Various types of insulators used for overhead transmission and distribution lines are described below:
Type # 1. Pin Insulators:
This type was amongst the earliest designs, and used for supporting line conductors. It provides the most economic, simple and efficient method of conductor and bus-bar support for voltages up to and including 33 kV. Modern pin type insulators are very reliable and inherent cracks in porcelain are very rare and never occur with toughened glass insulators. The life of modern porcelain insulators is relatively long (expected to be about 50 years). Pin type insulators are available for use up to 50 kV.
The pin type insulator is designed to be mounted on a pin which in turn is secured to the cross-arm of the pole. The insulator is screwed on the pin and the line conductor is placed in the groove at the top of the insulator and is tied down with soft copper or soft aluminium binding wire according to the conductor material.
To avoid a direct contact between the porcelain and the metal pin, a soft metal (usually lead) thimble is employed. The insulator and its pin should be sufficiently mechanically stronger to withstand the resultant force due to combined effect of the weight of the conductor, wind pressure and ice loading, if any, per span length.
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For lower voltages generally one piece type of insulators are used. For high-voltage transmission lines, stronger pin type insulators are used. The high-voltage pin type insulators differ in construction from low-voltage type in that they consist of two or three pieces of porcelain cemented together.
These pieces form what we call petticoats or rain sheds. The multiple shells or petticoats are provided in order to have an adequate length of leakage path so that the flash-over voltage between the line conductor and the insulator pin is increased. The petticoats or rain sheds are so designed, that even when the outer surface is wet due to rain, sufficient leakage resistance is still provided by the inner dry surfaces.
It is desirable that the horizontal distance between the tip of the lower most shell is less in comparison to the vertical distance between the same tip and the cross-arm, otherwise in case of an arc- over, the discharge will take place between the line conductor and cross-arm rather than line conductor and the pin of the insulator, thereby, the cross-arm will have to be replaced rather than the insulator.
Many engineers have a preference for one piece insulators up to and including 11 kV and use the multi-piece type for high voltages while others prefer multi-piece for all voltages. One- piece insulators are more prone to cracking, because of rain, and, therefore, their use may cause interruption of supply during the very first rain after the lines is commissioned.
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Multi-piece insulators have an advantage that a defect on one shed does not seriously affect the mechanical strength of the insulator, and it generally functions electrically at normal voltage until the defective unit is traced and replaced. For rural 11 kV lines, carrying less load and therefore, using light conductors, single piece insulators are employed for reasons of economy. For urban 11 kV feeders, specially feeding industrial loads, multi-piece insulators are preferred, being more reliable.
Two one-piece insulators are shown in Figs. 9.1 (a) and 9.1 (b). Leakage paths are shown by dotted lines in the Figs.
Two multi-part insulators for use on 33 kV and 66 kV are shown in Figs. 9.2 and 9.3 respectively. The flash-over distances are also shown when the insulators are wet and dry in Figs. 9.2 and 9.3 respectively.
Flash-over distances for dry insulators, as shown in Fig. 9.2, = a + b + c + d
Flash-over distances for wet insulators, as shown in Fig. 9.3, = a + b + c
Thus it is seen that flash-over distance in case of wet insulators is less as compared to that in case of dry insulators. The rain sheds are, therefore, made to keep the inner side of the insulator dry.
The main advantage of the pin type insulator is that it is cheaper. In many cases one pin insulator can do the work of two suspension insulators. Secondly the pin insulator requires a shorter pole to give the same conductor clearance above the ground since the pin insulator raises the conductor above the cross-arm while the suspension insulator suspends it below the cross-arm.
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However, its use beyond operating voltage of 80,000 volts becomes uneconomical. Pin type insulators become very bulky and cumbersome when designed for the higher voltages, and owing to the larger length of the supporting pin the bending moment near the point of attachment to the cross-arm tends to become excessive. The modern practice is not to use pin type insulators beyond 33 kV.
For these the ratio of the spark-over voltage to the working voltage (usually known as safety factor) must be high for low voltage than for high voltage. This is about 10, but these are so designed that spark-over takes place before these get punctured.
