Off grid PV with both thin film and monocrystalline questions

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• BB. Super Moderators, Administrators Posts: 33,079 admin

  September 2014 #2

  Re: Off grid PV with both thin film and monocrystalline questions
  
  Welcome to the forum Vangelis!
  
  Nice sized system there. A few comments...
  
  First, do not ground any of the PV Panel negative power terminals/wiring with a 2 amp fuse to ground. If these panels have metal frames (not electrically connected to the PV +/-), then ground the frames to your system ground just like the other panels are. You want any short circuit to the framework to go to the safety ground (not make the frames/mounting racks "hot").
  
  Because your battery bank main negative bus is already safety grounded (that is fine), you really do not need (or even want) any fuses in any of the negative/return wires. The fuses are (if everything is properly wired) in the negative leads are redundant. And worst case, if you have a return fuse blow, it still leaves the device powered from the "hot" + fused circuit. Ideally, you want the positive fuse to blow only... Then everything on that branch circuit is now "dead".
  
  I have to be a little careful here--Generally in the US, we used "RED" for "Hot" or + wiring--Yours is reversed (Red=negative). So any of the panels that have fusing in negative should be moved to the positive lead.
  
  Also, you show a bunch of wiring going to your battery bank and no fuses/breakers. With a negative grounded battery bank, you do not need any fuses/breaker in any leads to the - battery bus.
  
  On the positive bus, every wire that leaves the positive bus should have a fuse/circuit breaker that is rated at no more than the current capacity of that branch circuit wire leaving the bus. To charge controllers, to loads, to AC inverter, etc. Fuses+Breakers are there to protect the wiring from too much current and starting a fire.
  
  Note that the array string to charge controller does not really need any fuses/breakers.
  
  Next issue... For arrays with (typically) 3 or more parallel panels, each panel should have its own series fuse. If, for example the panel has a 15 amp series fuse rating, then each panel should have its own 15 amp fuse/breaker.
  
  Because you are using MPPT charge controllers, you can put two 36 volt panels in series and make for a 72 volt Vmp string--That will allow you to use smaller diameter wire with less voltage drop. And you will have fewer parallel panel connections--Fewer fuses/breakers per array.
  
  I cannot tell from your drawing--But charge controllers should be close to the battery bank (low voltage drop for accurate battery charging). Long wire runs should be from the array to the charge controller.
  
  AC inverter DC wiring from the battery bank to the AC inverter should also be short/heavy wire to limit voltage drop and save on the price of heavy copper cables.
  
  And regarding the ground wire from the battery bank negative bus to your Ground rod/ground bus--The battery ground wire should be able to carry the maximum amount of short circuit current that you would expect. Nominally, it should be as heavy as the largest DC circuit in your power system (typically the DC input to your AC inverter).
  
  That is a quick start--Your thoughts?
  
  -Bill

  Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  

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• vaggianak Registered Users Posts: 7 ✭

  September 2014 #3

  Re: Off grid PV with both thin film and monocrystalline questions
  
  First of all thank you for your time and kind advice.
  
  1. It was Solibro's advice to ground the thin film modules this way. The modules are frameless. They indicated "Q.SMART UF L modules are technically identical to Solibro’s SL2 modules and should be negatively grounded to prevent potential induced degradation (PID)". In their technical datasheet is claimed that I should use a 2A fuse for each module string. Every string has Isc = 1.6 A
  
  Let me see if I understand your advice right.
  

  Because your battery bank main negative bus is already safety grounded (that is fine), you really do not need (or even want) any fuses in any of the negative/return wires. The fuses are (if everything is properly wired) in the negative leads are redundant. And worst case, if you have a return fuse blow, it still leaves the device powered from the "hot" + fused circuit. Ideally, you want the positive fuse to blow only... Then everything on that branch circuit is now "dead".

  Do you mean that I should remove the ANL fuses from the negative wires and leave the ones in the positive?
  

  I have to be a little careful here--Generally in the US, we used "RED" for "Hot" or + wiring--Yours is reversed (Red=negative). So any of the panels that have fusing in negative should be moved to the positive lead.

  Fuses connected to the wires from the negative pole of the panels to the charge controllers must be placed in the positive wires between panels and charge controllers?
  

  On the positive bus, every wire that leaves the positive bus should have a fuse/circuit breaker that is rated at no more than the current capacity of that branch circuit wire leaving the bus. To charge controllers, to loads, to AC inverter, etc. Fuses+Breakers are there to protect the wiring from too much current and starting a fire.

