Engineering Considerations

Glenn Stapleton - Senior Electrical Design Engineer Lines

Overview

Transmission line terminology and concepts

Types of clearances

Electrical safety considerations in co-use

Underground high voltage cables

Structural engineering considerations

Background

Why do we have high voltage transmission lines?Population centres/loads a long distance from generation

Losses (thermal) due to current through wire resistance

Reduce losses with higher voltages over longer distanceExpressed in kV

110 kV, 132 kV, 275 kV, 330 kV up to future 500 kV

Terminology

Suspension Tower Tension Tower

Conductor

Earthwire

Foundation and Electrical Earthing

Insulators

Clearances

Voiding statutory requirements

Changing natural ground levelStorage on high objects on easements

Storage of metallic objects on easements

Tree planting

Vertical and horizontal clearancesConductor movement with wind

Maintenance access to conductors in span and access to structure to set up equipment

Exclusion zones are set by law

General Arrangement

Ground Clearance

Conductor

Tower A

Sag

Conductor Span (average 450- 500 metres)

Tower B

Sag as a Function of Temperature

Ground Clearance Statutory

Tower

Sag at Temp 1

Conductor Span

Sag at Max Temp

Easement Width and Blowout

Emt BdyEmt Bdy

Wind Direction

Towers(Plan View)

Cond Blowout

Safety Clearance

StillConductor

Conductor Blowout

Electrical Safety Considerations in Co-use

Transmission line operation

Considerations during normal transmission line operation

Earthing and safety during system disturbances

Insulator flashover (Pollution)Lightning strikes

Earth potential rise at earthed structures

Earthing Considerations in Co-use

Earth Potential RiseTouch Voltage Risk

Mitigate risks via correct earthing

Voltage with respect to remote earth

Electric and Magnetic Induction Effects

Magnetic and Electric Induction Issues

Co-use of corridors with other metallic infrastructure (phone, water, gas or rail)

Long parallel exposure between transmission line and metallic objects / service due to magnetic field coupling

Electric field coupling to unearthed metallic objects on easements

Easily mitigated but requires co-operative review with Transmission company and service designer, operator and maintenance staff

Common activities for engineering review

Fences and gates

Induction / Transferred PotentialNew services to developments

Installed parallel to easements

OH and UG service crossingsClose to substation boundaries

Phone and LV Power

Major and minor pipelinesUnapproved swimming pools on easement

Pipeline or long parallel metallic services

Underground transmission cables

Out of sight out of mind

Still require engineering considerations for co use situations

Clearances are still mechanical and electricalStill have issues regarding alteration to ground profile

Can still cause magnetic induction issues with long parallel exposure to other metallic service

Restrictions of what can be done on easement (e.g. tree planting)

Structural and Mechanical Effects

Importance of transmission line foundations

Excavation, ground level changes can impact on structural design capacity

Transmission structure foundations can be significant depth and raked which restricts what can be done in a zone surrounding the structure