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Powerwall2 and Enphase Microinverters

Discussion in 'Tesla Energy' started by wiredmeyer, Oct 28, 2016.

  1. miimura

    miimura Active Member

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    If the inverter is only rated for 5kW Continuous and 7kW Peak, you're not going to be able to charge a car at 10kW, no matter how full the battery is or how much the solar is generating at that moment. Now, if you want to dial the car down to 20 amps (4.8kW) and let the rest of the stuff on critical circuits run in the house, you could do that while the sun is shining and the battery is full...
     
  2. Nevetsyad

    Nevetsyad Member

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    If the AC Powerwall 2 is chosen, there's two inverters, not just the solar panel'S inverter. That's why I'm saying you can run more during an outage or if running an off grid setup, if using a Powerwall 2 AC version.
     
  3. miimura

    miimura Active Member

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    Sure, but you didn't state that clearly in your previous post. This also highlights how important it is to coordinate the output of separate inverters in an off-grid system. If the Tesla battery was full and the solar was generating a lot of power, it would have to have somewhere to go or it would have to be curtailed in order to keep the system stable. Not all inverters designed to be grid interactive have a mechanism to be curtailed. They simply assume that the grid can absorb whatever they can generate. When a PowerWall is set up to be a backup power supply, these things have to be engineered in. Otherwise, the grid interactive inverters would have to be put on the utility side of the transfer switch and their generating ability would be lost during a utility outage.
     
  4. tga

    tga Supporting Member

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    Can you explain this? What's special about a grid-tie inverter as a voltage source? The electrons don't have to go anywhere. A voltage source (AC or DC) doesn't self destruct when the load disappears. Nothing bad happens if I disconnect the load from a solar panel, 12V car battery, or my server's UPS.

    Clearly I'm missing something...
     
  5. miimura

    miimura Active Member

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    An AC grid needs to maintain voltage and frequency. A grid-tied inverter synchronizes to the grid and it's job is to vary the DC voltage and current (MPPT) so that it pushes the maximum amount of AC energy into the grid that the panels can deliver. The grid is relatively infinite compared to one inverter, so the voltage and frequency change very little based on how much power one inverter is pushing into it. When you have a micro-grid you have to have clear mechanisms in place to ensure the stability of the grid. Something has to balance the generation and consumption so that the voltage and frequency are maintained within allowable limits. Normally, the battery inverter in an off-grid system does this job and any AC coupled solar inverters must be smaller than the battery inverter so that they can't push out more power than the battery inverter can absorb. This requirement can be avoided by using the solar to directly charge the batteries through charge controllers. Those controllers perform the solar MPPT function and are programmed with the maximum battery voltage so that they can curtail the solar and avoid over-charging the batteries.

    It's all about regulation. If someone pushes you, you have to be strong enough or heavy enough to push back or else you'll fall over. Most solar inverters don't have proportional control for how hard they can push. If any parameter goes out of range, they just shut down. I'm obviously making generalizations here. There are some inverters like SMA Sunny Boy that can interpret changes in the frequency as signals to curtail production in a more elegant way.
     
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  6. Pinot.Noir

    Pinot.Noir Member

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    Has anybody successfully installed a PW2 with an existing solar system with Enphase inverters? Thanks.
     
  7. doubleohwhat

    doubleohwhat Member

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    I haven't heard of an install using enphase micro-inverters yet.

    The powerwall 2 should technically be compatible with almost any existing inverter provided you can export to the grid or will have enough powerwall capacity to soak up all excess solar energy generated.

    I think the areas where compatibility *might* be any issue are:
    A) when the grid is down and the powerwall(s) need to keep the inverter online.
    B) when the powerwalls are full and grid export is not desired.

    Enphase could absolutely issue a firmware update if compatibility issues do arise. It's just a matter if they'll do that (if needed) now that they are pushing their own batteries.
     
  8. arnolddeleon

    arnolddeleon Supporting Member

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    It seems pretty clear with my interactions w/ Tesla/SolarCity that the Powerwall 2 will "signal" the solar inverters (most likely by moving frequency, most common guess by several people) when they're full and unable to soak up the power. At that point the PV systems will just shutdown. They were really pushing hard to change my design to 3 Powerwalls based on the size of my PV systems because they're afraid of PV production exceeding the charging rate that the Powerwall can sustain resulting in the PV shutting down even when the batteries are not fully charged yet.
     
  9. Saghost

    Saghost Well-Known Member

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    Interesting. How much solar do you have?
     