  Agree. Concerning the wires from the batteries to the inverter, the cable is 35 mm2, so the appropriate fuse is 125 A. The fuse is chosen according to the wire diameter, right?
  

  Next issue... For arrays with (typically) 3 or more parallel panels, each panel should have its own series fuse. If, for example the panel has a 15 amp series fuse rating, then each panel should have its own 15 amp fuse/breaker.

  Each panel is rated with Isc = 1.6 A. The technician from Solibro advised me to use a diode if I connect more than 3 panels in series, but in case of one by one parallel connection, it's not obligatory to use fuses or diodes. Would you agree?
  

  I cannot tell from your drawing--But charge controllers should be close to the battery bank (low voltage drop for accurate battery charging). Long wire runs should be from the array to the charge controller.

  These wires are 2 metres at most.
  

  And regarding the ground wire from the battery bank negative bus to your Ground rod/ground bus--The battery ground wire should be able to carry the maximum amount of short circuit current that you would expect. Nominally, it should be as heavy as the largest DC circuit in your power system (typically the DC input to your AC inverter).

  There you have a point. I decided to replace the 16 mm2 wire with a "heavier" one, just in case.
  
  Well, it's the first time I ground the negative pole of the batteries, and the first time I use thin film panels of which the negative pole must be grounded. All these, in addition to metal parts etc. are connected to the same grounding system. This confuses me a little. But the grounding system must be common according to our regulations. Is it ok?
  Also, what if someone touches the metal part of the tilted mounting construction of the panels (which is also grounded) and the positive pole of a battery?
  
  What about my question about the voltage drop of the batteries when the immersion heating resistance goes on? Is it normal?
  
  Your help is valuable, thank you
  
  Vangelis

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• inetdog Solar Expert Posts: 3,123 ✭✭✭✭

  September 2014 #4

  Re: Off grid PV with both thin film and monocrystalline questions
  
  Partial answer: If your panels are in parallel, then the common point of the negative leads from all of the panels need to be grounded and the panel frames must also be grounded.
  If you put two panels in series, the negative lead of the first panel in the string will be grounded and both of the panel frames must be grounded.
  
  If you touch a panel frame and a positive battery lead at the same time, you will feel the corresponding voltage. If you bridge them with a wire, you will blow the fuse in the positive battery lead. An additional fuse in the negative battery lead will not provide any additional protection.
  
  The rule is that all ungrounded conductors need a fuse or other overcurrent protection device (OCPD). If neither panel lead is grounded (as with some grid tie inverters) then you need OCPD in both leads.

  SMA SB 3000, old BP panels.
  

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• BB. Super Moderators, Administrators Posts: 33,079 admin

  September 2014 #5

  Re: Off grid PV with both thin film and monocrystalline questions
  

  vaggianak wrote: »

  First of all thank you for your time and kind advice.
  

  1. It was Solibro's advice to ground the thin film modules this way. The modules are frameless. They indicated "Q.SMART UF L modules are technically identical to Solibro’s SL2 modules and should be negatively grounded to prevent potential induced degradation (PID)". In their technical datasheet is claimed that I should use a 2A fuse for each module string. Every string has Isc = 1.6 A

  
  It probably depends on the solar charge controller. Most (but not all) Solar Charge controllers, the negative Vpanel input connection is directly connected to the Vbatt negative output connection.
  
  So, connecting the battery negative bus to your ground rod/cold (metal) water pipe, etc. -- And the negative wiring to the charge controller/negative solar connection should be just fine.
  
  I have no idea what a 2 amp fuse per module or string (remember, 7 panels in parallel can easily blow a 2 amp fuse if something goes wrong.
  
  I believe that this can also cause other issues--You have two parallel paths back to the battery--One through the array negative leads, and a second through the 2 amp fuses + grounding connection. You could easily pop the fuses due to voltage drop difference between the two parallel wire runs--The ground fuses--if they blow, you will not really see any changes (unless you inspect the fuses). The system will run about the same, either with or with blown fuses.
  

  Let me see if I understand your advice right.
  
  Do you mean that I should remove the ANL fuses from the negative wires and leave the ones in the positive?

  
  If you have a "solid" (large gauge copper) ground wire from the battery negative bus to the building Ground Rod/Cold Water Pipe and the rest of the building safety ground--Then the negative battery bus should even get much different than zero volts relative to grounds--And therefore, a short between negative and grounded metal (sink, conduit, etc.) will have near zero current flow.
  

