L OS NÚMEROS RACIONALES

There are two main categories of alternative routes for overhead lines that require investigation during the planning stages: -

ƒ Alternatives which are investigated by the electricity company in the normal course of selective route planning, using the same criteria for assessment as those used for the selection of the preferred route for which it is proposed to seek consent.

ƒ Alternatives suggested by objectors to the preferred route which have not necessarily been subjected to the same criteria as those applied to the preferred route. These must be investigated and assessed in the same manner as the electricity company’s preferred proposal and a reasoned statement made about their suitability or otherwise based on engineering, wayleaving (where appropriate) and amenity considerations In the planning of routes for the larger transmission lines, where considerable distances can be involved, it is not generally feasible to obtain consents along both the preferred route and also any alternatives being considered. The case in the UK is that outline consent is usually sought from the individual landowners until they are all obtained and then final consent being obtained from the government. In the planning of short routes the situation is different in that it is more normal to obtain consents for all its length before requesting consent from the appropriate government department.

2.3.8.1 Technical Feasibility

The alternatives(s) must achieve the same standards as the electricity company’s preferred proposal in respect of the quantity, quality and security of supply.

2.3.8.2 Cost

If its is to be considered seriously, the cost of any alternative must follow the general level of costs incumbent on the electricity company’s preferred proposal. A thorough appraisal of the number and types of structures to be used on each route is required in order to compare costs. This requires a ground survey on the alternative routes unless an aerial survey has already been flown which covers this (Refer also to Module 3). When comparing the costs of alternatives, consideration has to be given to the vulnerability of such a route to undergrounding either because of a physical constraint such as an airfield or for amenity reasons. Any additional problems associated with access for construction and maintenance should be calculated and included, together with the extra construction costs caused by the need to pile drive foundations or excavate in rock where this is required.

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2.3.8.3 Amenity

Any assessment of the effect on amenity will include the following comparisons: Visibility overall and prominence of the alternative routes

1. The values of the countryside along each route 2. The amount of tree felling if required

3. The effect on existing properties and their environment

2.3.8.4 Future Known Developments

These will include considerations of the following 1. Urban development.

2. Road proposals.

3. Mineral working areas.

4. Changes in land designation e.g. land set aside for a future new town or amenity park.

5. Future proposals in the area to be made by the electricity company

2.3.8.5 Agriculture

The overall effect of the number of structures per kilometre of line upon agricultural operations should be considered particularly where aerial crop spraying is adopted. It may be useful to discuss the general merits of alternative routes with representatives of the appropriate government ministries when they are broadly similar in all other respects.

Where alternative routes cover wide areas, it is sometimes convenient to fly along them in a helicopter to eliminate, where possible, those routes that have hidden physical constraints, such as properties in woodland, which are not visible from the surrounding roads. The actual selection of the final route should be done on the ground where the contours are more readily apparent and the effect of the proposal on the countryside as it will be seen by people can be assessed. Aerial and Ground surveys are addressed later in this module. (Refer to section 2.5)

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2.4. Types of Objections to be Considered when Planning Routes

The greatest visual impact of any overhead line is caused by the supports, whether steel or wood pole, and the associated insulators. The visual impact of wood pole lines is over a very limited area and they are generally capable of being assimilated into the landscape without becoming a major intrusion. The impact increases, however, when a number of wood pole lines are sited close together or when there is associated top-hamper such as pole-mounted transformers and cable sealing ends.

The larger steel tower lines, however, are generally out of scale with the intimate countryside covering most of the landscape of countries in Europe and elsewhere where there is a large amount of fauna. The heights of indigenous trees are rarely as tall as main transmission towers. Unfortunately, the parts of the tower that cause the greatest visual impact are the cross-arms and the insulators that are by necessity located in the upper part of the tower. They cannot thus be screened effectively. The proportions and design of the upper sections of steel towers can only vary within the engineering standards prescribed and the line planner must therefore concentrate on reducing the number of towers with multiple insulators and heavy cross-arms to a minimum. (Refer also to Module 3)

In Europe, experiments have been made by painting steel towers with other than “battleship grey” in an effort to reduce their effect on the landscape. However, the changing seasonal colours render this exercise somewhat futile and the general consensus is that the weathered galvanising or grey paint is the best compromise.

In the UK the Electricity Supply Industry has discussed new tower designs with the Royal Fine Arts Commission and, over the years, this has helped to produce such improvements in design as waisted or eiffelised towers with a more pleasing appearance.

Porcelain, toughened glass and polymeric/resin (composite) are the three insulating materials used on overhead lines. Variations in the colour have been employed in a number of countries. Porcelain insulators have been given a silver- grey finish instead of the more normal brown colour and insulators made of green instead of clear glass have been employed on some instances. There is a tendency, however, for glass insulators to reflect in sunshine and become very much more visible.

The number of strings of insulators on some suspension towers is reduced from the normal two to one per phase by making a slight overall increase in the size and load bearing capacity of the remaining single string.

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The increase in the physical size of lines necessitated by the increases in operating capacity has been of considerable benefit. The multiplicity of smaller lines required to provide the same power transfers at lower voltages would have resulted in an unacceptable level of “wirescape” and interference with land use. Many objections to towers and their attendant insulators can usually be overcome by a minor re-positioning along the line and an intelligent appreciation of the surrounding environment.

2.4.2 Conductors

To increase power flow through each route accomplished, both the line voltage and the number of conductors need to be increased. At 400 kV quadruple conductors in each phase are normally employed. The four conductors are joined at intervals along the span by spacers to maintain conductor separation in all conditions. Some 400 kV lines employ double conductors again with spacers. Both these arrangements keep corona discharge within acceptable limits it being inversely proportional to the effective emitting area. (Refer also to Module 5) at the same time as increasing the capacity.

However, this quad and twin type construction also increases the prominence pf the line spans, although at a distance, the twin and quadruple conductor formations merge to become one entity visually. In order to reduce the visual impact of the quadruple conductor formations, conductors were developed with larger cross-sectional areas for use in twin formation per phase on the heavy duty 400 kV lines.

When planning the route profile, the design engineer should, wherever possible, endeavour to provide a sympathetic match of the conductor catenary to the physiographic qualities of the area.