AMAZON multi-meters discounts AMAZON oscilloscope discounts
...
--1 INTRODUCTION
Before any design or planning work on an overhead line is contemplated the national and regional authorities must first be consulted in order to ensure no statutory regulations are being contravened with regard to:
- _ safety factors on supporting structures and conductors;
- _ maximum conductor working temperatures;
- _ clearances between accessible ground and conductors;
- _ minimum separation between the overhead line and railways, telegraph lines and pipelines;
- _ planning permissions.
The logical sequence for the design and planning of the routing of an overhead line is shown in FIG. 1. It is assumed in this example that the client, or his consultant, carries out the preliminary routing and includes this information in a tender specification such that competitive tenders may be received from a variety of design and construct contracting organizations.
The contractor will then carry out the detailed line routing and profile work.
By careful preliminary routing the effect on the environment may be minimized. In addition, the client is in a position to narrow the choice of the tower and span design to the most economic. At the same time the client is able to take into account a strategy for minimum maintenance costs.
===
Power demand location (B) PWRR and optimum line casting Tower spotting optimization Final tower and foundation design Try alternative conductors Try alternative voltages Reconsider type of conductor originally selected Review original conductor tension choice Ruling span optimization Foundation designs Soil survey Preliminary tower configuration Tower top layout Shield wire selection and location Flashover risk analysis Operational and maintenance constraints NATIONAL REGULATIONS Statutory clearances Finalize conductor selection Insulation selection and coordination Voltage drop and loss calculations Loading conditions Select only from standard conductors Preliminary conductor selection Optimize conductor selection Preliminary ruling span Family of towers Profile analysis Ground profile survey Route optimization Environmental impact studies Preliminary route selection Environmental constraints Climatic conditions Select only from standard voltages Voltage selection Optimize voltage selection [A] SOURCE OF POWER SUPPLY Decision to build transmission line from [A] to [B] External influences System planning Electrical Engineering influences Civil/ structural Survey/ geological/ geographical
FIG. 1 Logical sequence for overhead line design, planning and routing.
===
--2 ROUTING OBJECTIVES
The preliminary routing work determines the physical constraints involved and allows the establishment of the least-cost solution for the overhead line.
Estimated quantities for the towers, foundations and conductors may then be included in tender documentation for the supply and/or erection of the overhead line.
The detailed routing survey and profile allows the towers to be located in the most economic manner. It will take into account proximity restrictions and maintenance of specified design parameters such as electrical clearances, wind spans, angles of deviation, etc.
--3 PRELIMINARY ROUTING
--3.1 Survey Equipment Requirements
1. Good maps for the expected line route and adjacent areas to a suitable scale; typically 1:10,000 for cross-country work.
2. Good survey quality compass and compass bearing monocular. These greatly assist orientation, spotting tower positions and suitable road or river crossing locations, and in the production of cross-section projections along the route.
3. Theodolite and level may be worthwhile but they are not essential for the preliminary survey.
4. 100 m tapes.
5. Hammers, identification paint and pegs.
6. Ranging rods to enable checks and recording of location relative to centers of proposed angle towers.
--3.2 Aerial Survey
Simple aerial survey photographs, or if available, satellite photographs, greatly aid the routing designer and reduce the amount of time taken for the ground survey. The proposed route is indicated on the photographs in conjunction with maps of the area if these are available. However, corridor mapping can now be provided as a complete service using LiDAR (light detection and ranging) in conjunction with GPS (geographical positioning system) satellite systems. Specialist contractors can provide CAD (computer aided design) drawings of the full route plan and profile, on the basis of a helicopter overflight of the planned route (see Section 4.3).
--3.3 Ground Survey
The exact route may differ considerably from that proposed by studying maps and aerial photographs. This is because of the difficulty in obtaining wayleaves, overcoming local planning requirements and ensuring that any specific local landowner requirements that may be accommodated are considered.
FIG. 2. 400-kV single circuit twin conductor overhead line crossing the Zagros
Mountains in Iran
--3.4 Ground Soil Conditions
It may be possible to route the overhead line such that the chosen ground conditions favor low foundation costs. However, in practice huge savings are unlikely since considerable deviations are likely to be necessary and this will in turn increase the overall materials cost of the overhead line route.