  Fuses connected to the wires from the negative pole of the panels to the charge controllers must be placed in the positive wires between panels and charge controllers?

  
  For a negative grounded battery bank--You would need one fuse (or breaker) between the panel and the + solar bus.
  
  Using "typical" 100-200+ Watt panels, you will find that the Series Fuse requirement is usually about 15 Amps. And the panel Isc is around 8 amps or so (short circuit current). So, if you have one or two panels in parallel to the charge controller, no fuses were needed.
  
  If you have 3 or more panels (or panel strings with two or more panels in series), each string of panel(s) would need a 15 amp fuse/breaker. This is to prevent a short circuit in one string from being feed too much current from the other 2 or more parallel strings.
  
  For "some" Thin Film panels the Series protection fuse may be 10-15 amps, but the Isc is only 1.5 amps or so you may be able to put 5-10 panels in parallel before you would need to install series protection fuses.
  
  Anyway, from the manual:
  
  http://solibro-solar.com/uploads/media/Solibro_Installation_and_Operation_Manual_SL2_Modules_G1-5_2014-07_Rev06_EN.pdf
  

  Parallel connection

  • Only connect modules of the same type and power class.
  • Do not exceed the maximum permissible reverse current as given in the respective data sheet. In the event of reverse currents (caused by module defects, ground leakage or shading), modules can be placed under strain.

  
  In order to safely account for reverse currents, we recommend the
  following fuse variants:
  
  Layout with limit on the number of strings operated in parallel

  • Without further measures for current limitation, only a maximum of 3 strings of modules may be operated parallel on an inverter.

  
  Layout with string diodes

  • When more than 3 strings are connected parallel, a maximum of 3 strings respectively must be protected against reverse currents from the remaining system with a shared string diode.

  
  Layout with string fuses

  • In this case, each string of modules must be protected with 5 A on the plus and minus side.

  
  
  I don't know enough about these panels--But you may have to use a Diode per string. Some thin film panels do not tolerate being in shade while having their Vmp voltage across them (I think--Otherwise, it does appear that a 5 amp series protection fuse is required for these panels if diodes are not used).
  

  Agree. Concerning the wires from the batteries to the inverter, the cable is 35 mm2, so the appropriate fuse is 125 A. The fuse is chosen according to the wire diameter, right?

  
  Basically--Yes. Details do matter. Type of insulation, ambient temperature, if the wiring is in conduit/piping, etc. all matter (in US code, we "derate" the current rating of the wiring for high temperatures, etc.).
  
  Also--You size the copper wiring diameter based on how long the wire run is. For a 12 volt battery system, you want about 0.5 to 1.0 maximum wiring drop (from battery to AC inverter or other load). If you have longer wire runs, you may need heavier copper wiring to prevent too high of voltage drop.
  

  Each panel is rated with Isc = 1.6 A. The technician from Solibro advised me to use a diode if I connect more than 3 panels in series, but in case of one by one parallel connection, it's not obligatory to use fuses or diodes. Would you agree?

  
  The manual seems to say differently (link above).
  

  These wires are 2 metres at most.

  
  You can find a voltage drop calculator online or a table for your wiring. I am in the US--So mm2 is not something that I use a lot.
  
  Here is one that may work for you:
  
  http://www.epanorama.net/index.php?index=calc_cable
  

  Well, it's the first time I ground the negative pole of the batteries, and the first time I use thin film panels of which the negative pole must be grounded. All these, in addition to metal parts etc. are connected to the same grounding system. This confuses me a little. But the grounding system must be common according to our regulations. Is it ok?
  Also, what if someone touches the metal part of the tilted mounting construction of the panels (which is also grounded) and the positive pole of a battery?

  
  Different countries do have different customs regarding grounding. And even in the US--Groundingis one of the most complicated questions we deal with here.
  
  A true floating power system will not usually cause a shock if one lead is touched. If you ground reference that output (battery, transformer, etc.), then touching ground and a "hot lead" gives you a complete circuit--And a shock (or worse) is possible. In the US, floating power sources are not common (for building scale power systems).
  
  For a 12 volt battery system--You will not generally get shocked if you touch +/-.
  
  For 24 volt battery system--If your hands are sweaty, you can get a "nip" touching +/-.
  
  For higher voltage systems, the shock hazard increases--And 60 VDC is the maximum that any safety standard I have seen be called "touch safe" (again, with the right connections, you can get a fatal shock with less than 60 volts).
  
