Overhead or underground?
This debate continues. Both designs have advantages (see Table 1). The major advantage of overhead circuits is cost. An underground circuit typically costs anywhere from 1 to 2.5 times the equivalent overhead circuit (see Table 2).
But the cost differences vary wildly, and it’s often difficult to define “equivalent” systems in terms of performance. Under the right conditions, some estimates of cost report that cable installations can be less expensive than overhead lines.
If the soil is easy to dig, if the soil has few rocks, if the ground has no other obstacles like water pipes or telephone wires, then crews may be able to plow in cable faster and for less cost than an overhead circuit.
TABLE 1 – Overhead vs. Underground: Advantages of Each
|Cost — Overhead’s number one advantage. Significantly less cost, especially initial cost.||Aesthetics — Underground’s number one advantage. Much less visual clutter.|
|Longer life — 30 to 50 years vs. 20 to 40 for new underground works.||Safety — Less chance for public contact.|
|Reliability — Shorter outage durations because of faster fault finding and faster repair.||Reliability — Significantly fewer short and long-duration interruptions.|
|Loading — Overhead circuits can more readily withstand overloads.||O&M — Notably lower maintenance costs (no tree trimming).|
|Longer reach — Less voltage drop because reactance is lower.|
In urban areas, underground is almost the only choice. Too many circuits are needed, and above-ground space is too expensive or just not available. But urban duct-bank construction is expensive on a per-length basis (fortunately, circuits are short in urban applications).
Aesthetics is the main driver towards underground circuits. Especially in residential areas, parks, wildlife areas, and scenic areas, visual impact is important. Undergrounding removes a significant amount of visual clutter.
Overhead circuits are ugly.
It is possible to make overhead circuits less ugly with tidy construction practices, fiberglass poles instead of wood, keeping poles straight, tight conductor configurations, joint use of poles to reduce the number of poles, and so on.
Even the best though, are still ugly, and many older circuits look awful (weathered poles tipped at odd angles, crooked crossarms, rusted transformer tanks, etc.).
TABLE 2 – Comparison of Underground Construction Costs with Overhead Costs
|Single-Phase Lateral Comparisons|
|NP||Overhead||1/0 AA, 12.5 kV, phase and neutral||8.4||1.3|
|NP||Underground||1/0 AA, 12.5 kV, trenched, in conduit||10.9|
|APL||Overhead||Urban, #4 ACSR, 14.4 kV||2.8||2.4|
|APL||Underground||Urban, #1 AA, 14.4 kV, trenched, direct buried||6.6|
|Three-Phase Mainline Comparisons|
|NP||Overhead||Rural, 4/0 AA, 12.5 kV||10.3||1.7|
|NP||Underground||Rural, 1/0 AA, 12.5 kV, trenched, in conduit||17.8|
|NP||Overhead||Urban, 4/0 AA, 12.5 kV||10.9||1.6|
|NP||Underground||Urban, 4/0 AA, 12.5 kV, trenched, in conduit||17.8|
|APL||Overhead||Urban, 25 kV, 1/0 ACSR||8.5||2.2|
|APL||Underground||Urban, 25 kV, #1 AA, trenched, direct buried||18.8|
|EP||Overhead||Urban, 336 ACSR, 13.8 kV||8.7||6.1|
|EP||Underground||Urban residential, 350 AA, 13.8 kV, trenched, direct buried||53.2|
|EP||Underground||Urban commercial, 350 AA, 13.8 kV, trenched, direct buried||66.8||7.6|
Underground circuits get rid of all that mess, with no visual impacts in the air. Trees replace wires, and trees don’t have to be trimmed. At ground level, instead of poles every 150 ft (many having one or more guy wires) urban construction has no obstacles, and URD-style construction has just padmounted transformers spaced much less frequently.
Of course, for maximum benefit, all utilities must be underground.
There is little improvement to undergrounding electric circuits if phone and cable television are still strung on poles (i.e., if the telephone wires are overhead, you might as well have the electric lines there, too).
While underground circuits are certainly more appealing when finished, during installation construction is messier than overhead installation.
- Lawns, gardens, sidewalks, and driveways are dug up;
- Construction lasts longer; and
- The installation “wounds” take time to heal.
These factors don’t matter much when installing circuits into land that is being developed, but it can be upsetting to customers in an existing, settled community.
Underground circuits are more reliable. Overhead circuits typically fault about 90 times/100 mi/year; underground circuits fail less than 10 times/ 100 mi/year. Because overhead circuits have more faults, they cause more voltage sags, more momentary interruptions, and more long-duration interruptions. Even accounting for the fact that most overhead faults are temporary, overhead circuits have more permanent faults that lead to long-duration circuit interruptions.
This can partially be avoided by using loops capable of serving customers from two directions, by using conduits for faster replacement, and by using better fault location techniques.
Underground circuits are much less prone to the elements. A major hurricane may drain an overhead utility’s resources, crews are completely tied up, customer outages become very long, and cleanup costs are a major cost to utilities.
However, underground circuits are not totally immune from the elements. In “heat storms,” underground circuits are prone to rashes of failures. Underground circuits have less overload capability than overhead circuits – failures increase with operating temperature.
In addition to less storm cleanup, underground circuits require less periodic maintenance. Underground circuits don’t require tree trimming, easily the largest fraction of most distribution operations and maintenance budgets.
The CEA estimated that underground system maintenance averaged 2% of system plant investment whereas overhead systems averaged 3 to 4%, or as much as twice that of underground systems.
Underground circuits are safer to the public than overhead circuits. Overhead circuits are more exposed to the public. Kites, ladders, downed wires, truck booms — despite the best public awareness campaigns, these still expose the public to electrocution from overhead lines.
Don’t misunderstand; underground circuits still have dangers, but they’re much less than on overhead circuits. For the public, digins are the most likely source of contact. For utility crews, both overhead and underground circuits offer dangers that proper work practices must address to minimize risks.
We cannot assume that underground infrastructure will last as long as overhead circuits. Early URD systems failed at a much higher rate than expected. While most experts believe that modern underground equipment is more reliable, it is still prudent to believe that an overhead circuit will last 40 years, while an underground circuit will only last 30 years.
Overhead vs. underground is not an all or nothing proposition. Many systems are hybrids and some schemes are:
Overhead mainline with underground taps
The larger, high-current conductors are overhead. If the mains are routed along major roads, they have less visual impact. Lateral taps down side roads and into residential areas, parks, and shopping areas are underground.
Because the mainline is a major contributor to reliability, this system is still less reliable than an all-underground system.
Overhead primary with underground secondary
Underground secondary eliminates some of the clutter associated with overhead construction. Eliminating much of the street and yard crossings keeps the clutter to the pole-line corridor. Costs are reasonable because the primary-level equipment is still all overhead.
Converting from overhead to underground is costly, yet there are locations and situations where it is appropriate for utilities and their customers. Circuit extensions, circuit enhancements to carry more load, and road-rebuilding projects — all are opportunities for utilities and communities to upgrade to underground service.
Reference // Fundamentals of Distribution Systems by T. A. Short