f’

.’ \

_Questions at a glance : rl,

What $a Transformer and what is its principle of ”
operation?

TRANSFORMER
Why is the efficiency of the transformer so high? 8

What is meant by Rating of transformer? 9

What is the Power factor of a transformer? 1n

What is the Efficiency of a transformer? 11

What is meant by Regulation of a transformer? 1 I-

What is the normal rated current that can be drawn 12'

from a transformer?

What are the essential requirements which the 14

design of a transformer should fuhil?

What is meant by Graded insulation? 1’

What are swithcing surges and restriking voltages?
1; i

! What is the cause of insulation breakdown? 15

Explain the manner in which heat produced inside a 17
transformer is dissipated?

What is Therm0 syphon act&r? 19

What are the standard symbols used for describing
different methods of cooling .transformers?
What determines the life of a transformer and how 20
does it age? 6
How is the temperature rise measured? 23
 I
‘\
What are the fittings and accessories which should 2 4 32. Describe briefly the operation of the on load tap 41
be provided on a transformer? changer?

What is the difference between type tests and 25 33. How heavy a single phase load can be connected to - 41
routine tests? I a three phase transformer?

How is the winding resistance measured? 26 34. Can you operate 3 transformer on a frequency 43
different from that for which it is designed?
How is the ratio test conducted? 27 35. Some transformers make a lot of noise in operation. 43
What is polarity and how it is tested? 28 Why is it so?

What is Impedance Voltage? 29 36. What is the standard terminal marking on 3 4%

transformer?
How do you determine the no load losses? 29
37. What are the essential conditions for satisfactory 45:
How do you find out load losses? 30 parallel operation of transformers?
How is the insulation resistance measured and what 3 1 38. Is it necessary for the rated capacities of two 46'
should be its minimum value? transformers-to be the same for parallel operation?
What is an Auto Transformer and what is its main 3 3 39. What is the meaning of phase sequencand how do 46; b
advantge? you alter it?
What is meant by a variable transformer?
35 40. Explain how phase angle difference can exist 47!
between the primary and the secondary of a
What is an Induction regulator? 36 transformer?
What are Automatic Voltage Boosters? 37 41. How can you paralle1 3 transformer in GROUP I 49
with GROUP II?
How will you compensate for voltage variations on
the H.T. mains input into 3 transformer? 39 42. How can you operate 3 transformer belonging to
GROUP 3 with one in GROUP 4 in parallel?
49
The Indian Electricity rules allow a maximum
variation of & 12.5% on the H.T. Supply and of k 4 () 43. Describe the precautions required and the correct 49
5% on the consumer side voltage. How is it then practical test procedure for paralleling two
possible to manage with only k 5% and L i.F% ' transformers? ,
tappings which have been standardised for
transformers? 3
2
 i?i!
44, What are the hazards against which a transformer 51 58. What is the objection against open circuiting of the 6 5
requires protection? secondary of a CT?

45. What Protection is required for a small 230 v What maintenance work is required for 66
transformer. (say for an inspection hand famp). 52 transformers and at what intervals?
What are the points to be checked if the oil level
46. How do you protect an indoor distribution 52 I (3 60- tends to fall down?
66
transformer installed in a small substation?

What protection is required for outdoor pole 53 61. What attention is required on bushings and ,?I
47.
mounted distribution transformers? insulators?

54 62. Why are the transformers fitted with qil 72

48. Describe the protective equipment required for a
conservators?
large transformer in a major substation.
54 * 63. What is the function of a Breather? 72 ‘
49. How do temperature operated devices function?
64. Considering the very small quantity of water vapour 7 3
50. How does gas get formed in a transformer? 55 ’
in the atmosphere, are breathers really necessary?
51. Describe the function of a Buchholz relay. 56 What inspection is required of Earthing? 23.
65.
52. What is the function of the overcurrent relay? 57 66. What are the different methods of drying out a 7 ,.i
transformer?
53. What is a differential relay? 58
Explain how an Earth Leakage relay system 59 67. How long should the drying operation be 75
54.
functions. continued?

62 I 68. What are the qualities required of a good 75

55. What are instrument transformers and why are they , transformer oil?
used?
69. What are the different methods of purifying 768
56. What are the main differences between current 62 (filtering) and drying out transformer and switch
transformers and potential transformers?
oils?
Compare and contrast them.
63 70. Describe the streamline oil filter briefly. 77
57. What is meant by secondary Burden of a PT or a
CT? 5
4
-“.-A .- ^- ,^“,,__
 Dissolved analysis by GAS Q.1. What is a Transformer and What is its Principle of
CHROMATO~k’H - (A modern tool to detects () operation?
incipient faults in power transformer) is the need of
today. Comment. ;‘&e transformer is theoritically a simple device i.e.,
How to inspect a’transformer on arrival at site for a35 one without moving parts, which is used for stepping up
expansion project? or st ping down alternating currents or voltages. It
z!!
Ours is a small scale industry. We wish to operates on the principle of electro magnetic induction.
commission a 500 kva transformer which has
arrived last month, can you elaborate as to how we 87 .,<
Basically, a transformer consists of a closed
will go about it? magnetic core, made uI) of thin silicon iron or special
&&sheets or starnpings, with one or more coils of
insulated wire placed on its limbs. The winding which is
ANNEXURES : &$Jected to the AC supply source is called the primary
i; y
and that, which is connected to the load is called the \
Conversion Tables 3
&&dary. The current passing through the primary
National. Standards of Transformers and other
connected items. ing produces an alternating field in the core, the
PP
Values for safe insulation resistance of windings &lie of which is given by the following equation:
ri 2 -b
and fuse ratings for power transformers.
t- -8 volts . ..(A)
Basic duration of overload for oil immer*ed \ter;
transformers. = frequency in cycles per second
f
Permissible concentration of dissolved gases in c’ c? i number of turns of winding
transformer oil. ; 5 total magnetic flux in the core
(JZ
Rationalisation of capitalisation f o r m u l a f o r
transformer losses. The voltage induced in the secondary is proportional to
Form B - packing slip
;; $e number of turns and is given :
Order format

