when positive ground is used in pv

sam csam c Posts: 16Registered Users
I have a set of panels charging batts, the array is too large for the charge controller, the set up and question is ---one string of 3 panels @ 48vdc tied to the batts 10 golf cart @12vdc--- 2 strings (6 panels) 48vdc through a c40 controller, total watts about 1800,--- the panels are pos. ground, how is this going to effect every thing down stream from the panels-- the inverter is a dr1512 thanks sam

Comments

  • BB.BB. Posts: 27,588Super Moderators admin
    Re: when positive ground is used in pv

    What do you mean by Positive Ground? Generally the panels are not grounded at all--and require you to make the ground connection...

    If you have the Sunpower panels, which require positive grounding (read this thread for technical details and links to Sunpower documents)--yes, you need to positive the ground the "+" terminal from the string. If you don't and use a "-" ground, you can lose multiple ten's of percent worth of output because the panel accumulates charge. The charge can be quickly removed by positive grounding (hours?).

    As far as I know, the other standard silicon panels can be positive or negative grounded without issue.

    You can positive ground some MPPT charge controllers (I think the MX-60 can be positive ground--but read the manual)--you have to be real careful--much "car type" DC equipment assumes negative ground and the sheet metal chassis will now be "hot"... Positive ground is used by telephone companies and for cathodic protection systems (to prevent, for example, buried metal gas pipes from corroding).

    I would avoid positive grounding and off-grid system unless you have no other option (or need positive ground reference for some reason) and have checked out each piece of equipment you plan to use and understand where chassis ground vs positive grounded battery bus are each referenced.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: when positive ground is used in pv

    i'm confused here. what is the battery system voltage you are using?
  • boBboB Posts: 949Solar Expert ✭✭✭✭
    Re: when positive ground is used in pv

    This should work with a C40 EXCEPT that you will not be able to ground the battery plus at the same time because the C40 (and MX60) work
    between the positive input and output terminals. Connecting both input and output positives to ground would short the two together.

    The negative ins and outs are already connected together internally.

    Normally, when someone wants or needs to do a positive grounded system, like in a 48 volt telecom installation, they just ground the battery positive.

    These Sunpower modules can be a real problem this way.

    So, I have wondered how you would use these modules if they were installed on, say, a mobile RV application, or at the extremes, on an airplane or an earth orbiting satellite where there isn't a real "ground". I have looked on the Sunpower web site for an app note for this and haven't found one, but I haven't looked in quite a while now.

    boB
  • sam csam c Posts: 16Registered Users
    Re: when positive ground is used in pv

    the info is a good read--- batts are 6vdc series parallel--- 12vdc
    if the + - is a problem why not use a diode in one of the ground wires to stop feed back sam
  • BB.BB. Posts: 27,588Super Moderators admin
    Re: when positive ground is used in pv

    It is not a feeback issue... It is trying to run a positive ground (referenced through the controller) out at the panels and a negative ground reference out at the battery bus (normal 12 volt operation)...

    It cannot be done with any standard solar charge controller... The inputs and outputs are not isolated as is done with a Grid Tie inverter (by a transformer).

    You can find things like DC to DC voltage converters with internal input to output isolation... Here are a couple specifically for 12 volt DC positive ground to 12 volt DC negative ground conversion.

    Obviously, they add cost and losses to your system and are not usually a good solution because of that.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sam csam c Posts: 16Registered Users
    Re: when positive ground is used in pv

    now I get the picture, the thread spoke if high voltage, 600 to 1000 v, Iam at 48v and a little less 39v I think, I have not grounded the panels, the out put ,Iam ok with, (near 50amps) I will ground the frames,(lighting) and let the rest open, unless their is a reason to tackle this + ground problem
  • Solar GuppySolar Guppy Posts: 1,959Solar Expert ✭✭✭
    Re: when positive ground is used in pv

    The C40 is not a down-converting charge controller. If you have 48V panels and a C40, you effectively running at 1/4 the wattage as the C40 just shorts the panel to the battery ( bulk mode ) and on/off at 400 hz for absorb and float

    You need a true mppt charger, MX60, XW-60 ect for your setup
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: when positive ground is used in pv

    you will have to try and either lower the voltage from the pvs or get another controller that can downconvert as you are wasting so much pv power going through a c40 from 48v down to 12v.
  • BB.BB. Posts: 27,588Super Moderators admin
    Re: when positive ground is used in pv

    Sam,

    You also did not tell us the manufacturer of the solar panels--If SunPower brand, then they must be positive ground or you will lose lots of power (besides the issue that Solar Guppy said about using way to high of solar panel voltage for a PWM controller--just throwing 3/4ths of your power away).