  With good grounding practices (heavy enough copper cable between "neutral or negative" and "earth/safety ground")--A short circuit (broken wire shorted to aluminum frame, metal kitchen sink, etc.) will pass enough current and pop the fuse/breaker.
  

  What about my question about the voltage drop of the batteries when the immersion heating resistance goes on? Is it normal?

  
  Lead Acid batteries will have a depressed output voltage if hit with a heavy load... But it is all relative.
  
  For flooded cell storage batteries (C is the AH capacity of battery at 20 Hour discharge rate--For example, a 100 AH battery discharge at 5 amps will go dead in 20 hours):
  

  • C/20 is the typical average current draw for a cabin (5 hour per night, 2 nights, 50% discharge).
  • C/8 is the typical maximum continuous discharge rate
  • C/5 is the typical short term maximum discharge rate (hour or so)
  • C/2.5 is the typical maximum surge current discharge rate (seconds to minutes--starting a well pump, etc.).

  
  AGM and GEL batteries have much better surge capabilities--C/1 to even as high as C*4 discharge rate (drain battery dead in 15 minutes--Used for computer UPS power systems, etc.).
  
  Making sense?
  
  -Bill

  Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  

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• inetdog Solar Expert Posts: 3,123 ✭✭✭✭

  September 2014 #6

  Re: Off grid PV with both thin film and monocrystalline questions
  
  I disagree on the series fuse question.
  If the panel is rated for an Isc of 1.6A, the panel nameplate probably specified a maximum series fuse size of 5A or less.
  The situation is that with only one panel in parallel, the maximum back current through a damaged panel would be ~1.6A, and it should handle that fine.
  Add just one more panel in parallel and the back current becomes 3.2A, which may be more than the damaged panel could take. Hence the general rule that with more than three panels or strings in parallel they must all be fused.
  If the nameplate data says that you can use a 10A series fuse (what does it say?) then you might think that you could group as many as say 5 panels on a fuse, but if you have a total of 10 panels, in two groups of 5, then one damaged panel can see 5 x 1.6 + 4 x 1.6 without a fuse blowing. Not a good thing, as the total current will be 14.4A which is more than 10A.
  This is a rare occasion where I disagree with what I think that Bill said.

  SMA SB 3000, old BP panels.
  

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• BB. Super Moderators, Administrators Posts: 33,079 admin

  September 2014 #7

  Re: Off grid PV with both thin film and monocrystalline questions
  
  I was using a general example (for crystalline and some of the older thin film panels).
  
  In the above case, I looked up the manual from the mfg. for these panels and quoted them.
  
  The part I am not sure about is if these panels in parallel do they need blocking diodes to prevent damage if one (or more) panels are in shade.
  
  These panels are suseptable to PID (Potential Induced Degradation, PID) -- And for this mfg., the panels may be/are permenently damaged if they are not negative grounded (via the PV - lead):
  
  http://solibro-solar.com/en/news-downloads/documents/
  http://solibro-solar.com/uploads/media/Solibro_Product_Information_EN_02.pdf
  

  The risk of Potential Induced Degradation can be prevented for the future by installing negatively
  grounded inverters in place of inverters that are not negatively grounded. If degradation
  has already occurred, the damaged solar panels must be replaced. Alternatively, it may
  be worth considering replacing only the inverter, because a limited reversibility of the degradation
  has been demonstrated, if an only positive potential is applied. Since not all panels in
  a photovoltaic system are subject to power loss due to the degradation effect described, it is
  impossible to predict to what extent the overall performance of the photovoltaic system will
  be affected by this and whether replacing the inverters will turn out to be economically
  worthwhile. When replacing the inverters, please follow the installation recommendations in
  our installation manual, which can be downloaded from www.solibro-solar.com.

  
  -Bill

  Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  

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• vaggianak Registered Users Posts: 7 ✭

  September 2014 #8

  Re: Off grid PV with both thin film and monocrystalline questions
  
  Technicians from Solibro :
  1."Please secure this grounding of the negative pole with a fuse as described in our installation manual on page 17. You are using frameless modules – if your installation is ground-mounted you can use a maximum 2A fuse."
  2."As a reverse current protection, the use of diodes as described by Mr. Matthes is usually required. However, since the “strings” are only composed of one module each, this point may be neglected. Please make sure, however, that you fulfill all national safety and electrical regulations."

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