Micro inverters, is it a viable option?

South Africa
South Africa Solar Expert Posts: 295 ✭✭✭
in Solar Beginners Corner #1
Hi,

I came across these recently. For mismatched panels, I am wondering if this is an option for Grid tie?

Or, if you do not go grid tie, it was suggested to me to let these inverters together feed a UPS power so that if there is a break, then UPS takes over.

Attachment not found.

The 2nd question, to go grid tie (I know enough to be very respectful of it) the supplier says to me:
“The PMI-250 can be connected to each other with an AC cable, do not need any cable accessories, you can connected it to the grid directory. But in order to avoid damage caused by power failures on the power system, it is recommended to install circuit breakers, fuses and surge protectors before connected to the grid, specifications based on solar power system capacity to choose. “

As far as I am aware, this cannot be all that is required? I mean, when the grid goes off, to protect people working on it, you have to have a auto disconnect function that stops ALL feedback!?

Can one have a relay installed to stop feeding if the grid goes off.

Last question (sorry if it is stupid thing to ask, but I am SURE others (read newbies) may have the inclination to wonder same) : :-)
Can one connect a micro inverter directly to a wall socket where you then have live, neutral and earth all together to not only earth the micro inverters and panels, but also feeding power into the house circuits? (Yes, a lightning strike will destroy the house electrical components and everything on them.)
5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.


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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #2
    Re: Micro inverters, is it a viable option?

    Yes essentially micro inverters are for grid tie.
    They require the grid be up to output anything, it is called anti-islanding, a safety feature.

    No it is not safe to hook them up to an outlet, it can cause over amperage conditions on the plug in line . The proper method of attachment is basically the same as any other grid tie inverter.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #3
    Re: Micro inverters, is it a viable option?
    South Africa wrote: »
    Or, if you do not go grid tie, it was suggested to me to let these inverters together feed a UPS power so that if there is a break, then UPS takes over.

    Micro inverters are meant for grid tie, but you can use them in an off-grid setup IF you have a bi-directional inverter/charger that provides the reference voltage and frequency to effectively make it's own mini-grid. Some inverter/chargers that offer this are the SMA Sunny Island, the Victron multiplus, Outback GVFX, Magnum and more. You can then connect the microinverters to the output of one of the previously mentioned inverter/chargers like this:

    Battery
    inverter/charger
    230V AC
    microinverter ---- micro inverter
    etc.

    Since the loads and microinverters are all on the same 230V AC bus, if they can supply all the power needed by your loads then nothing gets taken from the battery.
    And it works in reverse too: if the microinverters provide more power than the loads demand then the inverter/charger puts the excess into the battery.

    Here's where it starts getting a bit complicated, because the inverter/charger when driven backwards like this cannot control the charging, so it just keeps dumping excess power into the battery regardless of the battery voltage. If you want to find out more about this mode of operation try a search for "AC Coupling".
    South Africa wrote: »
    “The PMI-250 can be connected to each other with an AC cable, do not need any cable accessories, you can connected it to the grid directory. But in order to avoid damage caused by power failures on the power system, it is recommended to install circuit breakers, fuses and surge protectors before connected to the grid, specifications based on solar power system capacity to choose. “

    As far as I am aware, this cannot be all that is required? I mean, when the grid goes off, to protect people working on it, you have to have a auto disconnect function that stops ALL feedback!?

    This protection is built into the micro-inverter. If it doesn't detect 230V 50Hz, then it switches off automatically itself. This is why you need an inverter/charger to provide the reference "mini-grid" for them to work in an off-grid setup.
    South Africa wrote: »
    Last question (sorry if it is stupid thing to ask, but I am SURE others (read newbies) may have the inclination to wonder same) : :-)
    Can one connect a micro inverter directly to a wall socket where you then have live, neutral and earth all together to not only earth the micro inverters and panels, but also feeding power into the house circuits? (Yes, a lightning strike will destroy the house electrical components and everything on them.)


    Can it be done technically? sure. AFAIK it's not legal to do this in many countries because it's against the electrical code. Hopefully some of the more technically savvy members can explain why, personally I don't see that much danger with a single 230W panel and single microinverter plugged into a socket
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  • South Africa
    South Africa Solar Expert Posts: 295 ✭✭✭
    #4
    Re: Micro inverters, is it a viable option?

    Great idea Stephen, but for me that is a wee bit expensive to go for a bi-directional inverter/charger at tis time. UPS idea is out the door. :-)

    The background:
    I have 3 x 310w Tenesol panels that do not really fit anywhere with my controllers because a) they are too big or b) to make a bigger controller viable, I need more panels. Problem with more panels is that I cannot get any more, so I need to mix and match. A old mistake of buying the panels first, then the rest, that I made years ago. :-)

    So if I cannot sell the panels, and the micro inverters as per spec sheet in first post are a good reliable make (? anyone have experiences with them), the idea is to get them for the Tenesol panels and grid tie that power. It is much cheaper solution than any of the others I have contemplated lately. Just need to get municipal approval first for the 930w incoming which is way less than we use. :-)

    So, assuming they are ok, and not that I will attempt this myself, in principal, to connect a grid tie inverter to the house distribution board is as simple as connecting the live and neutral of the inverter to the live and neutral of the DB board via a 220v AC 2 pole breaker? Earthing is to the same earth as what is commonly available in the DB board?
    5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.


