A utility client told me that they were trying out covered conductors on a feeder in a forested area. This was the first time that this large utility tried covered conductors. The objective is to reduce the impact of tree contacts and falling branches that blow fuses and therefore result in permanent outages for customers. In this context, the great length of feeders and the high system voltage (25 kV) make coordinating reclosers and fuses difficult.
Covered conductors have a thin insulation covering – not rated for the full phase voltage, but sufficient to reduce the risks of flashovers and fire when a tree branch falls between phases, when a tree makes momentary contact with a conductor, or when an animal jumps to it. Covered conductors also allow utilities to use tighter spacing between conductors.
While covered conductors help with tree contacts, they also have a number of operational disadvantages:
- High impedance faults with a downed conductor are more likely, leading to public safety issues, especially since the conductor may not show arcing and may not look as if it is energized.
- Covered conductors are more susceptible to burndowns caused by fault arcing. Covering prevents the arc from motoring with magnetic forces along the wire, concentrating heat damage. Repair time and cost increase significantly.
- Covered wires have a larger diameter and are heavier, increasing loading, especially with freezing ice and high wind, which can likeliness of mechanical damages (including broken poles and cross arms), leading again to high repair time and costs.
- Covered conductors have somewhat lower ampacity at high temperature (worsened by the black color that absorb more heat from the sun), with more limited short-circuit capability. High temperature also degrades the insulation. This results in more design and planning constraints that may increase construction costs.
- Water can accumulate between insulation and wire at the low point between of a span, causing premature corrosion and weaken the conductor and can lead to failure.
- Covered conductors must be installed differently than bare ones. For instance, using conducting insulator tie can lead to partial discharges and radio interference.
- Finally, cost is an obvious issue – replacing conductors on existing lines is extremely expensive, possibly as much as $100k per km.
These issues got me thinking on how I could provide a better alternative. Replacing fuses with single-phase reclosers appears to be an interesting (if unlikely) alternative to covered conductors. Cutout-mounted single-phase reclosers can easily be installed in existing cutouts to protect lateral circuits. Those circuits are then protected against tree contacts without the disadvantage of covered conductors. Coordination with upstream mainline reclosers is eased by making the single-phase recloser faster than the mainline recloser. Cost is clearly lower than re-conductoring.
Full disclosure: I am employed by S&C, and S&C makes a cutout-mounted recloser.