Underground power line feasibility

Should the power wires be overhead or underground? Underground power line feasibility article talks on the feasibility on using underground lines instead of overhead lines. Both underground and overhead lines have their good and bad properties. Burying power lines is not cheap, it is said the cost would be $1M per mile to convert existing lines running above ground to underground.

Underground power line feasibility article tells that the electrical behavior of the underground wire and the electrical behavior of the overhead ground wires during a ground fault are very similar. There were differences however. The largest difference was the range of the earth surface potential rise: the potential around the underground ground wire is extremely widespread and its gradient is low. The electrical safety aspects needs to be carefully taken into consideration.

Underground lines are strictly limited by their thermal capacity, which permits less overload or re-rating than overhead lines. Long underground cables have significant capacitance, which may reduce their ability to provide useful power to loads. From a line loss perspective, the underground circuit can be more or less efficient. Underground cables need a larger conductor to handle the same amperage as a smaller overhead conductor. This is due to the difficulty of dissipating heat to the earth. Larger conductors means higher cost. The cost of construction for an underground transmission line is 5 to 10 times that of an equivalent overhead circuit.

Underground power line feasibility article says that underground cabling idea would eliminate many risks (for example wind and ice-related risks) but can increase the risk of outages due to storm surge or flooding. Another consideration is that when a power line is buried underground, heat due to IR losses is generated due to electricity flowing through the line that is not as easily dissipated as it would be in free air. Excavation near power lines can be dangerous and damaging to the power transmission system.With underground power cables the electrical and magnetic field is radiates to environment around the wiring are lower than with overhead cables, but very near the cable the magnetic field can be considerably higher than with overhead line.

According to Power Underground promotion underground placement of power lines would guarantee that storms and other natural events no longer deprive millions of people of electric power. And it would end the ugliness of poles and wires which pollute the appearance of our streets and scenic views.

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Underground power line feasibility article comments say for example the in many areas the ground water level is such that any underground cables will be submerged most of the time, which will cause problems where there are leaks in the cables, which there always are a few. The outages in underground lines are rare, but outages usually last 3 to 5 times longer than above ground and cost considerably more to repair. However, overall, their cost has not been considered all that bad. Overhead circuits can often be worked on while they are still energized. Nearly all work on underground circuits is performed while things are de-energized and grounded. Repairing underground cable takes normally considerably longer than repairing overhead line.

Underground power line feasibility article comments tell that there is a big increase in capacitive reactance, both between phases and from each phase to ground, when the wires are burried compared to overhead lines. Comments also tell that you need to consider environmental aspects on other parts than just cable. For example on low areas where flooding is possible, if the electrical power distribution gear is placed into the subbasements (like in N.Y. city), they can be easily flooded with seawater, a killer for anything electrical. If you want reliable operation after flood, any transformer and switching gear have to be elevated to the second or third story level. Areas subject to flooding have had outages when the water fills underground vaults and shorts out the cables.

There are groups like Power Underground that promote burying wires underground. Singapore and The Netherlands are 100% underground power, Belgium banned overhead power lines in 1992 and new policies in France are putting 25% of their power lines underground.

In Finland there are plans to increase the amount of underground cabling to reduce power outages. Building underground power cable is around three time more expensive than overhead line. Vattenfall plans to use cables instead of overhead lines in all new installations. They say it is not considerably more expensive and it will be cheaper on the long run.

I think there is still a good place in the world for both overhead and underground power distribution systems. It needs to be carefully considered which one is the right answer to which application at given time. For low voltage lines (230/400V, 220/110V etc.) buried lines are a vast improvement over above ground. For medium voltage distribution the cables are very practical solutions in urban and sub-urban areas. For high voltages transmission over long distances overhead lines are very good, possibly DC high voltage lines underground would be an answer to that on some applications.

10 Comments

  1. Tomi says:

    Overhead line versus buried cable
    http://www.50hertz.com/cps/rde/xchg/trm_de/hs.xsl/1571.htm?rdeLocaleAttr=en&rdeCOQ=SID-CCD886CD-5CB06FEB

    Overhead lines or buried cable? In this controversial debate, the supposed advantages of laying underground cables as opposed to constructing overhead lines are frequently cited. However, positive ecological side effects from laying 380-kV underground cables are not to be expected. Quite the opposite: This technology involves restrictions for land owners as well as irreversible ecological damage.

    Overhead lines make up a large part of the interconnected system. They ensure low-loss transmission at 380-kV extra-high voltage, and thus guarantee reliable energy supply. Cables, in contrast, are predominately used in medium- and low-voltage networks, as well as for power distribution in densely built-up areas with high electricity demand. However, this technology is also becoming increasingly important in rural areas. Nevertheless, underground cables have, in many cases, ecological and legal disadvantages which are frequently neglected in the public debate.

    When burying cables, the soil must be exchanged. Furthermore, not only do the cable routes need to be kept free from deeply rooted plants, they may not be built on for any other purpose. In addition, underground cables radiate heat. This has an effect on soil humidity, which, for example, can lead to drainage or drying out of marshes. The laying of underground cable also requires the construction of cable jointing structures every 500 to 700 meters, as well as compensation facilities along the cable route.

    An extra-high-voltage power line is a space-consuming structure – regardless of whether it is built as an overhead line or underground cable. Laying an underground cable will affect the rights of the owners and occupants of the land on which the power line is built and used, to a degree similar to the erection of an overhead line.

