Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

120V AC Supply to Pull Boxes in station yard (Design Problem)

  1. Aug 8, 2013 #1
    We have ~78 underground pull boxes (12ft by 12ft) scattered around our 700m by 1500m yard that are passing 12kV cables through underground ductwork.(i.e. pipes)

    See figure attached for detailed drawing of layout including pull boxes, station service transformers and control buildings.

    Recently, water has started seeping into these pull boxes, and this water needs to be pumped out. Only problem is, there is no AC power available inside these pulls boxes to power the pump needed to pump the water out!

    So the design challenge we’re faced with is to come up with a simple, efficient and cost effective solution that will provide each of these ~78 pull boxes with AC power such as a standard 120V outlets and lighting.

    The outlets will only need to handle loads as large as the pump and the necessary lighting, nothing else.

    So far we’ve come up with a couple ideas:

    1) Take the 600V (line-line) feed from the station service transformers (denoted SST1 and SST2) and route these to the nearest pull boxes where each individual pull box will have its own separate transformer needed to step down the voltage to the desired 120V.

    2) Route the 600V (line-line) feed from the station service transformers (denoted SST1 and SST2) and to a couple strategically placed 600V/120V transformers and supply multiple pull boxes from this one transformer. With this option we could have issues with voltage drop across the 120V cables depending on how far the distances are from the 600V/120V transformer to the pull box.

    3) Take the existing 120V AC feed from each of the control buildings within the yard (denoted CBF1, CBF2, CBF3 etc…) and somehow connect this to the surrounding pull boxes. However, there is a problem with this. The cabling for the control buildings is in cable trenches, while the cabling for the underground pull boxes is in underground cable ducts. (i.e. pipes) So in order for us to get a cable from the control building into the pull box, it would require for drastic changes to existing cable trenches in order to route the cable inside the trench to the pullbox. (Unless there is an easier way?)

    We do have 12kV cables at our disposal coming out of the two switchgear buildings 1 and 2 (denoted SGB1 and SGB2) but transformer needed to get a 120V AC from 12kV may be larger and more expensive than practical.

    Does anyone else have any other creative/inventive ideas in order to solve this problem, or see any advantages/disadvantages with the three options we’ve come up with so far?

    I think some group brainstorming is always a good idea!

    Thanks again!

    EDIT: I just thought of another idea. How about rectifying the 600V AC (line to line), running it into the pull boxes as DC and converting it back into 120V AC? One rectifier feeding all the DC cables and one inverter per pull box. Just popped into my head and figured I'd throw it out there.
     

    Attached Files:

    Last edited: Aug 8, 2013
  2. jcsd
  3. Aug 8, 2013 #2

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    How often [STRIKE]to[/STRIKE] do you access these pull boxes? Can't the 12kv cables be in water? Is the water coming through the underground ducts?

    If the cable insulation can be in water, just pump out the boxes when access is required and save a butt load of money.
     
    Last edited: Aug 8, 2013
  4. Aug 8, 2013 #3

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    It's generally frowned on to run high voltage lines through water. The possibility of shocks, short-circuits, damage to the lines and other equipment, make it prudent to keep the ducts dry.

    Sounds like somebody cut too many corners with the original installation. For the plant's own protection, both from a technical and legal standpoint, I would urge them to seek a more professional solution than posting the problem on an anonymous board and seeing what the members throw at it. There are certainly more things going on here than what has been described.
     
  5. Aug 8, 2013 #4

    dlgoff

    User Avatar
    Science Advisor
    Gold Member

    Totally agree
     
  6. Aug 8, 2013 #5

    etudiant

    User Avatar
    Gold Member

    This seems a lawsuit in development.
    If water is seeping into the pull boxes, then surely the underground ducts and pipes are also at risk.
    So the integrity of the system is threatened even if the pull boxes are pumped out.
    As a first stab at this, would it be more efficient to use 600V pump motors? It surely would cost more for the pumps but might be rather simpler to install than the stepdowns or rectifiers discussed above, as 600V lines are available.
    Separately, would it be possible to keep water out of the pull boxes by filling then with some insoluble material such as poly foam? That would not work if frequent access to the boxes is required, but as a one time fix, it might be simple and cheap enough to look at.
     
  7. Aug 8, 2013 #6

    wirenut

    User Avatar
    Gold Member

    First I have a few questions.
    1) where are you located?
    2) what type of cable is installed in these boxes?
    3) what type of boxes are these?
    4) what is the 12kv for?
    5) what are the pipes made of? rgc, pvc, other?
    6) is the water infiltration a new problem?


    Now a couple of concerns with your ideas.
    1)Options 1,2,3 violate the nec.
    2)the protective equipment required to just access these boxes while energized is expensive and bulky.

    If the cables are correctly installed and terminated there should not be a concern if they end up under water, the power company underground conduits that carry up to 35k are not water tight and thus always have water in them. Also tap boxes installed correctly have drainage provisions. No underground conduit can be made 100% water tight. Do you follow arc flash procedures when you access these? If not, PLEASE TAKE AN ARC FLASH COURSE.
    Why can't you use a small generator on the back of a truck to run the pump and a light tree? or a similar set up?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: 120V AC Supply to Pull Boxes in station yard (Design Problem)
  1. AC and DC power supply (Replies: 3)

Loading...