--3.5 Access and Terrain
FIG. 2 shows an example of a 400 kV single circuit overhead line crossing extremely mountainous terrain in the Zagros mountains of Iran between Reza Shah Kabir Dam and Arak. Overhead lines often cover areas without good communications access. In such cases the construction of overhead lines is greatly assisted by the use of helicopters. FIG. 3 shows helicopter assisted conductor stringing for the China Light and Power Company in Hong Kong.
FIG. 3 Helicopter assisted conductor stringing _ Hong Kong
--3.6 Optimization
--3.6.1 Practical Routing Considerations
The sending and receiving ends of the transmission line from existing or future substations or tee-off points are first established and are usually well defined.
The straight line between these two points must then be investigated to see if this really represents the cheapest solution. In practice, wayleave availability (refer also to SECTION 3, Section 3.2.7), access, ground conditions, avoidance of populated or high atmospheric pollution coastal or industrial areas, difficulties for tower erection and maintenance almost always require deviations from the straight line option. Further economic considerations involving parameters which are difficult to equate in purely financial terms such as impacts of the line on the environment must be considered. If a 400 kV tower costs approximately d 100,000 to design and detail then the use of a limited number of standard designs may well prove cheaper than having a large number of special tower types necessary to achieve the more direct line route.
Lines should not be routed parallel to pipelines or other similar services for long distances because of possible induced current effects. Where this can not be avoided minimum distances of, say, 10 m should be maintained between the vertical projection of the outer phase conductor of a 145 kV over head line and the pipeline. Similarly, precautions should be taken with regard to proximity to gas relief valves or hydrants. Gaz de France sets threshold levels of maximum AC-induced currents in pipelines at 100 A/m^2.Corrosion effects from AC should be negligible because of the current reversal but research shows a small polarizing effect which could lead to corrosion in the very long term. Oil companies also require minimum clearances between over head line counterpoise (if installed) and buried steel pipes of, say, 3 m for the first 5 kA of earth fault current plus 0.5 m for each additional kA.
--3.6.2 Methodology
Once the terminal points for the line have been established, they are linked on the maps avoiding the areas mentioned in Section 3.6.1. Angle or section towers may be provided near the terminal points in order to allow some flexibility for substation entry and slack spans or changes to the future sub station orientation and layout.
The proposed route is then investigated by walking or driving along the whole of the route. Permissions to cross private property must first be obtained from the landowner. The purpose of this thorough investigation is to ensure that the route is feasible and what benefits could accrue from possible changes.
The feasible preliminary route is then plotted on the maps to at least 1:10,000 scale. The approximate quantities of different tower types (suspension, 30_ angle towers, 60_ angle towers, terminal towers, etc.), conductor and earth wire are established. Suspension towers will often account for more than 80% of the total number of towers required on the overhead line route and quantities must be optimized and accurately assessed. Ground conditions are recorded during the field trip in order to estimate the different tower foundations required (piled _ and, if necessary, is access for a piling rig possible? _ screw anchor, 'normal', rock, etc.). At the same time, an estimate of the difficulties likely to be encountered in obtaining the required tower footing resistance and the need for a counterpoise, tower earth rods, etc., is made.
The cost of the line is proportional to the tower steel and foundation loads. The tower weight, , may be approximated from Ryles formula:
5 U U
????? p where tower weight of steel (kg) constant tower overall height (m) tower overturning moment under maximum loading conditions at ground level (kg m)
Tower heights and their overturning moment are established for a variety of basic spans. This then allows the engineer to concentrate on the total number of intermediate towers required. Such an iterative procedure is, of course, entirely suited to computer analysis. However, the computer will place towers in inconvenient or impossible locations without the knowledge resulting from the field survey described. The costs of suspension insulator strings, fittings and foundations are then added to the estimated number of towers in order to derive the basic span and the first approximation to the cheapest overhead line routing solution.
A constant ratio is applied to each basic span in order to obtain the wind span. This constant (typically 1.13) is necessary to allow for some flexibility over uneven ground. A factor of 23 over the basic span may be used as a guide to the weight span (which does not greatly affect tower design) and this will allow for tower spotting and wind spans to be optimized.
The average span is the basic span multiplied by an efficiency factor which takes into account the nature of the ground and varying span lengths envisaged from flat to hilly terrain. The estimated quantities for materials may be derived from the average span. Finally, the technical specifications for the overhead line are drawn up for use in tender documentation.
--4 DETAILED LINE SURVEY AND PROFILE
--4.1 Accuracy Requirements
The objective is to draw up a plan and section so that further refinement of the tower distribution may be made. The party carrying out this work will depend upon the type of contract being let by the electricity supply authority in charge of the works. An explanation of the suitability of different types of contract for different areas of transmission and distribution work is explained in SECTION 22.
