TES-P-122.07 : Transmission Structure spotting Pads and Roads :

ransmission line plan and profile drawings are essential throughout the design and construction process. They begin with route surveys to map features along the proposed line, guide design tasks like structure placement, assist in material procurement and construction, and serve as permanent records for future operations and maintenance.

Drawing Preparation :

    Adequate control of field survey, including ground check (in case of aerial survey) and proper translation of data to the plan and profile drawings are of utmost importance. Errors which occur during the initial stage will affect the transmission line design because a graphical method is used to locate the structures and conductors.

    Plan and Profile drawings shall be drafted on SEC Standard drawing sheets, per SEC Engineering Drawing Standard SEEDS-I and SEEDS-II.

    The scale of the drawings shall be as below :

    Horizontal

    1 cm = 20 meters for 69kV to 230kV transmission lines 1 cm = 25 meters for 380kV transmission lines

    Vertical

    1 cm = 2 meters for 69kV to 230kV transmission lines 1 cm = 2.5 meters for 380kV transmission lines.

    Start the plan and profile drawing so that stations which are multiples of 20 or 25 meters (depending upon the scale) will coincide with the vertical lines of printed portion of the sheets. Increase in stationing and structure numbering shall proceed from left to right with the profile and corresponding plan view on the same sheet. The change in station (station equations) shall be avoided as far as possible. Profile stationing shall start from the proposed power source increasing towards the supplied point. Usually the source point is a take-off point, a first P.I. (point of intersection) or a tap point.Choose the starting profile station so that negative values are avoided during adjustments. Use horizontal ground distance for profile stationing calculations. If a control traverse shows a different distance, adjust the profile stationing proportionally.

    Features of Plan and Profile Drawings:

    Generally the Plan and Profile drawings include all the necessary information about the line route such as ground line showing chainage, elevations and major features traversed. However, for complete and accurate design of a transmission line certain additional information shall be required. The engineer responsible for detailed design shall physically traverse the line route in the field and collect all such information. This information shall include, but not be limited to, the following features:

    SAG TEMPLATES : https://www.the3dutility.com/pssag.html

    1. General

    The sag template is a tool used to find the vertical position of conductors and ground wires. It helps show where and how tall structures should be on drawings to meet design rules for clearances and spans. The sag template is a tool on drawings. It shows conductor and support heights to help design the line for clearances and spans. It also allows testing different layouts to reduce costs. Conductor sag curves usually guide the design. And the ground wire template shows its position compared to conductors in special spans or changed setups.

    Sag Template Curves :

    The sag template shall include the following sag curves based on the design ruling span:

    1. Cold Curve

    This curve shall indicate initial sag at minimum temperature of -1°C, no wind and no ice conditions. This shall be used to check for uplift and insulator swing.

    1. Normal Curve

    This curve shall show final sag value of conductor at everyday temperature at no wind conditions. This shall be used to check normal clearances and insulator swing. Every day temperatures for various SEC Operating Areas shall be as in Table 07-1 below:

    Table 07-1: Every Day Temperatures

    SEC Operating AreaEvery Day Temperature, oC
    Central25
    Eastern27
    Western30
    Southern25 & 30
    1. Hot (Maximum Sag) Curve

    This curve shall show final sag value at maximum design temperature at no wind conditions.Use this to check minimum vertical ground clearances. Table 07-2 below gives the maximum design temperatures causing maximum sag for these clearances.

    Table 07-2: Maximum Design Temperatures

    SEC Operating AreaMaximum Design Temperature, oC
    Central80 for ACSR conductors
    Eastern85 for ACAR conductors 93 for ACSR/AW conductors
    Western80 for AAAC conductors
    Southern80 for AAAC conductors 93 for ACSR/AW conductors

    Ground Wire Curve:

    The curve shows ground wire sag at normal conditions to check its position relative to conductors and mid-span clearances.

    Ground Clearance Curve :

    This curve shall indicate the specified ground clearance over open terrain from the maximum temperature (Hot) curve. This will be equal to the vertical offset distance below any point on the hot curve.

    These curves help locate sag low points and determine the vertical (weight) span length of conductors. A sample of typical sag template indicating all these curves is shown in Figure TE-2207-0100-00. The intersection of curves with the vertical axis line represents the low point position of the sag.

    Sag Template Design:

    æ

    S = ç

    L ö 2

    ÷

    (SRS )

    (Eq. 7-1)

    è LRS ø

    Where:

    S           =          Sag of other span in meters

    SRS       =          Sag of the ruling span in meters L          =             Length of other span in meters LRS   =        Length of ruling span in meters

    change in any one of these factors may affect the design characteristics of the template. For steep slopes with large elevation differences, the sag must be calculated and sag curves drawn instead of using the normal template.

    Sag Template Preparation:

    The sag template shall be made of dimensionally-stable transparent plastic or celluloid material of about 1mm thickness. A contrasting colored material such as red may be helpful when the template is used to check Plan & Profile drawings which are blue prints. The curves are first plotted on paper using correct scales and then reproduced or copied on the plastic material. To cut a template, the transparent material is fastened securely over the sheet and the centerline and upper curves are etched lightly by a sharp-pointed steel scriber. The outside edges can easily be broken out and the edges sanded smooth. Structure height scales may also be drawn or etched on the template or a separate template may be made for determining structure height required for each type of structure used. The etched lines shall be filled with ink to make them easier to see when the template is used. Conductor size, design tension and loading conditions ruling span and descriptive data for each curve shall be shown on the template

    Structure spotting :

    General:

    Structure spotting is the design process which determines the height, location and type of consecutive structures on the plan and profile sheets. The efficient location of structures on the profile is an important component of line design. Structures of appropriate height and strength shall be located to provide adequate conductor ground clearance and minimum cost. Actual economy and safety of the transmission line depends on how well this final step in the design is performed. The structure spotting shall closely conform to the design criteria established for the line. Constraints on structure locations and other physical limitations encountered may prevent spotting of structures at optimum locations. Success of the effort to minimize or overcome these special conditions can be judged by how closely the final line layout follows the original design parameters.

