Download Transformers plays a crucial role in the power

Transformer Basics:
Transformers plays a crucial role in the power distribution network and any failure creates
disruption of power supply to consumers and the extent of effect depends upon the rating of
transformers. For this a well balanced design meeting the GTP and capable to withstand all
special tests / type tests / Acceptance tests is essential in first stage.
In second stage the quality of materials used play a greater role in deciding the life of
transformer. Well-established systems should exist to ensue that right quality of material is
released for manufacturing.
Further, duringmanufac-turing, the following practices are recommended in every stage to
ensure that the quality is build and transformers give a satisfactory life.
CORE ASSEMBLY
Prime quality /grade core need to be used
Burr free core slits are available for further processing.
Core assembly should be neatly done
Assembly should ensure perfect magnetic circuit
Ensure smooth surfaces of core assembly on all the sides. This can be achieved by maintaining
perfect slit widths as per the design.
CORE CLAMPS /CHANNELS
Avoid sharp corners / edges and ensure that the welds are uniform
WINDINGS
Ensure defect free enamelled or paper covered conductors from suppliers.
(Check that the conductor sizes are meeting the design data and the tolerance limits provided in
ISS.
Good quality of insulation (edge strip, runners, press board, Kraft/ ED paper) be used as per
design details.
Windings are to be made tight maintaining the designed overall dimensions.
All leads are to be well insulated at the bendings.
Pressing & bonding is recommended for rectangular coils to maintain phase to phase
clearances.
In power transformers coil compression process be adopted for eliminating slackness in
winding.
In case of multiple strips for IV coil and if the strips are placed one above the other, the
transposition is must.
CORE COIL ASSEMBLY
Check that the coils & core assembly dimensions are well within design dimensions. Ensure
that coils are not too tight during assembly.
Maintain the phase to phase clearances are as per the design / voltage class.
Check that the insulations provided are as per the drawing for maintaining phase to hhase and
phase to earth clearances.
Complete the yoke assembly so that the magnetic circuit is perfectly closed.
Tighten the core coil assembly for providing the mechanical strength and eliminating the
slackness
DRYING PROCESS
* Core coil assemblies are dried in heating / vacuum oven to eliminate moisture. Heating cycle
is to be run to ensure that temperature attained by active part is between 80UC to 100IJC.and
water collection level comes down to less than 50 ml for at least two consecutive hours.( in
case of vacuum drying oven)
TANK FABRICATION
# Inside positive tolerances are to be maintained for ensuring required phase to earth
clearances.
Ensure that sharp corners and unnecessary of projections inside the tank are avoided.
Qualified welders to be used in tank fabrication and all welds are to be made uniform.
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Before tanking operation., the core coil assembly has to be tightened for eliminating the
slackness resulted during the heating cycles.
CCA to be well placed and secured in the tank for ensuring the earth clearances based
on design / voltage class.
Transformer oil filling to be done under vacuum level less than 5 tar, if possible.
As transformer oil plays critical role in the life of transformer, it is to be ensured the oil
characteristics meets IS-335 standard and oil filling is done at 60 Deg.C with a
minimum oil BDV of 60kV
All the gaskets used to be placed in position & tightened for eliminating the leaks at
gasket portions.
Transformer filled with oil is to be pressure tested at 0.35 kg / Cm as per IS-1180 for
ensuring that there are no leaks.
The quality of all accessories should be uniform.
In case of conservator mounted transformer, air is to be released from the bushing
pockets before testing.
TESTS
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A transformer can be stated healthy, if it passes all routine/ acceptance tests as per IS
2026.
However impulse and short circuit test on a sample transformer may be carried out in
any of the independent lab like CPRI, IREDA & NTH etc.,
The above measures in different stages will ensure service free transformers except for
preventive checks as per the standard procedures.
CAUSES OF FAILURE OF DISTRIBUTION TRANSFORMERS
The followings are some of the causes of failure of transformers in service, being attributed to
the users. Probable remedial measures have been highlighted to reduce the rate of failure,
which are assuming alarming in the present days distribution network across the country.
