Insulator of Overhead Transmission line
Introduction
The overhead line conductors should be supported on the poles or towers
in such a way that currents from conductors do not flow to earth through
supports. i.e., line conductors must be properly insulated from supports.
This is achieved by securing line conductors to supports with the help of
insulators.
INSULATOR
Insulator are required to support the line conductor and provide
clearance from ground and structure.
Insulator material-
High grade Electrical Porcelain
Toughened Glass
Fiber Glass
Choice of insulator material is govern by availability, price and ease of
maintenance.
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Properties:-
In general, the insulators should have the following desirable properties :
1. High mechanical strength in order to withstand conductor load, wind load
etc.
2. High electrical resistance of insulator material in order to avoid leakage
currents to earth.
3. High relative permittivity of insulator material in order that dielectric
strength is high.
4. The insulator material should be non-porous, free from impurities and
cracks otherwise the permittivity will be lowered.
5. High ratio of puncture strength to flashover.
Types Insulator
1) Pin type
2) Suspension type :- For voltage greater than 33 KV
3) Strain type:- For dead ends, corner or sharp curve
4) Shackle type:- For low voltage distribution lines & can be used either in
a horizontal or vertical position
Pin type:
The pin type insulator are normally used
up to 33KV,
In any case it is not desirable to use them
beyond 50KV as the cost of such insulator
increases much faster than the voltage.
Pin type
Suspension type
Most times we use suspension type insulator for voltages above 33KV,
because of the following advantage:-
Each insulator is designed for 11 KV and for any operation voltage a string
of insulator can be used.
In case of failure of on one of the units in string, only that particular unit
needs replacement rather than the whole string.
The operating voltage of the existing transmission line can be increased by
adding suitable number of discs.
Strain type
identical in shape with the suspension type insulator.
These strings are placed in horizontal place rather than vertical place.
These are used to take the tension of the conductor at line terminals, at
angle towers, at road crossings and at junction of overhead lines.
These insulators are known as tension or strain insulator.
According to their shape Insulator can be categorized
Disc Type
Strut Type
Disc type Insulator
It consist of central suitable shaped porcelain/ glass body like a disc with an
metal clamp on one side and metal ball pin on other side
Cap is made of malleable cost iron and the ball pins is of forged
steel.
Strut Type Insulator
It consist of several insulator disc cemented altogether without any
link.
It is rigid and can take both tension and compression load.
These are used for holding the conductor out of way of structure.
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INSULATOR STRING
Disc insulator are joint by their ball pins and socket
in their caps to form string.
No of insulator disc is decided by system voltage,
switching and lighting over voltage amplitude and
pollution level.
Insulator string can be used either suspension or Fig. single string
tension.
Two suspension string in parallel used at railways,
road and river crossing as statutory requirement.
Swing of suspension string due to wind has to be
taken into consider.
Fig. Double string
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Potential distribution over a string of suspension insulators
To increase string efficiency
a) Selection of m
b) Grading of Units.
c) Static shielding
Grading rings serve two purposes:
Equalization of voltage drop across the units
it protects the insulator string from flash over whenever an overvoltage
appears (under normal or abnormal conditions) between the tower
structure and the power conductor.
Insulator And Insulator String Mechanical Design Considerations
a. Everyday Loading Condition
Everyday load 20 to 25% of insulator rated strength.
b. Ultimate Loading Condition
Ultimate load on insulator to not exceed 70% of its rating.
This limit corresponds roughly to pseudo-elastic limit.
c. In addition, capacity of tension insulator strings at least 10 % more than
rated tensile strength of the line conductors.
