PLS-CADD Complete Professional Guide: Transmission Line Design, Spotting, Sag-Tension, Structures, and Engineering Workflow

PLS-CADD Complete Professional Guide: Transmission Line Design, Spotting, Sag-Tension, Structures, and Engineering Workflow

This guide covers the FULL engineering workflow using PLS-CADD, from survey import to final plan & profile sheets, sag-tension charts, structure loading trees, and construction deliverables.

Table of Contents

1. What is PLS-CADD?

PLS-CADD (Power Line Systems – Computer Aided Design and Drafting) is the industry-standard tool for designing overhead transmission lines. Utilities such as NGCP, National Grid UK, AEMO, FERC-regulated operators, Eskom, and RE/utility EPC contractors use it for:

  • 3D survey terrain modeling
  • Tower spotting
  • Sag & tension computation
  • Electrical/mechanical clearance checks
  • Structure loading and strength checks
  • Plan & profile sheet generation
  • Material take-off (MTO/BOM)

ASCII Diagram: Basic PLS-CADD View

  +---------------------------------------------------------+
  |                   PLS-CADD 3D View                      |
  |                                                         |
  |      Tower A                 Tower B                 Tower C
  |        /\                     /\                      /\ 
  |       /  \                   /  \                    /  \
  |------/----\-----------------/----\------------------/----\-------
  |     Conductor Sag Curve (per temperature & load case)
  +---------------------------------------------------------+

2. Importing Survey Data

PLS-CADD supports multiple survey formats:

  • LiDAR point clouds
  • Drone photogrammetry
  • Total station or RTK CSV files
  • DEM/contour files

Steps:

  1. File → Import → Survey Data
  2. Select format (XYZ, LAS, DEM, TXT, CSV)
  3. Assign point types: ground, road, vegetation, structures
  4. Generate the terrain surface

ASCII Terrain Model Diagram

   Elevation Profile
   ^
   |        /\            /\         /\       x (Tower Location)
   |       /  \          /  \       /  \
   |______/____\________/____\_____/____\__________________→ distance

3. Creating the Terrain Model (DTM)

A DTM (Digital Terrain Model) is essential for:

  • Clearance checks
  • Tower heights
  • Spotting optimization
  • Sag behavior above ground

Procedure:

  • Survey → New Surface From Points
  • Set TIN (triangulated irregular network)
  • Filter vegetation using classification tools

4. Defining Alignment and Centerline

The centerline is the exact path of the transmission line.

Steps:

  1. Import KML/shape alignment
  2. Assign stationing (0+000 etc.)
  3. Define PIs (Points of Intersection)

ASCII Alignment Path

   (PI1)-----\____(PI2)______/(PI3)---------Straight Line---------

5. Design Criteria Setup

This determines sag behavior, structure loads, and safety clearances.

Key Input Required:

  • Conductor type (ACSR, ACCC, AAAC)
  • Temperature cases (min, max, everyday)
  • Wind and ice loading (ASCE 74 / IEC 60826)
  • Safety clearances:
    • Ground clearance
    • Road/river crossing
    • Building clearances
  • Electrical constraints (SIL, blowout)

6. Conductor and OPGW Modeling

Important Properties to Enter:

  • Diameter
  • Weight
  • Modulus of Elasticity
  • Thermal expansion coefficient
  • Creep characteristics

ASCII Conductor Model

Conductor ------------------ OPGW ------------------ Conductor

PLS-CADD uses these inputs to calculate the correct sag for each temperature & load case.

7. Tower Spotting (Automatic & Manual)

PLS-CADD can automatically optimize tower locations using:

  • Minimum/maximum span limits
  • Slope limitations
  • Clearance constraints
  • Tower families and height options

Automatic Spotting Procedure

  1. Line → Spot Structures → Automatic
  2. Select tower family
  3. Set span limits (e.g., 200 m – 400 m)
  4. Run optimizer

ASCII Tower Spotting Diagram

      T1        T2         T3               T4
      /\        /\         /\               /\
-----/--\------/--\-------/--\-------------/--\---------
   210m     260m       235m            290m

8. Sag-Tension Calculations

Sag-tension is performed for:

  • Initial (post-stringing)
  • Final (after creep)
  • Maximum temperature
  • Minimum temperature
  • Wind/ice loading

Ruling Span Formula Used by PLS-CADD

LR = Σ(L³) / Σ(L²)

Sag Curve ASCII Illustration

Tower A                               Tower B
   /\                                   /\
  /  \_________________________________/  \

9. Structure Loads & Load Trees

PLS-CADD generates loading trees for:

  • Transverse loads
  • Vertical loads
  • Longitudinal loads
  • Wind-on-conductor
  • Wind-on-structure

ASCII Load Tree

          (Top)
           ↓  Wind
      +----------+
      |  Tower   |
      +----------+
       ↑   ↑   ↑
    Vert Long Trans

10. Plan & Profile Sheets

Plan & Profile sheets are required for construction and regulatory compliance.

Generating PNP Sheets

  1. Reports → Plan & Profile
  2. Select template (NGCP, IEC, utility standard)
  3. Include:
    • Tower positions
    • Span lengths
    • Sag profiles
    • Ground clearance
    • Crossings (roads, rivers)

ASCII PNP Snapshot

| Station  | Elev | Tower | Sag | Clearance |
|  0+000   |  56  |   T1  | 14m |   OK      |
|  0+240   |  61  |   T2  | 12m |   OK      |

11. Foundation Outline

PLS-CADD can generate foundation reactions (DXF compatible) for:

  • Pier footings
  • Pad foundations
  • Pile foundations

12. Bill of Materials (BOM)

Automatically compute:

  • Conductor length
  • Insulator sets
  • Tower heights
  • Hardware fittings
  • Foundations volume

13. Clearance Checks

Clearance checks include:

  • Ground clearance
  • Vegetation
  • Road/river crossings
  • Building proximity
  • Substation entry

14. Engineering Reports

PLS-CADD can export:

  • Sag-Tension Reports
  • Loading Trees
  • Tower Spotting Report
  • Plan & Profile PDFs
  • KMZ/KML files for Google Earth
  • DXF files for CAD drafting

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