Surface leakage current is due to the accumulation of dirt and in order to reduce its value the insulators are given a long leakage path by providing two or three petticoats or sheds. Insulators of this type are used on intermediate poles on straight run. These do not take tension.
Type # 2. Suspension Insulators:
With the increase in operating voltage, the insulation required increases. Transmission lines use extremely high voltages, 400 kV, for example. At these voltages the pin type insulators become bulky, cumbersome and costly. Besides, the pin which must hold it would have to be inordinately long and large. In order to meet the problem of insulators for these high voltages, the suspension insulator was developed.
Suspension type insulators consist of a number of porcelain discs flexibly connected in series by metal links in the form of a string. The suspension insulator hangs from the cross- arm of the supporting structure and the line conductor is attached to its lower end. Because there is no pin problem, we can put any distance between the cross-arm and the conductor just by adding more insulators to the “string”.
The entire unit of suspension insulators in called a string. How many insulators the string consists of depends upon the working voltage, the weather conditions, the type of transmission construction, and the size of insulator used. It is worth noting that in a string of suspension insulators one or more insulators can be replaced without replacing the whole string.
The main advantages of suspension type insulators over pin type ones are enumerated below:
(i) Suspension type insulators are usually cheaper in cost for operating voltages above 50,000 volts.
(ii) Each unit of suspension type insulators is designed for comparatively low voltage (about 11,000 volts) and can be used by connecting them in series, the number depending upon the working voltage.
(iii) In the event of failure of an insulator, one unit, instead of the whole string, can be replaced.
(iv) Suspension type insulators give more flexibility to the line and mechanical stresses are reduced in this arrangement. The connection at the cross-arm is such that the insulator string is free to swing in any direction, and thus takes up a position where it experiences only a pure tensile stress.
(v) The suspension type insulators, when used in conjunction with steel supporting structures, has the advantage of rendering the conductor less liable to be affected by lightning disturbances. At every point of support the wire is hung below the earthed cross-arm, thus enabling the tower to function as a lightning rod.
(vi) In case of rapid increase in load on transmission line, the increased demand can be met by raising the line voltage than to provide another set of conductors. With suspension type insulators, additional line insulation required can be obtained easily by adding one or more discs to the string.
(vii) In case of long spans (river or valley crossings) where heavy conductor load is to be sustained, two disc insulator strings can be yoked. Such an arrangement is not possible with pin type insulators.
The disadvantage of suspension type insulators is that large spacings between conductors are required than with pin type insulators due to large amplitude of the swing of the conductors, but this is not a serious disadvantage.
There are three types of suspension insulators generally used namely, Hewlett or interlinking type, cemented-cap type and core and link type.
(a) Hewlett or Interlinking Type Suspension Insulators:
The Hewlett suspension insulator is shown in Fig. 9.6 and is one of the earliest designs. Each disc consists only of one piece of porcelain, the central bulbous portion of which is provided with two curved tunnels lying in planes at right angles to each other. The short steel strips forming the connection between individual discs are threaded through these tunnels and thus loop through each other, being separated by a layer of porcelain which is totally in compression.
The main advantages of Hewlett type suspension insulators are:
(i) Simple in design.
(ii) High mechanical strength since the porcelain in between the two tunnels is under compression only.
(iii) No risk of breakage owing to the difference in expansion or contraction of the connecting links and the insulating material.
(iv) No risk of interruption to the service in case the porcelain between the links gets accidently broken, since the links keep the other units held together.
However, the Hewlett insulator appears to be rather more liable to puncture than other types of suspension insulators, owing to the high electrostatic stress in the material between the links.
(b) Cemented-Cap Type Suspension Insulators:
Cemented- cap type suspension insulator shown in Fig. 9.7 is the most commonly used type and consists of a single disc-shaped piece of porcelain grooved on the under surface to increase the surface leakage path, and to a metal cap at the top, and to a metal pin underneath. The cap is recessed so as to take the pin of another unit and thus build up a string of any number of units.
The cap is secured to the insulator by means of cement. A very uniform distribution of the electrostatic stress in the material between the connecting links is obtained in such insulators.