  10. miimura

    miimura Active Member

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    We discussed it in the other PowerWall thread here: Powerwall 2 "waiting list"
     
  11. mongo

    mongo Well-Known Member

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    Huh, the Powerwall should regulate it's charge current and the PV should limit how high it drives the voltage. I suppose if the Powerwall stops charging at say 245V (totally made up) and the PV inverters will output up to 250V (totally made up), that senario could happen. If so, a separate disconnect/ disable for part of the array would fix it. Or easier, a high power load (resistive heater) that is turned on at high line voltage.

    It would be interesting to know how the line frequency regulation works in that setup. The PV inverters expect something else to do the frequency regulation. The Powerwall expects to either follow utility, or run at 60Hz internally generated. If the PV is charging the Powerwall, who is providing the regulation?
     
  12. Got2BTru

    Got2BTru Member

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    Read through most of these posts and wanted to clarify. I had a powerwall2 reservation, but Tesla was unable to fulfill since I wanted to be completely off-grid (new construction). I was told by 2 different techs that they had suspended off-grid sales due to a "software" issue where they can't shut off the solar panels to prevent the batteries from over-charging.

    Very disappointed to hear this...after waiting since December! Now looking into other options...the last resort is to go on-grid for a cost of $16k because we're 1300feet off the road.
     
  13. mongo

    mongo Well-Known Member

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    #53 mongo, Jul 19, 2017
    Last edited: Jul 19, 2017
    An Outback battery backup inverter system may work for you http://www.outbackpower.com/downloads/documents/flexcoupled/app_note_acc_new.pdf
    From the app note, it looks like the micro inverters can follow the battery powered inverter's isolated side.
    May be able to configure charge points to use reclaimed Tesla pack modules.
    However, it seems like a load control could be added to prevent powerwall overcharge. I wonder if a powerwall feeding the grid side of an interactive outback inverter would work (with minimal standard batteries).

    edit: enphase app note: http://www2.enphase.com/global/files/Enphase_Application-Note_AC-Coupled-Battery-Based-Systems.pdf
     
  14. doubleohwhat

    doubleohwhat Member

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    So basically, for now at least, they can't install powerwalls unless they can dump excess solar energy to the grid? Obviously that affects off-grid but that also affects people with solar-unfriendly utilities where export isn't feasible.

    I'm very glad I canceled my powerwalls a couple of months ago and went with a solaredge system. I would have been pissed to finally get them to certify an installer here and have them start on the project only to find out that since I can't export to the grid, the powerwalls can't be installed.

    An AC-coupled Outback Radian might work. You would have several battery options. Ex:
    1) Lead Acid / AGM
    2) Lithium-Ion batteries from ebay (Tesla car battery modules work well)
    3) LG Chem 48v battery: LG Chem RESU10 Residential ESS Energy Storage Battery System
    4) Any 48v battery really. The Outback Radians can be tweaked for almost any ~48v battery
     
  15. Saghost

    Saghost Well-Known Member

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    Huh. This is for an installation with microinverters, right? (Based on the thread we're in.)

    I'm surprised Tesla's having trouble with this, because there are a bunch of things that AC couple with Enphase systems already, and a simple, well established way of forcing the microinverters off when needed - you just push the pilot frequency they're matching to out of range (below 59 Hz or above 61 Hz) and they drop into anti-islanding protection.
     
  16. mongo

    mongo Well-Known Member

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    From the second app note I linked to, Enphase recommends against the Hz shift technique due to the ability to adjust the settings on their microinverters (apparently that can be set for a larger operation range, so it is not guaranteed wheat frequency is needed to cause a disconnect)
     
  17. Saghost

    Saghost Well-Known Member

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    If that's the only concern, a simple feedback loop in the software that's trying to shut them down would address it - shift frequency> did I get the power reduction I need?> Shift further.
     
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  18. mongo

    mongo Well-Known Member

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    That is way to provide stages of supply shedding also. Although too much messing with the frequency can have adverse effects on electronics (well analog clocks at least). Nice and clean would be a control line from Powerwall to an external contactor to disconnect the solar input.
     
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  19. liuping

    liuping Active Member

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    Did they say the suspension was permanent, or just until they solve the software issue?

    Having the panels operate during a blackout is extremely important part of they system. Without a shut down mechanism, they will need to add a load shedding device. It seems like this would be an issue for not only microinverter systems, but also with most regular inverters.
     
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  20. miimura

    miimura Active Member

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    A system that is designed for off-grid is going to have a lot more solar than a grid-tied system. The chances of the solar over-generating in a backup situation are relatively minor and it wouldn't be a big deal to handle it in an inelegant way. However, an off-grid system will bump into that every summer day, so it should be handled in the best way possible.
     

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