Voltage Rise with String Inverter

DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
Just want to show the brain trust here the voltage rise I see with a Fronius string inverter. I record this voltage rise on especially sunny days. I use the TED system to monitor system dynamics. Posting a screen shot to show the effect I see. My suspicion is that the utility transformer is undersized for the amount of production of my array. Just wondering if anyone has experienced this effect on their grid tied array? The red line indicates the voltage. Green line is my usage and dark blue line is net of solar production.

Inverter has never shut down on high voltage since it has never exceeded a 5% rise (ie 126 volts) but it has come close as seen on the chart. I'm adding a second array which will double current in full sun.
23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Voltage Rise with String Inverter

    This problem has cropped up before on the forum, and you're quite right: it's not your inverter it's the utility power. Sometimes they run higher than normal Voltage, sometimes the skimp on wire/transformer size. Et cetera.

    I recall one case of it being the distance in wiring from the inverter to the service panel allowing too much Voltage rise. And maybe a case where an inverter had to be adjusted to allow the extremely high Voltage because the utility solution was not forthcoming.

    Grid Voltage allowance is +/- 10% nominal as I recall, which means it should allow 108 to 132 on half phase, 216 to 264 full.

    As a disclaimer I should like to point out that the hippocampus is in fact a variety of Swiss cheese and as such contains a great many holes that memories can fall out of, and other things (like mice) can crawl in to. :p
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Voltage Rise with String Inverter

    132v is way to high and no good. if i remember rightly as it has been some time since i complained about here, it is 126v as a max. you may need to watch for voltage sags along your wires too as that can cause you troubles.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Voltage Rise with String Inverter

    Niel;

    He's getting 126. I quote 132 as what some utilities deem an "acceptable" high. Wouldn't recommend it myself. :roll:
  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    Re: Voltage Rise with String Inverter

    I see this all the time with my TED. This month the high was 126.4 half phase.
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Voltage Rise with String Inverter

    As you are finding out, voltage regulation on the grid is not a trivial problem. Its pretty amazing to think utilities can keep it as tight as they do. With loads cutting in and out unpredictably, long distances between generation and loads, old equipment all throughout the grid, and now - distributed intermittent solar generation all over the place. No wonder all the talk about a smart grid to add additional regulation throughout the network. I think SMA is working on a new inverter that will have a small amount of storage capacity to enable it to help out with short term regulation of the AC voltage. Solar systems can and should be part of the solution to this grid stability problem.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Voltage Rise with String Inverter

    Thanks guys for the responses, you confirmed my suspicions.

    I talked to my local REMC utility and they confirmed that my transformer was undersized. I am supplied by a 10kW transformer but my house is a 200 amp service. I always wondered why my lights would dim when motors would kick on etc. I always thought it was typical or due to the wiring in the house. Now I know better.

    The good news is that the utility is upgrading the transformer to at least a 15kW, but I'm going to try to talk them into a 25kW. That should do it.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Voltage Rise with String Inverter
    DanS26 wrote: »
    Thanks guys for the responses, you confirmed my suspicions.
    I am supplied by a 10kW transformer but my house is a 200 amp service. ...
    The good news is that the utility is upgrading the transformer to at least a 15kW, but I'm going to try to talk them into a 25kW. That should do it.

    Yes, when you consider that a 200A 240V service comes up to 48kW, they have to be doing a much lower estimate of what your actual load will be.
    Your best bet to persuade them toward the higher numbers will be to generate a credible table of your actual maximum loads based on currently installed appliances, etc including planned purchases in the immediate future (such as an electric vehicle fast charger, for example). Then remind them that the difference in voltage drop will be proportional to the sum of that maximum load and the maximum backfeed you will be producing if you turn off all of your optional loads at some time during the day.

    That is, if your peak load will be 19kW (80A at 240V) , and your peak summer noon solar production will be 8kW, the difference in load between the two situations is 27kW.
    The transformer will only have to deliver up to 19kW, but the difference in voltage between the two extremes based on resistive losses only will correspond to a 27kW power consumption difference.
    SMA SB 3000, old BP panels.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Voltage Rise with String Inverter

    Thanks inetdog, I think a 25kW is the minimum transformer needed. You're ahead of me because I am headed for an EV in the near future. Charging only when the sun shines. I sell to the the REMC at wholesale and buy it back at retail at night, so it is to my advantage to use as much of my production as possible internally.

    I built my first array in late 2011 and I am doubling the capacity this spring. The cost of the second array is $5k less than the first array but with higher capacity panels. I like this cost curve but now will be maxed out as a residential producer since my REMC does not allow production over 15kW.

    I have a positive ROI on my PV investment, but only because I have done all the work myself. I'm still waiting for the day when PV becomes a "no brainer" investment for the average American.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Voltage Rise with String Inverter

    Doing a little more research, I found that utility companies will regularly undersize transformers because they know that usage is intermittent, ie the high watt draws will only last for seconds or minutes, not enough to overheat their undersized transformers.

