Maintenance
of Transformer : The Complete Guide | One of
the passive electrical device is the electrical transformer. It
is used to transfer the electrical energy. In this article we are going to see
about maintenance of transformer. The guide is completely covered about
transformer maintenance. So, read carefully without skip any paragraph.
Precautions
For Paralleling Transformer
The
following precaution should be taken:
(i) Study
the nameplate particulars, voltage ratio, percentage impedance, vector diagram
and group number, terminal marking etc. and make sure that all conditions for
parallel operation are fulfilled. If the name plate or terminal marking is
missing a much more elaborate procedure is required i.e. the transformer
windings have to be lifted out and the external connections traced and
identified, if necessary.
(ii) Even
when everything seems right, it is still advisable to observe a definite
testing procedure as any erroneous connection may result in a short circuit. Do
not forget to connect the transformer tank body solidly to earth.
(iii) For
conducting a phasing out test, it is not necessary to apply the rated voltage
side. A much lower voltage is more convenient. For example, a 6.6 kV/ 400 V
transformers primary cloud be connected to a 400 volt supply. The secondary
voltage will be corresponding lower i.e. about 24 volts which make it much
safer and easy to work with.
(iv) If
full rated voltage has to be applied, it will be necessary to use potential
transformers. The terminal markings and polarity of the potential transformers
also will have to be checked. Remember the possibility of 1800 phase difference
if the connections are not correctly made to lie terminals.
(v) It is
necessary to link up the two secondary by one common connection, so that
voltmeter readings may be taken. If the secondary star connected and the
neutrals are available, no neutral is available and the secondary leads of the
two transformers are completely isolated, connect any terminal of one
transformer of what appears as the corresponding terminal of the other
transformer. Even if it is connected to the wrong terminal, no harm is done and
the tests will reveal the error.
(vi) In
using a voltmeter for the test, a fundamental precaution to be taken is to put
its range switch to a much higher scale before carrying the test, i.e. at least
twice the excepted voltage. If the secondary voltage is 400 V, the voltmeter
range switch should be set to 1000 V, for the first set of measurements. After
the terminals are identified and the corresponding terminals show little or no
voltage, the selector switch should be put to a lower scale say 0-50 volts or
less to make an accurate measurement of the voltage different between
corresponding terminals. This should be negligible.
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| Maintenance of Transformer |
Inrush
Current and Its Remedy
When a
transformer is switched off the line, the primary circuits open at zero
current, but the secondary current takes a sort time to decay to zero. This
leaves a residual flux in the core of the transformer. It is a fundamental
principle of the transformer that the voltage applied to the primary is nearly
counterbalanced by the back e.m.f produced by the alternating magnetic flux in
the core.
At the instant
of switching in a transformer the back e.m.f which develops is dependent upon
the value of the residual flux which may either be in opposition to the flux
produced by the impressed voltage as it should be, are it may be as assisting
it.
Therefore
at the instant of switching in the current flow is determined by the value and
direction of the back e.m.f. and, therefore, upon the residual flux in the core
at the time. Under the worst condition, this
current is limited only by the air core reactance of the windings since
the core would be saturated and its impedance is greatly reduced.
The
current inrush would be as high as five times full load current and persist for
as long as two seconds. Since the current flows only through the primary
winding of the transformer and there is no corresponding outflow on the
secondary side, it has the same effect as an internal fault as the differential
relay is concerned.
This
unavoidable current such at the time of switching in a transformer has always
been a source of difficulty especially with high speed differential relays the
inrush current is essentially a pulsating direct current which rapidly decays.
It contains a large percentage of harmonic components. This fact is utilized to
block the operation of the differential relay during the current inrush period
by providing restraining coils (RC) and harmonic restraint, comprising series
and parallel tuned filter circuits (TF). To make the differential relay inoperative
during the current inrush, but leaving unimpaired its ability to act instantly
for false within the transformers.
Read About : Electrical Substation Equipments
Insulation
Co-ordination
A grid
network comprising extensive overhead transmission lines, switch gear and
transformer is subject to over voltages which may cause by external atmospheric
sources such as lightning discharges, or internally produced surges due to
switching operations or faults. When such voltage disturbances occur, it is
naturally the weakest link in the system which breaks down. In the interest of
continuity of supply it is obviously necessary to protect the system against
such over voltages by the installation of lightning arresters or spark gaps
whose characteristics should be such as to bypass the surge safely to earth.
The
breakdown voltage of the arrester should be carefully chosen: it should be
appreciably lower than that of the overhead line insulators, switch gears or
the transformers, but much higher than the normal operating voltage.
