A PROCESS FOR MAKING CLASS H CAST RESIN DRY TYPE
TRANSFORMER
FIELD OF INVENTION
The present invention relates to a manufacturing process of Thermal
Class H Insulated Epoxy Resin Cast Coils adaptable to Cast Resin Dry
Type Transformer making.
BACKGROUND OF THE INVENTION
Presently epoxy resin cast coils are made using various types of
epoxy resins that are suitable for insulation class up to F only. The
class F insulated coils can have a maximum continuous operating
temperature of up to 155 Deg C only. By using Class H insulation the
maximum operating temperature is allowed up to 180 Deg C. This
enables the same volume of transformer to handle more power,
thereby making possible a compact and cost effective equipment.
The higher temperature operating ability also caters to overloads
during contingencies. The present invention describes a process for
making class H insulated epoxy resin cast coils and manufacturing of
cast resin dry type transformers using the same.
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OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a
manufacturing process of thermal Class H insulated epoxy resin cast
coil adaptable to cast resin dry type transformer making which
eliminates the disadvantages of prior art.
Another object of the present invention is to propose a
manufacturing process of thermal Class H insulated epoxy resin cast
coil adaptable to cast resin dry type transformer making which helps
in operating the transformer to a maximum allowed temperature
upto 180 Deg C.
A still another object of the present invention is to propose a
manufacturing process of thermal Class H insulated epoxy resin cast
coil adaptable to cast resin dry type transformer making which
enables the same volume of transformer to handle more power,
thereby making possible a compact and cost effective equipment.
A further object of the present invention is to propose a
manufacturing process of thermal Class H insulated epoxy resin cast
coil adaptable to cast resin dry type transformer making which
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enables higher temperature operating which in turn also caters to
overloads during contingencies.
SUMMARY OF THE INVENTION
The various methodologies used to make dry type transformer are
i) Resin impregnation by dipping transformer coil in insulting
varnish and using VPI process.
ii) Coil caking by using semi-curd resin impregnated insulation
as layer insulation and winding the coils with it then baking
the whole coil.
iii) Resin Casting, using moulds for holding the winding, into
which resin is poured to encapsulate the complete wound
coil.
Low cost distribution and power transformer coils, it is the third
method that results in the most robust ad maintenance free coils for
making coast resin transformer. A process has been developed to
make Class H insulated transformer coils, using the third
methodology mentioned above.
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The electrical current carrying winding, for transformer coils, in this
process can be made using aluminum or copper sheet foil or profiled
conductors. The conductors for the winding are provided with Class
H insulation and are wound on steel former with requested diameter,
as per the cols design. The Class H insulation used at this stage is
fibrous and compatible with Class H fibers to ensure homogeneity of
the winding. The HV and LV coils can be wound separately or
together. After winding, the former along with the insulated winding,
is placed inside a mould of desired dimensions and encased in a
removable jacket.
The winding inside the mould is pre-dried in hot air oven and then
put under vacuum for complete removal of moisture. The moulds
are also kept under vacuum, at a suitably high temperature, so that
no air bubbles are left in the cast coils. Special Class H epoxy resin
mixture is used, with or without a filler, to fill the mould and
encapsulate the winding. The epoxy resin mixture used, is a
combination of liquid Class H epoxy resin, hardener, flexiblizer and
accelerator in desired proportions.
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DRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure-1- shows a schematic view of the process plant for making
Class H cast resin coil for producing Class H insulated cast resin dry
type transformer.
BRIEF DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As shown in fig-1 the resin mixture is prepared in a thin-film
degassing mixer, (1) under vacuum. The resin for making class H
epoxy resin cast coils used is specially formulated bisphenol a epoxy
resin with carboxylic anhydride curing hardener and tertiary amines.
The proportion of each constituent of the resin is finely tuned to
ensure proper cross-linking and polymerization during curing. The
basic resin is stored in Resin stage tank (3). Hardener is stored in
hardener storage tank, (2). The flxiblizer is stored in flexiblizer
storage tank, (11). All the three storage tanks are provided with
vacuum, heating and cooling facilities. The resin mixture
constituents are added in exact proportion to the thin film De-gassing
mixture (1), by using hardener dosing pumps (9), flexibilizer dosing
pump (12) and resin dosing pump (10). After all the constituents
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have been pumped in the mixture, (1), the mixture is started and
thorough mixing of the resin, hardener and fiexibliser is done. The
accelerator is weighed separately and added in the mixture (1),
manually. The accelerator accelerates the reaction between resin
and hardener. The complete mixture is then allowed to flow, via rein
pouring nozzles (13), into the moulds, kept in the vacuum casting
chamber (4).
After complete mould is filled with liquid epoxy mixture, it is removed
from the vacuum chamber and placed in the hot curing oven (5), for
curing of the resin. The resin filled coil is cured with a fine tuned
temperature cycle. The temperature is initially kept at a higher value
so that slow curing is initiated. The reaction of resin and hardener is
exothermic, and to have uniform heating the curing oven (5), is
provided with hot air circulating fans (6), which maintain uniform
temperature inside the oven, this ensures uniform curing. The
temperature is then slowly raised, from a value of 100 Deg C to 140
Deg C, to further cure the resin filled inside the molds. A total curing
time of 20 to 24 hours is taken depending upon the mould or coil
size.
