How to trick Enphase Microinverters???

Thanks to wind and ice storms in Kentucky, it is no longer unusual for people to be out of power for a week or more, even in cities the size of Lexington. For backup power, I presently use a Kohler 8.5 RMY which is powered by natural gas, with a propane backup. It's tied to the grid via a Kohler automatic transfer switch. Now it's time to add solar to the mix in order to have more backup power - and also to do my little part to stop burning coal and bulldozing down the beautiful mountains of Appalachia!!!

Because of shading and PV module placement problems, it seems most practical to use Enphase Microinverters to do the power conversion. 99% of the time, the output will be to the grid for the net metering benefits. However, during prolonged power outages, it could be critically important to have the PV output directed to providing power for the house.

Question: How can the Enphase Microinverters be tricked into putting out when the grid is down. A rep at Enphase said that by putting a high-grade sine wave into the system (via a dedicated inverter), the Enphase units would come alive and begin putting out. That's the easy part, and I had that figured out. However, what I cannot ascertain is how to do this in such a fashion (automatically) that no power will backfeed into the grid (or into the generator). Has anyone had any experience tricking out an Enphase to accomplish something similar. Any comments would be most appreciated.

P.S.: I am acutely aware that a conventional PV system, using a Xantrex or Sunny inverter package, would do everything I want (in most settings). However, because of extremely limited options for PV module placement, a conventional system will either not work or be inefficient at best. In fact, the top installer in the area advised that it simply wasn't practical to install a system here. Help!!!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,611 admin
    Re: How to trick Enphase Microinverters???

    Yes, you can "back drive" an off-grid inverter with GT inverters. Issue revolves around the fact that you have to setup some sort of battery charge controller to prevent over-charging your batteries. Not impossible--but certainly not an off-the-shelf product at this time.

    How much power / what voltage battery bank would be powering? You could simply add a whole bunch of 12 volt panels in parallel and connect them to a charge controller. Each panel that is in full sun will output its maximum power to the charge controller.

    You could do the same with higher voltage panels (and placing panels in Series / Parallel strings) to power a higher voltage battery bank. All without the issues of having to do the micro inverter path.

    The bigger issue is the shade--you will probably end up with a hyper expensive set of solar arrays. Probably not worth it unless you do a bit of clear cutting and/or find another place to mount your solar array.

    A better place to spend your money (at least for now)--is conservation...

    If you can reduce your power needs enough--then you can run a smaller genset and save on fuel costs / storage issues for when you are with out power (as well as reduce your overall power usage too).

    I can power my home easily with a 1,600 watt Honda eu2000i (probably around 400-500 watt average power usage; less than 2 gallons of gas per day)--no A/C requirements). Either convert to natural gas or propane, or look for the multi-fuel Yamaha or used RV genset.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: How to trick Enphase Microinverters???

    Thank you, Bill, for replying to my questions. If possible I would, at this time, prefer to have the system operational without a battery array. Certainly some manner of battery would be necessary to generate the sine wave to trick the Enphase inverters into operation, but I assume it would not require much capacity. If the system only provided solar electric during daylight hours (when the grid was down), I would be quite happy. This would keep the generator from running during the brighter daylight hours. If you have any suggestions about info sources to accomplish this "unique" design, I would be most appreciative.

    I agree with you fully about conservation being the primary path to take. We are already doing much of that, but power use can only go so low without spending thousands of dollars on new appliances, super insulation, etc. I plan on putting up a fairly large PV array, not only for an emergency backup, but so that I can "sell" power back to the local utility. Both goals will minutely help reduce the amount of coal mined from our beautiful hills.
  • BB.BB. Super Moderators, Administrators Posts: 32,611 admin
    Re: How to trick Enphase Microinverters???

    Usually, it is better to put thousands into conservation vs thousands into solar to generate the power. But--past a point--yes you will eventually need some power.

    Basically, you will need an inverter that is large enough (at least) to consume all of the AC power being generated by the GT Inverter.

    The Off-Grid inverter (probably) needs to be a standard Off-Grid True Sine Wave inverter (Outback, Xantex, etc.).

    Your battery bank needs to be large enough to consume the load (at least for a little bit), then you need either a "dump" load--basically, an electric heater that takes the excess energy from the battery charging and turn it into heat (such as an electric hot water heater).

    And/or you can put a solid state relay in the AC out of the GT inverter, and when the batteries are "charged"--it kills the GT inverter (for a minimum of 5 minutes) then when the batteries "need charging" again, it turns on the solid state relay and allows the GT inverter to charge again. Some GT inverters may have a little "inhibit" connection that you can turn off instead.

