Types of Insulators in Transmission Lines
By Natasha Gilani , eHow Contributor
Transmission line insulators
separate, contain and suspend transmission line conductors.
Transmission line insulators are
devices used to contain, separate or support electrical conductors on high
voltage electricity supply networks. Transmission insulators come in various
shapes and types, including individual or strings of disks, line posts or long
rods. They are made of polymers, glass and porcelain--each with different
densities, tensile strengths and performing properties in adverse conditions.
1.
Pin Type
Insulators
o
Pin type insulators are used for the
transmission of lower voltages. A single pin type insulator is used to transmit
voltages up to 11 kV (kilovolts) and higher voltages require two-, three- or
four-piece pin insulators. They are not economically feasible for 33 kV and
higher transmission lines. Pin type insulators are secured with steel or lead
bolts onto transmission poles. These are typically used for straight-running
transmission lines.
2. Suspension Type Insulators
o
Suspension type transmission line
insulators suspend and support high voltage transmission lines. They are cost
effective for higher voltage transmission, typically replacing multiple pin
type insulators. Suspension type insulators have a number of interconnected
porcelain discs, with each individual unit designed to support a particular
voltage. Together, a system of these discs is capable of effectively supporting
high voltages. There are three types of suspension insulators: cemented cap
suspension insulators; interlinking or Hewlett suspension insulators; and link
or core suspension insulators.
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3. Strain Type Insulators
o
Strain type insulators are
horizontally suspended suspension insulators. They are used to handle
mechanical stresses and take the pressure off a conductor at the end of a
transmission line, at a sharp corner or curve or over long river crossings.
Strain insulators are typically used for higher voltage transmissions.
4. Shackle Type Insulators
o
Shackle type insulators, similar to
strain type insulators, are used on sharp curves, end poles and in section
poles. However, unlike strain insulators, shackle insulators are designed to
support lower voltages. These insulators are single, round porcelain parts that
are mounted horizontally or vertically.
5. Stay Insulators
o
Stay insulators, also called egg
insulators, are primarily used to prevent stay wires from becoming energized
from accidentally broken live wires. They, therefore, function to provide
insulation between stay clamps and transmission poles. Stay insulators are
mounted at a height of at least 3 meters (118 inches) from ground level.
Material
Insulators used for high-voltage power transmission are made from
glass,
porcelain or
composite polymer materials. Porcelain
insulators are made from
clay,
quartz or
alumina and
feldspar, and are covered with a smooth glaze to shed water.
Insulators made from porcelain rich in alumina are used where high mechanical
strength is a criterion. Porcelain has a dielectric strength of about 4–10
kV/mm.
[1]
Glass has a higher dielectric strength, but it attracts condensation and the
thick irregular shapes needed for insulators are difficult to cast without
internal strains.
[2]
Some insulator manufacturers stopped making glass insulators in the late 1960s,
switching to ceramic materials.
Recently, some electric utilities have begun converting to polymer
composite materials for some types of
insulators. These are typically composed of a central rod made of
fibre reinforced plastic and an outer
weather shed made of
silicone rubber or
EPDM.
Composite insulators are less costly, lighter in weight, and have excellent
hydrophobic
capability. This combination makes them ideal for service in polluted areas.
However, these materials do not yet have the long-term proven service life of
glass and porcelain.
Comparison of suspension insulator string with other
insulators:
Advantages of
suspension type insulators
Less expensive than pin-type insulators - For high
voltages pin-type insulators are
Uneconomical since the cost increases very rapidly as
the working voltage is increased.
In a suspension insulator system, each insulator is
designed for a comparatively low working voltage, usually about 11,000 volts,
and the insulation for any required line voltage can be obtained by using a
'string' of a suitable number of such insulators. 8 In the event of a failure
of an insulator, one unit - instead of the whole string - has to be replaced.
T he mechanical
stresses are reduced, since the line is suspended flexibly; with pin-type
insulators, the rigid nature of the attachment results in fatigue and ultimate brittleness
of the wire, due to the alternating nature of the stress.
Also, since the string is free to swing, there is an
equalization of the tensions in the conductors of successive spans.
In the event of an increase in the operating voltage
of the line, this can be met by adding the requisite number of units to each
string, instead of replacing all insulators, as would be necessary with
pin-type.
Disadvantages of suspension type insulators
Owing to the free suspension, the amplitude of swing
of the conductors may be large compared with that on a pin-type insulated line
and the spacing should therefore be increased.
Construction of suspension system is, of necessity,
associated with high electrostatics tresses in the porcelain immediately
between the links, so that the liability to puncture is greater than with other
types.
Advantages of the insulator grading method:
Good results
can be obtained by using standard insulators for the most of the string and
large units for that near to the line conductor. T here won’t be any lightning threats like in graded ring method
For very high voltage lines the
insulator grading method is very economical. This is very easy and very simple method which uses in very high voltage
transmission systems. String with identical unit and graded ring method
is very simple method. It can be easily taken the same voltage distribution
throughout the string accurately than string with graded units method. Since
there is an equal voltage distribution throughout the insulators in the string, no insulator will be over stressed.
Insulators
in Transmission Lines Practical Errors
Reasons for differences between practical and theoretical values
1) The capacitors which we used aren’t ideal so they may not be represented
the actual values.
2) The components which we used for this practical can be worked with
errors.
3) Human errors could be occurred when taking the readings
4) Errors could be occurred
when reading from the meters with naked eyes.
5) Connecting Wire resistance wasn’t included in calculations
6) Errors caused due to internal resistance of the meters.
7) Internal capacitive components of used equipments may have affected to
the readings