    If you have other brands of solar panels, you can just safety ground the "-" battery bus to earth ground and you are pretty much done. (assuming that "+" panel/battery is tied to ground somewhere else. Using a 12 volt brake light bulb between the "-" bus and earth ground will show if you have current flow or not (if the light turns on--then you have a positive ground somewhere in the system).

    A floating DC system is not recommended--you can build up static electric charges and get a few hundred volts potential between your DC battery bus and earth ground (can cause shock and equipment problems).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sam csam c Posts: 16Registered Users
    Re: when positive ground is used in pv

    they are sunpower. a ck with an amp probe shows 50amp to the batt from the controller, what am i missing, ohms law? 12v @50 amps vs 48v @50amps big differnce in wattage
  • BB.BB. Posts: 27,588Super Moderators admin
    Re: when positive ground is used in pv

    Sam,

    What model/rating of panels do you have... SunPower panels (at least the once I can find on their website) are around 210 watts at Vmp=40 volts and Imp=5.25 amps.

    Unless they are the 305 watt commercial panels which are Vmp=54.7 volts at Imp=5.58 amps... Which sounds closer to your 6 panels at 1800 watts...

    If you are getting 50 amps for your 12 volt batteries, that would be 50/6=8 amps per panel assuming all are in parallel...

    If these are truly 1,800 watts = 6x305watt each, then if you were using a MPPT type charge controller, one would expect:

    6x305watts / 17 volts (charging batteries plus controller drop)=107 amps peak

    So, the 50 amps does not quite make sense--that would be over the expected current from 6x5.58amps=33.5 amps through a PWM controller like the C60...

    And, since the panels are ~54.7 Vmp panels--putting two strings of 3 panels in series would not make any sense at all with a C60--Your measured 50 amp would seem to confirm that.

    And if you were using 6 in parallel with a MPPT controller (positive ground capable) and you would expect upwards of 100 amps on a good day (probably normally somewhat less than 80 amps because of overall efficiency)...

    A PWM controller is nothing more than an electronic switch, opening and closing very rapidly between the solar panels and the batteries.

    A MPPT controller is basically a switch mode DC to DC converter... It can efficiently "transform" from higher voltages/lower currents to lower voltages/higher currents (think of a an "electronic" transformer) through the use of internal inductors, FETs, diodes, and capacitors.

    So--when you have a solar panel (or string of solar panels) with Vmp>>Vbatt--using a MPPT controller will be much more efficient.

    So, from the information you have given us so far--the C60 setup still does not make sense (to high of current, and no panels should be in series).

    There are probably other Sunpower panels that I don't know about--but I don't have any specs. for them--so I cannot do much else.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar GuppySolar Guppy Posts: 1,959Solar Expert ✭✭✭
    Re: when positive ground is used in pv

    Also note, running into a 12V battery bank, your going to need more than one true Mppt controller. A MX-60 or XW-60 is limited to 60 amps @ ~12V or 720 watts. If the charging voltage is upto 14V is a little bit more 840 watts but your still 2X over a charge controllers rated amps with 1800 watts of SunPower Panels
  • TelcoTelco Posts: 201Solar Expert ✭✭✭✭✭
    Re: when positive ground is used in pv
    boB wrote: »
    So, I have wondered how you would use these modules if they were installed on, say, a mobile RV application, or at the extremes, on an airplane or an earth orbiting satellite where there isn't a real "ground". I have looked on the Sunpower web site for an app note for this and haven't found one, but I haven't looked in quite a while now.