Prolonged overloading
Single-phase loading Un-balance loading Faulty termination Power theft & hooking Wrong
earth connection Prolonged short circuit Less or no maintenance ' Operation of tap-switch on
load Poor quality of L.T cable
1. IMPROPER INSTLLATION
It has been discussed, in brief, each of the above causes to surface out the reasons of failures as
well as few remedial measures to curb such unhealthy, pre-matured failure of transformers in
service.
2. PROLONGED OVERLOADING
Distribution Transformers are generally not recommended for continuous overloading. l
iowever overloading for a short duration cannot be avoided. It is one of the prime
respons¬ibilities of the utilities to keep a check on the loading of a transformer and should
record the loading Patton on a history card. In case of overloading the additional loss generates
more heat, which effects the burning of winding insulation, causing ultimate failure of the
transformer..
In most cases, it has been observed that the Line Operators use higher rating of re-wirable fuses
to avoid frequent breakdown of supply caused by overloading necessitating replacement of
fuses.
Proper sizes of fuse elements on both H.T and L.T side will definitely reduce the probability of
failure of transformer due to overloading. Measurement of load current with a Tong Tester at
frequent intervals, especially during peak loading hours, will give a fair idea about the load
demand of the locality. In case the overloading is more than 20% during peak hours and if it
persists for days together, it is recommended to replace the existing transformer with a bigger
rating, or else a second unit may run in parallel.
Furthermore, the Line Operators should have some elementary knowledge of rating-wise
correct size of fuse elements. The utilities should make their own standard of rating - wise
fuse elements and should communicate the same to the Line Operators for use in the field.
Moreover utilities must ensure the availability of such proper size of fuse elements at the
appropriate time. It has been seen in many occasions that the Operators use two of thin fuse
wires instead of one correct fuse element, because of non-availability. This procedure is wrong
and must be discouraged.
3.SINGLE PHASE LOADING
A distribution transformer should ideally be loaded uniformly on all the three phases. But there
are few occasions, especially running irrigation pumps in the state of Punjab, where a 3-phase
heavy-duty pump is made to run by initial mechanical priming with a single phase supply form
a 3-phase transformer. As a result the load on one phase goes drastically high causing
operational problems thereby leading to failure of transformers.
Such like abuses of transformers for no fault of the manufacturers should be stopped. It is
recommended to educate the ultimate users the effect of such single phase loading on the
performance of transformer.
It has been seen that in most cases, the agricultural customers using power, are the cause of
single phase loading on a 3-phase transformer.. It would be advisable to encourage such
customers to own their transformers including the overall maintenance responsibility as has
been introduced by PS.E.B recently. Only then, the failure due to single-phase loading will
considerably be reduced, except for those when the system runs on single-phase inadvertently
without the knowledge of the users.
4. UNBALANCE LOADING
If Delta/Star connected transformer with earthed neutral system, it is recommended that all the
three-phases are uniformly be loaded. This is infact very difficult to achieve with the
distribution network available in the suburban and metro cities. However an unbalance loading
upto 10% may not be that serious to create an operational problem for a delta/star connected
transformer. For a 3-phase balance load, the potential on L.V neutral is zero. In case of unbalance loading, a voltage is generated on the neutral and will remain floated between neutral
and earth. Since the neutral is solidly earthed through external link, a circulating current will
flow through the loop of delta winding. This additional circulating current will superimpose on
the main branch current of the delta winding and will cause additional heat, which may lead to
the failure of the winding insulation.
It is recommended to keep a check of load current including the current flowing through the
neutral at frequent intervals and should be recorded in the history card. As long as the neutral
current is within 10% of the load current, the matter may be ignored. In case the neutral current
exceeds 10% limit, the remedial measure should be taken to bring down the neutral current
within acceptable limit.
