INSULATING MATERIAL
3.1 Introduction
Solid, liquid, and gaseous insulations have been
used in the electrical industry for different apparatus. They form one of the most important
components in all electrical apparatus, including Busbar Systems.
Porcelain (and glass to a limited extent) has
been used as an insulator for indoor and outdoor applications ever since the
commercialization of electric power. A
porcelain insulator manufacturing industry requires significant capital
investments. The process is labour
intensive with large lead times and consequently, the products are relatively
expensive. Despite that, even today,
there are applications where porcelain is the only accepted insulation
material. The faith and confidence that
it generates in a user can be compared to a cult following. It commands and gets a premium price over
other insulating materials for similar applications.
Disc Insulators made of glass have been used in
transmission lines. One of the
advantages of glass as an insulating material is that it can be recycled. Since water condenses on glass easily, its
use is limited to specific applications and geographical areas.
Oil has been extensively used in transformers
and continues to be the prime insulation medium in power & distribution
transformers and high voltage instrument transformers. Oil had also been used in bulk and minimum
Oil Circuit Breakers for providing insulation and efficient quenching of the
arc. Oil is still used as an insulating
medium in oil-filled cables (OFC) in extra-high voltage (EHV) systems. (Oil also acts as a cooling media in Power
and Distribution Transformers).
Over the last century, with the demand for power
increasing exponentially, there arose a need to develop a suitable insulating
material, for medium voltage switchgear and instrument transformers, dry-type
distribution transformers, and motors, that not only had good dielectric and
thermal properties but was also suitable for moulding to different shapes to
meet the requirements of supporting arrangements. Considerable research in the field over the
past century in organic insulation has resulted in the development of synthetic
resins (epoxy, polyester, cycloaliphatic, polyurethane) that had the desired
properties. The components with new
insulating materials with shorter lead times were cost-effective. The industry was quick to accept, adopt and
encourage the new insulating material.
Glass reinforced resin components such as dough moulding compound, sheet
moulding compounds, pultrusions, and filament wound components were
developed. These had excellent
mechanical properties and desired dielectric properties. Composite Insulators were developed for use
in outdoor service.
Consequently, the porcelain insulator
manufacturing industry has seen a steady decline in its market share despite
having good dielectric properties, higher temperature-withstand capability,
better resistance to corrosion & ultraviolet radiation, and longer life. Composite insulators are replacing them
increasingly in outdoor applications as in post insulators, housings of switchyard
equipment, and transmission line insulation.
Films and tapes were developed to meet the
ever-increasing demand for better thermal and dielectric properties in the
transformer and motor industries. These
have now been adopted in the switchgear and Busbar Systems industry
as well.
Developments in vacuum and Sulphur hexafluoride
technologies had led to their use in switchgear. Vacuum bottles are used in medium voltage circuit
breakers in distribution and generator connections. Both
are now used as Insulating and arc quenching media in medium voltage switchgear.
The minimum thermal class of insulation in Busbar Systems should be class ‘B’
so that it meets the general requirements of International Standards (ANSI/IEEE
& IEC).
A majority of all failures in a Busbar
System can be traced back to the failure of the Insulator or the insulating
medium.
Insulator cost is a significant part of the
overall cost of the Busbar System.
The insulating material
in Busbar
Systems can be broadly classified as under:
Porcelain and Glass Synthetic
Resin with/without Glass
Polyester Films Insulating
Gas
The following major
properties are to be considered when selecting an Insulation System.
3.1.1 Electrical Properties
Good electrical properties are the most
important criteria for insulating material. Among others, these include high breakdown
voltage (BDV), low loss angle (Tan Delta), and high surface resistivity (for
solid insulation).
3.1.2 Mechanical Properties
Good mechanical properties of insulating
material will result in a compact insulator. Among others, these include a
combination of high tensile, compression, cantilever, torsional, cross-breaking
and impact strength, and mouldability.
3.1.3 Corrosion & U/V Resistance
Insulating material must be suitable to
withstand the ambient atmosphere which may be corrosive. Insulators exposed to the atmosphere must
also have resistance to ultra violet radiation. Most insulators have metal
parts embedded at either end, for connections.
While the insulating material is susceptible to degradation due to ultraviolet
radiation and corrosion, the embedded inserts and metal parts are susceptible
to corrosion. Thermal cycling should not
result in the failure of the bond between the insulating material and the metal
parts, due to differential thermal expansion.
3.1.4 Thermal Class
The thermal class of insulation must meet the
requirements of the envisaged temperature of the conductor during operating
conditions. For most Busbar
Systems conforming to international specifications, class B insulation
will meet the requirements. Selecting a
higher class of insulation, believing it to provide a higher safety margin,
does not improve the life or performance of the product, and only serves to
increase the cost.
In transformers and motors, the class of
insulation has a significant influence.
It allows the current density to be increased making the equipment more
compact and perhaps less expensive.
Maximum allowable temperatures of various
classes of insulation as per IEC are as under:
NEMA and UL have added Class N, R, S for the
enhanced temperature-withstand capability of the insulation reaching up to 240
IEC presently designates the class of Insulation
by the numerical value of the temperature withstand capability in degree
Celsius. For example, class 250
designates an Insulation that can withstand continuously a hot spot temperature
of 250
Most of the manufacturers still follow the
alphabetical representation of the class of insulation. Each class of insulation is associated with
the typical organic insulating material.
3.1.5 Expected Life – Organic Insulations
An insulator made of organic material will decay
and has a definite life. The life of
organic insulating material is around 20 – 25 years. This figure is arrived at on the assumption
that the insulation material is operated below the designated temperature of
the class of insulation it belongs to.
Any thermal overload reduces its life.
Based upon the Arrhenius law of the
chemical reaction of Time Vs Temperature, it is generally accepted that an increase of the
operating temperature of 8 - 10 degrees Celsius over the rated value reduces
the service life by 50%.
Similarly, an insulation system is designed for an operating
voltage. To verify the suitability of
the insulation, international standards have specified a short time power
frequency voltage withstand test (Routine Test) and an impulse voltage
withstand test (Type Test) corresponding to a rated voltage of the Busbar
System.
If organic insulation is subjected to
continuous/repeated power frequency overvoltage or surges, it could impact its
life significantly and can lead to failure.
It is generally accepted that an
increase in the operating voltage by 8 – 10% over the rated voltage reduces the
service life by 50%.
This is not valid for air or gas insulated bus since the upper limit of the permissible conductor temperature is well below its thermal class of Insulation. However, in the case of a sandwich bus or conductor embedded in solid insulation as in a cast resin bus or solid insulated bus, the class of insulation should be capable of withstanding continuously, the envisaged hot spot temperature of the conductor under specified rated current.
In the conservative power industry, some
customers are prepared to pay a premium for the Busbar Systems with
Porcelain Insulator for a perceived, better performance and comfort. However, newer materials have proven to be
more than adequate for most applications.
Continued..........
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