6
The induced voltage serves two different functions : mechanical friction loss. The only losses are:
4 In the primw, the induced voltage is in opposition
to the applied voltage and slightly less than it, and a. Iron losses due to eddy current and hystersis in the
the difference between the two causes the flow of magnetic core. (No load loss)
current. This is called the EXCITING or Copper loss, representing I*R loss in the primary
b.
MAGNETISING current and its function is to and secondary windings. (Load loss)
maintain the magnetic flux in the core under no
load conditions. It does not exceed 5% of full load Both are comparatively low in value and therefore
current for selectively small transformers and in the efficiency of a transformer is of the order of 98
large Transformers, it is as low as 1 or 2% and on to 99% which is very high, as compared with that of
very large power transformers it is less than 1%. a motor-generator set which is of the order of 80 tu
85%.
b) In the secondary, the induced voltage feeds the load
circuit. When a load current flows though the Q3. What is meant by ‘RATlNG’of a Transformer? ,
secondary, it produces a magnetic flux which is in
opposition to that produced by the primary and
The “rating” of transformer refers to the nominal
tends to demagnetise the field. Since the induced
voltage in the primary becomes correspondingly4 parameters that the transformer is expected to withstand
reduced, a greater amount of current will flow from continuously over the expected life of the transformer.
the mains through the primary as much as is
These are
necessary to restore the magnetic flux to its original
normal-value, and a state of equilibrium is once (1) KVA = Rated Voltage x Rated Ampere per phase
again established. This is how the load current on
(2) Rated Voltages
the secondary side gets transferred to the primary
and is drawn from the mains. Although the two 4 Primary
b) Seccondary
windings are insulated from each other, tie
magnetic field acts as the invisible link which (3) Rated %ents
transfers energy from one winding to another and
from one systemvoltage to another. That is why the (4) Rated Temperature rise
whole unit is called a ‘TRANSFORMER”. i
The KVA/KW rating of any electrical equipment is
Q.Z. Why is the Efficiency of the Transformer so high? the maximum active and reactive power which can be
drawn from it, without the temperature riSe in the
This is because the transformer is a static windings exceeding the safe limits, for the particular class
equipment with no moving parts. There is therefore no of insulation employed. The temperature rise in the
m ,,v# __T -
windings of a transformer is determined by the heat the no loud. P.F which is very low. Hence it is necessary
balance, where the following two factors equal&: not to select too large a transformer capacity for the
a. Rate at which the heat is generated inside the required load. Which may result in a low p.f
transformer. This is made up of iron losses, which
are constant at constant flux and copper losses, Q.5. What is the ‘EFFICIENCY’ of a Transformer?
which vary as the square of tile load current. b)
Rate at which the heat is dissipated into the The efficiency of tranformer is given by :
atmosphere. Heat is dissipated by convection and
output output in watts x 100 . ..(C)
radiation. These, in turn, depend upon the ambient = --L----------------------------------------
? = --------
temperature of the atmosphere and the method of input output in watts + Total loss in watts
cooling employed. It is quite obvious that the
greater the efficiency of the cooling of a Usually, in smaller sizes of transformers below 100
transformer, the greater is the load which it can
KVA, the efficiency will be 96 to 97% and 98 to 99% in
deliver without becoming unduly hot.
larger size transformers. Efficiency also depends upon
4.4. What is the ‘POWER FACTOR’ of a Transformer? the load. For example, at l/4 load, the efficiency of even
a large transformer may come down from 98% to perhaps
PF is the ratio of the active power delivered to the 96%. Therefore, in specifying the characteristics of the
load (KW) to the apparant AC power (KVA) supplied by transformer, the load factor should also be taken into
the Source- PF = KW/KVA = KVA Cos 9 account.
when $ is the angle by which the load current lags h
or leads the phase Votage. The efficiency will be the highest at a percentage x
On no load, the power factor of a tranformer is very of the full load
low and lagging. On load, the power factor is nearly equal Ironloss
X = 100 ____----__---___ . ..(D)
to the power factor of the load, which it is carrying. The
Copperloss
power factor is denoted by p.f. and its angle is denoted by J
cosine. Q.6. What is meant by ‘REGULATION’ of a
Since the no load powerfactor of the transformer is Transformer?
very low (approaching o) the effective powerfactor on the
primery side of the transformer is almost the same as that Regulation is the percentage Vottage drop on the
of the load at high loads. But at lower lot& it approbles secondary of the transformer expressed as a percentage
10
of the no load Voltage to the voltage when the KVA 0.577 KVA
transformer is loaded. Unlike in a rotating machine Current output from secondary = ------- = ---------
where the intermal reaction in the machine is very high, flKV KV(Sec)
the reactance in a transformer is relatively low and the
The secondary equivalent Secondary &rent
consequent voltage drop due to varying loads is relatively
Current input into primary = _---____-----_- - -
small. The usually low voltage condition usually met with
efficiency
in the most systems is more due to the reactance the
transmission/distribution line reactance rather than due O&-KV4
to the inherant transformer reactantiitself. Current for 96% efficiency = --I-- ... F
Regulation of a tr&former refers to the % KV.
Although the efficiency is assumed somewhat as
variation of secondary voltage from no Joad to full. load.
At unity power factor, low as 96%, it gives a convenient and easy thumb rule to
:
remember.
Copper loss x 100 (percentage reawce)’
= ----------------v--m + ----_-____-_-_-_________ . ..E Thus, the current drawn by a lKVA/4OOV
‘2o(-J . / ..l , .I
output Transformer will be 0.6/0.4 = 1.5 A The currents drawn
I
.;_ by a 1000 KqA transformer at other voltages are given
In the large siie tranSformers,.the % regula&mis of
below, based on the thumb rule :
the order of 2 to 4%
..,
Currerit = 6OOlKV
Q.7. What is the normal rated ‘curf&t’lat can be
_----___---____-----___________________I-------__
d r a w n fi-om a transfWmy?
1 - Supply voltage (KG) Current in Amps ~~
The current that can be drawn contim~ously from a
transformer is dependent upon:& KVA rating and the f; - 272.0 183.0
6:6 ’ 91.0
secondary voltage of the transform$ within the limts of 11.0 54.6
the permissible temperature rise.. The” name ‘plate 22.0 27.2
provided on the transfonfler ft.&i&es the primary and ’ 33.0 18.3
66.0 9.1
secondary current ratings. T&y can,, however, be 132.0 4.55
calculated as under for a three phase Transformer: 220.0 2.72
400.0 1.5
12 --------_____- --Lz- -
Q.8. What are the essential requirements which the usually gets broken, when it passes through zero. In the
design of a transformer should fulfill? next negative half cycle, when the contacts are not yet
fully open, the voltage crest may restrike’ between. the
i. The dielectric strength of insulation and the contacts, thereby producing a (back EMF) voltage
terminal arrangements should be adequate and double that of the normal crest voltage, on account of the
effective,to withstand not only normal variations in
operating voltage but also over voltages and surges electro static charges remaining on the system. If this
due to system faults, switching operations and restriking takes place during subsequent voltage crests
lightnings. also, restriking voltages could be 4 to 6 times the normal
ii. Adequate coil ventillation by proper design of the voltage depending on the reactance/PF of the load. The
coil and oil ducts to aid circulation of oil, so that, the restriking voltage is unidirectional (D.C) in nature and
hot spot temperature is never exceeded;
causes voltage transients resembling lightning surges.
Q.9. What is meant by “graded insulation”? Frequently, circuit breakers give rise to surge 2 to 3 times
the normal system voltage, especially when breaking
If in ‘a transformer, one end of the winding is heavy short circuit currents.
earthed. The insulation towards the earthed and could be
Q.ll.What is the cause of “insulation breakdown”?
reduced or graded to reduce the cost. This becomes
worth while only in extra high voltage transformers. Application of a voltage exceeding the insulation
IS:2026 allows graded i n s u l a t i o n f o r Y a n d strength of a liquid or solid medium results in the
interconnected Y windings, if the neutral point is formation of streamers i.e., movement of loose ions
earthed, for transformers above 66KV rating only. In existing in the medium. These streamers start from the
other transformer up to 33KV class uniform insulation is high voltage electrode, at a point which has maximum
normally provided. (ie) the enter winding is insulated to electro static voltage gradient, such as at a sharp point,
earth for the full power freqency test level specified, (eg) and move towards Be other electorde, reaching if after a
28 KV for 11 KV, 50 KV for 22 KV 70 KV for 33 KV-etc short interval of time, when a complete breakdown
including the neutral of the Winding if any) occurs. The streamer dies down if the voltage of the D.C.
surge drops down greatly before it reaches the other
Q.lO.What are switching surges and restriking voltages? electrode. The wave shape has a great effect on the
breakdown value, Quite.often, the breakdown occurs in
When an AC circuit breaker trips, the current
15
14
c
the wave tail, after the crest has passed, because of the
time taken for the streamer to form and reach the other point, having a high potentialgradient.There is, however,
electrode. one important difference between solids and liquids as
regards the effect of the streamers. In a liquid, the
streamers disappear when the applied voltage is
removed., In a solid, it leaves a permanent mark on the
insulation, which gets extended on a subsequent exposure
to a higher voltage or for a longer duration. The
breakdown value is quite high for the insulating material
itself, but the formation of streamers is greatly influenced
I
d-
by the porosity of the medium and the moisture content &
J-+- b
- mpr Vi‘) t other impurities in the oil surrounding it. The creepagc
One other point .to be remembered is that, the path is therefore kept as adequately long, and surface ’
presence of air bubbles materially reduces the deposits like dust and sludge should not be permited to
breakdown value as ionisation can easily start. It is, accumulate.
therefore, necessary to allow some time to elapse, after
centrifuging the oil by a purifier, to ensure that all air Q.12.Explain the manner in which heat produced inside
a transformer is dissipated?
bubbles disappear, after which the transformer may be
charged. Another point to note is, that, the dielectric Heat is produced in the winding on account of 12R
strength of oil is much higher than that of air and the losses, and in the core, of eddy current and hysteresis
clearance between the live parts and the tank interior is losses. In small% dry type transfomers, heat is dissipated
correspondingly less. Therefore, a high voltage 1 directly to the atmosphere. In oil immersed transformers,
transformer should not be charged with full voltage heat is dissipated by therm0 syphon action. The oil serves
unless it is completely filled with oil and all air or gas T as the medium for transfering the heat produced inside
bubbles inside are extracted or eliminated by appropriate th.e transformer to the out side atmosphere. The coil heat
means. first passes through the layers of insulation and is taken
The
- breakdown of transformer winding insulation up by the oil. The temperature difference between the
also is initiated by the formation of streamers, at a local hot conductor and the exposed surface of the coil is
16
17
-d
_
dependent upon the amount of heat generated and the Q.W.What is thermo-syphon action?
thermal resistance of the insulation employed.
Thermo-syphon action refers to the circulating
It is given by the following equation:
currents set up in a liquid because of temperature
Temperature drop in ‘C= PxRt = P x b . ..H
difference between one part of the container and another
part. When oil becomes hot, it becomes lighter and
where P = Heat produced in watts
Rt = thermal resistance in “C per watt therefore rises up, drawing in its wake colder oil from
t = thickness of insulation in inches below. The rising current of oil takes the heat away horn
K = thermal conductivity of material in the coil surfaces to the top of the tank. From there, it
watts per square inches per ‘C per
inch thickness. passes down the radiator tubes, where the heat is
A = Cross section of path in square inches radiated/convected out into the atmosphere. As the oil
The temperature drop through 0.02 inch thick gets cooled, it becomes heavier and sinks to the bottom of
paper at 1.0 watt&q. in would be 1 x .02/.004x 1 = 5’C the tank. What causes the convection current is the
(4vW difference in the weight of the column by the side of the
winding and that of a similar column of oil by the side of
The difference between copper temperature and the tank where the radiator tubes are connected. TO
winding surface temperature is assist radiation, the surface area of the cooling tubes is
made as large as possible by providing fins. To reduce
Heat loss in copper in watts Thickness of insulation friction, cross section is also kept as large possible and a
__-------_-__--_______I___________ x __---_ --_ _--- _-- ___ ________- number of tubes /.channals are provided to give parallel
Thermal conductivity of Exposed surface of paths. As the size of the transformer becomes large, the
insulation winding
rate of oil circulation by therm0 syphon action becomes
The thermal conductivity (k) for different type of insufficient to dissipate all the heat produced, and
materials is given as under : artificial means of increasing the circulation has to be
__--___--____-____________I_____________---------------------~---- adopted, viz., forced oil circulation by electric pumps,
Insulation # provision of large radiators with forced air draft cooling
_____-_---_---_-----____l__l___________^-------------- --m-v----
by electric blowers which are automatically switched on
Fibre sheet .007
Mica .015 and/or switched off depending on the loading of the
Paper, oil soaked transformer. In very large transformers special oil
Transformer oil :E4 coolers with water circulation may have to be employed.
______-_____-___--_________________I____------ ----s-
19
18
Q.M.What are the standard symbols used for describing well-worked transformer by a reputable manufacturer
different methods of cooling transformers? can be about 20 years, if it is well looked after. It can
i
indeed serve much longer, perhaps 30-40 years, if it is
The following symbols are usually employed: operated with care.
= Air cooling for dry type transformer
fi = Natural cooling by convection currents only It also follows that it may ‘die’ prematurally and its
B = Air blast cooling life span may get reduced to 10 years or even one year by
= Oil immersed (mineral oil) cooling gross negligence. Excessive temperature rise and
:: = Water cooled
= Forced oil circulation by pump humidity are like poisions. It has been estimated that,
; = Synthetic liquid used instead of oil. every g°C rise above the permissible limit halves the life
of the insulation. Abnormal over loading (Hot
Two or more of them are joined up as may be
, spot temperature exceeding 140’ - 250’ C ) destroy the t
applicable eg., ONAN; ONAF; OFWF, etc.,(Refer IS
insulation in a very short time can certainly char the
2026)
insulation. Therefore. overloading beyond capacity can
Q.15 What determines the life of a Transformer and how be a very costly business indeed!.
does it “age”?
1. Exposure to high temperature over long period
Transformer is a static device without any moving results in progressive deterioration of the *
insulation; it’s mechanical strength reduces
parts. Therefore there is nothing to wear in it as in a gradually until it becomes quite brittle.
motor. Copper and iron can last indefinitely. It is, Eventhough the dielectric strength may be still
however, subjected to many strains and stresses, such as high, the insulation fails in the mechanical sense;
the transformer ceases to be reliable and will be
by the, application of impulse and power frequence
unable to withstand short circuits. The ageing
withstand over voltages after manufacture, and after process is greatly accelerated at high temperatures.
commissioning by natural occurrences like lightning ii. Everytime a transformer is subjected to an external
surges, short circuits and over voltages, each of which short circuit, the coils and turns do suffer some
leaves its indelible mark on the winding structure. Yet, mechanical displacement which in course of time
becomes noticeable. Unless the resulting slackness
being a sturdy equipment, a transformer does survive all
is taken up periodically, the.mechanial wear by
these and lives a long life, if it is installed, operated and repeated short circuits and vibration may lead to an
maintained intelligently. The normal l i f e o f a inter-turn or inter-layer failure.
20
 -

...
111. Presence of moisture in the insulation, even in very Q.16 How is the temperature rise measured? - .-
small quantities has surprisingly great effect in Although the temperature rise in the oil can be
causing decomposition and deterioration of the
insulation. Oil should, therefore, be maintained in measured by a thermometer, it will not be a true
a high state of dryness and purity. . indication of the average value for transformer windings.
iv. Sludge and impurities close up the passage ways A better method would be to measure the resistance of
and ducts and cause obstruction to the free either the primary or the secondary, preferably the one
movement of oil and effective dissipation of heat. which has a high resistance. One reading should be taken
This will result in ‘hot spots’ and deterioration of
insulation. Some varnishes used for oil for each winding. This should be done before and after
impregnation react with oil and accelerate sludge the heat-run test. A wheat stone brige gives accurate
formation. Copper is also one of the most active results and is also convenient. Temperature can be
catalysts.
calculated from the follo&tg formula : -X
V. Air dissolves in oil to some extent. Its oxygen T2 = R2 (235 + Tl)/Rl - 235 . . . J
content reacts with the cellulose of the insulation,
and leads to the formation of sludging. The where Tr & T2 represent initial (cold) and final
primary objective of providing an oil conservator
tank is to reduce the area of oil exposed to oxygon (hot) temperature in ‘C.
and incidentally maintain this exposed surface at a Rl & R2are initial (cold) and final (hot) resistances
low temperature. The constant breathing in and in ohms.
out of air replenishes the dissolved o.xygen in the oil
and therefore promotes. ‘ageing’. The silica gel A rough and ready formula for finding out the
breather prevents only humid air from approximate rise in temperature in degree C is given
contaminating the oil, but it does not prevent below :
oxygenation of the oil. There is a modem tendency
to omit the oil conservator and the breather, by % rise in resistance
Temperature rise = ____________-_______--------
completely sealing the transformer. Sufficient gas
space is left above the oil in the tank, and the only 0.4
For eg., if the hot resistance is 20% higher, the
effect of temperature variation is correspondingly
pressure variations in this gas space. Sometimes, approximate temperature rise in 20/0.4 = 5O’C.
the top space is filled up by nitrogen, by connecting One point to remember when conducting this test is
it to a nitrogen cylinder kept out side the
transformer tank through a suitable pressure the cooling of the‘transformer, from the time the current
regulating device. But such arrangements are is switched off to the time the measurement is made. This
expensive and are suitable for only very large may t&e anything from 2 minutes to 4 minutes if well
transformers. organised, as the transformer should first be
22
23.
disconnected both on the primary and secondary, the Item To which fitted
instrument connected and balanced. During this period
the temperature may fall from 2’C to 4’C. The resistance
It. Conservator with air SO KVA and above for a
values have to be corrected for the fall in temperature release device rated voltage of 11 KV &
below and all ratings above
duing this period. This is done by taking cotiecutive 11 KV
readings at intervals of about a minute for another 10 to
15 minutes. If the readings are plotted-on a graph paper, 12. Jacking lugs Transformers above 3 tons
in weight
the curve can be extended backwards and the correct
reading at the instant of shut down could be estimated 13. Filter valve Transformers above 1CXX
with reasonable accuracy. (for exact determination refer KVA

to IS 2026-Part I)
,
Q.l’I.What arethe fittings and accessories which should In addition to the above, a few of the other fittings
be provided on a trarsformer? which could be provided, if required, are filter valves, ‘
rollers, winding temperature indicator, Marshalling box,
The Standard fittings provided on a transformer are explosion vent, gas and oil actuated relay, inspection
given below : (Ref&r to IS 2026) cover and skids.