5. FAULTY TERMINATIONS
In many occasions we have the experience to witness heavy electric sparks coming out from
the bushing termination joints, especially on L.V connections. In most of the cases, these
sparks are because of loose terminations during the course of installation or else have been
loosen due to bad service conditions. Once spark occurs at the cable termination, it causes
melting of the busing sealing gaskets, effecting oil leakage from the bushing top, resulting
failure of transformer in due course of time because of low oil level
The incoming and outgoing termination should be done through proper connectors. ISS and
REC have recommended such details with drawings of lugs and connectors. Users must ensure
that the cable connections have been done with proper lugs and connectors. Direct Connection
of cable/conductor to the bushing terminal stud, should be avoided. During termination, we
should not forget the effect of bi- metallic action. If aluminium cable or conductors are to be
connected with brass/copper terminal or vise-versa, a proper bi-metal should remain in
between. Otherwise due to bi-metallic action, a milli-volt will generate causing a localized
current and may deteriorate the current carrying thread.
6. POWER THEFT AND HOOKING OF MAINS
We have the.experience to witness the stealing of power by hooking the system. This has been
a regular phenomenon in the unauthorized colonies to use electricity without paying the cost of
it. Power utilities do not pay much attention to such occurrences. But ultimately the
transformer manufacturers are paying the dividend. Since such hooking make the transformers
to run in overload/unbalance load and may cause failure in due course of time. This is one of
the major causes of failure of distribution trans-formers for which the manu-facturers are
blamed all the time. This is a serious national problem and the power utilities must think upon
stopping such power theft by hooking
Lot has been written and debated in various forum at national level. Almost all utilities are
taking number of measures to curb the menace of theft but the results do not appear to be upto
the mark. However we suggest that firstly, meters are required at every substation to find out
how much pilferage is actually taking place and what amount is really lost due to technical
problems related to transmission. Quite a few SEBs claim that they are in a process of
installing tamper proof electronic meters. These meters record the time at which theft has been
committed and how much has been stolen. But it needs huge invest-ment. And there are other
ways of checking power theft. A significant part of the revenues loss on account of theft is due
to inadequate billing. This can certainly be rectified by minimum investment. All that needed is
more will and sincerity on the part of power engineers. Regular energy audit and surprise raids
can mitigate the evil to some degree. The best way to con:abat theft is to privatize power
distribution. Take the case of Mumbai/Calcutta where there are private distribution agencies
have been doing business since long. The losses on account of T & D and pilferage together
are only 10 to 12% less than half of national average. Similarly in the case of Noida Power
Company limited, a subsidiary of Calcutta Electric Supply, who undertake distribution of
power in Greater Noida where the total losses including the theft is less than 5%. These are
very old private utilities. Recent privatization of SEBs/utilities in Delhi & Orissa etc. which
come have come up for the last 3 to 4 years have not indicated promising results. Unless open
access is introduced the monopolistic attitude of DISCOMS cannot be changed.
7. WRONG EARTH CONNECTION
The earthing of IV neutral will prevent the presence of any voltage above the normal appearing
in the IV circuit and therefore the possible danger to human life will be reduced to minimum.
Secondly, the earthing to the neutral point eliminate the possibility of an arcing fault to earth
and therefore of fire risk, while it also ensures the rapid disconnection of faulty apparatus from
the system without undue delay.
Due to discharge from high voltage the tank may be charged to an abnormal floating potential
causing danger to life.
In both the occasions provision of solid earth is extremely essential. As per Indian Electricity
Rule the tank body is to be earthed of two diagonally opposite points. The purpose is to ensure
that in case one of the earth points fails to act, the other will serve the purpose. The earthing
should be done with G.I strips having a minimum cross section of 25 x 3mm.
Moreover painted surface in and around the earth bolts should be cleaned to ensure proper
earthing. It is advisable to check the earthing at regular interval.
8. PROLONGED SHORT-CIRCUIT
In case of an external short-circuit on IV side, a fault current approximately 20 to 25 times the
rated current will flow through the windings. The windings are designed to take such fault
current for 2 to 3 seconds. If the fault persists more than 2 seconds, we are not sure about the
fate of the transformer. The transformer is bound to fail on such occasions of prolonged shortcircuit beyond permissible limit.