It is, therefore,
necessary to co-ordinate and grade the insulation level of the various
components parts of a system intelligently, the principle being that the
greater the cost of an equipment and the more its vulnerability the higher
should be it’s insulation level to reduce the risk of failure. Thus, in a grid network,
the surge diverters should have the lowest insulation level(Figure ), followed
by suspension insulators, isolating switches, circuit breakers and finally the
transformer which should have the highest level since they are costliest.
The expression ‘insulation co-ordination’
refers to the steps taken to correlate the insulating strength of the different
electrical apparatus to prevent their being damaged by over-voltages and to
localize flashovers and thereby achieving reliability of supply.
Effect
On Insulation During Start Point Earthing
In the
three phases UN earthed system fig. if one leg say C gets earthed, the other
two lines A and B will be at a potential equal to full line voltage above the
ground. Therefore the protective devices including lightning arrestors should
be based on full rated line to line voltage, with respect to earth. In a three
phase system containing a star winding, whose neutral point is connected to a
perfect earth.
One having
no earth resistance, the lines A and B will theoretically remain at a potential
equal to 1/ 3of the line voltage above earth, if line C is earthed, 57.6
percent of the normal value. In practice, however, the earth connection will
necessarily have some resistance which will raise the potential of the neutral,
when line c is earthed, because of IR drop in the earth resistance.
The effect
of this is that all insulators on the line A and B will be subjected to a
potential which is somewhere between 57.6 percent of full line voltage as in a
perfectly earthed system. And a maximum of line-to-line voltage as in the
unearthed system. Thus earthing has a direct bearing on the insulation level,
and is taken care of by what is called ‘co-efficient of earthing’. The system
which is considered as effectively earthed if the co-efficient of earthing is
80 percent. The insulation level is always with respect to earth, although it
is indicated against the three phase line-to-line voltage.
Transformer
Maintenance Schedule
The
transformer is one of the most reliable items of electrical equipment,
requiring relatively little attention; yet often even this minimum of attention
is not given, and no wonders they do sometimes breakdown because of neglect.
The
programme of inspection and maintenance is governed by the size of the
transformer, where it is installed, whether indoors or outdoors, if in a
substation is it manned or unattended, the operating conditions and so on. The
degree of attention required depends greatly upon how heavily or lightly the transformer
is loaded the maintenance schedule in table 1. Covers oil immersed,
naturally-cooled transformers of three categories.
1. Major
installations at large grid substations which are manned all the time, with unit
size of 1000 to 10000 KVA or more. Quite often the transformer loading is quite
high.
2. Medium
installation in small substations with unit capacities of 250 KVA to 1000 KVA.
3. Minor
installation up to 250 KVA unit capacity which are unattended and often at
remote points, such as pole mounted transformers, and rural distribution
transformers.
The
interval of inspection are indicated as; H for hourly, D for daily, W for
weekly, M for monthly, 3-M & 6-M for 3 monthly and 6 monthly, Y for yearly,
2-Y for, 5-Y for, 2yearly and 5 yearly.
In major
and medium installations a stand by transformers should always be available to
be switched on at a moment’s notice, in place of the largest unit installed. It
should be maintained to the same standards as the other units, and periodically
put to use to keep it in a good and healthy condition.
In major
installations, using year blast cooling or water cooling, a daily check should
be made of the ancillary installations like air blowers, water pumps and the
connected protective devices, on load tape changes etc.
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| Transformer Maintenance |
Action
To Be Taken While Transformer Oil Temperature Rises Unduly
In every
large well regulated system, a close watch is always maintained of the loading conditions,
and the oil temperature. Excessive oil temperature is the result of
overloading. This should not be permitted to occur, by taking timely action to
switch on a larger unit for operation or to put another unit in parallel to
share the load. Remember, excessive temperature reduces life and therefore, if oil
temperature rises unduly for any reason the transformer should be put out of
service immediately, but the cooling fans. If any, should continue to run until
the oil temperature falls to normal.
Points
To Be Checked By Oil Level Tends To Fall Down
Transformer
oil may leak at several points, i.e. oil level gauge, cork packing below the
top cover, oil conservator, connection, drain cock, gasket, bolt where a cable
box is bolted in, and welded joints. Every oil leak should be traced to its
source and remedial action taken to stop the leak. If the gasket leak cannot be
stopped by tightening the bolts, the gasket should be renewed.