Q.18 What is the difference between type tests and

Item To which fitted
routine tests?
1. Inspection cover 1000 KVA and above
2. Rating plate All transformers Type tests are those which are restricted to a single
3. Terminal marking plate -do- transformer of one type and are intended to check the
4. Two earthing terminals -do-
5. Lifting lugs -do- design characteristics not covered by routine tests.
6. Drain valve with plug 25 KVA and above
7. Dehydrating breather 25 KVA & above for a rated
voltage of 11 KV & below A prototype test usually relates to the first-unit
and all ratings above 11 KV. manufatired by a firm to a given specification, after
8. Oil level indicator with
marking of minimum level All transformers successfully passing which manufacture of other units
9. Thermometer pocket All transformers may start. Routine tests are individual tests which are
10. Oil tilling hole and cap All transformers
25
24
 made on every transformer before it leaves the factory, to ready and easy method. A heavy current of 50 amps and
ensure that it is in accordance with the spdfication It is a sensitive meter would give an accurate result. Tests
presumed that, every such transformer would also may be made when the transformer is cold or hot. What
comply with the type test, since its design is identical. is important is that the winding and oil temperature
should have equalised and be recorded when the
Routine test Type tests measurements are made. Sufficient time should be
Winding resistance 1. Impulse voltage withstand allowed for the current to stablise since the winding is
; Ratio test 2. Temperature rise highly inductive and the DC measuring current will take
3: Polarity test several seconds to reach its final steady value.
4. Phase relationship
5. Impedance voltage
6. No load loss and current Q.2O.How is the ratio test conducted?
7. Load losses
Insulation resistance
E Induced over voltage withstand Although this test could be done using a good
lb. Separate source voltage withstand calibrated voltmeter, it is best done by a special
ratio-testing equipment called ratiometer designed for
Special test
the purpose. This consists essentially of small portabie
1
(1) Short circuit test transformer witha fixed primary and a secondary winding
(2) Zero sequence test having a large number of taps at equal intervals,
(3) Noise level test connected to two electrical switches, one coarse and the
other fine, so that any voltage desired could be obtained
A certificate of tests made on each transformer and read off directly. The HT mains supply say 400 V or
should be obtained from the manufacturer. 230 V and the induced voltage on the secondary winding
is compared with the voltage output of the ratiometer
Q.19.How is the winding resistance measured?
after ensuring &at the two voltages are in opposition.
Accurate readings are obtained by an ammeter
Accurate measurements of winding resistance can .
connected between the windings, so that the circulating
be made with good wheatstone bridge or preferably a
current, if any, due to the difference in potentials may be
kelvin bridge. Voltmeter ammeter method is alright for
detected. 27
some ratings of transformers, and is a relatively rough and
26
 Ratio test is the first to be conducted on every
and Bl will be double that of a single battery. This
transformer even before the core and windings are
corresponds to series connection and the polarity is said
lowered into the tank, as any ratio error detected in the
to be additive.
windings may be readily rectified. The permissible
tolerance is +- 0.5% of the declared ratio. This test Q.22.What is impedance voltage?
should be conducted for every tapping position.
When impedance voltage is applied across oile
Q.21.What is ‘Polarity’ and how it is tested? winding of a transformer it produces normal full load
current to flow through the secondary winding when its
The expression polarity refers usually to the
terminals are short circuited. It is expressed as a
positive and negative terminals of a D.C. supply. percentage of the applied voltage. For example, in a
Consider two batteries as shown below in which the 1 lKV/440V transformer having an impedance voltage of
terminals A & B are negative and terminals Al & B1 are 5%, full load current will flow in the 400 V winding, if 550
of 11 KV is applied across the 11 KV windings, and the
400 V winding is short circuited at the terminals. The
assumption that there is complete magnetic coupling
between the primary and secondary windings is an ideal
condition. In practical transformers there is always a
certain amount of leakage flux which results in a slightly
lower flux linking the secondary than the flu in the
primary. This results in a voltage drop inside the
If like poles A & B are connected together the transformer. This, in addition with resistance drop in the
votage across Al & Bl will obviously be zero since the windings is know? as the leakage impedance of the
voltages cancel out. This corresponds to a parallel transformer.
connection. When voltage builds up in the same direction
from the common link, the polarity is said to be Q.23.How do you determine the no load lossess?
subtractive. If, on the other hand, the batteries are
connected in reverse, i.e., A to B the voltage across Al The iron and core losses are determined by
measuring the watts input and current when normal
28
29- - I
-_rc_. -I
voltage is applied to the primary winding, keeping all Load losses get reduce-d as the size of the
other windings open circuited. The power factor will transformer becomes larger. The following table shows
necessarily be very low. The maximum permissible some typical values at normal voltage and frequency for ’
variation from the guranteed figures of no load losses, is transformers upto 11 KV.
15%. The exciting current on no load varies usually
between 1 %.and 5% depending on the design. It is very ---------__--I------_l________l

rich in harmonics and necessary allowance should be No load loss Full load
KVA rating (w-1
made when taking the readings.(The tolerance on the
magnetising current is 30 % as per IS 2026)
160 425 3000 5
570 3300
Et 620 3700 5”
Q.XHOW do you find out load losses? 315 8tXl 4600
500 1100 6500 :
630 12Ou 7500 5
This mainly comprises copper loss and is 1000 1800 11000
determined by measuring the watts input into the 1600 2400 15500 2
transformer, when impedance voltage is applied, and full 2000 3000 20000 6
--------------------------------------------------------------------
load current is allowed to pass through the other winding
by short circuiting it. Another method adopted is to The maximum permissible tolerance on the load
circulate full load current through the transformer losses is _t 10%. of the guranteed values for load losses
without an external load, by what is called back to back and + 15% for no load losses as per IS 2026.
arrangement, provided two identical transformers are
available. The primaries are then connected across the Q.25.How is the insulation resistance measured and
supply and the secondaries connected in opposition what should be its minimum value?
through an ammeter. By adjusting the tappings available
This is best measured by a megger. The megger
on the windings differently i.e., one up and the other
reading gives a good picture of the amount of moisture
down, a circulating current equal to the full load current /
! absorbed by windings. If the readings are recorded at
can be set up. This arrangement is also suitable for
i regular inervals, they would be very useful for finding out
conducting a heat run test.
30

I . .
when drying out should be taken up. Remember that as the winding gets fully charged, when it will show the
temperature has a direct bearing on the insulation correct reading. If the cranking speed at any time is
resistance obtained and that it comes down with a rise of reduced slightly, there will be a reverse flow of current
temperature. For class A insulation, the insulation from the accumulated charge, and the needle will shoot
resistance in megaohms gets halved for every 10’ to 15’C up to infinity, giving an incorrect reading.
rise. Therefore, the temperature should be noted and
The above difficulty will be overcome if a motor.
recorded every time insulation resistance is taken.
driven megger iswsed, which will be a distinct advantage
As regards its minimum permissible value, there is when testing large high voltage transformers. Make sure
no definite rule, since it varies with the size of the that, the test leads themselves are in first class condition
transformer and the voltage rating. A safe general rule is and show high insulation resistance.
t
to aim for about 1 megaohms for each 1OOOV of operating
Q&What is an ‘Auto-Transformer’ a n d what is its
voltage, when the temperature is (6O’C). The following
main advantage?.
values may be considered as satisfactory when the
transformer is warm (45OZ): An Auto-transformer can be conceived of as a
___________________-_______ __________--_____I____________________I_ conventional transformer gth two windings, but ‘with
Voltage of Winding Minimum insulation Voltage of.
resistance in megaohms megger the secondary winding connected in series~ with the
primary. It may be used to step up a voltage or to step it
400 V o l t 2 500V
2.2KV 3 5OOV down.
1lKV 20 1OOOv
33KV 25OOV
132Kv :2 25OOV
------------------------------------------------------------~-----
When using a hand megger, keep turning the
handle at full speed and ensure that the voltage generated
is steady. There will be appreciable capacitance in the
windings and the same precautions, as when measuring
the insulation resistance of a condenser, should be taken.
The IR valve will be low initially, but will rise up quickly
32- 33
 If the connections are so made that, the Polarity is is much less, short circuit currents will be much higher
additive the output voltage will be more than that of the than in a double wound transformer. The short circuit
primary, by any amount equal to the secondary voltage. stresses to which the windings will be subjected to are
To step down the primary and secondary terminals are correspondingly larger. In a three phase transformer, the
interchanged as above. phase relationship will be shifted with respect to the
Primary.
It will be observed that the load current pass&
directly from the supply mains through the series Q47.What is meant by a ‘Variable Transformer’?
winding. The common winding only carries a small
proportion of the load current depending upon the ratio A variable Transformer, which is known by several
of the secondary to the primary vohage. Because of this, trade names such as variac, Dimmerstat etc., is also an
the effective size of the transformer is significantly auto transformer, but it has a continuously variable tapso
reduced. For example a single phase transformer that any outPut voltage desired could be obtained. It is a
required to boost up the mains voltage from 5500 V to very useful piece of equipment in any laboratory or test .
6000 V, with a load current of lOOA, if a conventional room. The primary windL !, consisting of enamelled
double wound transformer were used, its rating will ha$ copper wire, is wound round c. circular core and bedded
to be (6OOOx100/100) = 600 KVA, and if, an Auto on a flat insulated circular strip on one side, where each
transformer were used, its approximate equivalent frame turn is braced. Over this, a contact arm can slide along
size need only be (6000- 5500) lOO/lOOO = 50 KVA i.e., from one end to another making contact with any part of
1/12th of 600 KVA for a corresponding double wound the winding. The slide arm is connected to the output
transformer. An Auto transformer is, thus, of much terminal and can be moved and set at any point desired,
smaller size, has lower cost, lower losses and much higher
by an insulated external knob. It is, thus, possible to
efficiency.
adjust the output voltage continuously from 0 to 260v.
The slide arm is fitted with carbon contacts to limit the
The auto transformer, however, suffers from some
disadvantages. Firstly, the secondary circuit is in metallic circulating current in the few turns short circuited by the
continuity with the supply line to which the primary is slider contact piece.
connected. Secondly, as the impedance of the secondary
34 3.5
 Variacs are available for 230 V/single phase in stator winding, which is connected to the supply mains
various sizes up to 100 KVA Three phase units could, and creates a rotating magnetic field in the core as in an
however, be readily built up by mounting three single induction motor. Inside the stator is a movable rotor, also
phase units one above the other and coupling their containing a 3 phase winding, which can be rotated and
spindles to a wrnmon control wheel. Variacs are rated by set to any position by an external hand wheel set. The
rotor winding is connected, in series with the sipply
their current capacity which is independent of the output
mains, to the output terminals, As the rotating magnetic
voltage, since the size of the primary conductor
field cuts the rotor windings, it induces a voltage where
determines the maximum current which it can carry. A
phase angle can be altered by rotating the rotor with
500 VA variac can, thus, give about 2A output at all
respect to the stator. The magnitude of the voltage
voltages from 0 to 250 V, whereas, a double wound
remains constant, but its phase angle with respect to the
transformer of the same rating could be designed to give incoming supply can be altered, and because the
an output, say, of about 21 A 24 V or 250 A at 2 V. secondary winding is in series with the mains, it can both
Another point worth remembering is that, the output t boost up or buck the output voltage, over the entire range
leads are in metallic connection with the primary and not of Vs + Vr and Vs - Vr where Vs & Vr are the stator and
insulated from it, as in a double wound transformer. Care rotor voltages. Since these are relatively Costly for
is, therefore, required in making the connections to the equivalent power out puts variacs are more-or less
mains. The terminals of such a transformer consist of a universally used these days.
set of 3 terminal, per phase where the input line should be
QL?B.What is an Automatic Voltage Booster?
connected between the two fixed terminals (line ex
‘L
neutral) and the out put taken from the neutral and the AVB is essentially an auto transformer consisting of
ih
moving terminal. primary winding c?nne@ed in parallel with the circuit
and secondary winding connected in series. Taps are
Q.28.What is an ‘Induction Regulator’? provided on the series winding. ’ The tap changing
mechanism is operated by a driving motor. When a
This works both on the principle of a wound rotor
voltage is applied across the primary winding, voltage will
induction motor and a booster transformer and is used for
be induced in the secondary winding due to the magnetic
voltage regulation purposes. It consists of a hxed 3 phase
37
36
flux linking it. The amount of the induced voltage is The AVB improves the voltage as well as reduces
controlled by means of the taps provided on the series line losses. Effect of AVB is predominant in voltage
L
winding. Voltage/load senstive controls provide drive to improvement as voltage boost upto 10% can be obtained
motor which operates the tap changer.. The AVBs are which is difficult to obtain with shunt capacitor.
voltage sensitive and the voltage is obtained -from
secondary of a P.T. Q30.How will you compensate for voltage variations on
the HT mains input into a Transformer?
Transformers are usually provided with taps on the
HT windings to permit voltage adjustments of + 2.5 and
+ 5% on the highvoltage side. Tap changing can be done
by an externally operated switch. In a 3 phase
I
transformer, the tap operating mechanism is ganged, so
that, by one operation of the external handle, the contacts
Rate of Booster : KVA rating of the Voltage r are changed over on all the three phases simultaneously.
Booster = Rated Amps of AVB x Rated range of boost in
K.V.
Tap changing should never be done on load, but
For 6600 Volts, 10% booster whose curret carrying
capacity is 100 Amps KVA rating of booster = 100x 6600 only after switching off primary supply and taking the
transformer out of service: This is because, the switch
x lO/lOO = 66 KVA.
contacts, which are inside the transformer tank are oil
Effect ofAVB: When a booster is installed on a line, immersed, are only designed to carry the full load current
it causes a sudden voltage rise at its point of location and t continuosly without overheating, and are not designed
improves the voltage on the line beyond the location of for breaking any current, even the small magnetising
current. I
AVB. The % Voltage improvement is equal to % boost
setting of AVB. The increase in voltage will cause
If any attempt is made to operate the tap changing
reduction of losses in the line beyond the location of
gear on load, the consequence may be disastrous, as the
AVB.
38 resulting arcing across the tap change switch contacts may
39
because a short circuit between adjecent tap windings and
Q32.Describe briefly the operation of the on load tap
there by cause a short circuit between adjacent tap changer?
windings are thereby cause severe damage to the The great advantage of an on load tap changer is
equipment. To prevent such inadvertant operation, that, it permits the output voltage of a transformer being
provision is made to lock the switch in position anti increased or reduced as required, without interrupting
unauthorised operation of the tapping should be totally the supply, either to correct for the fluctuations in the
prohibited. incoming supply or to compensate for the voltage drop in
the outgoing lines. The principle is one of using a make
Q31.The Indian electricity rules allow a maGmum before break type of tapping switch with a currenl
variation off 12.5% on the HT supply and off 5% limitting device between the adjacent taps to prevent
on the consumer side voltage. How is it then
excessive circulating current when the tap-changer is
possible to manage with only f 5% and f,2.5% ‘
tappings which have been standardised for bridging adjacent taps.
Transformers?
Q33.How heavy a single phase load can be connected to
Normal specificatio;i caters only for standard a three phase transformer?
transformers. It is for the supply authorities to he Every transformer can feed a single phase load to a
appropriate measures to ensure satisfactory voltage limited ,extent. The determining point is the relative
value of the single phase load as compared with the
regulation on their system. --This can be achieved by
capacity of the transformer and the load on it. For
several methods, such as by providing synchronous
condensers static VAR compensators for counteracting
impedance drop in the transmission lines, by providing a
larger number of taps on transformers if required, and
using automatic voltage regulators in conjuction with on
load tap changers. It should also be remembered, that,
there will usually be more than one transformer between
the high voltage network and the consumer’s mains, the
tappings on which are ordinarily sufficient to look after instance, a 20 KVa single phase load can be certainly
the voltage fluctuations on the distribution system. taken by a 100 KVA/ 3 Phase Transformer, but a 40 KVA
/ 3 Phase Transformer will be overioaded even if a 20
40
KVA single phase load is put on it, since the capacity on be restricted to keep the unbalance on the system to the
each of the three windings of the 40 KVA / 3 transformer minimum.
~
is only 40 / 3 - 13 KVA approximately. t

