The Ideal Transformer Encapsulant
This paper was written by our good friend and
mentor, the late Mr. T. W. Kolator P.Eng whose knowledge and wisdom helped us to
develop some of our best products for the encapsulation of instrument
transformers.
This page of our web site and this article is
dedicated to his memory.
Tadeusz Kolator wrote:
I thought that it might be useful to outline some
properties of an encapsulant for use in casting Instrument Transformers or
bushings. I shall list properties and give short explanations or reasons for
their desirability in these applications.
-
Dielectric Strength about 400 v/mil
This value, although it may seem quite low as
compared with many insulating materials of today, is quite sufficient for our
application. Higher values would not be of any advantage, as a matter of fact,
it would be very difficult to utilize it. Designs with higher stresses than
400 v/mil would create even greater problems with partial discharges and radio
interference. Also, because of low creepage and jump distances in the air,
external clearances would still have to be maintained regardless of the
dielectric strength of the encapsulant itself.
-
Dielectric Constant As Low As Possible
Ideally it would be best to have dielectric
constant equal to that of air (namely K=1). However, as this is impossible, we
have to accept fairly high values, but the lower they can be the better it is.
The higher the value of K the more difficult it is to meet required ionization
levels. Also, it may cause surface flash-over in some configurations.
-
Low Viscosity at Pouring Temperatures (say about 3000
cps or lower)
This is
important for the following reasons:
-
ease of
filling all small crevices and spaces thus providing good electrical
insulation.
-
ease of releasing trapped air bubbles
-
the time of pouring can be shortened
-
pouring can be done through small openings
-
better homogeneity of mixture can be obtained
CAUTION: settling of filler has to be kept to
minimum
-
Low or No Shrinkage
This is extremely important in the case of
casting Current Transformers. Electrical steel characteristics (losses and
magnetizing current) change with pressure exerted on the core effecting intern,
accuracy of the transformer often shifting it beyond limits of acceptance.
Also, with no shrinkage there wouldn't be any stresses in the castings and as
a result, there would be a lower chance of cracking.
-
Flexibility - Elongation
Same reasons as under point # 4 above. Stresses
on cores can be eliminated if the shell cast is flexible enough, there would
be no cracking. The desired elongation is in the order of 40%+.
-
Thermal Expansion
Ideally it should match that of electrical
steel (core).
-
Tensile Strength
Preferably should be above 2300 psi or perhaps,
still better, it should have low tensile strength but have high flexibility
elongation with a very high ultimate yield point (similar to rubber).
-
High "Green" Strength
Sufficient strength should be reached by the
encapsulant prior to de-moulding otherwise internal damage, often not visible,
can be done to the encapsulant resulting later on, probably in service, in
cracking of the shell. High "Green Strength" would make de-moulding easier and
faster.
-
Low Temperature Cure (preferably R.T.)
-
energy saving
-
ease of handling
-
no need for oven facilities
-
Good Adhesion
This feature is extremely important in both,
transformers and bushings in order to prevent:
-
oil leakage
-
creating voids by separation (pulling away)
-
reduction in the strength of the moulded body
-
Compatibility with Transformer Oil
This is important not only with respect to
the cured encapsulant which may be used immersed in oil, but also prior to
cure. Some processes involve casting oil impregnated equipment and this oil
must not effect the curing encapsulant.
-
Tracking Resistance
This feature is important especially for
outdoor use. Having Alumina as a filler or part of the filler improves this
feature.
-
Resistance to Chalking (ultraviolet)
Chalking caused by exposure to sunlight
effects appearance and, as some claim, lowers the self cleaning abilities of
the exposed surface.
-
Gel Time
The preferred gel time is around 1-1/4 to
1-1/2 hours.
-
Boiling Point
In view of the fact that some encapsulations
will be carried out at very low vacuum, even below 1 mm of Hg, it is
essential that very little, if anything, should come out of the mix.
Stripping any ingredients from the mix could effect the properties, entrap
vapours and cause non-uniformity in the casting.
-
Surface Tension
The surface tension should be such that it is
easy to remove bubbles and the material will readily flow into small spaces.
-
Low Toxicity, Odourless
Safety reasons, ease of handling without the
necessity of excessive precautions.
-
Thermal Shock
The cured encapsulant should be able to
withstand temperature variations at an approximate rate of 340C/hr.
The usual temperature range is between -450C and +1000C
for a minimum of 12 cycles.
-
Outdoor use - Weathering
The fillers
contained in the encapsulant must be carefully chosen, selecting
closed-cell, non-absorbing filler types. Any moisture absorption initiates
erosion of the casting and can cause changes in the electrical
characteristics.
-
Aging
The aging
properties must be such that an acceptable level of physical properties are
maintained in service for a period of at least 20 years.
-
Flammability
It is
desirable for the product to be flameout or at least a type that does not
support combustion.
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This article is posted verbatim as given to us
by Mr. Kolator in April, 1980. The above concepts were instrumental in the
development of our CLC 15-150 compound and its derivatives for the
encapsulation of instrument instrument transformers.
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