The layout design process involves the selection of a suite of structure types, the location of these structures on a line corridor, the profiling of the conductors and the calculation of wind, weight and ruling spans. The layout design shall ensure the following outcomes are met:
• Acceptable electrical clearances to structures and ground for the voltage of line
• Maximum adjacent span ratio selected to ensure longitudinal loading on insulators and supports do not cause failures under adverse environmental conditions. The adjacent span ratio is typically less than 3:1 (where there is free movement of conductors on insulators) or 2:1 (where there is no free movement of conductors on insulators)
• Weight to wind span ratio greater than 0.7 to ensure there are acceptable electrical clearances on structures under wind conditions
• Acceptable clearance of structures and conductors alignment to objects (eg buildings, swimming pools, billboards)
• Set back on roads appropriate to the speed of the road. These set backs can be reduced where there are kerbing or natural barriers (drain or mounds)
• Suitable foundation integrity (eg avoid side slopes)
• Co-ordination with other Authorities and Services (Road, Rail, Water, Telecommunication and Aerial Operations)
Designers need to ensure that the ground and environmental conditions are factored into the layout process and need to consider for example the existence of steep slopes, existing and future services, heritage sites, sensitive environmental areas, etc.
Terrain
A 3-dimensional GIS-type (Geographic Information System) terrain model is suggested for its flexibility and compatibility with modern electronic surveying equipment and mapping techniques. Terrain data are normally collected electronically (total station, photogrammetry, lidar, etc.) and are subsequently downloaded into ASCII terrain files. A terrain model normally includes information about the location and type of a large number of terrain or above-terrain points. Above terrain points will be referred to as "obstacle" points. There are two ways to describe an obstacle point.
You can either: 1) describe the obstacle by its height above a ground point and the coordinates of that ground point, or 2) locate the top of the obstacle directly with its own coordinates.
Before generating a terrain, one should decide on broad categories of terrain or obstacle points which have unique requirements. These requirements include minimum code clearances to be met above or to the side of the points as well as symbols to be used to display these points on the final drawings. (See Table 3.7)
Code clearances depend on the voltage of particular conductors.
If a point having the feature code is an obstacle described by its height above the ground, whether to draw a line between that point and the ground or
If a point having the feature code is an aerial obstacle which your wires are allowed to pass under, whether to check vertical clearances both above and below that point. whether a point having the feature code is a ground point that will be used to draw a ground profile or a point that should be by-passed when drawing the ground profile (for example the top of an obstacle), minimum required vertical clearances above (and below for aerial points) points having the feature code and minimum horizontal clearances to the side of these points for the voltages selected
Terrain Model
The XYZ model includes points described by their global coordinates X,Y,and Z.
The PFL model includes points described by their Station (cumulative distance from an arbitrary reference point along the centerline of the line), Offset (lateral distance from the centerline) and elevation, Z.
Also included for each ground or obstacle point are optional surveyor's notes to be displayed on profile or plan views.
An XYZ file can be prepared and edited with a text editor or word processor or it can be created by downloading survey data from an automatic instrument. There are many tools and techniques available for importing and filtering XYZ terrain points data specially for LIDAR data which may contain many millions of points.
Survey Information
The survey requirements for an overhead line design may include:
Formatted: Indent: Left: 18 pt
1. Width of the line corridor to be surveyed (which may be different than the easement width)
2. Contour interval
3. Key features to be surveyed (fences, gates, roads, trees, railway lines, existing services) 4. Land use and limitations / constraints
5. Centreline and line deviations 6. Coordinate system and height datum
Alignment
The alignment (or alignments) of a project need to be defined before any engineering can be performed. In the plan view, the alignments consist of straight line segments between PI points (Points of Inflection). If you start with an XYZ terrain model, the alignments are defined in the plan view by selecting the PI points. This is not required when using a PFL terrain model since the alignment is implied (however, the PFL model is limited to a single alignment).
Once you have at least one alignment defined, you can create: 1) other independent unconnected alignments, 2) alignment branches, or 3) alignment loops.
When you have multiple alignments you can build lines on all of them.
Values for the Maximum Offset for Profile View (MOPV) and the Maximum Offset for Centerline Ground Profile (MOCGP) are to be selected. All ground or obstacle points within the MOPV (measured from the center-line) are displayed with the appropriate symbols in the various profile views, whether on screen or on a sheet of paper. Points outside the MOPV are not displayed in the profile views. In addition, any structure or wire with an offset greater than MOPV will not be shown in the profile view. Once you have an alignment defined on an XYZ terrain model, you can create an equivalent PFL model.
The center-line is defined in the plan view as the collection of straight line segments connecting alignment corners. The center-line ground profile is theoretically the intersection of vertical planes going through the center-line and the ground. However, because the terrain data maybe defined at discrete points within the line corridor, there is a need for rules to define how the profile is displayed on drawings. The ground profile line displayed is a line that joins all ground points within a specified offset from the center-line. That offset (MOCGP), is for two widths. The points are joined in ascending order of stations. For example, if one selects a MOCGP of 3m, then the profile line will pass through all the points within 3m of the center-line.