The required accuracy should be to 60.5 m in the horizontal plane and 60.1 m in the vertical plane. Greater accuracy is possible from survey data but in practice cannot always be easily transferred to the profile (but see comment in Section 3.2; LiDAR survey claims 'centimeter precision' and can be provided in a CAD form compatible with profiling). The location of angle and terminal towers is best specified in a contract document rather than allow a complete free hand to the overhead line contractor. This is because access may be an important parameter for the electricity supply authority if maintenance costs for the line are to be kept down.
The vertical profile ground line is surveyed from one angle or terminal tower to the next. When national maps of good quality are available the vertical survey data may be cross-referenced to bench marks of a known level.
Horizontal survey dimensions to tower centerlines are checked against the 1:10,000 map and differences investigated until resolved on site. In hilly terrain side slopes in excess of 60.3 m must be recorded together with all major features (angles of deviation, other power overhead lines or cable routes, underground services, roads, rail, river and pipeline crossings, buildings adjacent to the wayleave, etc.).
--4.2 Profile Requirements
--4.2.1 Vertical and Horizontal Scales
In order to keep the drawings to a manageable size, the detailed survey drawings are scaled to typically 1:200 vertical and 1:2,000 horizontal or as necessary in hilly terrain. On sloping sites it will be necessary to ensure that foundation depths are not compromised and individual tower legs may have to be adjusted to correct to the tower centre profile level. The profiles, whether computer generated or not, should be on graph type paper with a grid background. This greatly eases the reading of span lengths or clearances even when photocopy prints have slight distortions.
--4.2.2 Templates
Traditionally, the design line profile was prepared by hand using a sag tem plate made from perspex (B3 mm thick) with all the engraving on the back using the same scales as the ground profile. A template would show:
_ the maximum sag condition curve (usually at maximum temperature but could be under extreme loading conditions);
_ the minimum sag condition (usually at minimum temperature without ice loading);
_ basic span and cases up to, say, 620% above and below the basic span.
Normally the parabolic approximation will suffice for distribution work unless special long spans or hilly terrain with slopes .15_ are envisaged.
For transmission lines a full catenary calculation may be more appropriate (see Section 4.3). Using the parabolic approximation the tension for any equivalent span is then given by:
More than one technically acceptable solution for tower locations is always available and therefore the final test of acceptability is based on cost.
It is essential that tower fittings, extensions and foundations are taken into account. In addition, clearances must not be infringed.
--4.3 Computer-Aided Techniques
It is now normal practice to use computer aided techniques to prepare the overhead line profile and with modern computer tools the sag/tension relationship may be calculated using full catenary equations. Chainage, level, vertical and horizontal angles may all be transferred directly from a modern theodolite via a portable computer to an office power line survey and computer aided drafting and design (CADD) facility. This eliminates the need for completion by the surveyor of a field record book and any transcription errors that might occur. In addition, some packages allow details of type of ground, ownership, etc., also to be recorded electronically during the survey.
As a further feature such surveys may be linked into co-ordinates derived from geostationary 'geographical information system' satellites (see SECTION 3.2). It has been estimated that power utilities using such systems can achieve survey and data transfer time savings of between 40% and 50% and savings of 50% and nearly 80% for line design involving poles and towers, respectively ( , Vol. 3, March/April 1991).
Once field data has been transferred to the CADD tool the ground line profile may be automatically produced with all the annotations that the surveyor has included in the field. Overhead line structures may be 'spotted' at any point along the profile manually by the engineer or automatically by the computer and strung with any conductor type. The software library containing conductor, pole and tower information will then be used to calculate sag and tension for the given conductor, uplift forces on any structures and ensure ground clearances are not infringed at a user specified temperature.
Typical computer profiles may be generated using OSLs Software on an IBMs compatible personal computer workstation. This particular package also allows 3-D representation to detail such items as terminal tower down loads and jumper clearances. (OSL is a trademark of Optimal Software Ltd and produces a suite of programs including Powerline, PowerCad, PowerSite, Towerline, TowerCad, Towerlog, TowerSite, PoleLine, PoleCad, PoleLog and PoleSite. Optimal Software is an Integraph Third Party Software Partner. IBM is a trademark of International Business Machines. Intel is a trademark of Intel Corporation.) Another suitable program is PLS-CADD.
.