Q34 Can you operate a transformer on a frequency

Heavy single phase load on 3 phase circuits is not different from that for which it is designed?
desirable, since it produces unbalance on the supply
system. If heavy single phase loads are to be met, such-as Generally transformers designed for 50 HZ can be
those required for furnaces (Spin casting system) the best operated on 60 HZ of appropriate voltage, but using
thing would be to arrange them in threes, one connected
transformers designed for 60 HZ should not be used on
to each phase, so that they present a balanced load to a 3
50 HZ systems without proportionately derating the
phase transformer.
/ rated voltage (Appoximately 20% less)
i
If this is not feasible, and the load can only be
arranged in two circuits, Scott connected transformers 435 Some transformers make a lot of noise in
can be used. This requires two transformers, one of operation. Why is it so?
which is called main (some times called the base), has a
centre tap on the primary and wound for full line voltage Every transformer does make some noise, but it
and the second called the teaser (also known as height) is becomes conspicuous when ;he size is large. The noise or
wound for 0.866 times the line voltage. The secondary / hum is due to the fact that th 5 core, which is made up of
windings furnish two phase supplies No. 1 & No.2, to ! thin laminations, vibrates because of the alternating .
1
which two independent single loads can be connected. If magnetic flux and therefore expands or contracts ever so
the single phase loads are equal, the unbalance is greatly slightly,with each cycle of magnetisation. This causes the
minimised on the supply system. If, however, the loads audible hum. The Contraction caused due to
i
are not equal, the unbalance on the supply system
magnetisation is called “Magnetostriction.”
becomes higher. If the single phase loads are extremely
heavy, the unbalance caused on the supply system will
To reduce the hum, the core must be tightly
make the system voltage unequal and unsymmetrical,
which will adversely affect other consumers connected to clamped together. The flux density should also be
the same system. It will also produce current unbalance reduced as much as possible, but there is a practical limit
, to this. Sometimes, under unfavourable conditions or
which will adversely affect the alternators at the
,
generating station. Therefore single phase loads should poor design or construction, the hum gets multiplied by

42 43
 Q36.What is the standard terminal marking on a
100’s of times due to ‘resonance’. This should not be
Transformer?
permitted.

The noise and hum produced by large transformers

in urban areas reach such high values as to be a source of
disturbance and annoyance to the people living nearby.
The noise level decreases quickly with distance is by some k-9 b+++
A B’ c
6 db (decibel) for every doubling of the distance, so that at QRMINAL MARKIF~GS.
about 200 feet the reduction could be about 26 db.
Transformers therefore should be located as far away The arrangement and marking of terminals is quite
from residential areas as possible. important for identification of terminals, especially when
the transformers are tobe paralleled. Normally, the high
When they have to be installed /located in heavily voltage terminals are shown in capital letters, N standing
populated places, one method adopted to reduce the for Neutral and A,B & C for the Phases. Corresponding
noise is to install the transformer inside an enclosure small letters are used for the low voltage terminals. In
which is made sound proof. Brick walls lined with single phase transformers, the symbols A2, Al, a2, al are
acoustic materials are quite effective and all connections used in descending order from left to right. Please refer to
should be by underground cables. When the size is large ‘part IV of IS 2026.
and the voltage high, the radiators could be mounted
outside the enclosure and connected to the transformer 437 What are the essential conditions for satisfactory
by flexible pipe connections. The transformer itself is parallel operation of Transformers?
mounted on resilient pads. While there is little that can be
The five essential conditions to be satisfied are that
done to the inherent 100 HZ noise of the transformer due
they shquld have the same.
to the magnets striction of the core proper, any external
noise due to loose components on the transformer such
i. Polarity
as loose diagram plates, marshalling lockes, or other
fittings shorted be located and can be easily located and ii. Voltage ratio for all transformers
eliminated by proper tightening ...
lu. %impedance
44
-.. 45 A
 iv. Phase sequence
anticlockwise rotation as normal and clockwise as reverse
(rotation)
for all polyphase phase rotation. In a 3 p h a s e circuit, p h a s e
V. Identical vector diagram transformers
rotation/sequence can be reversed if any pair of leads is
and phase displacement.
changed over.
vi If the transformeri ark fitted
with on-load tap changers the _ Q.4O.Jhplain how phase angle difference can exist
control circiuts of the tap changers
between the primary and the seconw of a
should also be compatible with
each other. transformer.
Q38.1~ it ncvessay for the rated capacities of two
transformers to be the same for parallel operation? This diktly arises out of the internal CoMections
of the windings. Even in a single phase transformer, the
No. When two transformers of differe,nt capacities secondary and primary windings can be or the same
are paralleled, they should share the load in proportion to polarity of opposite polarity (0’ or 1800) depending on
their capacities. This can only be ensured if the % how the terminal connections are made. In 18O’a 3 phase
impedances of the two transformers are nearly the same. transformer too the same thing happens. A YN
i.e., the deviation should not be more than- + 10% from transformer having the similar vector diagram for

E I the average of the two. Normally transformers of

differing capacities, where the smaller unit is of capacity
less than 30% of the larger unit should not be operated in
parellel.
primary and secondaty side, can be connected in two
different ways internally. In the first case, there is a 0’
displacement between the primary and secondary
whereas in the second case mere is 180’ displacement,

In addition to the above, ;? plus 30’ or minus 30’

Q39.What is the meaning of phase sequence and how do displacement is possible in a 3 phase transformer, when
you alter it? the vector diagrams are different i.e., or . For parallel
operation, transformer secondaries must have the same
Phase sequence refers to the cyclic order (RYES or
phase angle displacement with respect to their primaries,
RBY) in which successive current or voltage waves reach
so that there may be no phase difference between the
their peakvalues. The accepted convention is to consider
terminals of the secondaries themselves. To help
46
47
identification, transformers are classified under four
different groups as shown and a three digit vector symbol
is adopted to designate them like Yy 6, in which :
a. the first digit in capital letters may be D,Y or 2
stand&g for delta, star or inter connected star
connection of the primary winding.
b. the second digit in small letter may be d, Y or Z
standing for delta, star or interconnected star
comieclion of secondary winding.
c. the third digit represents the phase displacement
between the primary and the secondary. The
convention employed is to describe it by the hour in
a clock, in which the higher line toneutral voltage is
represented by the minute hand set to 12 ‘0’ clock
position and the lower line to neutral voltage is
represented by the hour hand. Since 12 hours
represent 360’ of a full circle, each hour represents
a 30’ phase difference, 1 stands for minus 30’
displacement, 6 for ~~&us 180’ and 11 for minus
330’ or plus 30’ displacement, as referred to the
standard counter clockwise vector rotation.

The table on ‘he next page shows four different 3 Ydl

groupings of transformers. Transformers in the same YZl

group can be operated in parallel by connecting together
Dyll
the terminals which correspond both physically and i
EL

alphabetically, pr?vided all other conditions are satisfied Ydll

including standard terminal markings, Transformers in

Groups 1& 2 cannot be operated in parallel with those in
groups 3 & 4 as there is an inherent phase angle
difference which cannot be corrected.
 c

QAl.How can you parallel a transformer in GROUP 1

with GROUP 2 ?
In this case, there will be a phase difference of 180’
between the two secondary windings. This can only be
corrected by changing the internal connections of all the
three windings of one transformer i.e., either of the
primary or the secondary but not both. This will be clear
from the figure on the previous page in which the internal
connections of the secondary will obviously have to be
changed. It will not be sufficient to change over the
fntemal connection of one phase winding alone as it will
result in an entirely different arrangement. It is of no use
changing internal leads.

Q.42.How can you operate a transformer belonging to

GROUP 3 with one in GROUP 4 in parallei?

Here the phase difference between the secondaries

will be 60’.Transformer belonging to GROUP 3 could be
operated in parallel with those in GROUP4 orviceversa
by transposing two of the external high voltage
connections of one of the transformers and also the two
corresponding low voltage external connections. It is not
necessary to change any of the interna connections.