    Now--because this system is "dangerous"--i.e., a failure in your dump load controller circuit or a failure in your AC GT output controller can leave full current charging your batteries (and eventually cause a fire, or at least, a puddle of plastic and sulfric acid)--NEC / Building Codes would require two independent control circuits so that if one fails, the other is there to back it up.

    For a long life battery bank... I would guess that you would want the maximum charging current into the battery bank to be around C/10 (or possibly C/5) where C=20 Hour Rate (battery capacity in Amp*Hours--i.e., it will take 10-5 hours to charge the batteries from 0-100% capacity).

    There are some other issues (about multiple charge controllers in the system, and overlapping charging algorithms, etc.)--It is possible.

    But, don't you still have the shading problems (or is it OK if you move your array 400' from the home)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: How to trick Enphase Microinverters???

    "Basically, you will need an inverter that is large enough (at least) to consume all of the AC power being generated by the GT Inverter."

    i have to correct this statement as the inverter does not consume the power generated by the gt inverter, only the loads do. the loads have to be sufficient enough as to properly drop the higher voltages induced by the gt inverter.
  • boBboB Solar Expert Posts: 1,007 ✭✭✭✭
    Re: How to trick Enphase Microinverters???
    Speleoguy wrote: »
    Thank you, Bill, for replying to my questions. If possible I would, at this time, prefer to have the system operational without a battery array.

    Since the Enphase inverter is at present, most likely a current source inverter, just go inside of it and convert it to a voltage source inverter. Just draw a schematic diagram and then re-program the FPGA chip to do this. :D

    You would need a large enough battery running a sinewave inverter to make sure the impedance seen by the Enphase is low enough to make it think that it is seeing a stiff enough grid and not trip offline.

    boB
  • BB.BB. Super Moderators, Administrators Posts: 32,611 admin
    Re: How to trick Enphase Microinverters???
    niel wrote: »
    "Basically, you will need an inverter that is large enough (at least) to consume all of the AC power being generated by the GT Inverter."

    i have to correct this statement as the inverter does not consume the power generated by the gt inverter, only the loads do. the loads have to be sufficient enough as to properly drop the higher voltages induced by the gt inverter.

    Actually, in the case when you connect a GT Inverter and an Off-Grid Inverter together... The GT Inverter will output its maximum power (watts) based on the size and sunnyness of the solar array. And it will follow the Off-Grid inverter for voltage and frequency.

    If the "true loads" are more than the GT inverter's output--then the Off-Grid inverter will make up the difference.

    The GT Inverter output (in watts) is greater than the "true loads", the Off-Grid inverter will actually be "back driven" and start charging the battery bank.

    That is why, worst case, if there are no true loads, the Off-Grid inverter + battery bank need to be able to "absorb/consume" the maximum output of the GT inverter.

    If you have known, fixed loads (say a 2kW hot water pre heater), a 3kW GT Inverter+solar array--then in theory, you will only need an Off-Grid inverter that supplies 2kW (to manage the Hot Water Heater load). With the load on, the GT 3kW max - 2kW Heater Load = 1kW max to be absorbed by the Off Grid inverter+battery bank.

    Which leads to another possible solution--your "dump" load is on the AC side of the inverters... Use this as your dump system to limit current into the battery.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: How to trick Enphase Microinverters???

    Just to show how much I don't know about grid tie,, why couldn't you energize the enphase on the grid side with a small sine wave generator,,, like a Eu honda,, tricking the enphase into thinking that it is getting grid power. (Assuming that you use safe transfer switching etc).

    Then you could match the load from the PV/inverter,, although I would think that that would be quite difficult except into a resistance heating load.

    I'm sure that you guys will tell why this is a crazy idea.

    Tony
  • BB.BB. Super Moderators, Administrators Posts: 32,611 admin
    Re: How to trick Enphase Microinverters???

    You can only do this (just guessing) if the GT inverter output is equal to, or less than, the true load's watt requirements.

    A "standard" A/C generator could possibly sink some current (but how?--it would not turn faster as it is syncronous to the frequency). And if it turned faster, then the GT inverter would have a frequency fault.

    A Honda eu2000i family generator would meet the frequency and (probably) the sine wave requirement. The question is, what would happen if there was energy driven back into the Honda? It has an inverter--so it would be possible for it to sink current--but then the question is what happens to that energy--I don't think it can drive the engine faster (need power to be dissapated somewhere).

    Also, the Honda's can sincronize with other Honda gensets... So, I assume they have some way of synchronizing frequency (without one being a master, and the other being a slave)--and I would guess that they "droop share" the load (i.e., 120 VAC 0% output, 115 volt 100% output). Droop share works pretty well (stable) in many applications.