Necessary protections such as O.C.B on HV side, A.C.B o IV side, fuses etc., are
recommended. In case of small transformers, H.R.C fuses of appropriate size should be
provided. But under no circumstance, ordinary thick wires are not to be used.
9. LESS OR NO MAINTENANCE
A well maintained transformer always enjoys a longer life. Transformer is capital-intensive
equipment and must be, provided regular maintenance. However the frequency of the
maintenance mostly depends on the type of installation, its KVA rating, connected load and
place of installation. Users should prepare their own maintenance schedule along with their
periodicities.
10. OPERATION OF TAP-SWITCH ON LOAD
Approx. 2 to 3% of failures of transformers are caused due to failure of tap-switch for its poor
quality. In few occasions the failure occur due to faulty operation of tap-switch on load by the
in-experienced operator. In case of low oil below tap switch, it may also cause voltage failure.
If the selection of tap position with respect to the input voltage does not properly match, the
transformer may fail due to over excitation.
Manufacturers should own the responsibility of failure of tap-switch for its bad quality and
should look into for further improvement in their future supply. Failure due to faulty operation
and low oil level should owned by the utilities.
In case it is necessary to operate the off-circuit tap -switch during service, the transformer
should be disconnected from the incoming supply source before operation. Further, the
operating handle should be moved from position 1 to 5 (or more) and back before putting the
switch to the dsired tap position. This will eliminate the alignment problem in the male-female
contacts of the tap-switch and also clean the metal surfaces for better contact. The utilities are
very much aware of the fact that they seldom make use of such tap-switch for voltage
adjustment during service, especially for medium sized distribution transformers upto 630
KVA and below. It is therefore a matter for the users to re-think on the requirement of tapswitch for their future tenders. It will not only eliminate the cause of failure of transformer due
to tap-switch, but also the overall cost of the transformer will be reduced by 3 to 4%.
Customers like BEST, Bombay has stopped using tap-switch upto 990 KVA transformer,
UHBVNL (formally known as HSEB) have already withdrawn the requirement of tap-switch
for their 200 KVA transformers. RSEB don't use tap-switch upto 250 KVA ratings, UPPCL
has withdrawn tap-switch upto 160 KVA transformers. Other SI?B's may also look to this
proposal for eliminating tap-switch upto atleast 630 KVA transformers. Even if the utilities
want to incorporate tapings on HV side to regulate secondary voltage, it is recommended to use
link-Board (instead of rotary switch) as has been made mandatory by Calcutta Electric Supply
(CESC) for last few decades.
It is a glass- fiberboard having a minimum thickness of 10 mm, fitted above the yoke with the
help of four studs. Taping of each phase are terminated on the board and two of the tapings of
each phase are shorted with an external link.
Studies on use of rotary tap-switch vis-a-vis their failure revealed that although the switches
are the easiest means of changing tap at site, their operation are seldom used. On the contrary,
loose contact due to sludge deposition on the contacts are very common which involve not only
regular maintenance at site, but at times lead to failure of transformer.
Tap link board instead of tap changer switch was introduced by CESC in 70's to avoid failure
of tap changer switch due to inconvenience, involved during tap changing operation. With the
introduction of tap link board, the failure rate of the transformers due to tap¬switch at CESC
has gone down to less than 0.2% as against 3% on National average.
11. POOR QUALITY OF L.T CABLE
In multiple occasions, it has been seen that the transformers fail due to poor quality as well as
under-rated PVC IV cables as the PVC insulation melts or get charred due to heat, causing
dead short-circuit in the transformer. Poor quality of PVC cable sometimes affects the
insulation resistance of the IV circuit also. Users should make a note of it while selecting the
size of cable. Effect of bi-metallic action may also be looked into while connecting aluminium
cable with brass copper terminals or vise-versa. Cable supports and clips should be provided in
such a way that the cable should not create unnecessary load on the bushing terminals. It has
been seen in few occasions that the L.V cables are hanging against the terminal bushings
without any support, pulling the terminal down due to its own weight and affecting leakage of
oil from the sealing gaskets of the bushings. Such octurrences are very common in most of the
Metros and Suburban cities (except Mumbai and Calcutta).
