Using an ammeter to monitor a grid tie PV solar inverter?

Discussion in 'The Projects Forum' started by RandySea, Sep 9, 2011.

  1. RandySea

    Thread Starter New Member

    Sep 9, 2011
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    We are putting in a 5kw, grid tie photovoltaic system with a Silicon Energy Aurora DC to AC inverter. We will generate a max of 5kw of 240v AC, running to our house electric panel. The run is 260'. That's a long way to walk in winter and deep snow to check on the inverter.

    We've been told that digital monitoring equipment compatible with this inverter costs $550 to $2,000, plus the additional conduit and ethernet cable that would go in the power trench between the inverter and our house. We'd have a readout device that would give us all kind of nifty figures and statistics, alarms and emails, etc. At the high end price, we'd be able to monitor over the internet.

    All we really want is to be able to tell from the house that the system is working approximately as it should.

    Here's my idea: Buy an old fashioned ammeter on eBay for $10. Mount it in a subpanel next to the main power panel and run the incoming AC from the inverter through it. It should read 0-20 amps, depending on time of day and brightness of the sun. In case of any unexpected deviation, we could trek out to the inverter to check the LED readout.

    Should this work, or am I missing something?
     
  2. #12

    Expert

    Nov 30, 2010
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    Maybe you could just put a light bulb out there. If it's dim, low current. If it's bright, high current.

    Oh wait...our government has outlawed incandescent light bulbs. They will not be available much longer.

    My bad or theirs?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    You could mount a wireless nanny-cam web camera to watch your cheap ammeter. No walking in the snow.
     
  4. mcgyvr

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    Oct 15, 2009
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    Doesn't it have a RS485 port that you could capture data off of.
     
  5. Bernard

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    Aug 7, 2008
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    Do you need two monitors, or is there no neutral? Slip current transformers on to the two hot leads, bring results inside; assuming that grid tie is at house.
     
  6. RandySea

    Thread Starter New Member

    Sep 9, 2011
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    Thanks, mcgyvr, This is already a bit beyond what I am familiar with. The inverter specs say there is an RS485 port. It is unlear to me whether I could use that without buying at the least an RS485 to ethernet converter, or possibly an RS485 to serial, and running 260' of conduit and cable. Then at the house end I might need a serial to USB converter. I'd also still need a computer hooked up whenever I wanted to check on the inverter. I don't even know if this would work. It certainly wouldn't give me a way to do a quick visual check to see if things are working.
     
  7. RandySea

    Thread Starter New Member

    Sep 9, 2011
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    The grid tie is at the house. Why do you think using two current transformers (and presumably two volt meters) is better than a simple ammeter on one hot lead? There is a neutral, but with a grid tie, wouldn't the current be equal on both hot leads from the inverter? Before you answer, I did try to figure this out myself with a little internet research. Maybe you could tell me if I'm correct. There are cheap (under $10) current transformers on eBay. They mostly go to 100 amps with a .33v AC max output. So it sounds like the idea is to just dangle one of these around the insulated section of a hot lead and connect it to an ac voltmeter. Or one each around both hot leads. The benefit I see is not running the full current (max 20 amps @ 240v) through the analog meter. You're not connected directly to the ac at all, and just a low voltage signal comes out. The cost is that it's more complicated than just one simple dial meter. Oddly (or not), eBay has lots of cheap analog ammeters for sale. AC voltmeters are mostly digital and require a dc power supply to work. Plus I'd have to create some kind of conversion scale between AC volts at the meter and current produced by the inverter. This might be a chart posted next to the volt meter. Sort of defeats the feature of having a digital readout. Any comments appreciated.
     
  8. Bernard

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    Aug 7, 2008
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    My inverter processes each half seperatly, generally 119V & 121V, so there is some unbalance allowing some neutral current. Just to monitor general operation monitoring one leg is probably OK.
    Is there any reason that the inverter cannot be placed adjacent to grid panel?
     
  9. RandySea

    Thread Starter New Member

    Sep 9, 2011
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    The solar panels are 260' from the main power panel in the house, where we make the grid connection. Running 240v AC we can use #4 aluminum wire for a >2% line loss. I hate to think what size wire we'd need to run DC over that distance.
     
    Last edited: Sep 10, 2011
  10. RandySea

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    Sep 9, 2011
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    I meant <2% line loss.
     
  11. Bernard

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    It seems to me that the only difference in line loss between AC & DC would be inverter efficiency; or is your panel low V, say under 300V?
    AT 48V there would be an unacceptable difference.
     
  12. RandySea

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    Sep 9, 2011
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    Ok, so here I show my ignorance. There will be 24 panels each with a nominal output of 24v DC. The inverter has two parallel inputs, with one output of 240v AC. I don't know for sure how they are connected.

    The inverter specs say Dual Maximum Power Point Tracking, so each array feeds it separately, I guess. The specs also say Operating MPPT Voltage Range: 90 – 580VDC (360VDC nominal). This suggests 12x24=288v DC output from each array.

    At 288v DC, it does seem like the inverter could be at the house. The # of wires would have to be doubled to feed the arrays separately into the inverter's two inputs.

    As I said, I don't understand all the logic behind this. What I do know is that the inverter is sold as a unit (waterproof panel) with the two DC inputs (switch boxes with thermal protection), inverter, output switch box, and meter base.

    I don't have much choice in inverters since they must be manufactured in Washington State. Which brings me back to the original question of whether I can use an ammeter on the incoming 240v AC as my rough test of solar performance.
     
  13. Bernard

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    Aug 7, 2008
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    To your last question- I see no reason that the 240V AC line cannot be monitored with ammeter.
    I believe that my inverter, Sunny Boy 5000US is similar to yours, & that mine, much to my surprise, hard wires the two hot DC lines in parallel from two 12 panel arrays 0perating @ about 348V & 8A. If the same then the panels could be paralled at the source requiring single + &- 16A to 20A lines, about the same as the AC I.
    PS: system rated 5.5 kW DC, but only 4 occasions saw 5 kW AC, but really enjoy the $8.oo, inc. taxes, / mo electric bill. Enjoy.
     
    Last edited: Sep 11, 2011
  14. RandySea

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    Sep 9, 2011
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    I've got an answer. In theory we could have 576v DC by putting 24 panels in series. That would really reduce the current. However, the inverter panel is designed for two inputs, max 3kw each. So we'd have to run two DC circuits at 288v each. Plus we'd need an additional pair of DC breaker/cutoffs, although I'm not sure why.
     
  15. Bernard

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    Can you check inverter mfg. literature to see if inputs can be paralled?
     
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