3.4 Resin
Porcelain insulators have to be manufactured in bulk for a commercially viable price and require a long lead time dictated by the process. With the porcelain insulators and bushings getting expensive and requiring longer lead time, they began to get substituted gradually by synthetic resin insulators and bushings. Due to their low cost, lightweight, better lead time, and acceptable mechanical and electrical properties, several manufacturers of Busbar Systems have opted for insulators and bushings manufactured with synthetic resins. Synthetic resins can be formulated and synthesized to deliver the desired performance for a given application.
Natural resins are viscous liquids, exuded from
trees and plants that are insoluble in water and flammable. They tend to solidify over time. Among many different applications, resins
have been used as adhesives for many centuries.
Synthetic resins are chemically synthesized organic polymers with
properties similar to that of natural resins.
These are thermoset polymers.
Common terminologies associated with the resin
manufacturing and processing industry are listed below.
·
Fillers are added to the resin in a specified proportion to obtain desired
mechanical properties. Anhydrous quartz and silica of specified particle size
are some of the most commonly used fillers.
·
Pigment pastes, that are
predominantly inorganic, are added to the mix and homogenously stirred to give
the desired colour.
·
Catalysts
– Initiators – Activators
are added to the resin in a one-part system to commence
the process of polymerization.
·
Hardener is a part of a two-part system (the other being resin) to react
with resin for chemical bonding.
·
Accelerator is added to the resin
mix to accelerate the reaction and reduce the process time.
·
Glass
Transition Temperature (GTT) on when tested under controlled conditions, under mechanical load,
as defined by the standards.
·
Plasticizer is added to the resin
mix to predominantly give adequate flexibiity during the moulding process and
usage. Excess plasticizer reduces glass
transition temperature by 5 to 10
·
Pot Life is the time taken for
an initial, mixed viscosity to double from the time the product is mixed and is
measured at room temperature. The
definition may vary for low viscosity products.
·
Working
Life is the time a resin retains low enough viscosity that it can be used
for a specific application. (Pot life can act as a guide in determining the
working life).
·
Gel
Time is the time interval between the introduction of the catalyst and
the formation of a gel. It is an indicator of pot life and working life.
·
Exothermic
Reaction is a chemical reaction that results in the liberation of heat.
(Endothermic reactions result in absorption of heat).
·
Thermoset
materials are those products that undergo a permanent transformation at a
threshold elevated temperature.
(Thermoplastic materials regain their original properties when cooled).
·
Curing is the process of cross-linking and attaining the desired
parameters after the process has been initiated. Curing time is the time taken for the
completion of the process and is temperature-dependent.
·
Potting
– Encapsulation is a process of providing a protective cover to offer protection
against moisture/water and a corrosive atmosphere. It can also be used for electrical insulation
and protection against ultra-violet radiation.
·
Impregnation is a process by which a reinforcement/filler is completely encapsulated
/wet with a suitable/compatible matrix
for imparting certain properties.
Resin Reaction
Synthetic resins are commercially available in
solid and liquid forms. The resin products are manufactured for a one-component
system or a two-component system.
One Component System
In the one-component system (normally with
polyester resin) resin mixed with the initiator in the desired proportion is
packaged in a common container. The
component needs to be heated to a specific temperature to initiate a reaction
for bonding and kept for a specific duration for it to attain the mechanical
properties. Reactions for some of the
resins mix can also be initiated by ultraviolet light. Modern-day, commercially available resin
systems necessarily use an accelerator to speed up the reaction along with the
initiator at room temperature. Various peroxides are used for curing the one
component (polyester) resin system at an elevated temperature when the process
demands it.
Two-Component System
In the two-component system (normally with epoxy
resin) the resin and the hardener are mixed to trigger polymerization. This process is associated with an exothermic
reaction resulting in cross-linking.
Depending upon the formulation, the curing can take place either at room
temperature or at elevated temperatures.
Products manufactured with the two-component system generally exhibit
superior properties than those manufactured with a one-component system.
Any manufactured resin component must be allowed
to cure for a week to attain the desired properties before it is taken for
testing. Alternatively, the component can be subjected to a post-curing cycle
at recommended elevated temperature, to attain the properties.
Resin Classification
For the Busbars
Systems, the resin components are classified based on the raw material and
process. Each of them has specific application
and cost implications.
Epoxy Resin Epoxy Resin with
Glass reinforcement
Polyester Resin Polyester Resin with
Glass reinforcement
Polyurethane Resin Cycloaliphatic Resin
Composites
3.4.1 Epoxy Resin
The process of making an epoxy resin cast
component is comparatively simple. Epoxy resin and filler (of correct sieve)
are thoroughly mixed in the recommended proportions. The filler is either quartz or alumina. Since both the filler elements are
hygroscopic, care should be taken to ensure that they are separately heated to
remove any moisture content. The cost of
the product is significantly dependent upon the mix ratio. The mix is heated to a slightly elevated
temperature and stirred continuously to avoid settling of filler. Hardener and plasticizer are added to the mix
in the desired ratio and stirred.
