The overhead line conductors are bare, and insulation and there is need to insulate the support and the cross-arm from the conductor. The support could be in the form of a lattice tower, wooden pole, concrete pole etc. As one walk along the street, you will notice that the overhead line conductors sit on top of disc-like material resting on something that looks like an outstretched arm. The arm-like element is called the cross-arm. The brownish moulding is the insulator and is made of porcelain while the transparent one is made of glass.
Source: Suspension insulator
Both glass and porcelain are suitable insulating material that comes with their respective advantages.There is also need to provide clearance between each conductor and metalwork; the conductor is secured insulators attached to a cross-arm.
The level of insulation is dependent on the level of voltage involved which means that a lower voltage needs a low level of insulation while a higher voltage needs a higher level of insulation. The insulation must not deteriorate against the highest level of voltage it is required to shield irrespective of the atmospheric condition it is subjected to.
Pin type insulators can be made from either glass or porcelain, but it is often used for distribution because it is not economical and lacks mechanical strength at a voltage exceeding 33KV. Beyond 33KV, suspensions offer more flexibility and economics.
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The most exciting thing to be observed about overhead line insulators is that some appear like an individual disc that is strung together. And many would wonder why it is so? Is it done to improve the aesthetics or is there a technical reason behind arranging it in that manner?
Such design of insulators containing individual disc stringed together is called suspension insulators, string insulators or disc insulator depending on choice. The single disc is made up of porcelain mounted one above the other. Each disc is furnished with a metal cap at the top, and a metal pin is attached under it to be able to hook up another one, and the chain continues depending on the level of working voltage.
The conductor is suspended below the point of support using the insulator string. The sting is flexible and can swing in the air. This ability to pivot freely makes it crucial to give enough clearance between conductors and between conductors and metal work. If the allowance is not there, the conductor may clap together during heavy wind leading to short circuit (spark over).
Technically, the design of the suspension insulators to be composed of individual discs comes with numerous advantages which include:
Each unit of suspension insulator (insulator disc) is designed for comparatively low voltage (say 11kv) and the insulator strength can be increased by connecting the insulator disc module in series. The number of insulator discs built together depends on the operating voltage. Let us say we have a 66 KV system requiring insulation. If there is an individual disc of 11Kv, the only thing left is to divide 66KV by 11KV giving us six discs. The six discs will be stringed together giving us the required 66KV insulation.
Suspension type insulator provides more flexibility to the line and mechanical stresses due to the wind and owing to the method of connection of the cross arm which allows it to swing in any direction where it can absorb the wind load and stress generated by the loading.
In case of an unforeseen increase in pressure on the transmission line, the increased demand can be accommodated by raising the line voltage than to provide another set of conductors. With suspension type insulators additional line insulation requirement can be obtained by merely adding one or more disc to the string, according to the demands of insulation.
The disadvantage of suspension type insulators is that there is an uneven distribution of voltage throughout the insulator string. Assuming in the chain of suspension insulators that there is six individual disc secured together, one end of the disc must be secured to the tower through the cross-arm, the other end will be secured to the line. The disc closest to the line is made to receive the highest stress, and if nothing is done to equalise the pressure, the insulator closest to the line instead of handling 11KV as in the above example may be subjected to handle up to 20 KV. The endpoint will be an eventual collapse of string closest to the line, with the one closest to the cross-arm wasting away. This unequal voltage distribution is why string efficiency of suspension insulator is hardly 100%.
The individual disc of the capacitor could be seen as a capacitor. Notice that a metal cap and pin is separating the single disc from the next disc. Also, recollect there exist a level of capacitance between two conductors separated by a dielectric and in this case the dielectric is the disc insulator while the conductor is the metal cap and pin. This capacitance existing between the individual disc and metal cap and pin is what is called the mutual or self-capacitance, and it is equal throughout the string.
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Equivalent circuit of 6-string suspension type insulator showing mutual and stray capacitances
K=C/C^'
C= Capacitance per insulator
K= can be assumed constant for a particular string
C^'= Capacitance between each link-pin and earth.
Some methods can help to improve the string efficiency thereby avoiding the wastage of insulating material. They include:
By increasing the length of the cross-arm and by so doing the effect of the stray or shunt capacitance will be reduced. There is a limit to increasing the length of cross-arm owing to mechanical strength, the height of tower and cost constraint.
The use of larger discs (capacitance grading) closer to the line and with smaller sizes closer to the tower tends to equalise the voltage stress. This method is seldom used because it lacks standardisation requirement and it is difficult to have spares of the resulting different sizes of discs.
Static shielding: This technique introduces capacitances at the various joints and line utilising a large metal ring called grading ring. The effect of this is to reduce the effective capacitance of the units closest to the line.
The string efficiency is improved under wet conditions because when the insulator is wet, the potentials are considerably equalised.
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Pin type insulators can be made from either glass or porcelain, but it is often used for distribution because it is not economical and lacks mechanical strength at a voltage exceeding 33KV. Beyond 33KV, suspensions offer more flexibility and economics.
Another disadvantage of suspension type insulators is that large spacing between the conductors is required than with the pin type insulators due to the large amplitude of the swing of the conductor.
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