The belt
material for gasket is cork, rubber sheet 5mm thick. This consists of fine
granulated cork and synthetic rubber as the binding medium. The surfaces
between which the gasket is provided should be quite flat and smooth without
any burrs or irregularities.
They
should be perfectly clean and free from scale, rust, old paint, remains of glue
and old gasket, grease or oil. The surface should be thoroughly cleaned and
washed with trichloroethylene, alcohol or other gases solvent. If it is glossy
surface such as bakelite, it should be made rough by emery cloth or sand paper.
The gasket should preferably made slightly narrow than the flange width so that
it may not swell beyond the edge when compressed. Though the cascade makes good
oil tight a joint, it is an advantage if the gasket joint surface is coated
with a thin layer of special glue obtainable from the manufacturers.
Inspection
covers, which may require to be opened whenever required, may be coated with
ball- bearings grease over the surface which comes in contact with the gasket.
The cover can then be opened without damaging the gasket.
Care
should be taken to tighten the bolts uniformly going from one bolt top the
next, over the entire lot several times so that the surface pressure is uniform
all over. The compression should be limited to 2/3 of original thickness. The
retightening operation should be repeated sometimes after the transformer is
put into use, as the gasket tends to settle down after sometime. This should be
done, say two days after renewing the gasket, and again a week later the bolt
lighteners should also rechecked during a six monthly inspection.
Sometimes
slight oil leakage occurs at the welded joints the exact point of the leakage
outside the tank can be discovered first by cleaning the surface thoroughly
with a grease solvent or petrol or denatured alcohol, and then coating the surface
with a thin layer of chalk, cement plate in water or white wash and allowing it
to dry. Leakage of oil is then readily revealed by the dark patch it forms.
As for the
cure much depends upon how bad the leak is. Formation of a few drops over a
period of time is not of any consequence and occurs in most transformers. If it
is bad, cracked or spongy well may be repaired by welding in a metal patch. A
small hole in the weld can often be put right by peeping with a cold blunt
chisel until the hole is closed, and then finally using a ball peen hammer to
complete the seal.
Special
solders which have a great affinity for iron but which are unaffected by oil
could also be used to stop oil leaks through porous weld threaded bolt
fixtures, internal welds making use of a blow torch; take care to guard against
any explosions. If a hole exists in a casting it may be drilled and a tight brass
plug driven in. it is no use using shellac as a filter as shrinks considerably
when it dries. After completing the repairs, the deficiency in oil level should
be made good and a careful note made of the oil level for verification at the
next inspection.
Attention
Is Required On Bushings And Insulators
It is
essential to examine porcelain bushing and other insulators, such as busbar
supporting insulators and surge-diverter at frequent intervals. The porcelain
surface should be thoroughly cleaned with a wet cloth and polished, after
removing every trace of oil or dust. In most insulator urban areas there is
heavy atmospheric pollution, and fine dust, soot, and industrial deposit settle
down on the insulator skirts. If not removed at frequent intervals they tend to
form a very hard curst.
When dew
or moisture appears on the insulator, the combinations make a conducting path
which could easily result in a flashover. Very often, the heat produced by the
leakage path damages the glazed surface of the insulators. The position is
aggravated is coastal areas subject to salt deposition. In U.K. extensive
failures have occurred when snow deposition on insulators starts melting in
early spring and eases up the dirty deposit of soot, salt and dust.
Extensive
research has been made as to the methods of overcoming the trouble. One of the principles
employed is to apply a water repellent on the porcelain surface such as thin
coat silicone grease. Dust and coil will no doubt stick to the surface and the
insulator may present a very dirty appearance, but the function of the
insulator remains unimpaired, as the oil or grease engulfs the contaminants and
insulator one particle from the other, inhibiting and tendency of from leakage
path. It is of course necessary to point is reached, and to apply a fresh coat
of thin grease.
Another
method employed for live-line cleaning, at substation which cannot be shut
down, is to wash the insulators by directing a high-pressure finely atomized
water spray, using high resistivity water and maintaining safe distances. In
India, none of these procedures appear to be called for. Hand cleaning by
wiping off the deposits with a wet or petrol soaked cloth will be found to be
fully efficacious it the glazed surface is finally polished with a dry cloth.
When
polishing, the bushing surface should be carefully examined for hair-line
cracks; when the transformer is put on load, the crack may develop rapidly to
dangerous dimension. At the same time it is as well to go over all
current-carrying parts with a spanner gently but firmly, to ensure good
contact. Remember that a poor contact will develop enormous heat when in
operation, which may ultimately cause a burnout or crack the insulators.
Final
Word
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