By Robin Scott, Powerline Designer, Vulcan Tubular, South Africa
No powerline system is ever completely stable as faults and incidents are likely to occur on the overall power grid at any given time due to the complexity of the high, medium and low voltage network – it is, after all, the largest and most intricate part of the power system.
Put simply, network resilience is the ‘give and take’ in the powerline system that carries or bypasses the dangerous ‘something’ that may cause a possible breakdown (partially or completely) in that section of the powerline network. If certain structures or hardware are found to be sensitive to interference in that part of the network then re-designed structures or hardware may absorb the interference.
What is commonly referred to as the national grid includes the generators of electrical power, the transmission network, and the distributor systems, which supply power to where it is needed. The overall system voltages range could widely from 720kV down to 11kV and to the lowest levels required by the end consumer. With power being sent over vast distances and the increasing collaboration between Africa’s regional power pools [Ed: refer to pages 70 & 71], it may be necessary to consider upgrading the earlier designs of lower voltage powerline networks into consideration when planning expansion and maintenance projects.
Resilience can be considered as the manner an existing and functional powerline installation operates normally and how it returns to normal operation after it has experienced an abnormal event. The list of causes is almost infinite and it is assumed that the initial conductor installation was faulty to some extent.
Most faults occur at the points where and when the conductor is terminated or supported. Any piece of hardware, be it a clamp or vibration damper, or even the conductor itself, can be a problem and may reduce the resilience of the system. In the same vein, and turning the cause around, the conductor could have been damaged on installation or where the section of line conductor has failed in some way during installation or during its life cycle.
The network’s resilience can be improved in such a way that any damage to the powerline installation will be radically reduced, if not completely avoided, by making additions or changes to the hardware. In the same way, careful selection and installation of conductors and hardware fittings will boost system resilience. Any items that fail in service must be examined to identify the cause of failure.
The ‘give and take’ of a powerline system can also be designed into the structures. Using more lightweight steel structures may give better support to the conductor, particularly in uneven terrain.
It also simplifies the transportation and installation of the structures. Special structural designs, particularly where an old structure has to be replaced, are often the best solution. This has been proven in practice when, for example, a new structure design fits in better than the old type of lattice structure.
With added distributed energy resources and renewables on the grid, new resilience considerations will come into play. However, it is eminently possible to plan for and invest in new technologies to overcome the threat to the existing network through smart, proactive planning. The benefits and costs should consider not only those directly felt by the utility but also those indirectly transmitted to interconnected infrastructure and socioeconomic systems.
The primary components of overhead powerlines
• Poles, which can be made up of wood, aluminium, steel, reinforced plastic and concrete.
• Cross arms and clamps supporting the insulators on the pole.
• Insulators supporting the conductors by withstanding sudden surges from lightning or switching.
• Conductors which, depending on the current transmitted, are aluminium conductor steel reinforced (ACSR), an aluminium-alloy conductor (AAAC), or copper.
• Lightning arrestors and Earth wires for discharging the excessive voltage that builds up in the line due to lightning.
• Vee guards used below the bare overhead lines to provide safety to the line in case it breaks.
• Bird guards to safeguard the conductors. ESI
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