Ungrounded PV

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  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Ungrounded PV
    BB. wrote: »
    On the DC Off Grid Side--It is still highly dangerous to put any sort of fuse/breaker between a "grounded return/neutral" and Earth Ground--It just breaks the whole single pole fuse/breaker of such a system design (because you still have a large battery bank as an energy source).

    I'm sorry, but did you read what I said in post #59? I suggested an alternative to the fuse.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Ungrounded PV
    ggunn wrote: »
    Sorry, but I must have missed it. Where is the 1200V differential coming from? ...

    Nowhere. He's wrong. I said that but I guess he's not listening. It's actually a great argument for why a GFDI device should lift the grounding bond for the grounded PV conductor(s), which is what he's saying is so terrible.
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Ungrounded PV

    As far as I know--NEC would allow to pairs of solar arrays to be carried in a single conduit/cable tray to two separate PV controllers (say two 5kW arrays and two independent 5kW inverters). One inverter is negative ground (+600 wrt ground) and the other has a hot to ground short that popped the 1 amp fuse causing the second array return to be at -600 VDC (worst case, Voc-cold)...

    I am not saying that this is the worst thing in the world--Just from my experience, if I designed a system with +/- 600 VDC power lines, it would be difficult to justify sharing the same conduit with 600 VAC rated insulation. I asked Wiles directly--and he avoid the question.

    I am listening. I just am not always at a full computer+keyboard (doing some stuff from my phone).

    Regarding #59:
    It seems to me that if charge controllers for OG systems used the same (dual ammeter) GFP method that ungrounded (non-isolated) GT inverters use, this whole issue might be solvable, without the issues Bill keeps harping on. The GFP device would be powered by the PV and would open both sides of the PV circuit if it detected a ground fault. The battery side and the battery grounding connection would be unaffected. As far as the NEC is concerned, it probably needs some language to clarify if the PV circuits in such systems should be treated as grounded or ungrounded (I think it should be the latter, even though I don't think the same for GT systems.) But it's already permitted by the current code.

    I have never needed to look at "precision" DC paired leakage detection circuits (or DC paired +/- circuits such as done with AC GFI systems and a simple current transformer)--But I have mentioned in earlier posts that such circuits as being easy or cheap to implement (DC current Hall Effect transistors tend to drift a fair amount over a few minutes of time--The other "obvious" way would be a pair of current shunts and use the sense leads to drive the disconnect circuit). It could get a bit messy--You would need to sense at the destination (GT inverter or Solar Charge controller that there was a miss-match of current flow in the +/- current lines, then send a signal back to the solar array to pop a dual pole breaker/relay of some sort. Not a very elegant solution.

    Yes--I could see the PV circuits as being at least "labeled" as ungrounded circuits.

    And, the current DG GFI system with a sense breaker/fuse--The location of the return to earth fault is dependent if the fault occurs at the array (where you can drive some current through the ground) vs a fault near the destination (where a return to earth fault simply bypasses the fault detection fuse/breaker).

    Regarding the DC Off Grid System side--Ryan "Halfcrazy" from Midnite tested my suggestion that a Hot to Earth fault will result in ground reversal on a DC power system (i.e., typical off grid system with battery bank/inverter for stored energy) which would deliver "excessive" current through a return wire (no breaker/fuses on grounded neutral return wires)--depending one where the fault would occur. So--It has been confirmed that such a setup is "not ideal".

    And, again from Ryan:
    Quote from: jtdiesel65 on January 04, 2012, 09:50:04 AM
    In terms of the classic hardware, I'm reading this as

    if I tie my ground bus to negative bus, it will defeat the gfp capability of the classic since the classic won't be able to detect voltage differences between ground and (-). Is that correct? Other than that, there is no impact to the classic. Is that also correct?
    That is correct. And is most likely how you would find my system with a big fat wire connecting battery negative and earth ground. rolleyes.gif


    In of my previous posts here--I asked what you guys would think if an AC Mains system for a typical North American Home we put a 1 amp fuse between the White Wire Bus and Earth Ground (instead of the standard Earth Bond at the mains panel, and at the pole transformer to its own ground rod)... Would that be safe (assuming we don't have a double pole breaker on the distribution transformer--that would be similar to how we wire DC Battery banks and Solar panels today).

    And would anyone want the entire power to their home (and the three or four other sharing the pole transformer) want to have their power killed if there was a "simple" hot to earth fault anywhere in those homes (in the DC GFI system, a hot to earth fault anywhere in the system--Solar panels, charge controller output, battery bank, loads, MS Inverters, etc.--Would pop that detection fuse/breaker).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Ungrounded PV
    BB. wrote: »
    I am not saying that this is the worst thing in the world--Just from my experience, if I designed a system with +/- 600 VDC power lines, it would be difficult to justify sharing the same conduit with 600 VAC rated insulation. I asked Wiles directly--and he avoid the question.
    Again, that would be a bipolar design, which may or may not be center point grounded. I have designed such systems and I never run the DC conductors from the upper (positive) subarray in the same conduit as those from the lower (negative) subarray. As a matter of fact, the installation manuals for the inverters used in such systems specifically stipulate that the conductors be in separate raceways.