If there is significant side slope (perpendicular to the line) the line profile may look jagged when it joins points of significantly different elevations on alternate sides of the center-line. If the jaggedness of the profile line is objectionable, one may draw separate side profiles. Or better, one may generate additional interpolated center line and side profile points using a Triangulated Irregular Network (TIN) model of the terrain or by using breaklines.
Triangulating an XYZ terrain
The XYZ terrain model consists of individual points with their coordinates and feature codes . The Triangulated Irregular Network (or TIN) model of the XYZ terrain is a surface made up of triangles having the terrain points at their apexes using Delauney triangles.
Formatted: Bullets and Numbering
The primary advantage of a TIN model over the basic XYZ model is that it is a surface and not a collection of points. That surface can be used to generate accurate center line and side profiles, to find the elevations of arbitrary points or to locate points at the intersection of latticed tower legs or guys with the ground. The TIN surface can be rendered in different colors to give a more realistic display of the ground, including elevations and light incidence. Bitmaps (aerial photographs) can be projected onto it to give an even more realistic appearance of the terrain.
Break Lines
Break lines (or break line segments) can be used to enhance XYZ terrain models. While break lines can be defined and displayed entirely by themselves, they are most useful in conjunction with XYZ terrain points and TIN models.
A break line or break line string consists of break line segments. Each segment is a straight line with known origin and end points. The location of each segment in 3-dimensions is fully known from the global coordinates X, Y and Z of its two end points. Break line segments which have one end in common are said to be part of the same break line string.
Using break lines to describe existing or planned facilities
Surveyors can provide data on portion of a larger terrain described by many thousands of break line segments and an even larger number of XYZ points. Some of the break lines correspond to yet unbuilt but planned road improvements.
PFL Terrain Model
The PFL terrain model requires that the center-line of the power line be defined first. The locations of terrain or obstacle points are then described relative to that center-line. The station of a point is the cumulative distance from an arbitrary reference point on the center-line to the projection of the point on the center-line and its offset is its lateral distance to the center-line.
Positive offsets and positive line angles are defined as follows; If one travels the line in the direction of increasing stations, positive offsets are to the right and positive line angles are clockwise. Prior to the days of electronic surveying and computers, the PFL terrain
representation was used almost exclusively in power line work. Therefore, by tradition, many of the early line design programs used that representation. However the XYZ model is more powerful as it allows the designer to easily change a line route and to move a structure in the plan view without being constrained by the existing center-line.
The data for a ground point in a PFL model include the feature code, an optional label or description, the point station, its offset and elevation, the line angle at the location of the point (if the point is on the center-line) and a zero obstacle height.
For an obstacle described by its height above a ground point, the data include the obstacle feature code, an optional label or description, the station, offset and elevation of the ground point directly below the obstacle, the line angle at the ground point (if on center-line), and the height of the obstacle above the ground.
For an obstacle described by its own coordinates, the data include the obstacle feature code, an optional label or description, the station, offset and elevation of the top of the obstacle, a zero line angle and a zero obstacle height.
Also included for each ground or obstacle point are optional surveyor's notes to be displayed on profile or plan views. Stations in a PFL file should be "true stations". They cannot be "equation stations".
Using scanned raster drawings to create PFL terrain model
There are basically two approaches to building models of existing lines. The better approach is to resurvey the terrain, the structure locations and the positions of the conductors with modern equipment, i.e. to create a XYZ terrain model. A limited and less accurate alternative is to get the locations of terrain, structure and conductor points from existing drawings or from scanned images of these drawings. These drawings can be displayed in the background of the profile view.
Once the drawings are properly positioned in the profile view you need only digitize at locations where you wish to create PFL points.
It is generally not recommended to use existing drawings as templates for building models of older lines because of the potential accumulation of errors at each step of the process. The original survey may have been inaccurate. The nature of the terrain below and in the vicinity of the line may have changed over the years. The as-built locations of the conductor attachment points may not be well reflected by the drawing. The catenary curves showing the positions of the conductors at some temperature may have been based on crude assumptions not reflecting actual sagging conditions and creep effects. These curves may have been drawn with templates not adjusted to the actual ruling spans in the lines. The digitizing process itself, through scaling and clicking on lines of finite thicknesses, will also add errors.
However, there are cases where one would want to quickly build a line model on top of a raster drawing. You should make sure that the scanned drawing clearly shows labeled station and elevation axes, with the station axis ideally labeled with true stations, as well as line angle locations. This can be done before scanning by overwriting the axes with a dark pen. True stations, that is stations measured from a point near the origin of the line can easily be calculated and marked with a pen, if they are not already shown.
XYZ or PFL?
Given the choice of working with an XYZ or a PFL terrain model, the XYZ model is much better.
The alignment can easily be changed on top of an XYZ terrain model. There is no simple way to change the alignment with a PFL terrain model as you do not have the ability to work in the plan view.
