CALCULATE THE BLOW OUT OF OXYGEN cONDUCTOR ON A 300 M SPAN SUBJECTED TO 500 PA WIND:
SWINGING INSULATOR LENGTH = 1.5 M
FROM A SAG TENSION CALCULATION, INCLINED SAG AT 500 PA WIND = 12 M TOTAL INCLINED LENGTH = 13.5 M
HORIZONTAL DISPLACEMENT = SIN (SWING ANGLE) X TOTAL INCLINED LENGTH
=
APPENDICES
11.6 Conductor Clashing
Under short circuit conditions conductors experience forces of attraction and repulsion due to electromagnetic force from the fault current in the conductors. If the fault current is large and experienced for a long enough time the movement can be substantial and cause conductor clashing (particularly on distribution lines). For phase-to-phase faults the conductor movement is more pronounced as the fault current is very high and the protection clearance times are typically long.
Primary Conductor Clashing
Primary conductor clashing may occur when there is a phase to phase fault on one of two overhead lines connecting the same substations. When a phase to phase fault occurs, one end of the faulted line will usually trip first and fault current will then increase significantly on the other unfaulted line. The increase in fault current may then cause the conductors on this line to clash, resulting in the loss of two overhead lines. Constructions which are prone to conductor clashing are underslung or suspension, flat pin and transitions from flat to vertical.
Secondary Conductor Clashing
Secondary conductor clashing may occur on a distribution feeder when a recloser trips and isolates an initial fault and the live oscillating conductors upstream of the recloser subsequently clash together. When the initial phase-to-phase fault occurs the faulted phase conductors repell each other due to the current in the phase conductors. When the recloser trips the fault current and thus the repulsion forces between the conductors is removed the conductors pendulum back into equilibrium. Both phase conductors pendulum simultaneously towards each other and if they get close enough they cause a secondary conductor clashing fault. Figure 2.5 below shows
diagrammatically the feeder, recloser and fault positions.
The repulsion forces can be great enough to exceed wind force design limits.
Conductor clashing has a higher probability of occurrence when the fault occurs on two adjacent conductors at the same height and the conductor has low weight. Conductor clashing can be avoided or mitigated by the following measures:
• Introduce a vertical spacing between conductors,
• Increase the horizontal spacing between conductors,
• Insert additional poles midspan between conductors,
• Install midspan spacers between conductors,
¾ Reduce protection clearing times
To determine whether a line is susceptible to conductor clashing the calculations can be
performed with formula provided in the following EPRI publication, Bathold L.O., Clayton R.E., Grant I.S., Longo V.J., Stewart J.R & Wilson D.D., Transmission Line Reference Book: 115-138 kV Compact Line Design, EPRI, 1978.
12 ROUTE SELECTION PROCESS
Appropriate consideration must be given at the route selection stage to the use of the land proposed for the power line corridor. There are zoning maps available from local government authorities which describe the land usage in the region.
Local jurisdiction planning instruments, particularly those regulating the clearing of trees, may also influence the selection of the most appropriate route for the power line. Some areas may be of high environmental significance such as aboriginal and cultural heritage or sensitive vegetation (mangroves) and the line route will need to avoid these areas where possible.
Where power lines traverse private property the approval of the property owner is required. This would normally take the form of a negotiated easement detailing any restrictions on land use necessary for reliable operation of the line.
On public land the agreement of the management agency must be obtained for the proposed line.
12.1 Risk Management Principle
The layout design process should include the identification and assessment of risks associated with the construction, maintenance and operation of the proposed line leading to the evaluation and implementation of risk treatment options which ensure that the residual risk is acceptable to the organization.
The risk management process used should align with AS/NZS 4360 Risk Management and companion handbook HB 436 Risk Management Guidelines
12.2 Prudent Avoidance Principle
Where potential risks with unproven consequences are involved a prudent avoidance approach is recommended.
The original recommendation related to electric and magnetic field exposures where prudent avoidance was defined as “doing what can be done without undue inconvenience and at modest expense to avert the possible risk“
12.3 Aesthetic Considerations
Visual amenity is now playing a major role in the selection of structures and other components on an overhead power line to gain community acceptance. Visual amenity can be improved by applying the following design principles:
- Locate power lines in corridors screened by vegetation or natural landscape
- Install “like with like” structures (if there is an existing tower line, select towers for the second line in the corridor)
- Use of low height and compact structures
- Avoid placing structures which dominate the skyline
- Use of non specular finish conductor
- Painting of structures (in particular poles) to match the existing landscape
Compacting the phase conductors will improve visual amenity but will increase the surface voltage gradient on the conductors and the noise (radio interference and audible). To offset the increase in electric field strength, a larger diameter conductor may need to be selected to ensure the surface voltage gradient is below the corona threshold level.
Non specular conductor will reduce the initial glare of the conductors and the high corona noise produced when the line is initially energised. Non specular conductor will make the conductor more hydrophilic to water and minimise the water drop corona effects.
12.4 Electric and Magnetic Fields
The principle of prudent avoidance has been adopted by the electricity industry for dealing with electromagnetic fields from overhead lines (refer Section XXX). Where there are vertically configured double circuit lines, electromagnetic fields can be minimised by diagonal phasing of the phase conductors.
Where there are 2 or more circuits installed horizontally on the structures, it may also be prudent to configure the phase conductors in a diagonal arrangement to minimise the electromagnetic fields.