    If the Enphase did "droop sharing" (or any GT inverter)--then this problem be much easier to solve--but at this time--GT inverter convert 100% of the available solar energy into electricity on the assumption that the "utility grid" is large enough to take all of the GT Inverter's output (up to rated current).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: How to trick Enphase Microinverters???

    Some weeks ago I spoke with an engineer who seemed to have expert knowledge about the Kohler genset that I use. According to him (and a few others I have spoken with) backfeeding the generator will destroy it and also probably do damage to the GT inverter(s). He told about equipment they use to sync multiple large generators, but indicated that the cost of this equipment was $70K +. Reps at Enphase told me essentially the same thing about the dangers of backfeeding a generator. (Note: I only questioned them about Kohler generators, not other brands.)
  • stephendvstephendv Solar Expert Posts: 1,571 ✭✭
    Re: How to trick Enphase Microinverters???

    You might be able to use the SMA Sunny Island inverter/chargers for this. They were designed to be charged from the AC side, but by design were only meant to work with other SMA GT inverters.
    Some discussion of this system here: http://forum.solar-electric.com/showthread.php?t=5047

    As Bill mentioned, you need a way to turn off the power from the GT inverters when the batteries are charged and the consumers are less than the power from the GTs. The SMA system does this by increasing the frequency: The Sunny Island increases the frequency and the Sunny Boy GT detects this frequency and reduces it's power output proportionally (produce 100% for a frequency below or equal to 51Hz, and scale down to 0% production at 52Hz). If you're not using Sunny Boy GTs then you might be able to trick the GT by configuring it to switch off completely if the frequency rises to XX hz. E.g. if the Sunny Island is configured to do it's funky power control between 51 and 52hz, then you could configure your GT inverter to shutdown at 51.5Hz.

    This is essentially the same system Bill is suggesting except that you'll be using the grid frequency as the switching signal instead of an external relay.

    As a backup solution/alternative solution you could also attach frequency dependent relays to the AC bus. They're called "Distributed Intelligent Load Controllers" towards the bottom of this page: http://www.windandsun.co.uk/Inverters/sunny_island_inverters.htm

    They're just relays that work based on grid frequency, and since the Sunny Island controls the frequency based on the available power you could either turn loads on or off depending on how much excess you have.
  • DaveH9DaveH9 Registered Users Posts: 13
    Re: How to trick Enphase Microinverters???

    You might try the ecojoule inverter instead of the enphase. http://terrawattpower.com/ this lets you use small strings of 2 modules each so shade is not such a factor, also claims to allow ac to flow to loads without batteries by using capacitors. It's radical and I don't have experience with these yet as they just got UL listed last month.
  • BB.BB. Super Moderators, Administrators Posts: 32,611 admin
    Re: How to trick Enphase Microinverters???

    Looks like an interesting inverter system... Grid Tied, Off-Grid, and Hybrid (needs external AC Charger / Generator control/transfer hardware?).

    Dave, have you tried it yet?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DaveH9DaveH9 Registered Users Posts: 13
    Re: How to trick Enphase Microinverters???

    Bill,
    No I have not tried ecojoule yet, but I would love to talk to someone who has! Small strings in parallel, building amps not volts. makes sense but strange stuff.
  • BB.BB. Super Moderators, Administrators Posts: 32,611 admin
    Re: How to trick Enphase Microinverters???

    The architecture is similar to some of the other Hybrid Inverter systems out there... But this one is significantly smaller than the others--hopefully less expensive too.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: How to trick Enphase Microinverters???

    Wow, this is a great idea. Just what I have been looking for. It makes so much more sense to be able to use the power from a grid tie without it having to shut down.

    Has any more information come out on this new GTI? Cost?

    It is now the TerraWatt.
  • RCinFLARCinFLA Solar Expert Posts: 1,453 ✭✭✭✭
    Re: How to trick Enphase Microinverters???

    Tricking a GT-Inverter is risky business.

    Most hybrid GT inverters will shutdown when they detect incoming current on their output port. It thinks someone hooked it up wrong connecting output to AC grid.
  • jhtimwilljhtimwill Registered Users Posts: 4
    I am having to do the same manipulation, but the difference is that I am off grid totally.  I can do it either by putting an inverter powered directly by a solar panel at the start of the GTmicro inverter system, or do as is suggested and put a battery supported inverter to provide the virtual grid.  Does the grid replicating inverter need to be  60 cycle 240 v (ie does the enphase use both L1 and L2), or is it seeing just one of the lines?