With an XYZ model you can better visualize the terrain. A terrain TIN surface can be developed and used for color rendering and the automatic display of contour lines. Maps and raster images can easily be superposed to the plan view. Raster images can be projected onto the TIN surface for realistic 3-d photo rendering of the terrain.
With an XYZ model, you can reference the locations of all your structures to the same coordinate system used for the management of your line (GIS, databases, etc.). You can integrate a computer model with other management tools used by your company.
While we highly recommend the use of the XYZ model over that of the PFL, you should understand that both models are just alternate ways to look at the same 3-dimensional terrain and
alignment information. In fact, you can convert an XYZ model to a PFL model or convert a PFL model to an XYZ model.
Side profiles, clearance lines, prohibited zones and special cost zones
Similar to the center line ground profile, side profiles are defined by an Offset from the center line and an Offset Tolerance. All adjacent points (in order of increasing stations) within the Offset Tolerance distance from the Offset line which are not separated by more than the Maximum Separation will be connected to form a side profile. Side profiles are only shown where there are terrain points within the specified Offset Tolerance.
A required clearance line (or several clearance lines if there are side profiles) can be displayed as a dotted line and dotted spikes above the profile. The line and spikes are displayed for the voltage specified. The clearance line consists of two parts. The first part is the basic ground clearance consisting of copies of the centerline and side profiles shifted upward by a specified value. The second part of the clearance line consists of vertical spikes indicating required vertical clearances above (or below) specific terrain points or objects within the Maximum Offset for Profile View.
Prohibited zones and special cost zones can be defined along an alignment These zones are only taken into account when optimizing the spotting of a line.
Equation stations
Once an alignment is defined, any terrain point has a station (distance along the alignment) and an offset (distance from the center line).
"True station" is defined as the total distance measured from the first P.I. in the alignment to which is added the designated station of that first P.I. The station of the first alignment point can be changed from the default value of zero to any value.
"Equation station" is defined as a relative distance measured either forward or backward along the alignment from an arbitrary point along the alignment. Unlike "True stations", "Equation stations"
are not continuous.
Design Criteria
Design criteria for power lines are often not the same in various countries and in different companies within the same country. These criteria also change over time. However, in spite of differences in particular numerical values, there are many similarities. General design check functions could easily apply to a wide variety of design practices, from very simple requirements for distribution lines to the most highly engineered processes for extra high voltage lines.
Modeling of wire system
One of the most complex parts of a transmission line is the wire system (conductors and ground wires) in a tension section (from one dead end structure to the next dead end structure). Questions arise regarding: 1) the handling of wind load which may not be uniform over the length of the section (wind on individual spans may be larger than the average wind over the section because of varying gust response factors and different wind incidences), 2) the handling of non-uniform ice loads, 3) the handling of the many phenomena that generate longitudinal loads (broken wires,
∑ ∑
= L
L L
3 RS
slack redistribution, etc.), and 4) the possibility of interaction between flexible structures and all wires in the tension section. Therefore, for practical design reasons, approximations and assumptions have to be made.
There are several modeling levels are available to determine the response of the wire system to some loading criteria. These levels are summarized as;
The simplest modeling level is based on the concept of the Ruling Span (RS) and it is sufficient in most cases. The most advanced modeling level (Finite Element) is based on a full structural analysis of the entire tension section, including detailed models of all supporting structures and all cables. Because it is computer time intensive and is not justified in most situations, FE should only be used in special cases where a very accurate representation of the interaction between the structures and the wires needs to be considered. You likely will never have the need for this advanced modeling capability (FE). Between RS and FE, there are some intermediate modeling levels. These are defined herein as Real Span (because it works with actual real lengths of wires in each span) or Finite Element (FE) modeling. The general assumptions used at these different levels are discussed in this section.
Ruling Span method (RS) modeling - Usefulness and practicality of method:
This is by far the most practical method and it is applicable to the overwhelming majority of line design situations. It should be used in all preliminary design situations. This is what you will use most of the time. This method works well with legislated design loads which are generally applied uniformly over a tension section. It should always be used at the preliminary design stage.
Assumptions:
1) The analysis involves a single wire (cable), in one or more spans, between dead ends, i.e. it is assumed that there is no interaction between the wire and other phases of the same electrical circuit or wires in other circuits.
2) The horizontal component of tension along the wire in all the spans of the tension section between dead ends is constant, i.e. all intermediate supports are assumed to be perfectly flexible in the longitudinal direction. This may not be very accurate in the case of rigid post insulators and short suspension insulators subjected to large vertical loads. It is usually considered sufficiently accurate in view of all the other uncertainties and approximations associated with line design.
2) The horizontal component of tension along the wire in all the spans of the tension section between dead ends is constant, i.e. all intermediate supports are assumed to be perfectly flexible in the longitudinal direction. This may not be very accurate in the case of rigid post insulators and short suspension insulators subjected to large vertical loads. It is usually considered sufficiently accurate in view of all the other uncertainties and approximations associated with line design.