Q.43.Describe the precautions required and the correct

practical test procedure for paralleling hvo
transformers.
The following precautions should be taken :
1. Study the name plate particulars, voltage ratio,
49
 percentage impedance, vector diagram and group transformers are completely isolated, connect any
number, terminal markings etc., and make sure that terminal of one transformer to what appears as the
all conditions for parallel operation are fulfilled. If corresponding terminal. of the other vansformer.
the name plate or terminal markings are missing, a Even if it is connected to the wrong terminal, no
much more elaborate procedure is required, ie., the harm is done and the tests will reveal the error.
transformer windings have to be lifted out and the
internal connections traced and identified, if 6. In using a voltmeter for the test, a fundamental
necessary. precaution to be taken is to put its range switch to a
much higher scale before canying out the test, i.e.,
2. Even when everything seems right, it is still at least twice the expected voltage. If the secondary
advisable to observe a testing procedure as any voltage is 400 V the voltmeter range switch should
erroneous connection may result in a short circuit. be set to 1000 V, for the first set of measurements.
Do not forget to connect the transformer body After the terminals are identified and the
solidly to Earth. corresponding terminals show little or no voltage,
3. For conducting a phasing out test, it is not necessary the selector switch should be put to a lower scale say
to apply the rated voltage on the high voltage side. O-50 volt or less to make an accurate measurement
A much lower voltage is more convenient. For of the voltage difference between corresponding
example a 6.6 KV/400 V transformer primary could terminals.
be connected to a 400 volt supply. The secondary
voltage will be correspondingly lower i.e., about 24 Q.JJ.What are the hazards against which a transformer
volt which makes it much safer and easj, to work requires protection?
with.
4. If full rated voltage has to be applied, it will be They may be classified broadly under the following
necessary to use potential transformers. The heads :
terminal markings and polarity of the potential
transformers will have to be checked. Remember 4 Over loading of transformers and short circuits
the possibility of 180’ phase difference if the h b) Internal faults like loose contacts, faults between
connections are not correctly made to likb adjacent turns and earth fault in the windings or
terminals. leads.
5. It is necessary to link up the two secondaries by one cl Improper design of transformer or auxiliaries. This
commod cormectio< so that volt meter readings can be taken care of by obtaining the equipment
may be taken. If the secondaries are star connected from a reputed manufacturer compliance with the
and the neutrals are available, connect the neutrals latest relevant specifications by ISI, successful
together to serve as the common link. If no neutral completion of routine tests prescribed, and careful
is available and the secondary leads of the two inspection before commissioning.
50 51
~._ . - - _^_^ ^
d) Improper maintenance : This can be avoided by mains voltages up to 11 KV. Such transformers are
employing qualified and trained engineers and controlled by a circuit breaker fitted with an over load
staff. *
1 tripping device, of which there are many types, namely,
4 Hazards of external origin like lighting surges, direct tripping by a series trip coil, a fuse shunting the
system over voltages. The only insurance against
this is to ensure proper insulation co-ordination and tripping coil connected in Series with a Current
to install surge arresters -having the correct Transformer, thermal over load release etc., If there is a
characteristics. bank of small transformers, each transformer may be
A transformer, however, is a robust and serviceable
protected by H.R.C. type fuses and the HT supply to the
piece of equipment which rarely gives trouble. But even
bus may be controlled by a circuit breaker. When the
such equipment can be damaged by negligence.
transformer capacity exceeds 250 KVA, it is advisable to
provide a three element inverse time overload relay.
QAS.What protection is required for a small 230%’
transformer (say for an inspection hand lamp)?
Q.47.What protection is required for outdoor pole
A double pole control fuse switch in the phase mounted distribution transformer?
conductor is sufficient. If the transformer is used as a
Since these installations are not manned and may
portable apprartus, a 3-pin plug socket and switch may be
also be in remote places, the most common method
used together with a 3-Core Cable, the third core being
employed is to provide drop out fuse swtiches on the high
used for earthing the body of the transformer. An earth
voltage side up to 33 KV. As its name suggests, a drop out
leakage minimum circuit breaker may also be used. The
switch fuse is one which acts as a fuse as well as a break
230 volt transformers for inspection hand lamps are
switch or link, with the added feature that, the fuse
normally required for use during annual shut down and ! element drops out of the normal position when the fuse
emergency breakdowns in continuous process industries
blows out and hangs vertically downwards, giving a clear
like paper/pulp, Cement and Steel.
visual indication of the fuse having blown. The fuse has
not only a high rupturing capacity (150 to 300 MVA) but
Q.46.How do you protect an indoor distribution
transformer installed in a small substation? also has an inverse time overload characteristic and a
The capacities of the transformers may vary from dependable current carrying capacity. The cost is also
SKVA to 500 KVA, with their primaries connected to HT low.
53
52.
 temperature rises slightly above a preset limit and sounds
Q.48.Describe the protective equipment required for a
large transformer in a major substation. an alarm to warn the operator, so that he may take
appropriate action to prevent further rise in temperature.
In a large substation working on 66 KV, 132 KV or If this warning goes unattended and the temperature
high voltages, the transformer rating may be anything continues to rise up to a dangerous limit, the second
from 5 MVA to 20 MVA or more, costing several lakhs of contact closes and trips the circuit breakers.
rupees each. The consequences of damage to such
transformers are so serious that the additional expense of The Thermal relay generally consists of a suiitable
providing a more elaborate system of protection is fuliy temperature sensing element surrounded by a heater coil
justified and indeed is essential. which carries a fraction of the load current. When
heated, the sensing element strip makes the necessary
There are three basic principles which are made use contact with the appropriate contact element. The trip
of for detection of faults in a transformer. contact closes when the temperature rise is abnormal.
a) Temperature rise : Thermometer, Thermostat, The Thermal re!ay is designed to simulate the actual
Thermal relay
b) Formation of gas : Buchholz relay thermal condition within the transformer windings.
c) Current inbalance : Over current relay, differential Relays are available with time constants between 15
relay, earth leakage relay
minutes to 2 hours. The heater, its dimensions and
Some or all of themmay be provided as may be proportions, and rhc amount of cooling which takes place
directed by the importance of the installation. A large are all carefully regulated by the designso that its thermal
annunciator panel is therefore provided on the substation capacity corresponds to those of the transformer.
control board, on which flag/lamp/focia indication is Thermal relays are particularly useful against sustained
given for each device which operates during any incident. overloads.

Q.49 How do temperature operated devices function? Q50.How does gas get formed in a Transformer?

The Dial type temperature Indicator, which is Any internal fault in a Transformer is almost
usually of the vapour, pressure type, is fitted on the invariably accompanied by evolution of gas. Any intense
transformer tank to be in contact with the oil. It is fitted heat produced inside the tank decomposes the oil or
with two contacts, The first closes when the oil
1: 5.5
,,,,,,,,,,, m 80’8 ,,A .d.<l,
 cellilose insulation and liberates gas. The rate of gas connecting pipe operates when there is a serious internal
formation is in proportion to the amount of heat fault like a short circuit between phases, coils or turns,.
produced. earth fault, arcing or puncturing of bushing insulator
inside the tank. This liberates a considerable amount of
QSLDescribe the function of Buchholz relay.
gas, and the displaced oil and gas surge through the
connecting pipe, and impinge upon a deflection plate
The Buchholz relay functions by making use of the
fixed on the lower assembly in line with the pipe. This
liberated gas inside the transformer due to an internal
tilts the Mercury switch which in turn trips the circuit
fault, to actuate an alarm or trip the circuit breaker
depending on the seriousness of the fault. The virtue of breakers.
Buchholz relay is that it detects an incipient fault i.e., in
The buchholz relay also detects accumulation of air
the very early stage itself before serious damage is done,
on account of low oil level, or faulty oil pumps. A pet cock
whereas other relays operate after the fault has occurred.
is provided on the top of the housing to draw samples of
Therefore repair measures are simpler. Example of
accumulated gas for analysis. An inspection window
incipient faults are short circuited core laminations,
shows the amount of gas which has accumulated.
broken down core bolt insulation, failure of inter turn
Provision is made for manual operation on the alarm and
I insulation and consequent local over heating, bad switch
tripping mechanisms to facilitate testing. A better
contacts and faulty joints etc..
method is to pump air through the pet cock.
The relay comprises a cast iron housing inserted in
the pipe connecting the transformer tank with the
conservator tank. It consists of two bucket mechanisms Q.52.What is the function of the oveYcurrertt relay?
each connected to a tilting head on which a mercury
There are principally two types of over current
switch is mounted. The upper assembly, traps small gas
, relays:-
bubbles generated by an incipient fault and if sufficient
gas has been collected, it closes a contact and sounds a
hooter in the control panel in the SCBSTATION. The
a> Instantaneous over current relay : This is intended
to protect the transformer against a short circuit
lower assembly which is directly in the path of the when a current of 10 to 20 times the normal value
may flow. This relay is usually of the magnetically
56
 operated type with an armature having negligible Differential protection is one of the most common and
inertia. The time lag is of the order of 40 to 80 dependable methods of distinguishing an internal fault
milliseconds. The relay acts as a back-up protection from one outside the equipment protected, and therefore
for external faults and functions earlier than the it is employed not only for protection of large transformer
Buchholz relay for internal short circuits.
of 10 MVA and above but also for other heavy eletrical
b) Inverse - time over current relay is of the induction equipments.
type. This is used where the principle of time CT connections for differential protection
discrimination is utilised by grading the time lags,
provided for tripping different circuit breakers in a
distribution system, so that the circuit breaker
nearest to a fault on the substation side alone trips
thereby reducing to the minimum the area affected
by the faulty section.

In this type of relay, the operating time is inversely

proportional to the current i.e., the greater the fault O/C cum earth fault protection with 3 Nos. CDG type relays.
current the shorter is the tripping time. This type of relay
can also be used for over load protection for small
transformers.

Q.53.What is a ‘Differential Relay?

A differential relay is based on the principle that the

QkXExplain how an earth ieakage relay system
current input into a transformer can be balanced against functions? -
the current output, if current transformer ratios on the
primary and secondary are appropriately selected. This It is well known that the metallic frames of all
balance will be upset if there is any internal fault in the electrical equipments should be solidly connected to
equipment, as the output will no longer be equal to the Earth.
input. When this happens, the relay trips the circuit
59
breakers tin both sides of the equipment instantly.
58
This JSrthing has two functions : Satisfactory operation of the above scheme
a To allow a heavy fault current to trip the circuit
depends upon a number of factors i.e., the variation of
breaker or to blow the fuse feeding in the
earth resistance should be kept as small as possible, no
equipment to ensure that the frame may remain at
earth potential and be safe for the operating other alternative path to the earth should be permitted
personnel. In an earth leakage relay, a current and the earthing lead must always be intact. Since failure
transformer is provided in the earth lead so that it of any of these will jeopardise the operation, it calls for
constitutes the only path to earth. The main high quality of maintenance.
transformer is kept insulated from ground
otherwise. If an earth fault occurs within the
b. A better scheme, especially for large transformers,
transformer, the fault current will flow through the
is what is called restricted earth fault protection.
current transformer to the ground, and operate the
The word restricted is used in the sense that the
relay contacts which in turn will trip the circuit
relay operation is restricted to internal faults and is
breakers. Under normal operation, no current
not responsive to through (external) faults. It is
flows through the relay coil R (see fig. below) and
applied both to the primary and secondary windings
the relay can be made as sensitive as required. The
and has to be connected to a separate fl Winding,
Primary winding of the current transformer should, other than that used for the differential relay. The
however be rated to withstand full fault current. principle employed is that, the currents of all the
leads into or out of the transformer normally
summate to Zero when healthy. In a star connected
winding, where the neutral is solidly earthed, a CT is
also provided in the neutral lead as shown in dotted
lines (hidden) which makes the relay more
sensitive, than the differential relay, to leakage
currents within the winding but keeps it impervious
even to heavy external faults. Instantaneous
tripping is provided both for earth leakage and
differential relays,,so that, if any one of them acts,
An earth leakage relay, however, has its limitations. the circuit breakers of both the Primary and
It will not detect other faults within the transformer such Secondary are simulatneously and automatically
as a turn- to-turn short, which will only be cleared by tripped out.
Buchholz relay. 61
60
Q.fKWhat are ‘Instrument Transformers’ and why are 4 A CT is connected in series with the supply line
they used? while a PT is connected across the supply lines.

