rated voltage.
 In NO case should a transformer be operated ata 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 primarytransformer with a 240 volt secondaryis operated at 240 volts, the secondary
voltage is reduced to 120 volts. If the transformer was originallyrated 10 KVA, the reduced rating would be 5
KVA, or in direct proportion to the applied voltage.
20) 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 byconnecting the primaryleads to any
two wires of a three phase system,regardless ofwhether the source is three phase 3-wire or three phase 4-wire.
The transformer outputwill be single phase.
21) 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.
22) Can Single Phase Transformers be used for Three Phase applications?