    Structure spotting shall be carried out using manual method or computer optimization method. In case the later method is used, the results shall be shown on Plan and Profile drawings and reviewed and finalized for construction along with structure list.

    1. Spans approximately uniform in length, equal to or slightly less than the design ruling span. Span utilization factor (defined as the ratio of average span to the design ruling span) shall be 90% or more.

    Actual ruling and wind spans must not exceed the design ruling span, even if the wind span is greater. Exceptions are allowed with proper justification, such as short stretches or single spans between tension towers. New templates and layout checks are required if ruling spans differ by more than 20 meters from the design.

    Preparation for Structure Spotting :

    The following data are required for structure spotting on a transmission line:

    1. Plan and Profile Drawings :

    Preliminary Reconnaissance of Plan and Profile :

    Before spotting the structures on the plan and profile sheets, review the entire line route on the Plan & Profile drawings and in the field, and specially note any unique features you encounter.

    You should identify areas prone to wind or water erosion. Note marshy, waterlogged, low-lying, and rocky areas, and try to avoid placing structures there, especially angle structures.

    Structure Spotting Procedure :

    The process of spotting shall begin at a known or established conductor attachment point such as a substation take-off structure or point of intersection (P.I.) and usually progresses from left to right on the profile. Hold the sag template vertically and place the ground clearance curve tangent to the profile. After setting the first structure and its height, determine the next structure’s height and location by scaling or using a structure template.

    The relationships of the ground clearance and conductor curves is also used for spans other than level-ground spans by shifting the sag template until ground profile touches or is below the clearance curve with the previously established conductor attachment point (normally, the left) positioned on the conductor curve. The conductor curve will then indicate the required conductor height for any selected span.

    Crossings:

    When crossing other power lines, the higher voltage line shall always cross over lower voltage line. Structures of the higher voltage line on both sides of the line being crossed shall be placed in such a way that the horizontal distance from the nearest conductor of the line being crossed to the center line of the adjacent crossing structure is not less than the height of the crossing structure.

    The crossing arrangement shall avoid use of special crossing structures with heights above 80 m for 380kV lines and above 65 m for 230kV and lesser voltage lines. The height of lower voltage lines being crossed by higher voltage lines may be lowered by placing gantry structures. Special crossing structures shall only be used in case no other options are left.

    Insulator Side Swing :

    Suspension insulators swing sideways from wind, reducing clearance and needing limits for insulation. They also deflect sideways at line angles due to conductor tension. The horizontal swing force equals half the wind pressure on adjacent conductors. The vertical force resisting swing is the conductor’s weight plus half the insulator’s weight. The length of conductor they support is the vertical or weight span.

    1. On rough terrains where each of the adjacent spans fall rapidly away from the structure, the conductor low points, as indicated by the sag template, may fall on the adjacent spans. However, you still consider the distance between the low points as the length of conductor acting vertically to hold the insulator from swinging.. Excessive low-point distance can cause a failure in the insulators, hardware or the structure. For all wind and weight span cases, you must calculate the Factor of Safety for insulators and hardware under maximum loading (high wind) conditions, and ensure it is not less than 2.
    1. You must calculate the wind span to weight span ratio and check it against the allowed minimum and maximum limits. In case it is beyond the

    allowable limits, following corrective steps are recommended in order of preference.

    1. Relocate structures to adjust wind to weight span ratio
    Grading and Uplift :

    The best layout has spans close to or slightly less than the ruling span, smooth conductor curves, and structures of equal height. Smooth conductor profiles show good design. Attachment points should form a smooth curve to balance structure loads, called grading the line, which is key in transmission line design.

    Uplift, a negative weight span, is calculated like a weight span and should be avoided if possible. Uplift may occur in a rough profile where the conductor supports are at different elevations. For example, refer to the three structures on Figure TE-2207- 0200-00. Conductor sag is shown at everyday temperature, but as temperature drops, the conductor contracts and the sag reduces.When the temperature reaches the minimum value, the conductor assumes the position indicated by the cold curve shown on the template. By placing the curve on the template between supports of alternate structures, you can see if the intermediate support is above or below the cold curve.. If the conductor support would then be below the cold curve, the conductor would exert an upward pull on the structure and this upward pull is the Uplift. Uplift at a structure will cause the conductor to pull the suspension insulators up into the cross-arm and with the horizontal post insulators; it might cause the conductor to pull away from the insulator.You can possibly avoid uplift by adjusting structure locations on the profile drawings to match the terrain and by using a taller structure at the uplift point.

    If these methods fail, you must dead-end the conductor. Designing to prevent or reduce uplift is similar to correcting excessive insulator swing, but you should avoid adding too much weight. Double dead-ends and certain angle structure can have uplift as long as the total force of uplift does not approach the structure weight. If it does, hold down guys are necessary, if applicable.

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