Con base en esto, di: ¿qué fracción representa la parte blanca del cubo?

The earlier sections for this module have addressed the consent requirements and the physical restrictions on the construction of an overhead power line. The technical constraints or the main items to consider are:

• The electrical voltage and current (load) requirements of the proposed overhead line

• The mechanical strength capabilities of the line components (conductor, fittings, steelwork and structures) used to meet with the environmental loads (wind, ice and snow)

• The components’ reliability and longevity in the field and the techniques or methodologies adopted for the construction and maintenance of the overhead line.

Each of these will be addressed in more detail in later modules. Most utilities will have standard tower arrangements for various voltage levels and conductor sizes. Figures 2.3 in section 2.2.4.3 on special towers were sketches of two 400 kV quad 0.4 sq inches Aluminium Conductor Steel Reinforced (ACSR) conductor tower support structures. Module 3 gives some further examples of steel support structures and wood-pole support structures at lower voltages. The examples in figures 2.3 are suspension towers. A different support structure is required called the tension tower.

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2.3.1 Section Lengths

The length of a section in an overhead power line is that continuous part of the conductors in suspension between two tension structures. Figure 2.4 is a photograph denoting a tension tower, two suspension towers and the meaning of a span.

Tension Tower

1 Span between Suspension Towers

Fig.2.4 Illustrating Tension Towers, Suspension Towers and Spans

Tension structures are required for one or more of the following possible reasons: -

• Wherever a line changes direction

• Where “uplift” of the suspension insulators would occur due to the tower being sited on relatively low ground in a valley and where excessive extensions would otherwise be required to overcome the problem

• Where the length of a straight section of suspension structures would exceed the design parameters of the line for construction and maintenance e.g. the maximum continuous length of a conductor that can be handled during these operations

• At points where the maintenance requirements necessitate the dropping of conductors where one line crosses over another and security for the lower circuits is required.

Consequently, section lengths are determined by the engineering requirements of line design and operation, by changes in direction caused by physical obstructions, and the availability of suitable sites for the structures.

It is one of the objectives of economic line planning to ensure that the minimum number of supporting structures is used. Each additional supporting structure adds to the cost of the line, may cause additional interference to the land on which it is sited, and can be considered an added visual intrusion into the area.

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The distance between line supports will vary with: -

™ The type of construction employed

™ The profile of the ground over which the line will pass ™ The cross-sectional area of the conductors being supported

™ The need to maintain the electrical safety clearances required from the

ground and from any buildings or other obstructions

™ Additional factors in design limitation concerned with the “sum” of adjacent

spans.

The latter limits the total length of any two adjacent spans to the design parameters of the type of line being constructed, e.g. referring again to figure 2.3 the normal individual span of the L6 line type is 366 metres.

Thus, by careful selection of the positions of tension towers, the use of extensions on steel tower lines where available and the avoidance of obstructions where practicable, a line can be designed where the sections provide the optimum run of conductor length supported by the minimum number of structures. Any larger steel tower lines, the stringing (erection) of conductors is a major operation and it is necessary to have the conductor drums at angle positions together with winches and other miscellaneous gear. The accessibility to those positions must be considered in an effort to reduce the problems of transporting heavy loads into a site. Additionally, the heavier angle towers are not as acceptable in the landscape as the lighter suspension towers and their siting should take into account the requirements of amenity as well as accessibility.

Increasing the height of any particular support in a steel tower line generally has a minimal impact overall and so consideration should be given during the planning stages to the inclusion of extensions on certain towers if this would avoid adding an additional tower into the section with its attendant constructional and amenity problems

The application for a toleranceprovision when it is submitted to the appropriate government department enables the constructing electricity company to adjust the line slightly in order to meet any last-minute wishes of the landowner and occupier’s affected subsequent to the original consent being granted and after a detailed survey has been carried out.

Tolerance provision – this is the UK term – other countries have similar procedures.

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2.3.2 Re-alignment of Route to Avoid Obstacles

Care has to be taken when planning the route for a proposal, to ensure that a situation does not arise where it becomes necessary to use a series of angle structures to deviate around an obstacle. This is both costly and is usually objectionable on the grounds of amenity.

In the case of wooden-pole lines, this inevitable leads to the need for stays which are an additional interference with uses of the land.

It is preferable to re-align the route in the early stages of planning in order to avoid the obstacle (Figure 2.5 illustrates this)