    But this is a thread about ungrounded PV systems, which is a different issue. Bipolar systems may or may not be grounded.

    I have met and spoken with John Wiles and had several email discussions with him. If he seemed to avoid your question, my guess that either he misunderstood the question or you misunderstood the answer. Simply avoiding the question would be very much out of character for him.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Ungrounded PV
    BB. wrote: »
    As far as I know--NEC would allow to pairs of solar arrays to be carried in a single conduit/cable tray to two separate PV controllers (say two 5kW arrays and two independent 5kW inverters). One inverter is negative ground (+600 wrt ground) and the other has a hot to ground short that popped the 1 amp fuse causing the second array return to be at -600 VDC (worst case, Voc-cold)...

    Now that you have finally spelled out that this requires two independent inverters or charge controllers, I finally get it. Still, for any electricity to actually flow at 1200V would require a second fault, and it would immediately trip the GFI fuse in the second inverter.

    Also, although the NEC doesn't require it, I've heard of some jurisdictions (state of Pennsylvannia, IIRC) not allowing multiple PV systems in the same raceway.
    It could get a bit messy--You would need to sense at the destination (GT inverter or Solar Charge controller that there was a miss-match of current flow in the +/- current lines, then send a signal back to the solar array to pop a dual pole breaker/relay of some sort. Not a very elegant solution.

    Why on earth would one have to "send a signal back to the solar array"? Non-isolated GT inverters just do this internally like isolated ones.
    In of my previous posts here--I asked what you guys would think if an AC Mains system for a typical North American Home we put a 1 amp fuse between the White Wire Bus and Earth Ground ... [blah blah blah]

    Why do you keep repeating this? It's a strawman that anyone here is defending the 1A fuse method for OG systems. Not to mention that protecting AC systems and PV source circuits is apples and oranges. It's patronizing and it clutters the thread.
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Ungrounded PV
    jaggedben wrote: »
    Now that you have finally spelled out that this requires two independent inverters or charge controllers, I finally get it. Still, for any electricity to actually flow at 1200V would require a second fault, and it would immediately trip the GFI fuse in the second inverter.

    Also, although the NEC doesn't require it, I've heard of some jurisdictions (state of Pennsylvannia, IIRC) not allowing multiple PV systems in the same raceway.

    So--A single fault could create a not-to-code voltage condition in an other wise generic NEC compliant system with several GT inverters sharing raceways/conduit Updating NEC to prohibit sharing of raceways would address that issue.
    Why on earth would one have to "send a signal back to the solar array"? Non-isolated GT inverters just do this internally like isolated ones.

    I thought you wanted to stop current from flowing if there was a fault by detecting differential current flow at the destination and placing a two pole disconnect at the array would do that without having a fuse between return and earth... But then that gets back to your next statement...
    Why do you keep repeating this? It's a strawman that anyone here is defending the 1A fuse method for OG systems. Not to mention that protecting AC systems and PV source circuits is apples and oranges. It's patronizing and it clutters the thread.

    OK--I am sorry, the original poster did not state if this was a GT or Off Grid system as most (all?) current GT inverters do not give an option of floating vs DC GFI PV Array configuration options. As near as I could tell, the question was worded, I would have guessed that floating/grounded PV was for an off grid system.

    And if you are accepting that the DC GFI as implemented by many manufacturers to meet the current NEC/UL/NRTL requirements are unsafe for off grid systems--That is fine with me.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mtdocmtdoc Solar Expert Posts: 600 ✭✭
    Re: Ungrounded PV
    BB. wrote: »
    And if you are accepting that the DC GFI as implemented by many manufacturers to meet the current NEC/UL/NRTL requirements are unsafe for off grid systems--That is fine with me.

    -Bill

    Bill, If I am undestanding this issue correctly, this is also the case for Grid tie with battery back up systems. Correct?
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Ungrounded PV

    Yes as we would be still discussing (typically) an MPPT type battery charger to battery bank to Battery powered Hybrid Inverter (there is AC isolation--So would not carry through issues like a MSW type inverter could).

    There is the option (like Sunny Island and a few others) where you use a GT inverter attached to the output of an Off Grid capable inverter--Then those solar arrays on the GT Inverter would be isolated from the DC battery bank (in terms of ground faults).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Ungrounded PV
    BB. wrote: »
    ... Updating NEC to prohibit sharing of raceways would address that issue.

    Yeah, maybe. A blanket prohibition on sharing raceways seems like using a sledge-hammer to put in finish nails, in my opinion. I'd say there should at least be exceptions for ungrounded systems and possibly other exceptions.
    I thought you wanted to stop current from flowing if there was a fault by detecting differential current flow at the destination and placing a two pole disconnect at the array would do that without having a fuse between return and earth...

    Yeah, and a relay inside the inverter or CC also accomplishes the job. This is how transformerless GT inverters do it.

