Page 2 of 2 FirstFirst 12
Results 11 to 14 of 14

Thread: 2017 690.7(A) Maximum Voltage

  1. #11
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
    Posts
    4,644
    Quote Originally Posted by ggunn View Post
    SMA does indeed record the maximum DC voltage and void the warranty if you exceed their maximum; I was in an SMA training session a couple of weeks ago and got it from the horse's mouth. Voltage does not change much with low irradiance; I don't consider it. As a designer and a PE my license (or at least my E&O insurance) is on the line when I stamp a design, so yes, I am conservative when it comes to Voc. Of course, as an installer you can change the string length in the field (sorry about your working conditions; I have been there and done that), but if you do it's you who is on the hook if things go sideways.

    That said, it's rare for me to have a maximum calculated string length so close to an integer that a few degrees difference either way would change it. YMMV, of course.
    I get that the person responsible for a decision will tend to err on the conservative side. I am sure most professionals do it. A heating guy doesnt want the furnace he spec'ed to be undersized so will tend to oversize the system, etc.....Of course there are industry accepted methods for many things that one can use to CYA. Just IMO there is excessive fear about the VOC and it as calculated more conservative than necessary. I think the NEC now allowing it to be modeled reflects that. Here is a somewhat dated article by Bill Brooks I have posted before, and he agrees with me. Being somewhat dated, he is discussing using ASHRAE temps instead of record lows, but I believe going from ASHRAE to modeled is analagous and an extension of that.
    http://solarprofessional.com/article...s#.W4yEes5KiM8

    Dont you run into situations where you get like 1015 volts using temp corrected STC VOC and want to get the extra "half" module in there?

    P.S. I have always wanted to see some accurate data on VOC vs irradiance - pretty much all the graphs one pulls up on google are pretty "coarse" and hard to get values for the VOC change. I found this which has some good data, table 4 page 284:

    https://ac.els-cdn.com/S187661021734...71caf3b8369f7e

    The 1000w/sq vs 800w/sq values in the paper agree with what I got by taking module data at STC vs NOCT values and temp corrected for the 20 degree difference: about .5 volts. Thats only about 9 volts difference for 1kv strings, but It starts to drop off more rapidly from there. Around 500w/sq looks like about 1.5 volts per module which would be about "half a module"
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  2. #12
    Join Date
    Nov 2015
    Location
    CA, USA
    Posts
    629
    Quote Originally Posted by electrofelon View Post
    And for the other point of view, I find Designers to be overly conservative with VOC. Irradiance will be quite low when these low temps occur. I dont know why everyone will count every volt from a 100 year cold event, but not count the volts gained from low irradiance. Go measure voltage some really cold morning and tell me what it is compared to the standard computed value - a few percent is all you need to get that extra 20 volts you were fretting over to get that extra module in. furthermore it is highly unlikely an inverter will smoke at +20,+40 volts even if it happens at all. Even if an inverter does have a warranty issue, and even if the inverter records the voltage, and even if the voltage has been slightly exceeded, the manufacturer still may not automatically disqualify the warranty. Even if they do, I imagine the labor and hassle are more of a concern than the actual repair/replacement cost....Maybe I am biased as I have been installing these systems in the 35 degree rain in mud 1 foot deep
    It's pretty safe, and conservative, to assume you will have STC Voc at 20% insolation. Current is very dependent on insolation, open circuit voltage much less so. I've heard the argument that the daily low occurs before sunrise so why worry about it, or the insolation is so low in the morning why worry about it. There is also cold soak to consider, and supercooling due to dark sky radiation that many people don't know about much less consider. Someone can go get a Ph.D. in physics and know down to the degree what is happening and still get it wrong because temperatures fluctuate and are not always what we think they are going to be. Yesterday's record low is broken today.

    Many people use the ASHRAE extreme annual mean minimum temperature. But notice the "mean" in there. That is the average of the low temperatures which means half the time the temperature will go below the number being used in the design. I usually use the mean minus one standard deviation, also given in the ASHRAE tables, that means about 80% of the time the temperature will be above my design low temperature. Belt and suspenders.

    It's one thing to make a boo boo when designing a 5kW residential system and popping an inverter, it's a little different when one is designing a 1MW+ systems and popping inverters will cost millions of dollars and the EPC will be dragging the designer into court to cover that loss.
    Last edited by pv_n00b; 09-05-18 at 01:13 AM.

  3. #13
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
    Posts
    4,644
    Quote Originally Posted by pv_n00b View Post
    It's pretty safe, and conservative, to assume you will have STC Voc at 20% insolation.
    It will in fact be about 11% lower. A big part of engineering is weighing risks with the likelihood of those risks. We just have a difference of opinion as to the likelihood and consequences.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  4. #14
    Join Date
    Oct 2009
    Location
    Austin, TX, USA
    Posts
    9,950
    Quote Originally Posted by electrofelon View Post
    It will in fact be about 11% lower. A big part of engineering is weighing risks with the likelihood of those risks. We just have a difference of opinion as to the likelihood and consequences.
    Likelihood, OK, but consequences? Some if not all inverters record the highest DC voltage they see, and some if not all the manufacturers of those inverters will void your warranty if you go over their published maximum irrespective of whether damage was done or whether a warranty issue is connected with the overvoltage incident that was recorded. That is a pretty severe penalty for a chance taken for a minor benefit, so I don't risk it. And yes, I agree, risk/benefit analysis is a big part of engineering. I don't take risks with minimal upsides and major downsides when there are uncontrollable variables like weather involved. As an engineer I believe it to be irresponsible to do otherwise, but YMMV.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •