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DC cable loss

  1. May 24, 2012 #1
    i doing the solar system design.

    seem like everything is on track now,
    but just the cable loss i not really know how to calculate,
    can anyone help me.

    i has formula on this
    (resistance on the cable (Ω/km)x length of the cable(L) x current of the solar panel(A)).

    for the equation above , is that correct for calculate the cable loss.
     
    Last edited by a moderator: May 29, 2012
  2. jcsd
  3. May 24, 2012 #2
    That calculation will give you theVOLTAGE lost in the cable.
    The POWER lost is given by r x I^2
     
  4. May 24, 2012 #3
    yes is for the power loss p=I2 x R.
    how about the voltage drop across the distance
     
  5. May 24, 2012 #4

    vk6kro

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    (resistance on the cable (Ω/km)x length of the cable(L) x current of the solar panel(A)).

    The total cable voltage loss would be double this because the current goes out in one wire and comes back in the other wire, so you get two voltage drops.
     
  6. May 24, 2012 #5

    what you mean 2 voltage drop is because of the positive and negative.
    if let say i has 3 input to the inverter , so that my voltage drop will be 6 time of the calculation i provide.
     
    Last edited by a moderator: May 29, 2012
  7. May 24, 2012 #6

    vk6kro

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    If you have a 2 wire cable and a load at the other end, then current leaves the positive side of the supply, goes down one side of the cable, through the load and then back down the other side of the cable to the negative side of the supply.

    So, you get two voltage drops in the cable and the load gets the supply voltage minus the two voltage drops.

    Why would you have 3 inputs to an inverter?
     
  8. May 24, 2012 #7
    for the SMA tripower is has 6 input on it.
    due to the voltage and current limitation ,
    so that i using 3 input.

    for the voltage drop x 2 is base on 1 input right.
    it wont be affect the 2nd input

    thanks
     
  9. May 24, 2012 #8

    sophiecentaur

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    The two wires from the array to the inverter are like two resistors in series, one in the positive lead and one in the return . You can just add the two together together to find the total additional series resistance (obviously doubled if the two wires are the same). If you want to know how many volts will be dropped across this 'loss' resistor, you need to know how much current will be flowing, then:
    Vdrop = I times Rseries

    It is not clear what you mean about Three Inputs. The Spec sheet seems to imply just a single DC input.
    Are you considering connecting several PV banks independently / in parallel? How feasible is this?
     
  10. May 24, 2012 #9
    the total power i need to achieve is 17000W,
    so that i need to connect the panel in series to achieve.
    total i has 3 series connect to the inverter.
    is mean i has 3 input and 6 wire.
    for the voltage drop with the formula i provide i need to be x 6 right ??
     
  11. May 24, 2012 #10

    vk6kro

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    Maybe there is an English problem. Could you draw a picture or a circuit diagram of what you intend to do?

    A 17 KW solar system with 3 phase output is a very serious project and you need to get it installed professionally.
     
  12. May 25, 2012 #11

    sophiecentaur

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    Professional certification would.be essential if there is a plan to connect to the mains.

    The total DC voltage drop would be due to the current times the sum of all series resistances. Their actual position in the 'loop' is not relevant.
     
  13. May 25, 2012 #12
    i has attach the file in PDF .
    i hope that you the image can give you a clear vision what i doing now

    Thanks for the helping
     

    Attached Files:

    Last edited by a moderator: May 29, 2012
  14. May 25, 2012 #13

    davenn

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    ok nice pic .... so each cable from each 250W panel array will have its own set of losses

    BTW how do you think you are going to get 17kW out of a 750W total panel array ??
    you cant create energy out of nothing

    Dave
     
    Last edited by a moderator: May 29, 2012
  15. May 25, 2012 #14

    sophiecentaur

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    Not really clear, I'm afraid. Does your DC switch just connect the PV outputs in parallel? Why would you want to connect only two out of the three panels at any given time?
    Your arithmetic is not clear, either as you have already confused Davenn with your missing zero (?)- and also me.
    From the spec sheet, it appears that the inverter can handle a wide range of input DC voltages but why exceed the normal operating range? (seems to go up to 800V on the sheet I have read but I may be looking at the wrong model)

    This is quite scary stuff. In the words of the prophet "I hope you know what you're doing" :smile:
     
    Last edited by a moderator: May 29, 2012
  16. May 27, 2012 #15
    beside that the diagram i draw above is a single line diagram.
    for this inverter is connect to the national grid for applying the FIT.

    in the mean while,my total panel generating output power is approximate 17250W +/- connect to the inverter .

    the inverter has a wide range of DC voltage is approximate 400- 800V.
     
  17. May 28, 2012 #16

    sophiecentaur

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    You are asking about the DC side? The diagram is not complete or clear. There can be no answer until you address my queries. I suspect that your electricity supplier may also want answers before he is prepared to connect you. They only do business with qualified contractors afaik and would require more information about the safety and control provisions on your DC side, I'm sure. (More than just your incomplete line diagram)

    If you are relying on just the forum for support then I question whether you should be involved with this project.
     
  18. May 29, 2012 #17
    The SMA inverter will "manage" most of the utility issues, but for the DC - Solar side each of the 3 Solar Strings should/will be calculated separately regarding losses - but you should also look at the voltage drop ( basically your first formula. -- for example if the 1st String is closer to the inverter and the 3rd string is farther away, the 3rd string will have a lower voltage at the inverter.
    Quite certain that model has boost on the DC inputs, allowing it to maximize energy "harvest" across the three strings.
     
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