3) In a CT, the secondary ampere turns are nearly

Instrument transformers are intended for use with
equal to the primary ampere turns; in a P.T., turns
measuring or recording instruments like ammeters, per volt on the secondary is nearly equal to that in
voltmeters, KWH Meters, Power factor meters and . the Primary.
frequency meters etc., as well as in conjuction with
41 In a CT, the Primary has one or more turns of heavy
protective relays. These are of two basic types viz.,
conductor and the secondary has a large number of
current transformer and potential transformer, and they turns of thin wire. In a PT it is the reverse i.e., the
serve three purposes : primary has a large number of turns of thin wire and
the secondary a small number of comparatively
thick wire. ‘
a> To insulate the instruments, relays and connected
wiring from line mains, especially if they are High
Tension, and make them safe to work upon. 5) The secondary of a CT should never be open
circuited but always kept short circuited and no fuse
should be inserted. In a Pf the secondary should
b) To transform heavy currents or high voltages to
never be short circuited and a fuse should be
lower values, within the range of ordinary
provided both on the primary and secondary.
instruments.

c) To achieve standardisation of voltage and current

ratings of instruments and relays, thereby Q.57.What is meant by ‘Secondary burden’ of a PT or a
simplifying the circuits and reducing their cost. A CT?
standard secondary rating of P.T’s is 110 Volts and
that of CT’s is 54, although occasionally 100 volts Secondary burden as applied to PT’s and CT’s
and 1A are also employed. referes to the maximum load in voltamperes (VA) which
may be applied across the secondary terminals, without
Q.56.What are the main difference between current
transformer and potenetial transformer? the ratio and phase angle errors exceedings the limits.
Compare and contrast them. The burden depends on the number of instruments or
relays connected and their individual ratings. Some of
1) The CT & PI usually step down from a higher value the Typical values are :
to a lower value.
62 63
\ ___________-----___-----------------------------------------------
Potential Current example, bring down the burden due to lead resistance
w.w (W from 50 to 2 VA
____________________-------------------------------------------------
Indicating voltameter I Ammeter 2to8VA .0.3 to 3 VA The rated burden could also be expressed in terms
_____-______________-------------------------------------------------
of impedance in ohms instead of volt amps. Thus a 15 VA
burden would be equivalent to a 0.6 ohm impedance
Potential Current
WV) 6%
burden vvith respect to a 5A secondary or 15 ohm
impedance with respect to a 1A secondary.
Watt hour meters and recording 3toSVA 5 to 7.5 VA
instruments Polarity is extremely important in making CT
Thermal Protective relays and 2 to 10 VA
over currents relays
connections. In calculating the impedance burden, the
________________________________________----------------------------- configuration of the different current coil connections of
meters and relays, whether series or parallel should be
kept in mind. For safety purposes, one of the secondary
The standard burdens for CIs & PTs are as follows. terminals should be connected to earth at one point only.
___________-________-------------------------------------------------
P.T C.T
____________________-------------------------------------------------
Q.Sg.What is the objection against open circuiting of the
Instrument transformers 10,15,50,100,24M 2.5,5, 7.5, 15,30
Protective transformers 50,100,200 25,5,7.5,10,15, Secondary of a CT?
, 30 w-0.7 lag)
A CT is designed to operate at a low flux density,
with a closed secondary circuit, i.e., with the secondary
For example, if a CT has to feed an indicating current flowing continuously and having a
(instrument) Ammeter (0.5 VA), a recording watt meter demagnetisation on the flux produced by the primary
(4.5 VA) and an over current relay (5 VA) the total current. In a CT, the leakage flux is of the same order as
burden would be 10 VA. If the resistance of the the working flux unlike a PTor a distribution transformer
connectin leads ( o and return) is 0.2 ohms, its burden where the coupling between the primary and the
3 4 secondary is tight and the leakage flux is negligible. A
would be I R i.e., 5 x 0.2 = 25 x 0.2 = 5 VA Therefore,
the total burden of the CT would be 15 VA. If, on the peculiar feature of the CTs is that, the voltage developed
other hand, the resistance of the cable connections from across the secondary is directly related to the value of the
the switch yard of a large substation to the control room is impedance of the burden across it. Since the current
considerable, say 2 ohms, the burden due to this alone ratio is fixed, if the load impedance is more, the voltage
would be 52,x 2 = SOVA which is excessively large and developed is more. As the load impedance across the
impracticable. In such cases, a lower secondary rated secondary becomes high approaching open circuit
current of 1A has to be chosen. A 1.4 secondary would for condition, the voltage across the terminals become very

1 64
65
c/
high and the magnetic flux approaches the saturation MAINTENANCE SCHEDULE FOR TRANSFORMERS
value for the core when the exciting current increases
very rapidly. This can result in altering the ratio-error and CATEGORY
consequent accuracy and reliability of the CT for future htAJOR MEDIUM MINOR
functioning. The voltage drop across the primary will
also be abnormally high, resulting in over heating. For a. Watch operating conditions, ic. ambwtt
the above reason, open circuiting of a CT should never be tcmpcrature, oil tcmperatuze, cumnt
radingand vacation of primary voltage
permitted. Check position of tap witch and rcadjwt if
“-3-y. H D M
Q.S!l.What maintenance work is required for b. lttspxt general condition, see if there 15 any
transformers and at what intervals? (Refer : IS unusual noise. check oil tml in tank and
bushing. D W M
l@w
c. Check Transformer bushing insulator and
examine for fine cracks, if any. check
The transformer is one of the most reliable items of A tightness of external clectncal connections. I
M 3-M 6-M
electrical equipments, requiring relatively little
attention. Yet, often even this minimum attention is not d. Check breathers and cnsurc that the passage
is clear. 3-M 3-M GM
given, and no wonder they do sometimes breakdown
because of negligence. The programme of inspection c. Test operation of all protcnm alarms and
and maintenance is governed by the size of the i-&q, batteries and airing. 3-M 3-M
transformer, where it is installed, whether indoors or I. Take megger tradings and nxord 3-M 6M Y
outdoors, if in a substation, is it manned or unattended,
& Check ground connections and record earth
the operating conditions and so on. The degree of resistance. 3-M 6-M Y
attention required depends greatly upon how heavily or
lightly the transformer is loaded. The maintenance ’ h. Test oil for dielectric strength, acidity. eolour
etc. Drain out samples of oil from drain plu&l
schedule shown below covers oil immersed, naturally at the bottom of the tank. Examine for water,
cooled transformers of three categoreis: sludge and acidity. Drain out until clean
sample is obtained. Replenish with gxd dry
oil. Carry out oil tiltration if required. 3-M Y 2-Y

i. Check tightness of all mver bolts 3-M Y 2-Y

j. Check lightning arresters. Check spark gap. 6-M 6-M Y

k. Thmughly overhaul after lifting up the core.

Repaint the tank, if necessary. 10-Y 7-Y 5-Y

66
 Major installations at large grid substations which cable box is bolted in, and welded joints. Every oil leak
are manned all the time, with unit sizes of 1000 to
10,000 KVA or more. Quite often the transformer : should be traced to its source and remedial action taken
loading is quite high. to stop the leak.

b) Medium installations in small substations’with unit If gasket leak cannot be stopped by tightening the
capacities of 250 KVA to 1000 KVA.
bolts, the gaskets should be renewed. The best material
c) Minor installations upto 250 KVA unit capacity for gaskets is rubber cork sheet of 5 mm thickness. This
which are often unattended and often at remote consists of fine granulated cork and synthetic rubber as
points, such as pole mounted and rural distribution the binding medium. The surface between which the
transformers.
gasket is provided should be quite flat and smooth
The intervals of inspection are indicated as : H for without any burr& ._ other irregularities. They should be
hourly, D for daily, W for weekly, M for Monthly, 3-M and perfectly clean and free from scale, rust, old paints,
4-M for 3 mothly and 6 monthly, Y for yearly, 2-Y and 5-Y remains of glue and old gasket, grease or oil. The
for two yearly and five yearly respectively. ’ surfaces should be thoroughly cleaned and washed with
trichloroethylene, alcohol or other grease solvent. It is a
In major and medium installations a standby glossy surface, such as bakelite, it should be made rough
transformer should always be available to be switched on by emery cloth and sand paper. The gasket should
at a moment’s notice, in place of the larges unit installed. preferably be made slightly narrower than the flange
It should be maintained to the same standards as the width, so that it may not swell beyond the edge when
other units, and periodically put to use to keep it in a good compressed. Though the gasket makes a good oil tight
and healthy condition. joint, it is an advantage if the gasket surfaces are coated
,
with a thin layer of a special glue obtainable from the
QdO.What are the points to be checked if the oil level manufacturers. (Shellac may also be tried). Inspection
tends to fall down? covers, which may require to be opened, whenever
required, may be coated with a suitable grease
Transformer oil may leak at several points, i.e., oil
compatible with Transformer Oil over the surface which
level gauge, cork, packing below the top cover, oil
comes in contact with the gasket. The cover can then be
conservator connection, drain cock, gasket, bolts where a opened without damaging the gasket. Care should be
68 69
taken to tighten the bolts uniformly going from one bolt and a careful note made of the oil level for verification
to the next, over the entire lot several times so that the during the next inspection.
stuface pressure is uniform all over. The compression
should be Z/3 rd of the original thickness. The Q.61.What attention is required on bushing and
re-tightening operation should be repeated sometime insulators?
after the transformer is put into use, as thegasket tends to
settle down after sometime. This should be done two It is essential to examine porcelain bushings and
days after renewing the gasket and again a week later. other insulators such as bus bar supporting insulators and
The bolt tightness should be checked during a six monthly surge - divertor insulators at frequent intervals. The
inspection. porcelain should be thoroughly cleaned with a wet cloth
and polished, after removing every trace of oil or dust. In
Sometimes, slight oil leakage occurs at welded most industrial urban areas, there is heavy atmospheric
joints. The exact point of leakage outside the ‘tank can be pollution, and fine dust, foot and industrial deposits settle
discovered first by cleaning the surface thoroughly with a down on the insulator skirts. If not removed at frequent
grease solvent or petrol or denatured alcohol, and then intervals, they tend to form a very hard crust. When dew
coating the surface with a thin layer of chalk, cement or moisture appears on the insulator, the combination
paste in water or white wash, and allowing it to dry. makes a conducting path which could easily result in a
Leakage of oil is then revealed by the dark patch it forms. flashover. Very often, the heat produced by the leakage
path damages the glazed surface of the insulator. The
As for the cure, much depends upon how bad is the
position is aggravated in coastal areas subject to salt
leak. Formation of the few drops over a period of time is
deposition. In cold countries when snow deposits on the
not of any consequence and occurs in most transformers.
insulator and starts melting, extensive failures occur.
It is bad, the cracked or spongy weld may be repaired by
welding in a metal patch. A small hole in the weld can To overcome this trouble, a water repellent such as
often be put right by peening with a cold blunt chisel until silicone grease may be applied on the porcelain surface.
the hole is closed and then finally using a ball peen When polishing, the bushing surface should be carefully
hammer to complete the seal. After completing the examined for hair line cracks. When the transformer is
repairs, the deficiency in oil level should be made good put on load the cracks, if any, may develop rapidly.
70 71
. .
Qdt.Why are transformers fitted with oil conservators? Q.e.Considering the very small quantity of water
vapour in the atmosphere, are breathers really
The object is to ensure that the transformer is necessary?
completely filled with oil at all times, and to provide for Yes, they are essential except for small sixes of
free expansion and. contraction of the oil due to transformers. Although the atmosphere contains very
temperature variations of the windings. It also ensures little water vapour, it should be remembered that the
that minimum surface of oil at lowest temperature is transformer is breathing in and out daily all the time. (24
exposed to the air. An oil gauge shows the variation in oil Hours a day, 365 days a -year for 20-25 years) The
level in conservator. The empty air space on top of the oil transformer oil is, moreover, extremely sensitive to the
conservator is connected to the outside atmosphere presence of moisture and the breakdown voltage of the
through a breather. According to IS 2026, oil oil falls down quite rapidly if ,the breather is not dry.
conservators are not required for transformers below 25 Breathers, therefore, should be inspected at least once in
KVA rating. three months and more often in the rainy season, the
date of inspection being painted on the transformer itself
Q.63.What is the function of a ‘Breather’? to facilitate the next inspection.
A breather is provided to prevent contamination of
transformer oil in the conservator by the moisture Q.65.What ispwtion is required for Eathing?
present in the air entering the conservator. The outside
In lighting distribution transformers, where neutral
air is drawn into the conservator through the breather,
is separately brought out, it should be connected to an
every time the transformer cools down. The breather is
independent Earth Electrode through a neutral link.
packed with some material such as anyhydrous calcium
Under a line fault on the system, short circuit current will
chloride or silica gel which has the property of absorbing
flow in the neutral. It is, therefore, of utmost importance
all water vapour contained in the air passing through it,
that the neutral earth should be particularly good, say less
thereby making the air quite dry. In silica gel breathers
than 1 ohm or better. This should be checked by a good
the charge should be replaced if it becomes pink. The old
earth megger and prompt steps should be taken to
silica gel can be reconditioned and used again by heating
improve the earth resistance, where necessary. Apart
it up to 150 - 200°C to drive out all the absorbed water.
from the above, however, the body of the transformer
72 itself should be connected to an earth system by heavy
73
earthing condluctors, having at least half the section of the of moisture absorbed by the windings. This is quite a time
main conductor. Earth continuity and earth resistance consuming process in a new transformer to be
should also be checked. commissioned for the first time, and may last from a few
In a large substaion, several earth electrodes should days for a small transformer to 3 or 4 weeks for a large
be provided at different points in the area and unit. And throughout this period the temperature has to
interconnected to form a common grid, to which the be maintained between 80’ to 85OC irrespective of
metallic frames of all electrical equipments as well as ambient temperature variations. Incidentally,
neutral point of the transformers may be connected. purification of the oil may proceed while the transformer
is in service on light load.
Q.66 What are the different methods of drying out a
transformer?
Q.67.How long should the drying operation be
1
continued?
New transformers are invariably sent fully dried
out, filled with oil and fitted with a breather. Some times, Whatever be the method employed, the drying out
large transformers are sent without oil, with the cooling operation should be continued till the samples from the
radiators separately packed, to reduce weight and top and bottom of the tank show high dielectric strength
facilitate handling. The cooling tube outlets are then sent and the windings show high insulation resistance.
blanked off and sealed, the tank itself being filledwith dry
gas, either air or nitrogen under pressure, so that the Q.68.What are the qualities required for a good
transformer oil?
windings may be perfectly dry when received; it will then
be necessary only to mount the cooling tubes and fill the The specifications for transformer oil are fully
tank with good transformer oil, preferably by the vaccum c o v e r e d b y IS:335 - 1983. For instructions for
suction process. Drying out period in the field will then maintaining the insulating oil, IS code of practice 1866 -
be greatly reduced. 1961 may be referred to. Briefly the following
characteristics are required.
The main problem in drying out a transformer is not Mineral oil grade B should be used.
1)
so much drying the oil -this is quite easily done by passing
2) It should have a high dielectric strength i.e., not less
it twice or thrice through a suitable filter - as the removal than 40 KV in drums and 30 KV in transformer
tank.
 3) It should contain negligible moisture content. A 4.70 Describe the stre&line oil filter briefly.
crackle test may be conducted by keeping a small
quantity of transformer oil in a test tube and heating Please refer to the pipe circuit diagram given
by a bunsen burner. There should be no cracking. below The following are the details of the stream line
Cracking shows presence of moisture.
oil filter:
4) The oil should be perfectly clear-and pale in colour.
5) Acidity content should be very low, as it will cause 1. Inlet valve 2. By pass valve 3. Outlet valve 4.
precipitation of sludge and corrosion of neutral T.Port valve 5. T. Port valve 6. Vacuum break valve cock
surfaces.
7. Vacuum receiver drain 8. Filter body drain 9. Heater
6) Flash point and Pour point should be as per
specifications laid down. vessel drain.