Accelerators are added in the right proportion before pouring into the
moulds. This is an exothermic reaction.
The mixing chamber is desired to be kept under vacuum for optimum mix
quantity that is void and moisture-free.
The process is also referred to as degassing.
A split/add-on mould is made with hard chrome
plated interior. Desired tapers are
provided for releasing the mould. The releasing
agent is wiped on the surface.
There are two methods of casting the epoxy
component. In both cases, the mixing is
carried out in a vacuum to remove air and moisture.
In one process, the mould is pre-heated and
placed inside a vacuum chamber. The mix
is then poured under vacuum into the mould very slowly, by gravity. Once the mould is filled, further heat is
applied to complete the curing. The component is demoulded after complete
curing is achieved. It should be noted
that the mix will shrink inside the mould, during the cooling process and
therefore, the mould design should cater to the same.
Alternatively, the mix is injected into the
heated mould under pressure. The
injection pressure must be maintained in the injection line so that any curing
shrinkage can be compensated for by the resin being pushed into the mould. The mould is taken out and the product is released,
after a defined time.
It is absolutely necessary that voids are not
present in an epoxy component. It will
not only impact the mechanical properties but will also lead to partial
discharge, resulting in the failure of the dielectric.
(There are formulations that resin manufacturers
market for the cold pouring of resin with similar additives. The preference in the electrical industry is
still for the hot setting compounds only)
In the cast resin insulators/bushings, the
inserts are attached to the mould before casting and they become an integral
part of the as-cast component. Here
again, the material of the insert such as plated steel, stainless steel, brass,
and bronze can be specified. The minimum acceptable thermal class of insulation
is Class ‘B’ to meet ANSI/IEEE standards or IEC recommendations for bus
products.
Epoxy Bushing
3.4.2 Glass Reinforced Epoxy Resin
Glass has been used as a reinforcing agent in epoxy
resin components to significantly improve mechanical strength. Depending upon the shape of the component,
the mechanical strength of the epoxy can be enhanced, several times, by
reinforcing it with glass. There are
various methods of reinforcements, depending upon the shape, size, and
application of the product.
Hollow cylindrical/conical components can be
made by filament winding. In this
process, a glass filament passes through a bath of a mix of epoxy resin and
hardener and is wound over a suitable mandrel.
The filament is kept under tension to ensure that there are no trapped
air gaps between the filaments. Woven
roving can be inserted during the build-up of the profile thickness to enhance
the cross-breaking strength of the cylinder.
The filament is wound to a coarse shape on the mandrel. The semi-finished
product is removed from the mandrel and cured in an oven at elevated
temperature. (The mandrel has a gentle
taper to enable the semi-finished product to be pulled out). After curing, machining is carried out to the
desired dimensions.
Filament wound cylinders are used as
housings for the composite insulators, transformers bushings, outdoor
instrument transformers, and lightning arresters. Filament wound cylinders are used in on-load
tap changer (OLTC), formers for winding HT & LT windings in a dry type and
oil-immersed transformer. They are also
used as bushing housing in Busbar Systems.
High-strength, long, profiled insulating
components are manufactured by a process called pultrusions. The process comprises a large number of glass
filaments, passing through a mix of epoxy resin & hardener and converging
to a profiled mould and pulled at the other end. Woven roving can be introduced at a convenient
location, to improve the cross breaking strength of the profile. (The process of pultrusions is such that it
has a far higher tensile strength in the direction of pull than perpendicular
to it. Introduction of woven roving
improves the strength in the radial direction.)
Epoxy sheets can be manufactured by pressing the
glass and epoxy resin mix at high pressure.
Woven roving or chopped strand mat is introduced to improve upon the
cross-breaking strength and other mechanical properties.
Glass-reinforced epoxy resin components are used
in Busbar
Systems as support blocks, bushings, phase barriers, and tie-rods.
3.4.3 Polyester Resin
Polyester resin is commercially available in
liquid form. It normally has a shelf
life of about one year and it is advisable to use the same within six months of
the date of manufacture.
Polymerization of polyester resin can be
initiated by a catalyst, methyl ethyl ketone peroxide (MEKP), that is mixed in
a specified proportion. The
polymerization process of polyester resin constitutes an exothermic reaction
and results in the release of a significant amount of heat. It is, therefore, necessary to ensure that
the reaction takes place under a controlled environment.
A Filler can be added to the resin as per the
desired ratio to deliver the specified parameters. (The filler should be heated to remove
moisture and stored in sealed packets). The filler should be thoroughly mixed
with the resin before mixing the catalyst.
Polyester resin is used for the encapsulation
and insulation of components. There are
various grades of commercially available polyester resins such as orthopthalic
resin, terephthalic resin, and isophthalic resin which can be used depending on
the end properties of the product and the costs.
Since most of the polyester resin components are
used for Low Voltage applications, the component need not be cast in a vacuum
as long as the mix is thoroughly homogenized and poured into the mould, taking
precautions to ensure that there are no voids.