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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Micro inverters, is it a viable option?
    South Africa wrote: »
    So, assuming they are ok, and not that I will attempt this myself, in principal, to connect a grid tie inverter to the house distribution board is as simple as connecting the live and neutral of the inverter to the live and neutral of the DB board via a 220v AC 2 pole breaker? Earthing is to the same earth as what is commonly available in the DB board?

    Connecting is simple. All you need is permission from the utility and the local inspector, the correct meter (so you don't get charged for surplus power you produce), enough 'room' on your service panel to accommodate the GTI (so that the bus bars aren't fed more power from each end than they can handle), micro inverters with panels approved for them (each company has a list of panels that will work properly with their inverters), wiring, mounting hardware, breaker, and of course miscellaneous.

    The output of micro-inverters is 240 VAC (or 230 VAC Euro standard); they do not have a neutral. Any that do connect to 120 (as in plug-in) are illegal junk and should be avoided. The reasons why have been explained on the forum numerous times but some people still don't listen. Otherwise the connection is straightforward right up until you run into a dim witted inspector who doesn't understand what he's doing.

    Setting them up with a micro-grid as Stephen outlined is more of a pain than most people imagine. Great for standard grid-tie though, and especially useful where you can't have a monolithic array pointing all in one direction, have shading issues, or wish to spread power distribution through the day by having arrays pointed in different directions. They also work with more than one type of panel so if you find yourself with panels that may not all work in one array they may work on individual inverters.
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  • South Africa
    South Africa Solar Expert Posts: 295 ✭✭✭
    #6
    Re: Micro inverters, is it a viable option?

    Have given this a bit of thought, hit me, I can take it. :-)

    Current system works as follows:
    - All equipment that the inverter powers, is on a separate circuit to the house.
    - This circuit has its own little DB board.
    - If 10% of batteries are used, switch box auto switches the circuit back to the mains of the house, inverter idles.
    - Inverter / mains power goes via online UPS to devices to protect them form our stable (sic) grid.

    To not go grid tie, can I connect the 220v AC coming in from the micro inverters to the output of the inverter? Because if I can it means that the 'grid tie' is actually tied to my solar system, not the utilities grid.

    Or option B: I have a online UPS in place, which can take a wide range of incoming voltages. Can I connect the micro inverters AC direct to its input? If there is no input from the inverters, a relay will be triggered to swap power back to mains to power UPS.

    Or am I playing with fire, yet again? :blush:
    5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.


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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #7
    Re: Micro inverters, is it a viable option?

    Sniff, Sniff ....
    What is that I smell burning?     :)
    SMA SB 3000, old BP panels.
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  • South Africa
    South Africa Solar Expert Posts: 295 ✭✭✭
    #8
    Re: Micro inverters, is it a viable option?
    inetdog wrote: »
    Sniff, Sniff ....
    What is that I smell burning?     :)

    Nope, not me.

    I thought it best to ask first before I get that oh so familiar smell ... AGAIN! :p

    Seems to me I have to wait for grid to become legal. :D
    5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.


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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #9
    Re: Micro inverters, is it a viable option?

    Any Grid Tie (utility interactive) AC inverter... They are designed to only "work" (turn on their output) if there is 120/240 VAC at 50/60 Hz output. If the AC voltage is not stable for 5+ minutes, the GT inverter will not turn on.

    So, there is no way a (standard) GT inverter can turn "on" and provide 120 VAC 60 Hz to the input of a UPS (or any other animal).

    However, you could take (for example) a 1,000 Watt AC off grid sinewave inverter--Connect it to the AC loads, and connect a GT inverter to the same AC "micro grid" bus (1,000 watts or less GT inverter in this case).

    Fire up the OG inverter and loads. The GT inverter sees 120 VAC 60 Hz for 5 minutes, turns on and starts feeding (current) the "micro grid".

    Some of the energy will go to the loads, and any excess energy will flow backwards through the Off Grid AC inverter and actually charge the battery bank.

    However--A standard sine wave inverter may or may not "run backwards". And a standard sine wave inverter has no battery bus voltage regulation, so you have to come up with some sort of regulator (battery dump regulator, control line back a relay on the GT inverter to shut it down when the battery if full, etc.).

    It does work with some known AC Off Grid inverters.

    And there are some AC Off Grid inverters that have been designed to operate in this mode... The OG inverter, as the battery bank gets "full" the OG inverter starts varying the 60 Hz by +/1 Hz or so--When the frequency goes out of range, the GT inverter turns off. Magnum and Schneider are two of the companies that have "AC Coupled" GT inverter support built into their newer OG and Hybrid inverters.

    SMA makes the Sunny Island system of OG and GT inverters that play well together (somthing like 0 Hz offset 100% GT output, +/0 0.5 Hz output, 50% GT output. +/- 1 Hz offset, 0% output--System is designed so clocks/AC timing devices are still accurate--very +/- Hz to average zero offset over time). (numbers are made up--I have no idea the exact values/offsets used).

    So--The more money you are willing to spend, the more fancy/integrated systems you can have.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • South Africa
    South Africa Solar Expert Posts: 295 ✭✭✭
    #10
    Re: Micro inverters, is it a viable option?

    Baie dankie.

    I think I will wait for legal grid tie. Seems much cheaper and easier. :-)
    5kVA Victron Multiplus II, 5.2kW array, 14kWh DIYLifepo4 bank, all grid-tied.


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