Fusing parallel strings in large array

freddyuk
freddyuk Registered Users Posts: 11 ✭✭
in Off Grid Solar & Battery Systems #1
Hi all first post hereon so be gentle.
I am helping install an off grid battery charging array of 6kw total. 8 separate strings of 3 x LG 250wp panels rated at 24 isc each string. So 6kw in all. Propose to use an 8 way combiner box to provide fusing and circuit breaker with cartridge fuses.(Schurter or similar din rail mounted) What size fuses for each string would you suggest? The cable is 6mm2 over 12 meters to charge controllers. Fuse holders are rated for 30amps each. I hope the batteries will be fused separately so we just protecting the cables to the panels and provide inbuilt string disconnect feature via fuse holders.
Thanks
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: Fusing parallel strings in large array
    freddyuk wrote: »
    8 separate strings of 3 x LG 250wp panels rated at 24 isc each string.

    Welcome to the forum.

    To answer your question, we must know the Isc, but the Isc of 24 amps for each string does not sound correct. How did you come up with that number?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Fusing parallel strings in large array

    Welcome to the forum.

    I think you've got your panel specs wrong; they will not have a string Isc of 24. When panels are placed in series the current does not go up, only the Voltage.

    Does this look like the specs for a single panel:
    250 Watts Vmp 31.1 Imp 8.05 Voc 37.8 Isc 8.28

    When you put three panels in series you'll have a string with specs like this (using the above single panel specs):
    750 Watts Vmp 93.3 Imp 8.05 Voc 113.4 Isc 8.28

    Each string would have a breaker/fuse on it of no larger than the maximum series fuse rating for a single panel. In this case 15 Amps.

    Eight of these strings in parallel would give you an array of:
    6000 Watts Vmp 93.3 Imp 64.4 Voc 113.4 Isc 66.24

    Be advised there is no single charge controller that can handle the output current of that array even at 48 Volts nominal, as it would be around 96 Amps @ 48 Volts. A MidNite Classic can only handle that current @ 12 Volts, and it would be four times greater at that Voltage level.

    This needs to be split into two separate arrays with two separate charge controllers.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #4
    Re: Fusing parallel strings in large array

    Are you sure it is ISC =24. In a 'string' you add the voltage not the amps so in this case you should have ~100 Volts and 8.37 A, total array would produce 100V and 66.96A. When you 'combine' strings you add the amps...

    spec at http://www.wholesalesolar.com/products.folder/module-folder/LG/LG250S1C-G2.html
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • freddyuk
    freddyuk Registered Users Posts: 11 ✭✭
    #5
    Re: Fusing parallel strings in large array

    The panels are in parallel ie. 3 x 250wp = 37voc / x 8amp x 3 = 24 amps. Each "array" will go to a combiner box where they are fused with disconnect. The controller input from the combiner box will be configured so it is within spec for each controller. There will not be one controller. Does that make sense?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Fusing parallel strings in large array
    freddyuk wrote: »
    The panels are in parallel ie. 3 x 250wp = 37voc / x 8amp x 3 = 24 amps. Each "array" will go to a combiner box where they are fused with disconnect. The controller input from the combiner box will be configured so it is within spec for each controller. There will not be one controller. Does that make sense?

    In a word, no.

    You say in your first post you have 6kW of panel. 3 * 250 is 750 Watts. Multiple by 8 strings you get 6kW.

    It does not make sense to say you have a string of three panels in parallel. Parallel is negative to negative and positive to positive. If you do this times eight what you have is twenty-four panels in parallel. You can not parallel three panels and put them on one fuse or breaker.

    Normally you would construct a string of three panels in series: (-)PANEL(+)--(-)PANEL(+)--(-)PANEL(+)

    And then parallel those strings at their negative and positive connectors, running each string through its own fuse/breaker.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #7
    Re: Fusing parallel strings in large array

    OK that make sense the way you describe it now as far as the amps go. However we would not generally call the group of 3 an array as it is not in series

    To clarify: you wrote "8 separate strings of 3 x LG 250wp panels "
    the term string means in series so it would look like this (-)---[]----[]----[]----(+)

    so to us you were originally saying 'I have 8 separate strings (groups) of 3 panels in series"
    and that the 8 strings are in parallel...

    cheers
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • freddyuk
    freddyuk Registered Users Posts: 11 ✭✭
    #8
    Re: Fusing parallel strings in large array
    westbranch wrote: »
    OK that make sense the way you describe it now as far as the amps go. However we would not generally call the group of 3 an array as it is not in series

    To clarify: you wrote "8 separate strings of 3 x LG 250wp panels "
    the term string means in series so it would look like this (-)---[]----[]----[]----(+)

    so to us you were originally saying 'I have 8 separate strings (groups) of 3 panels in series"
    and that the 8 strings are in parallel...