In case of transformer with L.T cable box, much care should be taken while selecting L.T
cables. It is safe to use the 31/2-core cable. If single core cables are used, the gland plate
should preferably be of non-magnetic material (either brass or aluminium) for transformers
ranging from 500 kVA and above. Otherwise it will create unnecessary induction heating due
to magnetic flux linking around the entry of the cable in the box. M.S gland plate cut and
further welded with non-magnetic material may also serve the purpose.
12. IMPROPER INSTALLATION
In case of pole-mounted transformer with exposed outdoor bushings, adequate earth clearances
are to be maintained. Breather, arcing horns etc. should be placed to their respective locations.
Generally the breather is sent separately in sealed condition. The adhesive tape being provided
to seal the air passage at the bottom plug of the breather is removed before putting the breather
in position. The oil tray inside the breather container should be filled with oil before energizing
the transformer. The colour of silicagel should be checked (which is essentially to remain blue)
In case there is a growing tree near the bushings, necessary precaution should be taken to
ensure that in near future it should not come in the vicinity of the bushings.
Since the bushings used in distribution transformers are of oil communicating type, the trapped
air inside the bushings should be released before energizing. The trapped air inside the tank
cover or in the radiator (for bigger transformer) should also be released through the air-release
plug provided for this purpose. Oil level of the transformer upto the normal marking should be
ensured only after the air-release operation is done. In case a transformer is installed after a
long storage (generally beyond one year), the dielectric value of oil may be checked in
conformity with IS-1866. Explosion vent diaphram should be checked for its soundness. All
the above including the pre-energizing insulation resistances should be recorded in the
HISTORY CARD.
If rollers are provided for pole-mounted transformer, the same should be removed before
installing the transformer on the H-Pole or should be locked in their position after installation.
If possible, after installation the leveling of the transformer may be checked with the help of a
spirit level.
13. CONCLUSION
With the increase in the market competition, it has been observed that the safety margins of the
distribution transforms have been brought down to bare minimum, just sufficient to satisfy
standard specification and ideal loading conditions. As a result users have been put to constrain
of running the plant within the specified capacity throughout the year. It is difficult for an
urban electric supply to maintain such an ideal network condition throughout the year due to
unpredictable load growth-during festive seasion or outage of an adjacent source in the thickly
populated area. In such a situation, two options are open to the utilities. Either under utilization
of the transformer would result into uneconomical network Or introduce safety margins within
the specification.
Considering the economical factor it is suggested to review the specification, which may
include the following:
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Reduce permissible oil and winding temperature rise from 50/55° (as specified in
IS¬2026) to 30/45° to take care of poor ventilation in some locations and to permit
marginal overloading.
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Stipulation on the grade and type of core material and the maximum flux density in
view of the switching surge due to load shedding and to permit a certain degree of over
fluxing. However further studies are necessary on the effect of over fluxing and the
possibility of specifying the maximum level is to be explored.
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Restriction on the number of HV coil sections per phase and limitation on the winding
current density to enhance the cooling and reduction of winding gradient and hot spot
temperature etc. are to be judiciously implemented.
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It has been observed that despite specifying flux and current densities, the loss figure of
the transformers vary widely. Since it is difficult to check the current and flux densities
of finished trans-formers, specifying loss figures including minimum weight of core
and winding material (Aluminium or Copper) instead of densities may be considered
for implementation in the specification.
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Temperature rise test may be carried out by feeding total loss corrected to 100° C.
Input by:
MR. B. LAL, Secretary General, ITMA, New Delhi.
MR. LJ. DASGUPTA, Director (Works)
East India Udyog Ltd, Ghaziabad.
MR. K. SAMBAMURTHY, VP (QA) & M.R,
Vijai Electricals Limited, Hyderabad.