The lower the NPV, the less impact or cost it has to the ratepayer which gives it the better ranking.
Figure 1 also demonstrates that the least-cost option depends upon what is assumed for the average annual load. The owning cost analysis shows “Drake” ACSR is the lowest cost for average annual load assumptions below 13% of the 1700A target capacity (less than 221A). This observation confirms that the original “Drake” ACSR investment was prudent for the loads that likely existed 40 years ago when the line was first designed. This also confirms that an appropriately sized ACSR may also be the lowest-cost option for new lines even today. However, there are now high-temperature lowsag (HTLS) options that offer double the capacity and reduced line losses at only slightly greater conductor first cost. The value of the capacity and loss reduction can easily justify an ACSS/TW option over ACSR even for new lines. A ranking of 1 indicates the least cost of ownership, whereas 4 denotes the highest cost of ownership.
Increasing the assumed annual load from 170A to 255A (orange bars) shows the effect of the “current squared” term on the line loss. At 255A, “Drake” is no longer the most economical option because the cost of losses has increased enough to overcome its advantage in lower first cost. 959.6 kcmil ACSS/TW “Suwannee” is the least cost option for assumed average loads of 15% and 25% of peak capacity. “Suwannee” combines modest first cost and moderate cost of line losses. At 33% of
capacity and above, the 1031.7 kcmil ACSS/TW/MA8 “Mississippi” conductor is the least cost option due to its lowest-in-class line loss and moderate first cost.
Regardless of loading assumptions, the 1025.6 kcmil “Drake/ACCC” is not the least cost solution at any load because of its high first cost and lack of an advantage in
line loss.
These rankings change in scenarios where the cost of
structure modifications is significant. In this scenario, all candidates meet the capacity requirement without exceeding the maximum thermal sag for the “Drake”
ACSR being replaced. The rankings will also change if there is a value
assigned to capacity above the 1700A nominal capacity target. 1025.6 kcmil “Drake/ACCC” meets the capacity target with a capacity increase of nearly 70%
compared to the original 795.0 ACSR “Drake”. The ACSS/TW options exceed the capacity of “Drake” by 99% for 959.6 kcmil “Suwannee”, and by 105% for
1031.7 kcmil “Mississippi”.