    cheers

    The guy wants to connect 3 panels together in parallel and have 24 amps from each array. There are 8 of these. Not going through one controller but separate controllers. I am only sourcing the cable and combiner box where he can disconnect each array via 8 breakers and have fuse protection. Why can't he connect three panels in parallel?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
    Re: Fusing parallel strings in large array

    you can parallel pvs, but any time there is more than 2 in parallel then each pv needs its own fuse or circuit breaker before being combined. you cannot combine 3 or more pvs then put 1 fuse or cb with a disconnect as something going wrong with one pv could pump too much power into that particular pv and create a bad situation not just for power production, but for safety concerns as well.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #10
    Re: Fusing parallel strings in large array
    freddyuk wrote: »
    The guy wants to connect 3 panels together in parallel and have 24 amps from each array. There are 8 of these. Not going through one controller but separate controllers. I am only sourcing the cable and combiner box where he can disconnect each array via 8 breakers and have fuse protection. Why can't he connect three panels in parallel?

    How many charge controllers? Usually you need one combiner box per controller. Thus: one array of panels is combined to make an output that goes to one controller. If you consider three panels to be an array, then you need 8 combiner boxes and 8 controllers.

    Each solar panel gets its own fuse (or circuit breaker) in the combiner box. The combined output of each combiner box should also have a circuit breaker.

    You can get better design advice from this forum if you tell us more about the system... which controller, how many, what battery voltage, etc.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • freddyuk
    freddyuk Registered Users Posts: 11 ✭✭
    #11
    Re: Fusing parallel strings in large array

    Sorry I got involved in this to supply a few bits and struggled to find out the whole picture. Thus I am trying to understand the possible scenarios. There is a chain and I am at the end. Due to time issues I needed to find solutions quickly which means looking a bit foolish at times. Apologies and thanks for the clarity. My question has always been why are we using such high amps to charge batteries as it causes all these issues. I still don't have the answer but told my contact that if we are to help we need to know the full story. I understand much of the kit has been bought but we should be able to re design the stringing to have series strings with higher volts and lower amps. Then we can parallel 2 strings of 3 panels into each breaker. There are 4 controllers but I still don't know what they are. The system may be 24 volts (I hope) but again I am awaiting clarification. So each controller will take 6 panels at roughly 100 volts/ 16 amps. Once we know what the controllers are we can decide if they can handle the load. I assume you can do all this into one box but the circuits are separate from there to each controller. My option would be a SMA Sunny Island but the kit has been bought already. You have been very helpful as until you have got involved in any project it is often difficult to understand the detailed semantics. I have taken in the information given so thanks for that.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: Fusing parallel strings in large array
    freddyuk wrote: »
    My question has always been why are we using such high amps to charge batteries as it causes all these issues.

    "High Amps" is a relative term.
    The amount of panel you describe (6kW) would supply over 90 Amps @ 48 Volts which is indeed high. It would be sufficient to charge a massive 900-1000 Amp hour 48 Volt battery bank. That's like 24kW hours of stored power.

    Like you, we don't really know what's going one here. It's difficult to give accurate answers without all the details. If there are four controllers, then the array is broken into four separate array and each one will have its own panels/breakers/combiner. The controllers can work together to charge the same battery bank, or separate banks supplying individual inverters or multiple banks available to one or more inverters.

    "The kit has been bought already." That is the worst possible news. It's known as "Ready, Fire, Aim!" around here.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #13
    Re: Fusing parallel strings in large array

    we could elaborate further such as if the 3 pvs in each array are in series for a 24v system then an mppt type controller would be needed to handle the higher voltages.

    it may be better to wait to hear exactly what was purchased and what he has in mind otherwise we are only speculating.
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  • freddyuk
    freddyuk Registered Users Posts: 11 ✭✭
    #14
    Re: Fusing parallel strings in large array
    niel wrote: »
    we could elaborate further such as if the 3 pvs in each array are in series for a 24v system then an mppt type controller would be needed to handle the higher voltages.

    it may be better to wait to hear exactly what was purchased and what he has in mind otherwise we are only speculating.

    Well I walked away as I could not find out any information so did not want to be involved or responsible for supplying the wrong parts. It seems that others had been involved and also decided not to help out! I could not even find out what the controllers were. Thanks for the good advice.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #15
    Re: Fusing parallel strings in large array
    freddyuk wrote: »
    Well I walked away as I could not find out any information so did not want to be involved or responsible for supplying the wrong parts. It seems that others had been involved and also decided not to help out! I could not even find out what the controllers were. Thanks for the good advice.

    I think you made the right choice.
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