Design of Overhead Transmission Line

The subject related to the design of overhead transmission lines is a complicated subject. There is a number of factors to be considered during the designing of a transmission line. We have discussed here in this article about some of the important aspects in brief. Video

Selection of Voltage Level of a Transmission Line

The transmission voltage level depends on different factors. Among them, the main factor is the expenditure on the construction of the line. In addition to that, the length of the line and the power carrying capacity of the line are two other main factors. Considering all the factors there is a formula for selecting the voltage level of a transmission line established on the basis of experience.

Where L if the length of the line in kilometer.

Overhead Transmission Line Support Towers

What kind of support will be used for a transmission line, depends on the voltage level importance, the topography of route and finally on the expenditure of the line. Up to 33kv overhead line, normally poles are used for the purpose. From 66kv onward supporting towers are used for the purpose.

For 33kv overhead lines, rail poles and PCC poles are used. The general height of the poles is from 9 m to 9.5 m. But in some special cases like on high road crossings and canal crossings, longer poles are used. In those cases, by connecting extension channels on the top of the pole, the overall height of the pole is made increased.

For 11KV overhead lines, 9 m high poles are used. 7.5 m to 8 m long PCC poles are also used for the purpose. In some congested places, 9 m long tubular steel poles are also used for the purpose.

Selection of Height of Supporting Structures

The minimum height of a supporting structure depends on six major factors.

  1. The voltage level of the transmission line,
  2. The weight of conductors, along with allowable sag,
  3. The ground clearance of the bottom conductor,
  4. The vertical distance between the top and bottom conductors,
  5. The vertical distance between the pole tip and the top cross-arm,
  6. The type of insulators used.

If pin insulators are used then the height of the pin insulator is subtracted from the total height calculated from the factors 1to 5.

If suspension disc insulator strings are used then the length of the string is added to the total height calculated from the factors 1 to 5.

The height of the pole (H) = sag + ground clearance of the bottom conductor + the distance between the top and the bottom conductors + the distance between the pole tip and the top cross-arm – the height of the pin insulator.

Normally one-sixth portion of the pole is inserted in the ground soil. Therefore

Where L is the total length of the supporting pole.

overhead supporting pole height

 

The height of a tower (H) = sag + ground clearance of the bottom conductor + the distance between the top and the bottom conductors + the distance between the tower tip and the top cross-arm + the length of the suspension disc insulator string.

tower height

Voltage Level Normal Span Support Height Sag
415/230V 45 – 65 m 7.5 – 9.0 m 0.3 – 0.6 m
11KV 75 – 105 m 8.5 – 9.5 m 0.4 – 0.8 m
33KV 90 – 125 m 9.0 – 11.0 m 0.6 – 1.2 m
132KV (SC) 305 – 365 m 20.0 – 25.0 m 6.0 – 9.0 m
132KV (DC) 305 – 380 m 25.0 – 30.0 m 6.0 – 9.0 m
220KV (SC) 320 – 380 m 20.0 – 30.0 m 7.0 – 9.0 m
220KV (DC) 320 – 380 m 30.0 – 35.0 m 7.0 – 9.0 m
400KV (SC) 350 – 420 m 30.0 – 35.0 m 8.0 – 10.0 m

Sag and Span

span and sag

When a conductor is strung between two supporting structures the middle portion of the conductor comes down. The vertical distance between the straight line connecting the two fixing points of the conductor and the lowest point of the conductor is called the sag. When the fixing points are at the same level, the lowest point of the conductor is in the middle of the span. The span means the distance between two consecutive supporting structures or towers. The span, sag, and height of supporting structures are related to each other. For a transmission line with lengthy spans has fewer towers, at the same time the sag of the conductors is increased therefore the towers have to be taller to compensate for the extra sag.

Conductors

The current carrying capacity of the conductor depends on the line voltage, conductivity of conductors and power factor of the load. The current carrying capacity of a conductor is the current which the conductor can carry without any permanent deformation and damage. The temperature of the conductor increases with its increasing current. An aluminum conductor can carry current without any permanent damage up to its temperature 75 degrees centigrade.

During the selection of an overhead conductor, in addition to its current carrying capacity and many other factors have to be considered. One main factor among them is the tensile stress that is imposed on the conductor after stringing it on the supporting structures. The tension acting on the conductor is due to the weight of the conductor and the pressure of the wind. If the span of the line is more, the tension acting on the conductor is also more. The tension on the conductor also varies with the variation of temperature. At lower temperatures, the tension is increased and at higher temperatures, it is reduced. At the lowest ambient temperature, and with the highest wind pressure, the tension acting on a conductor is maximum.