     Seems to me that there is a reasonable market opportunity to sell a virtual grid inverter for the times when the grid is out. It could have  a transfer switch capability so that when the grid is up it acts as a normal GT inverter and takes over as the grid when the grid is off but the sun shines. 
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    jhtimwill said:
     Seems to me that there is a reasonable market opportunity to sell a virtual grid inverter for the times when the grid is out. It could have  a transfer switch capability so that when the grid is up it acts as a normal GT inverter and takes over as the grid when the grid is off but the sun shines. 
    Then use a set of Tigo optimizers (to deal with shade) and a hybrid inverter with a battery, like a Radian or equivalent.  That's the right way to do it.
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    jhtimwill said:
     Seems to me that there is a reasonable market opportunity to sell a virtual grid inverter for the times when the grid is out.
    If you just want a little backup during power outages, then the SMA Secure Power feature will give you that.
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    To answer the question, yes, the Enphase needs to see 240VAC.    And monitor battery voltage - the off grid inverter may dump excess power from the GTIs into the batteries.

    I am available for custom hardware/firmware development

  • jhtimwilljhtimwill Registered Users Posts: 4
    jonr said:
    To answer the question, yes, the Enphase needs to see 240VAC.    And monitor battery voltage - the off grid inverter may dump excess power from the GTIs into the batteries.
    In my situation I had to evolve the system. For better or worse I started with a generator that charged batteries through a  charger/inverter, this was pending getting a grid connection which depended on an easement which got messy, so my equipment  was GT. Now I have to work that into a off-grid system.  I want the enphase power to go into the existing distribution panel as an alternative to the generator using an automatic disconnect so that when the PV is working the generator is disconnected, thus battery charging issues should be managed by the existing charger inverter.

    Does an off-grid inverter that would minimic the enphase 250 exist, having that would (I think) mean that the enphase outputs would shout down when radiation was inadequate. I was thinking that such an mpp inverter run off a single solar world 270 watt pv at the termination of the enphase could do the job better than a battery supported inverter.

    IS this a sensible idea and if so are there candidate inverters for the role of pied piper of enphase.
  • jhtimwilljhtimwill Registered Users Posts: 4
    jhtimwill said:
     Seems to me that there is a reasonable market opportunity to sell a virtual grid inverter for the times when the grid is out. It could have  a transfer switch capability so that when the grid is up it acts as a normal GT inverter and takes over as the grid when the grid is off but the sun shines. 
    Then use a set of Tigo optimizers (to deal with shade) and a hybrid inverter with a battery, like a Radian or equivalent.  That's the right way to do it.
    Bill,
    Thanks for this comment.  If I go this route what power output and storage would you think would be appropriate.  In the mean time I'll look up what Radian has that might match the situation as I see it.  Thanks

  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    jhtimwill said:

    If I go this route what power output and storage would you think would be appropriate.  In the mean time I'll look up what Radian has that might match the situation as I see it.  Thanks

    Those answers have to start from you.

    What is your peak power demand?  That's divided into two numbers - surge and steady state.  Steady state is easy; you can use a clamp-on ammeter to measure actual usage, or get rough numbers from nameplate ratings.  Surge is much harder since it's transient.  The right way to measure it is with a current sense transformer and a scope, but many people go by other people's experience (i.e. "I have a Radian GS4048A and have no problem starting my GE Armada refrig.")

    Now you know what size inverter you need.  The smallest inverter that can power your loads is generally going to be the right choice.

    Next comes energy.  Figure out how much energy (not power) you need each day in kwhr.  Then figure out how many days without sun/generator you want to run.  3 is a good minimum.  Then multiply by 2 because you shouldn't discharge lead acids below 50%.  Now you know your battery bank capacity required.

    Next figure out how much solar you need.  This will depend on your area, exposure and mounting options.  "Bad" areas (low number of sun hours a day) will require more solar.

    Now do a sanity check.  Is your charge rate more than C/13 and less than C/8?  If not, change things around so that you are not overcharging or undercharging your battery bank. 

    Next comes decisions about voltage.  If you have a big system you can run the normal ~600V strings and use either the Morningstar or the Schneider charge controllers.  For smaller systems you generally wire for about 100 volts and use a 150 volt charge controller; there are a lot of these out there from Outback to Midnite to Morningstar.

    You can then add optimizers if you want them.  For 100 volt systems they are not as important because you are only going to lose 3-4 panels when one gets shaded due to the shorter strings.  For 600 volt systems they would be more important.

    And of course you need a generator.  Again choose one so that you can charge at at least C/13.  You can go much higher in power since the inverter itself will limit battery charging to a safe level, and you can use the surplus to run bigger loads.

    Those are the big decisions; the rest are relatively straightforward.  Again, the above is for a hybrid system that will pretty seamlessly pump power back to the grid when it's available and charge your batteries when it isn't.  It will be expensive, but if 100% reliability is your goal it's the way to go.
Sign In or Register to comment.