After complete curing, the cast coils are removed from the moulds.
The coils fresh out from the mould are raw and have sharp edges.
These coils are suitably dressed by removing and grinding sharp
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edges. The HV and LV connections are cleaned and coils are
assembled in the core to make dry type cast resin transformers.
The process gives stable and robust transformer coils that can be
used to make different rating of cast resin dry type transformers.
The transformer coils produced can give continuous service at 180
Deg C, required for Class H temperature.
The finished coils are assembled concentrically with the core.
Suitable pressure is applied on the core and coils by steel end-frames
and tie rods. To maintain concentricity stepped fiber glass end
blocks are used. The end-blocks support the coils a top and at
bottom. Suitable padding of Class H, high temperature elastomer, is
provided on the end-blocks to take care of thermal expansion and
contraction of the coils during service.
The complete core-coil assembly is housed in protective Aluminium or
Steel enclosure for protection against contact with live high voltage
transformer coils. HV/LV terminations are brought out through the
cubicle using bushings/insulators.
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The availability of Class H epoxy cast resin transformer provides a
more compact and efficient solution for continuous growing
requirement of electrical energy.
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WE CLAIM
1. A manufacturing process of thermal Class H insulated epoxy
resin cast coils adaptable to cast resin dry type transformer
making comprises:-
- preparation of resin-mixture in a thin film degassing mixer (1)
under vacuum by addition of a predetermined proportion of
resin from resin storage tank (3) and a predetermined
proportion of hardeners from hardener storage tank (2) and a
predetermined proportion of flexiblizer from flexiblizer tank
(11);
- addition of predetermined weighted amount accelerator in the
mixture manually;
- Pouring the complete mixture through resin pouring nozzle (13)
into moulds being kept in the vacuum casting chamber (4);
- Removing the mould from vacuum chamber (4) after complete
filled with liquid epoxy resin mixture;
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- Placing the mould in a hot curing oven for curing of the resin
by soaking the mould at a temperature of 100 Deg C 140 Deg C
for 20-24 hours;
- Removing cast coils from the mould after curing;
- Dressing the coils by removing and grinding sharp edges;
- Connecting HV and LV points after cleaning and
- Assembling the coils in the core to make dry type cast resin
transformers.
2. A process of making transformer from cast resin dry type coils
comprises:-
- assembling the finished coils concentrically with the core;
- applying suitable pressure on the core and coils by steel end-
frames and tie rods;
- maintaining concentricity by stepped fiber glass end blocks
which support the coils at top and at bottom;
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- insulation of suitable padding of Class H, high temperature
elastomer, on end blocks to take care of thermal expansion and
contraction of the coils during service;
- housing the complete core-coil assembly in a protective
aluminium or steel enclosure for protection against contact with
live high voltage coils and END
- taking out HV/LV terminations point through the cubicle using
bushings/insulators;
3. The process of manufacturing of cast resin coil as claimed in
claim 1 wherein the resin mixture comprises a specifically
formulated bisphenol A epoxy resin with carboxylic anhydride
curing hardener and tertiary amines.
4. The process of manufacturing of cast resin dry coil wherein one
resin storage tank (3), hardener storage tank (2) and flexiblizer
storage tank (11) are provided with vacuum, heating and
cooling facilities.
5. The process of manufacturing cast resin dry coil in claimed in
claim (1) wherein the thin-film degassing mixer (11) are fed (a)
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resin by using resin dosing pump (10) from the resin storage
tank (3), (b) flexiblizer by using flexibliser dosing pump (12)
from flexibliser storage tank (11) and (c) hardener by using
hardener dosing pump (9) from the hardener storage tank (2).
6. The process of manufacturing cast resin dry coil as claimed in
claim 1, wherein the curing oven (5) in provided with hot air
circulating fans (6) which maintain uniform temperature inside
the oven, this ensures uniform curing.
7. The process of manufacturing cast resin coil as claimed in claim
1, wherein the coils are cast for HV and LV separately or
together as once single coil.
8. A manufacturing process of thermal Class H insulated epoxy
resin cast coils as substantially described and illustrated herein
with reference to accompanying drawings.
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9. A process of making transformer from cast resin dry type coils
as substantially described and illustrated herein with reference
to accompanying drawings.

The various methodologies used to make dry type transformer are:
i) Resin impregnation by dipping transformer coil in insulting
varnish and using VPI process.
ii) Coil caking by using semi-curd resin impregnated insulation
as layer insulation and winding the coils with it then baking
the whole coil.
iii) Resin Casting, using moulds for holding the winding, into
which resin is poured to encapsulate the complete wound
coil.
Low cost distribution and power transformer coils, it is the third
method that results in the most robust ad maintenance free coils for
making coast resin transformer. A process has been developed to
make Class H insulated transformer coils, using the third
methodology mentioned above.