3.4.4 Glass Reinforced Polyester Resin
Glass-reinforced polyester resins are used in
the electrical industry for low voltage applications such as laminated sheets,
insulators, bushings, and a large variety of components.
Filament winding of cylinders, Pultrusions, and
glass-reinforced sheets can be manufactured with polyester resin by the same
process as that carried out for the epoxy resin.
In general, polyester resins are less viscous
compared to epoxy resin. Further, glass
mixed polyester resins with pigments, fillers, and catalysts are readily
available from the manufacturers of the resins.
These are available in two specific glass compositions and are used,
depending upon the shape of the component, to be moulded.
One-part resin, dough moulding compound (DMC)
and sheet moulding compound (SMC) have been extensively used as insulators for
low voltage applications. The type of
compound (DMC or SMC) to be moulded depends upon the shape of the component.
In DMC, a dough is used with a predetermined
ratio of polyester resin, catalyst, and glass strands of approximately 25 mm
(1”) long. A predetermined amount of the
dough is injected into the mould and pressed at elevated temperature. The mould
should ensure that air pockets do not get entrapped inside the component. DMC can be used for moulding components with
difficult profiles.
In SMC, the impregnated glass mat with polyester
resin is applied in layers and pressed in a mould ensuring that there are no
air pockets. Sheet re-enforcement
provides much better cross breaking strength.
SMC is used where the component is mostly flat.
A manufacturer can use a combination of DMC and
SMC depending upon the nature and profile of the product. The percentage of
glass is 20 % to 40 % and the rest is filler, resin, pigment, and other
additives for imparting specific properties such as fire retardance, chemical
resistance, anti-static properties, etc.
Insert material made of plated steel, stainless steel, brass can be
specified and it can form part of the moulded component.
3.4.5 Cycloaliphatic Resin
Cycloaliphatic epoxy resins are non-aromatic and
have fully saturated molecular structures, they are suited for use in
applications requiring resistance to ultraviolet degradation (outdoor
application) and electrical arc-tracking.
Cycloaliphatic resins are also ideally suited for applications in severe
environments such as near sea coasts and areas of high industrial
pollution.
Among their most notable features are their
inherently low viscosity, coupled with excellent weathering and electrical
performance.
The inherent low viscosity of these resins
enables them to be formulated with higher levels of inorganic fillers like quartz
or silica. This enhances mechanical properties and electrical track resistance
for such components. Their low viscosity allows for rapid fibre wet-out
in commonly used processes that include filament winding, pultrusions, and
resin transfer moulding.
Formulations incorporating these resins can
exhibit high, glass transition temperatures in the range of 200°C. Their characteristics of low dielectric loss
and high electrical resistivity, up to or above their glass transition
temperatures, provide high performance and reliability in both AC and DC
circuitry.
3.4.6 Polyurethane Resin
Polyurethane has not found much acceptance in Busbar
Systems application unlike in the building industry where polyurethane
foam has been universally accepted as thermal insulation. It is used as a coating and bonding material
for a variety of applications.
It is used as a potting/encapsulating compound
for low and medium voltage instrument transformers, varnish for improving
anti-tracking properties, coat/paint for protection against corrosive
atmosphere.
Polyurethane resin for potting is commercially
available in a liquid state, pre-mixed with a filler element that is quartz or
silica. During storage, the filler tends
to settle down at the bottom of the container.
Resin with filler is thoroughly mixed to ensure homogenization and
hardener is added to the mix before potting.
The process is carried out in a vacuum/pressure gelation technique to
avoid any porosity. Low viscosity makes
the potting process quick and efficient.
Formulations are available in a two-part process system.
The properties of the polyurethane are very
similar to that of epoxy resin. It is
tough and has better impact resistance.
The thermal class of insulation is low and hence, its application is limited.
3.4.7 Composites
Composite insulations have found usage in
outdoor installations. A composite insulator comprises a glass-reinforced core,
covered with a silicone rubber casing.
The glass-reinforced core provides the necessary insulation and mechanical
strength while the covering protects against U/V radiations and atmospheric
pollutants. The covering can be profiled
for desired creepage distance.
The core can be a pultruded rod of glass epoxy
as in the case of transmission lines (string insulators) or can be a filament
wound cylinder as in the case of post insulators in switchyards. (Housings of high voltage oil-immersed CTs
and VTs also use composite insulation. The filament wound cylinder diameter can
be profiled from top to bottom for optimal design). Lightning arrester housings are also made of
composites since filament-wound tubes can withstand very high pressures and do
not shatter when absorbing the energy of an electrical surge. (Shattering of lightning arrestor housing can
damage expensive switchyard equipment in the vicinity of its installation.)
Metal parts or
flanges, normally made of spheroidal grey iron (SGI) can be glued and
attached as per the application requirements. The core can be inserted into a
silicone rubber skirt of the desired profile to provide
the necessary creepage. Composite insulators are significantly lighter than
their equivalent porcelain counterparts.
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