As per standard, the highest possible tensile stress withstand-capacity of an overhead conductor must be more than 350 kg. Although for LT line no, the span length is not more than 15 m and in that case, the maximum tensile stress withstand-capacity of the conductor is kept more than 150 kg. The safety factor of an overhead line must be kept at 2.5. That means after calculating the actual tensile stress of the conductor at extreme conditions means with the lowest possible ambient temperature and highest possible wind pressure, the value must be multiplied with 2.5 to get the maximum tensile stress of the conductor.

When a current flows through a conductor there is some voltage drop occurred across the conductor. The ratio of this fall of voltage to the full load voltage is called voltage regulation of the line. As per the standard, the allowed maximum voltage regulation of low voltage and medium voltage lines is 6%. For the high voltage lines, it is +6 to – 9%. For ultra high voltage transmission lines it is + 10% to – 12.5%. The voltage regulation also depends on the size of conductors of the line, length of the line, and the mutual distance between the conductors and the power factor of the load.

The line loss also depends on the size of conductors and the line length. The conductor size must be such that the line loss should not cross 10% of the load-carrying capacity of the line.

Insulators

For 33kV overhead lines, pin insulators are used up to 10 degrees diversion. For the angle of diversion from 10 degrees to 30 degrees, double-cross arm with double pin insulators are used. Otherwise, disc insulators should be used for the purpose. When the angle of diversion for a 33kv line is more than 30 degrees only disc insulators are used.

For 132kV and higher voltage lines, suspension type insulator strings are used at the tangent towers where the angle of diversion is from 0 to 2 degrees. At all other towers, i.e. angle towers and terminal towers strain insulator strings or tension insulator strings are used. The tensile strength of a suspension string must be more than 50% of the extreme tensile strength of the transmission conductor. The tensile strength of a tension string must be more than 90% of the extreme tensile strength of the transmission conductor.

Conductor Arrangement of Overhead Lines

The configuration of conductors of an overhead line may be either vertical, horizontal of triangular shape. For a 132kv single circuit overhead transmission line, the configuration of conductors should be of a delta shape. But for 220 KV or 400 KV single circuit transmission lines, the configuration of conductors is horizontal. For double circuit transmission overhead lines, the conductors are arranged in the vertical configuration.

Conductor Spacing of Overhead Lines

The spacing between two conductors of an overhead line not only depends on the line voltage. This spacing is decided also on the basis of span lengths, the configuration of conductors, the weight of the conductors, the maximum wind pressure and many more. The type of insulator used for the line is also a factor for deciding the spacing of conductors. The spacing of conductors for string insulators is always higher than that for pin insulators. Because for suspension type string insulators, the space for swinging the strings with conductors is to be provided. It is normal practice to consider the maximum swing of a suspension string is 45°. The required conductor spacing is also calculated depending on this configuration.

Sag and Tension

The sag of a conductor depends on four factors.

  1. The weight of the conductor
  2. The span length
  3. The tensile strength of the conductor
  4. The temperature

The sag of a conductor at a certain temperature and in the standstill wind can be formulated as follows.

There is an additional tension due to the wind pressure on the conductor. The tension is maximum when the wind strikes perpendicularly on the conductor.

The force acting on the conductor due to the wind is perpendicular to the gravitational force acting on the conductor due to its weight.

The sag is directly proportional to the square of the span length. Again the sag is inversely proportional to the tension of the conductor.

Due to a change in temperature, the length of the conductor is also changed. If the temperature is increased the length of the conductor is also increased. Because of that, the sag of the conductor is also increased. As a result, the tension on the conductor decreases.

Inversely when the temperature of the conductor decreases the sag is also decreased. Consequently, the tension on the conductor is increased. Therefore it is seen when the temperature is minimum and the wind pressure is maximum the tensile force acting on the conductor is maximum. The maximum tension that practically can act on a conductor must be less than 50% of the tensile strength capacity of the conductor. This is standard practice. During the maximum temperature condition and with zero wind pressure the sag of the conductor is maximum. The maximum sag must be such that the ground clearance of the bottom-most conductor of the line should be maintained. During stringing of the conductors the ambient temperature and the wind are not at their extreme maximum or minimum conditions. That is why it is mandatory to measure the ambient temperature during the stringing work. The sag and tension of the conductor must be maintained according to the ambient temperature during the work.

 

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