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
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)