    boB

    A body/earth ground is not really required, it's more of a convenience/cost savings issue. It is possible to do the ground as a wired network like the live system is, it just requires running a separate wire to the ground. Of course, this does double the wiring cost.
  • sam csam c Posts: 16Registered Users
    Re: when positive ground is used in pv

    well I guess I fly the flag of blissful ignorance,
    you guys are very helpful, and I hope you can walk me through this problem.
    here is what I have
    9 sunpower panels 215 watts vmp 39.8 imp 5.40 voc 48.3 isc 5.80
    10 golf cart batts @6vdc= 12vdc
    c40 charge controller
    dr1512 inverter
    this is a stand alone system, no grid any were
    this is what I did string 1 -3 panels in parallel tied straight to the batts
    string 2-- 6 panels tied in parallel to the c40, my thinking is that string 1 would act as a float charge, as 9 panels is too much for the c40,
    the c40 would act as the charge reg. with string 2 --6 panels.
    my ac load is such that string 1 would never over charge the batts,
    the pos. ground for the panels, from what I understand so far, is for a neutral bond to a grid system, I don't see how an earth ground would do any thing except for a safety ground. I know my batt pack is too small at this point, for the panels, and the c40 is inefficient, my back ground is in hydro gen. I thought the transition would be easy, it is not.
    Iam sure Iam wrong, but this should explain my thinking
    hope you guys can straighten me out, more then thanks for you're help SAM
  • Solar GuppySolar Guppy Posts: 1,959Solar Expert ✭✭✭
    Re: when positive ground is used in pv

    You need to purchase a Mppt charge controller than converts the ~40V vmp @ 5.4 amps into 12V @ 18amps ( each panel )

    Your total wattage is 1935 , divide that by your battery voltage ( 12 V ) 161 and thats how many amps your charge controllers need to be rated for

    The Two most popular Charger controllers are the Outback MX-60 and the Xantrex XW-60, both are 60 amp output capable units, you will need three of them to handle the Sunpower panels

    So, 3 panels wired in parrallel for each of the 3 MX/XW-60 that charge your batteries
  • mike95490mike95490 Posts: 7,723Solar Expert ✭✭✭✭
    Re: when positive ground is used in pv

    Do both controllers (Xantrex & Outback) support + gnd systems (for the sunpower panels ? )

    This is a LOT of amps for 12V but at 24 V, you could get by with just 1 or 2 controllers, and buy a new inverter for 24V, and rewire the batteries for 24V

    9ea x 215w x .8 (STC de-rate) = 1548W / 24v = 64A continuous at solar noon
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Solar GuppySolar Guppy Posts: 1,959Solar Expert ✭✭✭
    Re: when positive ground is used in pv

    There are NO charge controllers for positve ground systems, the charger would need to be an isolated design ( like a gridtie inverter ) for that.

    One can run the Sun Power panels negative ground, but may have a loss of 10-15% overtime.

    The OP has many issues, just trying to address the most obvious ones which is using a C40 in a 40V to 12V configuration which is really a 12V to 12V ( 66% loss of wattage ) configuration.
  • BB.BB. Posts: 27,588Super Moderators admin
    Re: when positive ground is used in pv

    Safety grounding is a good enough reason, many times, for grounding.

    With large systems, such as solar panels mounted on a 2nd story roof (or a wind generator on a tower), there are secondary for grounding.

    Besides lightning control ("safe" way to keep lightning from coming into your home) is the Earth's own vertical electric field...

    Typically around 100-300+ volts per meter. It can reach 3,000 volts/meter in stormy weather. And 3,000,000 volts per meter just before a lightning strike.

    Put a set of solar panels on a 2nd story roof (6+ meters up), and you are looking at a potential static charge of 600-1,800 volts on a normal day... You need some sort of grounding to prevent that static charge build up. You can get a nasty (if not deadly) shock from these voltages stored in the capacitance of your solar panels and wiring.

    The static dissipation ground does not have to be a solid copper connections--sometimes even a light bulb is used... If you have a 12 volt system you need a positive ground for--use a 1 amp 12 volt light bulb for the ground circuit. If the light is off--no problem... If the light turns on--you have a short between the negative ground and earth/safety ground--but not a large enough short to cause much damage (limited by the light bulb). Install a switch between negative and safety ground--and you can test the light bulb once a month.

    You can use DC Lightning Surge arrestors between the +/- and earth ground for better lightning protection.

    Regarding your Charge Controller--would it not be wonderful to triple the output of your current solar panels with just a $600 control (or two) change?

    And if you can up your system to 24 or 48 vdc and find an inverter that would run in a positive ground system (call Outback or Xantrex)--if you are using AC.

    If you are using 12 volt DC for a reason (powering local DC equipment)--there may be good reasons to switch to 120 VAC--one of which is to better isolate your equipment from lightning at your solar panels.