The main drawback of this type of insulators has been that coefficients of cubical expansion of the three materials-porcelain, cement and steel are different and no provision is made for their expansion and the sudden temperature changes occurring in service are sufficient to set up internal stresses which ultimately crack the porcelain, leading to electrical failure.
Furthermore, the cement itself, which is subject to volumetric changes depending on its moisture content, has often materially assisted in the process of failure of the insulator. The insulator manufacturers recognised these causes of ultimate failure of insulators and improved their designs from the point of view of reliability of service.
One way of improving the design is by way of substituting the cementing of the pin by purely mechanical fixing, such as the “spring-ring”. In this method of fixing a spiral spring ring of steel wire carried on the stem of the pin is forced into the interior of the insulator head which is of bulbous shape. The ring immediately expands and is locked in position, the interior being then filled with a lead alloy to prevent any movement of the various parts and protect the fittings from the weather.
(c) Core and Link Type Suspension Insulators:
Core and link type suspension insulator is illustrated in Fig. 9.8. Such an arrangement combines the advantages of both of the above two types of insulators and overcome their disadvantages.
In such a construction, each insulator disc is symmetrically placed and it conforms to the electrostatic lines of force, thus avoiding placing materials of different permittivity’s in series. The metal work consists of pressed steel spiders, the legs of which are fastened into the porcelain by an alloy having approximately the same coefficient of cubical expansion as the porcelain.
Thus high mechanical stresses on the porcelain, whether due to sudden temperature variations or to the employment of cement are completely eliminated. This is the recent type of construction and it allows discs to be formed out of quite thick porcelain thereby allowing the disc to be of one piece only. Added to the other advantages such an insulator has high puncture strength.
Type # 3. Strain Insulators:
Where there is a dead end of the line, or there is a corner or a sharp curve, or the line crosses river etc., the line is subjected to greater tension. Pin type insulators cannot be used in such situations because they cannot take conductor load in tension which often occurs in such situations.
For low-voltage lines (say, up to 11,000 V), shackle insulators can be used, but for higher voltage transmission lines strain insulators consisting of an assembly of suspension type insulators are used, as illustrated in Figs. 9.9 and 9.10.
Because of its peculiarly important job, a strain insulator must have considerable strength as well as necessary dielectric properties. Where the tension is exceedingly high, as at long river spans, two, three, or even four strings of insulators in parallel are used. The discs of strain insulators are employed in vertical plane whereas the suspension insulators are used in horizontal plane.
Type # 4. Shackle Insulators:
The shackle insulators or spool insulators are almost universally used on low voltage lines and provide a very neat, efficient and economical arrangement. Its construction is shown is Fig. 9.11. Every insulator is coated with an extremely hard, smooth glaze that reduces accumulation of surface deposits. The surface can be easily cleaned and it will not crack when subjected to temperature changes.
The wet flash-over and dry flash-over voltages for shackle insulators are 10 kV and 25 kV respectively while the puncture voltage is about 35 kV. Its operating voltage is 1,000 V. Its weight, transverse mechanical load and total creepage distance are 0.5 kg, 1,150 kg and 63 mm respectively. The tapered hole in the shackle insulator distributes the load more evenly and reduces the possibility of breakage when heavily loaded.
Shackle insulators may either be mounted horizontally or vertically, and the conductors are fixed in the grooves by means of soft copper or aluminium binding wire according to the conductor material. They can be directly fixed to the pole with a bolt or to the cross-arm.
The insulators are bell-mounted to prevent water being held in contact with the spindle. This type of insulator is used at all positions, either intermediate, terminal or angle. Where the angle exceeds 60° deviation they are generally used in conjunction with shackle straps.
Type # 5. Stay Insulators:
Such insulators are of egg shape, also called strain or guy insulators, and are used in guy cables, where it is necessary to insulate the lower part of the guy cable from the pole for the safety of people and animals on the ground. This type of insulator consists of a porcelain piece pierced with two holes at right angles to each other through which two ends of the guy wires are looped.
This is illustrated in Fig. 9.14. This keeps the porcelain between the two loops in compression and the guy wire remains in position even if the insulator breaks due to any reason. These insulators are provided at a height of about 3 m from the ground level. The size of insulator (small or large) used depends upon the tensile strength of stay wire.