Q.69 What are the different methods of puriging

(filtering) and drying out transformer and switch
oils?

The object of oil purification is to remove all

contamination such as water, carbon deposits, dirt,
,
sludge, dissolved moisture and gases. In transformer oil,
the most important quality to be preserved is the
dielectric strength. Transformer oils and the insulating
materials used in the transformer windings are
hygroscopic i.e., they tend to absorb moisture, which may
enter the transformer due to defective breathers, gaskets
or by addition of unfiltered oil. Dissolved air and
excessive heating cause oxidation of the oil and formation
of heavy sludge deposits at the bottom of the tank and on
the surface of the windings. It is essential to remove all A. Strainer B. Thermometer C. Thermostat D.
these impurities at least once in two years by filtering the Sight glass E. Compound gauge (filter body) F. Vacuum
oil, to maintain the equipment in a healthy state.

t
gauga G. Sampling Cock H. Safety valve.

76 77
 Brief description of streamline oil filter: through outlet valve No. 3. The float closes the outlet port
The vacuum receiver is evacuated by the vacuum before the oil level falls as low as the port, thereby
pump, which produces a low pressure, in filter body and keeping the port sealed, and preventing vapour from
heater chamber. Oilis pumped by the dirty oil pump, into passing out with oil. If, for any reason, the oil in the
the heater chamber where it is normally heated by 9 vaccum receiver raises too high, the float operates the
numbers of immersion heaters (3.6 KW per heater). The vaIve fitted above it and breaks the vacuum allowing the
outlet in the heater chamber is kept almost at the top of clean oil pump to deal with the surplus.
the champers, so that all the heaters are always immersed
in oil, when the filter is in operations. When the filter is started cold, with the oil irside,
the by pass valve No. 2 is opened and then imet valve No. 1
and outlet valve No. 3 are closed, so that the oil from the
The oil passes from the heater chamber into the outlet of clean oil pump is circulated back again through
filter body in which columns of filter packs of specially heater chamber until proper working temperature is
prepared filter paper discs, each held under compression reached, preventing the delivery of any oil which has not
by a spring at the top of its guide, are provided. Only clear been properly dehydrated.
oil can pass through the paper discs and all the solid
matter is retained at the outer surface of the columns. The solid matter accumulated on the filter pack is
The clean oil passes along the pack rod guide and springs removed periodically by reverse cleaning with
into the the vacuum receiver, where it is subjected to a compressed air, air being obtained from the vacuum
vacuum of 68 ems of mercury at sea level. The pump/compressor. The filter body is drained off oil
combination of vacuum and temperature causes through valve No. 8. The valves No. 4 & 5 are turned to
complete evaporation of any moisture which the oil may CLEAN position so as to open the suction of the vacuum-
contain. Water vapour, air and other gases are being pump to atmosphere and delivery to the vacuum receiver.
drawn out, by the vacuum pump, through T. Port valves Valve No. 7 is cleared after. draining the oil, so that
No. 4 & 5. pressure is built up in the vaccum receiver. The reverse
cleaning is carried by the sudden opening of the filter
The clean dry oil is drawn through the float
body drain valve No. 8. This allows the compressed air in
controlled valve by the clean oil pump and delivered i,
the vacuum receiver through the filter pack in the reverse

78 79 ..d
 direction of filtration, thus clearing away all the solid The faults in power transformers even at incipient
matter. stage may liberate gases slowly, but this will not operate
the buchholz relay or gas relay. Instead, the gases get
Q.71.Dissolved gas analysis (D.G.A.) by GAS dissolved in the oil. A qualitative and quantitative
CHRQMOTOGRAPHY - (A Modern tool to detect
analysis of this oil sample containing minute quantities of
incipient faults in power trakformers) is the need
of today. Cominent. gases, taken from different parts of the transformer,
could help in taking preventive action, much before the
Gas Chromatography is a specialised form of the fault occurs in the transformer. (Preventive maintenance
general Chromatographic technique, in which the mobile of transformers).
phase is a carrier gas and the stationery ph&e is a solid
adsorbent. The carrier gas, after pressure reduction, Formation of gases in oil filled transformers:
passes through the chromatographic column containing
.The Buchholz relays operate by the accumulation
the solid adsorbent. The solid adsorbents are fine
may be sudden. due to severe arcing fault or more gradual
granulated particles of chemicals like Alumina, Silia gel
as in the case cl’ slow deterioration of insulation. The
and molecillar sieves etc., The column is situated in a
I mechanism of gas formation in the transformer tanks can
thermostatically controlled oven.
be classified as given below:
The la&s in power transformers such as arcing, 1. Oxidatio?
Vapourisation
over heating, partial discharge, etc., result in abnormal i- Insulation decomposition
electrical and chemical disturbances. These 4: Oil breakdown
5. Electrolytic action.
disturbances are the main causes for liberation of gases.
The gas chromatography analysis technique for gas
OXIDATION :
analysis gives qualitative and quantitative information
enabling a more precise definition of the type of fault and The carbondioxide is liberated predominantly
its location. Further, a sensitive gas chrombtographic during the process of oxidation. This process begins
apparatus could detect, even low concentrations of gases
in gas mixture and in gases dissolved in oil.
81
80
when small quanitities of oil combine chemically with the monomide. Above 400°C, the gases formed are
dissolved oxygen in the oil, resulting in formation of relatively less.
traces of organic acids. These acids react with the metal
of the transformer, forming metal based soaps. These OIL BREAKDOWN :
soaps dissolve in the oil and act as catalysts that accelerate
The direct breakdown of oil by arcing results in
the process of oxidation.
cracking of the oil. The aromatic contents breakdown
into simple hydro-carbon gases and hydrogen of which
VAPORIZATION :
Acetylene (C2H2) and Methane (CH4) are major
The vaporization occurs for oil at about 280’~ and constituents. Other hydrocarbon gases may also be
for water at about 100°C. The false alarm of the liberated due to cracking, if the necessary temperature is
Buchholz relays may be attributed to the fact that the maintained for their stable formation. ‘

condensation of water vapour takes place, when the

excess moisture in the tank is vapurised by a heat source. ELECTROLYTIC ACTION :
False alarm can also occur, when the hydrocarbons, the Hydrogen and Oxygen are liberated during an
constituents of the insulating oil vapourise. electrolytic action. Presence of minute and small
particles of fibres within the oil leads to electrolytic
INSULATION DECOIWOSITION : action. Light hydrocarbon gases may also be present if
The solid insulators in power transformers are of the solid insulation is involved.
mainly cellular or resinous type viz., paper, pressboard, DETECTORS :
cotton, resins and varnishes. These Substances contain in
their molecular structure, substahtial amounts of The sensitivity of any gas chromatographic
Oxygen, Carbon and Hydrogen. Between temperatures instrument depends on the sensitivity of the detector,
of 15O’C and 400 “C, the insulation breakdown results in which is the most important part of the system. The two
liberation of hydrogen, carbon dioxide and carbon detectors in vogue for dissolved gas analysis are:

82 83
 a) l’hermai condriciivity detector or FLAME IONISATION DETECTOR :
KATHAROME3ER
b) Flame ionisation detector P
The flame ionisation detector is more sensitive than
KATHAROMETER the thermal conductivity detector. This detector utilises
The Katharometer measures the change in thermal an inflammable mixture of hydrogen, oxygen and
conductivity of a heated wire, due to effluent gases which nitrogen. The carrier gas on emerging from the column is
flow on it. The carrier gas flows at an evenly controlled burnt at a small jet connected to the column outlet. The
rate across the two filaments. One filament in the jet is protected in a draught free enclosure having a
chromatographic column and the other in a dummy collecting grid. An e.m.f, applied across the flame
column. The two filaments form the arms of a bridge changes its resistance. This. change in resistance is
circuit. A change in the specific heat and thermal i determined. Presence of any evolved vapour is indicated
conductivity due to the passing of evolved gases over bne by a large fall in the resistance due to the ionisation of
filament disturbs the balance of the bridge, resulting in i organic molecules in the flame.
change of v&age which is amplified and recorded. A
Most of the gas chromatographic instruments use
Katharometer with 4 - 12 volts for heating the wire is
t strip chart recorders which draw the finished
shown below.
CHROMATOGRAM on a moving strip of paper.

Q.72.How to inspect a transformer on arrival at site for

an expansion project?

, As soon as the transformers reach their destination,

observe the consignments carefully for any visible signs of
damage and/or pilferage.

85
84
Observe particularly the following : RECORD OF INSPECTION ON ARRIVAL

a. Transformer serial Number

a. Main tank and/or cooling tubes
b. Protruding fittings b. KVA
C. Bushings ,
C. Lorry Weigh Bridge RR Number
d. Oil sight glass Rly Receipt
(Lww
e. Oil leakage along welds, valves, inspection cover,
bushing covers etc., d. Despatched on
f. Pressure of inert gas filled in the tank
e. Received on
f5 Oil conservator
h. Breather pipe f. HAS OPEN DELIVERY CERTIFICATE been
i. Accessories which are received separately obtained by proper authorities.
j. Fan Control unit (OB type cooling) Condition of package
k. On load tap changer mechanism Extent of damage - a report

If anydamages/pilferages are noticed, register them Note :

with the transport authorities. Catain an open delivery The original may be returned to the manufacturer
certificate’ from the Railway authorities when the after inspection on arrival to report damage to the
consignment is received by rail. Insurance surveyor transformer, if any.
should also be informed immediately. Q.73.Ours is small scale industry. We wish to
commission a 500 KVA transformer which has
It is recommended that the transformer be checked arrived last month. Can you eleborate as to howwe
part by part and observations recorded in for ‘A’ and on will go about it?
the packing slip given in form ‘B’. The copies in Assuming - that the entire installation of the
duplicate should be sent to the manufacturer informing transformer has been carried out entirely by a First
the extent of damage, if any.
FORM ‘A’

87 ’
86
class/A grade contractor at the location meeting the
complete requirements of the local electrical
inspectorate. An electrical consultant may be assigned
the task of performing the commissioning of the
transformer and a commissioning report may be obtained
by the consultant after satisfactory cominissioning of the
above 500 KVA transformer. _.

You are advised to keep a record ofvarious checks,

made prior to commissioning. This will help avoid
hazards during commissioning and operation thereafter.

Once the checking is done, load is switched on and

operation is observed for a few hours. If the operation is
satisfactory, the transformer is kept on load and the
readings of voltage and current in all phases on both sides
(and in tertiary winding, if provided) are checked and
entered in the substation log book. A good and healthy
transformer operates with a healthy hum. Check the
temperature of top oil at regular intervals.

88
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!