    Again, the point of GFDI for PV circuits is to mitigate against the possibility of parallel arc faults by alerting the system owner to one half of such a fault (one pole to ground) before the other pole faults to ground and completes the short. For such purposes, it hardly matters if the circuits are opened near from or far from the array.
    OK--I am sorry, the original poster did not state if this was a GT or Off Grid system as most (all?) current GT inverters do not give an option of floating vs DC GFI PV Array configuration options.

    It's not an either/or, you can have a floating array with GFDI. All the leading GT manufacturers are now offering transformerless inverters that float the array (it's required to avoid a dead short) and they also all have GFDI and are UL listed. (It might benefit you now and then to pay a little more attention to what's going on in the GT-only world.)

    Now if you are taking about GT-with-battery-backup systems then I think you're right that products don't give an option. That part of the industry needs to catch up I think. And as far as I can tell they ought to be able to do so by simply putting the same type of GFDI into charge controllers. That is, the same type that the transformerless GT inverters are using. I don't know if it will cost more (on the GT inverters it's probably offset by saving on the transformer) but it doesn't seem like it would be cost prohibitive. I'm not an engineer who can make it happen, just putting out the idea hoping someone (you?) can look into making it work. Maybe there are obstacles I'm not thinking of.
    And if you are accepting that the DC GFI as implemented by many manufacturers to meet the current NEC/UL/NRTL requirements are unsafe for off grid systems--That is fine with me.

    Yeah, I will give you credit for raising my awareness of the issue. I'm trying to address the "as implemented by many manufacturers" issue and suggest an alternative. Seems like something UL should look at improving (rather than the NFPA).

    Generally speaking, I think moving towards ungrounded PV circuits is probably a step in the right direction safety-wise for many different types of systems. The GFDI fuse method has serious shortcomings for large GT central-inverter systems as well. And as you pointed out earlier in this thread, Europe operates pretty much a whole continent with ungrounded systems (PV and AC) and they haven't blown up yet. Here in the US we are kind of obsessed with grounding, to the point that people think ungrounded systems are inherently more dangerous, and that we have to sometimes explain to novices that electricity seeks to go back to its source, not to ground. It doesn't necessarily have to be that way, and with PV circuits the benefits don't seem to be there.
  • BB.BB. Super Moderators, Administrators Posts: 32,608 admin
    Re: Ungrounded PV

    In the US, using a center tapped transformer for 120/240 VAC power probably is what complicates the heck out the grounding requirements for North America... With three power circuits and two pole breakers (plus the center tap transformer connection which can over current the neutral if the Red/Black of the hots is not wired 180 degrees out of phase to cancel out current in the center tap lead)--What happens with the White Wire is critical for safety.

    But then Britain has its "loop" type power (or whatever it is called--two 230 VAC source breakers feed each end of a branch circuit... And then you have to have fuse/breaker inside the plug to avoid over current on each plug in device.

    However, having a known ground reference to the AC power did make defining barrier requirements a lot easier... In the US, you will never see > 120 VAC from wiring to ground. In ungrounded systems (define 600 VAC wiring max voltage; 3x that for proof testing, and you are done)--There is nothing (that I know of) that will "clamp" the AC mains to any voltage wrt to ground.

    In the Nordic countries, they had a very different idea on how to prevent AC mains energy from reaching the end user... In the US, you can use single barrier+ground or double insulation (barriers) and no ground.

    There is a third option with Barrier + Ground Screen between AC mains and low voltage output... Apparently, the earth grounding is a bit difficult because of the permafrost issues (at least what I remember from decades ago).

    I agree that floating AC (or DC power) is not the worst safety issue--But it is the baggage from the past that is causing us problems (such as Edison screw base lamps that make it very easy for somebody (or some kid) to touch a hot circuit). If we want "fool resistant" safety--There are some major issues that have to be addressed (exposed heat wires in toaster anyone?).

    Unfortunately, I think a "system level" (or high level document) like the NEC is where this stuff has to be defined/starts.

    Otherwise, you have different device specifications that can clash when implemented on a variable "system level" power system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Ungrounded PV

    Bill, I was just talking about grounding (or lack thereof) for PV circuits. The grounding of AC systems doesn't need to change.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Ungrounded PV
    jaggedben wrote: »

    It's not an either/or, you can have a floating array with GFDI. All the leading GT manufacturers are now offering transformerless inverters that float the array (it's required to avoid a dead short)...

    FWIW, there is at least one company (Ideal Power) who is offering center grounded transformerless bipolar inversion gear.
  • jaggedbenjaggedben Solar Expert Posts: 230 ✭✭
    Re: Ungrounded PV
    ggunn wrote: »
    FWIW, there is at least one company (Ideal Power) who is offering center grounded transformerless bipolar inversion gear.

    Good point. It would seem the dead short isn't a problem for a bi-polar PV system. I wonder how they handle GFDI.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Ungrounded PV
    jaggedben wrote: »
    Good point. It would seem the dead short isn't a problem for a bi-polar PV system. I wonder how they handle GFDI.
    Most bipolar systems are ungrounded; Ideal Power is the only company I know of whose bipolar system I am certain is ungrounded. They do GFDI the same way as ungrounded systems with differential ammeters.
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