    I was a bit pessimistic on running a positive ground system because I don't know your experience or needs... A positive ground can be done (just ground the positive battery bus--and nowhere else) and check that all of your equipment (charge controller, wiring, downstream DC equipment, etc.) does not tie negative battery to chassis/safety ground.

    You can check the link for the Sunpower documentation I provided earlier and see if the loss of output for negative grounded panels would be a problem for you (I would attempt positive grounding). You can monitor the solar panel current--and as long as it remains near Imp--then positive grounding it not going to change anything. If it starts to drop, then it will help.

    By the time you have addressed your current system's issues--you may end up with 2-4x as much solar power as you have now.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • boBboB Posts: 949Solar Expert ✭✭✭✭
    Re: when positive ground is used in pv
    Telco wrote: »
    A body/earth ground is not really required, it's more of a convenience/cost savings issue. It is possible to do the ground as a wired network like the live system is, it just requires running a separate wire to the ground. Of course, this does double the wiring cost.

    I suppose that what you mean by "it just requires running a separate wire to the ground." in a system where earth is not available, is to wire instead to the conducting "frame" of the system ??

    Is it enough for these Sunpower modules to add a conductive metal backplane behind the modules and connect the positive to that, to regain the efficiency ??

    boB
  • TelcoTelco Posts: 201Solar Expert ✭✭✭✭✭
    Re: when positive ground is used in pv

    Yup, all that is needed is to complete a circuit. Consider a light bulb and a battery, to make light you tie a wire from the positive pole of the battery to the tip of the light bulb, and from the negative pole of the battery to the body of the light bulb, which is soldered to the negative wire on the bulb element. In a system that uses the earth or vehicle body as the ground, they are just treating the earth or vehicle body as a gigantic, convenient wire where you just tie on where you are instead of tying in a second wire run all the way back to the generator/battery.

    However, the simplest way is not always the best, or safest way. Consider what the other guys are telling you here. Strongly look at converting to a negative ground 120V system as was recommended. While I've not been into solar all that long, I've been in electronics in one form or another for the last 20 years or so, and in all that time I've seen very, very few positive ground systems. Not many people know how to work on them, or at least correctly. And, this is the first time I've ever heard of a positive ground solar setup.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: when positive ground is used in pv

    "this is what I did string 1 -3 panels in parallel tied straight to the batts"

    do not do this as you have no regulation on the pvs to the batteries at all this way and seeing the pvs with a vmp of 39.8v to your 12v battery bank, your batteries will be toasted.:cry:
  • boBboB Posts: 949Solar Expert ✭✭✭✭
    Re: when positive ground is used in pv
    Telco wrote: »
    And, this is the first time I've ever heard of a positive ground solar setup.

    Positive ground is quite common even for solar PV with telecom sites, and of course comes from the telephone industry.

    I thought, from your title Telco, that you might be from that industry somehow.

    boB :D
  • TelcoTelco Posts: 201Solar Expert ✭✭✭✭✭
    Re: when positive ground is used in pv
    boB wrote: »
    Positive ground is quite common even for solar PV with telecom sites, and of course comes from the telephone industry.

    I thought, from your title Telco, that you might be from that industry somehow.

    boB :D

    I am, but I do remote testing of low level data circuits. The only field work I've ever done for the company was managing a very small field site for about 6 months, and the only reason I did that was due to the very specialized access that particular field site provided to the department I was in at the time. That site had all negative grounded systems. Not to mention that telecom is a HUGE field with thousands of different systems, and there's no way that anyone can know them all.

    I don't doubt that there are a lot of positive ground systems out there though. If I wasn't able to accept that I don't know it all I'd be gibbering somewhere in a room with padded walls.
  • richrich Posts: 62Solar Expert ✭✭✭✭
    Re: when positive ground is used in pv

    There are NO charge controllers for positve ground systems,


    Correction..I posted a charge controller that is 60 amp positive ground in the reviews section ..sms 60 out of korea or india (cant recall which)...i purchased one, works great! the only problem was eq voltage never passes 14.8...I need 15 or more for my traction batteries.....it works on a negative floating ground(how I used it)....I opened it up and was suprised to see how well it is made... for a 65 dollar unit.......the instructions call for posative ground...i did not use it that way....since then it has been replaced with a larger unit....this posative ground controller is rated for 12 or 24 volt use.....hope this helps a bit.
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