INSULATION RESISTANCE OF WINDINGS OF Basic Duration overload for oil Immersed Transformers
POWER TRANSFORMERS
ml minules pr time mnstant o[onz hour)
Rated wltage o: the Minimum safe insulation resistance in
wndmgs Mchohms at Temperature. COOLING - ONAN
(1) (2) (3) (4) (5)
Lad Ambient Temp 25’C maximum daity
WC 40°C WC .WC average tempclature 2o”c
65kV and Abuxc MO 300 150 75
33k V ml
6.6k V and Ilk V 400 For werload. Forwerload. with For overload
&low 6.6k V 200 startingwith the the transfonncr at follanng
H.C. FUSE RA’TINtiS FOR POWER TRAWFORMERS temperature Of the temperature MMlnUOuS running
transronncr no, conditions at nom.4 raied load.
Capacity or H.T. V&age exceeding the stated corrcqmnding to
TlXOSfO~~l5 buns ambient the fixed losses oniy
LVA kV tempxaturc (after continuous

I
(1) I (2) I (3) (4) ~ransformcr
65 21 unexcited)
3,ax) 20 I

C.M.R Cont. cont. Cont.

10% OL Cont. Gmt. Cont.
2a%OL 105 95 40
30% OL 65 50 1s
M% OL 35 2.5 5
75% OL 7.0 15 25

1m OL 15 10 1

Velue of time constant maw-cd in hours :

KVAl 1KV 33 KV
For ONrn 050 2.75 5.0
Transformen X-250 2.25
251-lcm 2.0 1::
looO-1woO 20 2.2s

Permissible duration of overload in minutes : Time constant X Basic

duration of OL in minute for time constlnt of 1 hr. IS 6600/1972 & Bs
CP 1010/1959, Guide to loading of oil immersed transformers which
gives detailed directions.

94 95
 Basic hwation overload for oil Immersed Transformers &sic Duration overload for oil Immersed Transformers
(7” minute for lime ~-0nsmt of 0m her) fin minutes for time constad of one hour
COOLING - ONAN COOLING - ONAN

Load Ambient Tcmp 40°C maximum daily Load Ambient Temp. 5kC maximum daiiy
aveI-ap tempcrarurr 39c avcragc tcmp-xature 4S°C

For overload. For overload, with For overload For Overload, For owxload, with
startingwith the the tlansrormer at- f0Ilciwing statingwith the the transformer at
tmperaturc Of the temperature C0ntmuous running temperature of the temperature
tlansf*mIcr not conditions at nomal rated load tlansfomw not conditions
cxceding the stated cormsponding to exceeding the stated mmspondiig t*
ambient the fued lmscs on& ambient the fIxed loves only
tempclature (after continuous temperature (after continuous
(Tran.Sformcr running on open (TlansfoImer running 011 open
UlEXCkd) circuit) unucitcd) circuit).

CM.R 80 55
CMK cont. cant cant
10% OL 50 /-- 30
10% OL 8s 6.5
20% OL 25 15
%OL , 50 3s
36% OL 25 I5
3G%OL 3.5 25 ,-
% OL 15 10
sow OL 20 1.5
7S%OL 10 7
75% OL IS 10
lwJ% OL 6 5
100% OL 10 7
Value of time constant meawed in hours :
Value of time constant measured m noutx :
KVAl 1KV 33Kv
WA 11 KV 33Kv 2 For ONAN 050 2.75 5.0
For ONAh 050 2.75 5.0 Trasformen S-250 2.25 35
Transformers 51-w) 2.25 35 251-1wO 2.0 25
2.51.lwo 2.0 25 lcm-lcco3 2.0 2.25
1x0.loo00 2.0 2.25

Permissible duration of overload in minutes : Time constant X Basic

Permissible duration of overload in minutes : Time constant X Basic b duration of OL in minute for time constant of 1 hr. IS MOO/1972 & BS
duration of 01, in minute for time constant of 1 br. IS 66&l/1972 & BS CP 1010/1959, Guide to loadlug +f oil immersed transformers which
CP 1010/1959, Guide to loading of oil immersed transformers which gives detailed directions.
gives detailed directions.

96 91
 18. Guaranteed no load
losses at 75 deg. C at
rated frequency and
TRANSFORMER (1) - rated voltage
(2) - 110% rated voltage
1. Manufactured by 19. Guaranteed load losses
2. Manufactuk’s type at principal tapptig,
designation rated current, rated
3. Type of transformer voltage, rated frequency
4. Type of cooling aad tiding tempera-
5. Rated KVA ture of 75 deg. C.
6. Rated voltage of 20. Total losses at principal
(1) - HV Winding tapping, rated output,
(2) - LV winding rated voltage, rated
7. No load voltages ratio frequency and maximum
8. Overload capacity attainable temperature
9. Thermal time constant at site.
10. Rated frequency 21. Tolerance on losses
11. Temperature rise a) Total
(1) - of oil by thermometer : b) No Load
(2) - of winding by c) Load
resistance 22. Percentage impedance
(3) - by hot spot between each pair of
temperature indicator : vidings at rated current,
12. Number of phases rated frequency and 75
w. Connection of deg.C.
(1) - HV Winding : (1) - Positive sequence
(2) - LV Winding (i) - at principal tap
14. Vector group referenrr: : (ii) - at maximum voltage tap
15. IS neutral accessible (‘) (2) - Zero sequence
16. Not Used (i) - at principal tap
17. Used ’ (ii) - at maximum voltage tap

99
98
23. Percentage readance 27. No load current and
between each pair of pOwer factor at
wimhgsatratedaure~ (1) - rated voltage and rated frequency
rated frequency and (2) - 110% rated voltage and rated frequency.
(1) - Principal tap 2.8. Conductor area for
(1) - HV Winding
(2) - maximum voltage tap
24. Percentage resistance (2) - LV winding
voltage drop at 75 deg. C 29. Current density for
and (1) - HV Wmding
(2) - LV winding
(1) - Principal tap
30. Nit core area
(2) - maximum voltage tap Flux den& in core at rated frequency and
31.
25. Efficiency at rated (1) - 100% kted voltage
voltage, rated frequency (2) - 110% rated voltage
at 7 deg. C and (3) - maxhun permissible primary voltage.
(1) - full load at 32. Core lamination mater-
(i) - unit power factor ial and grade as per Bs:
(ii) - 0.8 p.f. lagging 602
(2) - 75% full load at 33. Type of core joint
(i) - unity power factor 34. Type of windings
(3 - 0.8 p.f. lagging
(3) - 50% full load at t;; : ;;
(i) - unity power factor 35. Type of winding
(ii) - 0.8 p.f. lagging insulation
(4) - 25% full load at
(i) - unit power factor $1;;
36. Insulating materials
(ii) - 0.8 p.f. lagging
(1) - Turn insulation
26. Percentage regulation
(i) - HV
at 75oC and (ii) - LV
(1) - full load at (2) - Core to LV
(i) - unity power factor (3) - LV to HV
(ii) - 0.8 p.f. lagging (4) - Core Bolts
(*) (2) - 50% full load at (5) - Core Bolt washers
(i) - unity power factor (6) - Tappines
(ii) - 0.8 p.f. lagging (7) -Tapping co~ecti~~~

101
100
 37. Type of axial cd supports
3% Type of radial coil 43. Estimated weight of
supports (1) - core and coils
39. 1.2/M microsecond (2) - tank and fittings
wave impulse test (3) radiators (without oil)
voltage for (4) - oil
(1) - HV winding (5) - on-load tap changer
(2) - LV winding .\ (6) - complete transformer
40. One minute p o w e r (7) - heaviest piece of untanking
frequency withstand 49. Shiping information:
voltage for (1) - Heaviest piece
(1) - HV Wmding (i) - weight
(2) - LV winding (ii) - overall dimensions (length x breadth x height)
41. Extent of Ups reinforcement (2) - largest piece
42. Clearance in air (i) - weight
(1) - between phases HV/LV (ii) - overall dimensions (length x breadth x height)
(2) - between phases and earth HV&v 50. Are current transfor-
43. Clearances in oil mers provided as per
(1) between phasks (HV to HV) specification
(2) - bmcen pbses and earth (LV to earth) 51. A r e r a d i a t o r s detach-
(3) - behwen core and earthed clamping parts abie
44. Noise level o f t r a n s - 52. Lit of fittings provided
former on the transformer
45. Overall 53. Type of tap changiug
dimensions
(length x breadth x gear
height) 54. Number of steps for tap
46. Crane lift required for changer
untanking 55. Tap stop voltage
47. Thickness of tank 56. O n - l o a d t a p changiug
(1) - sides ge=
(2) - bottom (1) - make
(3) - cover (2) - rated volt+e
(3) - rated current
(4) - operation
(5) - voltage control
(6) line drop compensation

102
103
i
57. Are the foUowing
64. Switching surge with-
designed for oil filling
stand iioltage
under vacuum:
(I)-HVbw
(1) -tank
(2) - LV bushing
(2) - cQnservator
65. Corona discharge Vol-
(3) -‘radiators
tage
(4) - bW - HV bushing
(5) - fittings
66. Nature of the dielectric
(6) - on-load tap chaqing gear medium employed in
58. Type of bushings
the bushing
(1) - HV
(1) - HV bushing
(2) - LV
(2) - LV bushing
59. One minute power Permissible safe
67.
frequency dry and wet cantilever loading on _._ -- --
withstand voltage for HV bushing
(1) - MV bushing 68. Details of sensors
(2) - LV bushing provided for winding
60. 1.2/50 microsecond and oil temperature
wave impulse withstand local/remote indication
voltage for Details of temperature
69.
(1) - HV bushing scanner
(2) - LV bushing
61. Total creepage distance
in air for
(1) - HV bushing
(2) - Lir bushing
62. Protected af=page
distance in air for
(1) - HV bushing
(2) - LV bushing
63. Flash over voltage:
(1) - HV bushing
(2) - LV bushing

104 105
 pawnD+ nado (p)
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- luam=eldq at3evvqs - oogepscy eQ=ocrI (4)
30 =0-l- swwq 4v - fkpeowo pal!n3n3-wqs (e)

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30 noyzJq!A - Smieaq 01 anp sy?Ja33ereq3 .sassol paseamu1 (q)
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same3 w
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‘noyepluaa alenbapeo1
JO pue PO %!~ooa 30 uogeInz’f3 JadoJdmI Bugeaq la&g (3)
- amssald ~BLI~~u~. a,yssa3xg - jno UlOM
uogemxgaa (q)
smpeg - aLarep pueqaam/oo!so.uo~
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wogeiguan. alenbape?
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ikyeaq ~a= (a)
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qieuwqaa (9)
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EO 3o aAv@a? (=I

i
 ORDER FORMAT Commercial Terms
TO
M/s. ROWSONS TRANSFORMERS PVT LTD, Price Each Rs. 9,27,000/- Ex. works
86, Dr. Radhakrishnan Road, Excise Duty As applicable at the time of supply
MyIapore, Madras Central Sales Tax
Pincode : 600 004. Or As applicable
General Sales Tax
Dear Sirs, Delivery Period 12/14 Weeks from the date
of receipt of this order.
Sub : 2500 KVA,‘33KV/433V. Transformer
Site Address : J.P. Medico Equipments Limited,
With reterence to your quotation No.: 7547 dated 9th April 1994, and Madras - Trichy Road
also the personal discussion your Mr. V. Raviprakash had with our Madras.
Managing Director, we are pleased to place our order for the above
transformer as under :- Terms of Payment : B a n k Cheque N o . I
Dt. for Xs. 3,09,GUO
Specification and the balance payment will be
made after inspection at your
Quantity 2 Nos factory.
WA Rating 2.500 RVA
Voltage Ratio 33 KV 1433 v
Vector Group : DYN 11 Please acknowledge and send us your acceptance and also official
Installation Outdoor/Indoor receipt for the advance amount.
Material Copper/Aluminium Wound
Terminal Thanking you,
Arrangement H.V. side - Bushing or cable box.
L.V. side - Bus Duct Yours faithfully,
Tappings + 5 to -15% for J.P. MEDICO EQUIPMENTS LIMlTED,
ORload/Onhad Off Load
Temperature : 5o”c in oil Managing Director.
550 illwm+Iig
Impedance : 7.65 %
AccesSOrieS Buchholz relay
Oil Temperature indickx
with h%RP/RSD/A&T contacts

110 111

t -
 ON LOAD TAP CHANGER

RADIATOR
G.M. WHEEL VALVE-FLANGE TYPE

REMOTE TAP CHANGER

CONTROL CUBICLE

PRESSURE RELEASE DEVICE OIL LEVEL GAUGE

Fh4ASE BOARD ASSY

112 113