RISK REDUCTION THROUGH HAZARD TREE REMOVAL

To assess the potential of a hazard tree program to mitigate the risk of tree-line strikes an example for a 69 kV line will be used. This voltage, in the lower end of transmission service, is chosen because the tree-conductor clearances maintained will fall between those for higher voltage transmission lines and the lower voltage distribution lines.

Assume the following:

  • A 69 kV line is set on a 60 foot right of way.
  • The line is built on 10 foot crossarms and the average conductor height is 40 feet. Much of the line runs adjacent to a forest edge.
  • Typical tree height is 85 feet and tree density is 250 trees/Ac (620 trees/ha).
  • A hazard tree program on a five-year cycle removes an average of 30 trees per mile (one side only).

From this we can calculate that:

  • Clear width = 25 feet ((60 ft ROW – 10 ft crossarm)/2)
  • Trees up to 75 feet from the nearest conductor could strike the line (by triangulation)
  • The residual tree population per mile is 1515 trees ((50 ft X 5280 ft/mi)/43560 sq.ft/Ac X 250 trees/Ac)
  • The residual tree population is decreased by 2% through the hazard tree program (30 trees per mi./1515 trees per mi. )

A good hazard tree identification and removal program may substantially improve line reliability. However, the risk addressed by the hazard tree program is only that of weakened trees susceptible to failure under somewhat stressful weather conditions. The risk associated with the impact of lightning, severe wind and ice loading on healthy, structurally sound trees is not addressed by a hazard tree program.

A 2% reduction in the residual tree population examined in the context of typical tree mortality rates of 0.5% to 3% per year reveals the benefit of this hazard tree program will only be significant for a relatively short time. Most of the reliability gain will erode prior to the next maintenance cycle. Since we can’t predict which of the residual trees will next become decadent, the enduring outcome of the hazard tree program is no more than a 2% reduction in tree-line strikes risk.

Given the same tree characteristics, the residual tree population is lower for higher voltage transmission lines due to a greater maintained tree-conductor clearance and line height. Thus removing 30 trees mi-1 would yield a greater risk reduction. Conversely, for distribution lines, due to smaller maintained tree-conductor clearances and lower line heights, the residual tree population is higher and hence, removal of 30 trees mi-1 yields a smaller percentage change.

This example serves to illustrate the extent of the risk arising from off right of way trees is not meaningfully addressed, in an enduring fashion, through a typical hazard tree identification and removal program.

LINE STRIKE PROBABILITY CHART

Continuing with the example started above, of a 69 kV line, is there a clear width that optimizes line security for the maintenance dollar?

Yes there is. And further on we’ll look deeper into the economics to show what extent of widening has a positive net present value; that is a positive monetary return.

Let’s re-state the assumptions:

  • A 69 kV line is set on a 60 foot right of way.
  • The line is built on 10 foot crossarms and the average conductor height is 40 feet. Much of the line runs adjacent to a forest edge.
  • Typical tree height is 85 feet and tree density is 250 trees/Ac (620 trees/ha).
  • A hazard tree program on a five-year cycle removes an average of 30 trees per mile (one side only).
  • Clear width = 25 feet ((60 ft ROW – 10 ft crossarm)/2)

The Optimal Clear Width Calculator is used to generate the Line Strike Probability chart shown below. 

The chart shows that a clear width of 25 feet is still on the steep part of the slope. Where the curve is approaching horizontal there is no longer a substantial benefit in line security for the dollar invested in additional clear width. The optimum point would appear to be a clear width somewhere between 25 feet and 45 feet. Given that let’s consider increasing the clear width 10 feet. What would be the benefit in line security?