    But for us PV continuous generators, that assumption is not valid. We need a transformer that will handle continuous load without overheating the coils in the transformer. That is important on a hot July afternoon.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Voltage Rise with String Inverter
    DanS26 wrote: »
    Doing a little more research, I found that Utility Co. regularly undersize transformers because they know that usage is intermittent, ie the high watt draws will only last for seconds or minutes, not enough to overheat there undersized transformers.

    But for us PV continuous generators, that assumption is not valid. We need a transformer that will handle continuous load without overheating the coils in the transformer. That is important on a hot July afternoon.

    Yup, but on the flip side the full power output from your PV, even assuming that you turn off all of your loads, will usually be less than your maximum continuous draw without the PV.
    So the transformer would at an absolute minimum need to be good for 15kW, assuming you max out your system and turn all your loads off when you leave for the day.

    I think you will be able to make a case for the 25kW, but make sure that they also take their primary and secondary conductor sizing into account (voltage drop) as well as the size of the transformer itself.
    SMA SB 3000, old BP panels.
  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    Re: Voltage Rise with String Inverter

    I noticed yesterday the utility became unstable and the voltage was all over the place for about 2 minutes.

    Reaction, the grid tie inverters cut out and started the 5 minute cycle to determine the grid again.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Voltage Rise with String Inverter
    inetdog wrote: »
    Yup, but on the flip side the full power output from your PV, even assuming that you turn off all of your loads, will usually be less than your maximum continuous draw without the PV.
    So the transformer would at an absolute minimum need to be good for 15kW, assuming you max out your system and turn all your loads off when you leave for the day.

    I think you will be able to make a case for the 25kW, but make sure that they also take their primary and secondary conductor sizing into account (voltage drop) as well as the size of the transformer itself.

    Well the REMC replaced my transformer today with a 25kW unit. I think that is the right sized unit for me since I will generate 15kW on a continuous basis at max. I made my case to the utility based on the 125% rule even though the utilities do not have to comply with NEC rules. 15kW continuous production times 125% equals 18.75kW.

    Nice bright shiny new 25kW transformer ready for my solar upgrade.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Voltage Rise with String Inverter

    I've had the new 25kW transformer a little over two weeks and have been monitoring performance.

    All I can say is WOW, voltage is now very stable within a two volt range 121 to 123. No more lights dimming when the well pump or AC kicks on.

    I can't believe that I lived with that undersized transformer for over 30 years. My advice to anyone with dimming lights when large power draws occurs is to complain to the utility service, you may have an undersized service.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Voltage Rise with String Inverter

    could be bad service entrance wiring as well as other internal wiring problems within the house itself so check that before bothering the utility.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Voltage Rise with String Inverter
    niel wrote: »
    could be bad service entrance wiring as well as other internal wiring problems within the house itself so check that before bothering the utility.
    Service entrance wiring outside the meter is the utility company's problem as well.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Voltage Rise with String Inverter
    ggunn wrote: »
    Service entrance wiring outside the meter is the utility company's problem as well.

    yes, good point, but many could forget about the wiring between the meter and the service entrance panel. odds are if that looks shaky then the utility portion is most likely bad too.
  • DanS26DanS26 Solar Expert Posts: 250 ✭✭✭
    Re: Voltage Rise with String Inverter

    When I bought my house in 1971 it had a 60 amp service, you know with screw in fuses and ungrounded wiring. When I replaced all that with a new 200 amp service in the 80's I had all the wiring replaced from the transformer to service entrance. Yes, on my dime! My utility is an Indiana REMC and they do things a little differently than is normal in the rest of the country.

    I also replaced all internal wiring with no less than 12 ga throughout the house. Over the years I added two AC units and backup 240 volt in ground well pump. All wiring for those units were properly sized.

    But my lights would dim, especially when the larger AC unit kicked in. It just didn't dawn on me that the transformer was undersized.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring with PVOutput.org
  • peakbaggerpeakbagger Solar Expert Posts: 341 ✭✭✭
    Re: Voltage Rise with String Inverter

    My friend lives in a rural town and his inverter was kicking out frequently due to high utility voltage. Fronius gave him a password to change the allowable voltage variation and even with that change it still would trip out. The utility spent a couple of days and determined several flaws on their system. They fixed them and the problems went away. No need for a transformer.
  • solar_davesolar_dave Solar Expert Posts: 2,389 ✭✭✭✭
    Re: Voltage Rise with String Inverter

    Looks like my utility has been struggling with line voltage lately, the inverter kick out twice today.

    The yellow line is the voltage and the green line is the solar.
    The kick outs always happen as the voltage changes abruptly.

    Bet it screws the MPPT algorithm.
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