    Reply
  2. Tomi says:

    Lines Versus Cables: Consider All Factors
    http://tdworld.com/underground_transmission_distribution/power_lines_versus_cables/

    The choice of whether to use overhead line (OHL) or underground cable (UGC) must be consistent with safety, reliability and operational constraints to ensure that the capacity of the transmission grid efficiently matches the supply and demand of electrical energy. The choice between OHL and UGC is driven by technical, environmental and economic considerations.

    The operating costs over the life of an asset can be converted into an equivalent capital sum at the start of the project life and added to the capital cost of the investment. The environmental costs of a transmission facility can be quantified in terms of the burden on the built/developed/occupied land or territory. The presence of an electromagnetic field exceeding the value set by national laws (or rules or standards) may create a quarantined area of land unavailable for human activities or development. The economic impact to the land crossed by a transmission line can be estimated taking into account the loss of value of the rights-of-way.

    The article has a table on cable data vs overhead line (inductance, capacitance, resistance, etc..)

    Reply
  3. Tomi says:

    BURYING HIGH VOLTAGE #8
    POWER LINES
    http://retasite.files.wordpress.com/2011/07/fact-8-burying-updated-jan-2010.pdf

    Large-scale underground high voltage power line projects constructed since 1980

    Askon Consulting Group (2008):
    – Underground cables are much safer
    than overhead lines. No electric fields
    are emitted from buried cables, and
    most importantly, the magnetic field
    is greatly reduced both in intensity
    right over the line and in distance of
    impact.
    – Underground cables enhance power
    grid security and reliability and give
    improved performance, compared to
    overhead lines.
    – Underground cables are significantly
    more reliable.
    – Underground cables are significantly
    more efficient.
    – Underground cables are affordable
    compared to overhead lines. There
    is a higher initial capital cost, but this
    difference is cancelled out by the
    much higher and costly electricity
    losses from overhead lines over the
    life of the line. When this is taken into
    account, plus the lengthy planning
    delays, property devaluation, impacts
    on tourism and higher maintenance
    costs associated with overhead lines,
    costs of underground and overhead
    lines are almost equal.

    Reply
  4. Tomi says:

    Underground vs. Overhead Transmission and Distribution
    http://www.puc.nh.gov/2008IceStorm/ST&E%20Presentations/NEI%20Underground%20Presentation%2006-09-09.pdf

    National trends

    Municipalities have passed laws requiring new distribution facilities to be
    placed underground.
    - For aesthetic reasons
    - To increase property values (5-10% according to some studies)

    Cost covered by developers and ultimately paid by property owners.

    Provide better protection from storm damage and improve reliability of power supply

    Underground construction CAN improve the reliability of the electric power system by minimizing damage to the system from:
    - High winds
    - Ice and snow storms
    - Falling trees

    Underground Construction is NOT Immune from All Storm Damage
    - Flooding
    - Hurricane Damage
    - Earthquake Damage
    - Lightning Damage
    - Rodent and Human Damage (dig up)

    The main argument against constructing underground systems is usually financial. But costs are not the only limitation.

    The laws of physics limit how physically long a power line can be.
    - will severely limit high voltage underground cable systems (especially on voltages 100 kV and up)

    Capacitance causes current to flow even when no load is connected to the cable.

    Replacing overhead with underground lines will also change other characteristics of the line and connected power system.
    - Resistance will go down (probably beneficial)
    - Inductance (resistance to the flow of AC current) will also go down.
    - Voltage regulation will become much more difficult.
    - Line losses may increase and efficiency might become worse.
    - Time to repair the line will be much longer

    Underground transmission line construction may be 20 times the cost of overhead construction. (A $3 million/mile line becomes $60 million/mile.)

    Currently underground AC transmission is primarily used only where nothing else will do.

    A move to install underground transmission cables may require a conversion from AC to DC transmission.

    Reply
  5. Tomi Engdahl says:

    German Student Creates Device that Harvests Energy From Air
    http://www.designnews.com/author.asp?section_id=1386&doc_id=260486&cid=NL_Newsletters+-+DN+Daily

    A German student has designed an energy harvester that derives energy from electromagnetic fields in the air to recharge small batteries and appliances. Dennis Siegel, who is studying digital media at the University of the Arts in Bremen, Germany, outlines the design of the harvester on his website.

    With the energy harvested, it’s possible to charge a small battery in just a day depending on the size of the electromagnetic field.

    Reply
  6. Tomi Engdahl says:

    Animals ‘scared’ by bursts of light from power cables
    http://www.bbc.com/news/26548483

    Animals around the world could be scared away from power cables because these give off UV flashes invisible to humans, scientists have said.

    The findings, published in the journal Conservation Biology, claimed habitats and migration could be disrupted.

    The flashes, or corona, occur when charge builds up in a cable and is released into the air.

    The first animal to reveal its UV sensitivity was the reindeer. And, as the researchers explained, reindeers’ avoidance of the power lines running across the Arctic tundra was part of the inspiration for this project.

    coronas “happen on all power lines everywhere”

    Reply
  7. Tomi Engdahl says:

    Sniffing out an intermittent high-voltage fault
    Application Update: Troubleshoot a high-voltage electrical fault and see recommended solutions.
    http://www.controleng.com/single-article/sniffing-out-an-intermittent-high-voltage-fault/4ca385e18ca1cf5a65456d65e1f2ed7b.html

    Blown fuses on one of its 12.5 kV distribution feeders had shut down part of the plant. Unfortunately, it happened around noon on two days in a row. Power had been restored without finding any problems on both days. Pressure was mounting to find and correct the problem before noon the following day.

    All the evidence suggested this was the arc location and a classic example of what one might expect to find in the aftermath of an arcing fault.

    Reply
  8. Johnk106 says:

    A big thank you for your article.Really thank you! Cool.

    Reply
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