Micro Inverter at power outage

aleman83

Hello everybody,

I´m looking into a setup for my parents and have a few questions.

The ideal solution for them at the moment would be to go with Enphase Micro Inverters since they want to start small and add on whenever they can. They are on the grid at the moment using power costing 55 cents a KW/h (so it pays in no time).

Also we live on an Island where power outages are very common. Is it true that the enphase micro inverters only boost the grid and would not be able to provide power when there is an outage?

The Micro Inverters put out 240VAC. How does that work on a 120VDC Grid?

Looking forward to your expertise

Aleman

Cariboocoot

Re: Micro Inverter at power outage

All standard grid-tie inverters produce power only when the grid is 'up'. This includes micro-inverters.

No grid, no power. If you want back-up power, buy a generator. Otherwise you need a battery-based system (at least for critical loads) which is more expensive.

The household power is not 120 VAC, even though that's what comes from the outlet. It's actually 240 VAC "split phase" where the center tap is the neutral and the two legs supply 120 VAC each.

westbranch

Re: Micro Inverter at power outage

I can only relay what I have read and that is you probably need to read up on the Sunny Boy, Suny Island systems, german made and pricey f you want Grid tie power during an outage..

hth

inetdog

Re: Micro Inverter at power outage

westbranch wrote: »

I can only relay what I have read and that is you probably need to read up on the Sunny Boy, Suny Island systems, german made and pricey f you want Grid tie power during an outage..

hth

There is also a (still not yet released?) SunnyBoy GTI model which can deliver something like 13 amps at 120 volts straight from the GTI without any batteries. But that output is separate from the normal GTI output, cannot use the full output of the panels, and would have to be directly connected via cords to critical loads rather than running through the normal house wiring. The Emergency Power feature is an option, and I have not seen what it will to the cost of what is already in the high end price range for GTIs.
There will be 2Kw, 4Kw and 6Kw GTIs, (or is it 4, 6 and 8??) all with the same limited off-grid power capability.

BB.

Re: Micro Inverter at power outage

And, if your parents' home only has 120 VAC (not 120/240 VAC split phase power), then most GT inverters will not work--They need around 208 to 240 VAC 50/60 Hz. There are a few smaller ones that will run at 120 VAC--But most of those are the import "plug and play" types.

The normal answer for emergency backup power is a battery bank charged with solar panels (if that is your desire), and a Hybrid Inverter which does both Grid Tied and Off Grid (think of a UPS with the battery bank recharged by solar panels). A "true" hybrid system will push power back to the utility and turn the meter backwards. In most areas outside of US (with lots of permits and fees, plus many smaller utilities will not allow "net metered" grid tie systems) and Europe (sometimes less permits), Grid Tied solar is not legal and not supported by the utility (billing issues, they will tell you safety issues, and sometimes grid stability issues).

So--You can go back to the tried and true systems. Battery bank + AC Inverter + AC battery charger--Basically a large UPS system. And then add solar+charge controller to the battery bank for longer off grid power and less charging costs (could even power loads when grid is up and, possibly, save some money).... Note that in the US, a back of the envelope calculation shows around $0.45 per kWH just for hardware and battery costs (including maintenance and battery replacement every X years). So, usually, it is difficult to "save" money by adding solar--But once you have a stable battery+inverter+AC charger (plus automatic transfer switch or equipped AC inverter), they can certainly try adding solar. The additional cost for solar may not be that much (depends on local costs) because they would already have/need the battery bank+Inverter.

-Bill

NorthGuy

Re: Micro Inverter at power outage

For solar panels to provide power during grid outage, you must have a battery bank. You can have a smaller battery bank, but you still need it.

inetdog

Re: Micro Inverter at power outage

NorthGuy wrote: »

For solar panels to provide power during grid outage, you must have a battery bank. You can have a smaller battery bank, but you still need it.

Only because of the limitations of the available equipment for now. There is no inherent reason that you cannot go directly from panel to inverter if the inverter is designed for it AND you are willing to accept some serious power limitations compared to the panel power AND you have loads that you do not mind losing power to when a cloud goes by. :-)

ggunn

Re: Micro Inverter at power outage

BB. wrote: »

And, if your parents' home only has 120 VAC (not 120/240 VAC split phase power), then most GT inverters will not work--They need around 208 to 240 VAC 50/60 Hz. There are a few smaller ones that will run at 120 VAC--But most of those are the import "plug and play" types.

It is unlikely that the "grid" is 120VAC, but if the supply to the house is indeed 120V, you can use an autotransformer to connect a 240VAC inverter to it.

NorthGuy

Re: Micro Inverter at power outage

inetdog wrote: »

Only because of the limitations of the available equipment for now. There is no inherent reason that you cannot go directly from panel to inverter if the inverter is designed for it AND you are willing to accept some serious power limitations compared to the panel power AND you have loads that you do not mind losing power to when a cloud goes by. :-)

... AND if you assume that outages only happen during day time

aleman83

Re: Micro Inverter at power outage

Thank you so much for your help. This is already a big help. As far as I can see the installations here are split phase, so it should be all good. To start with it is supposed to lower the electricity bill, since as I told you with a KW/h costing over 50 cents there is not much calculation needed. The power outage thing still has me thinking though. It is not very important, but would be nice to just get over a 5 h outage on a monthly basis. Would it theoretically be possible to use the Panels with Micro Inverters under normal conditions. But have a small battery bank with a inverter for backup that is still boosted by the micro inverters?

Again thank you so much for your help.

Aleman

Cariboocoot

Re: Micro Inverter at power outage

What you are referring to is called "AC coupling" where the GT inverter(s) sync to a standard off-grid inverter to supply it with charging power for the batteries and extra Wattage capacity (during daylight hours).

Yes, it can be done. It's not particularly cheap or easy. Only certain off-grid inverter will work. Two that are known to are SMA's Sunny Island (very good, very expensive) and Xantrex's XW series (also costs money).

There are other alternatives, depending on the particular GT set up, such as reconfiguring the array to temporarily feed a charge controller during power outages. In either case you end up needing an additional inverter and batteries.

As such you need to consider carefully the likely frequency and duration of power outages. Often it is more economical to buy a back-up generator for such instances, even with a GT system already installed.

ggunn

Re: Micro Inverter at power outage

Cariboocoot wrote: »

As such you need to consider carefully the likely frequency and duration of power outages. Often it is more economical to buy a back-up generator for such instances, even with a GT system already installed.

If you do that, the interconnection point for the GT PV should be outside the transfer switch so that the PV shuts off when the grid is down. Otherwise (if the PV inverter accepts the generator output as grid power) you run the risk of backfeeding the generator. Grid tied PV is meant to offset the electric bill, and the contribution to the bottom line that the PV would make during a grid outage is unlikely to justify the added headache and cost of building in protection for the generator.

niel

Re: Micro Inverter at power outage

imo the hybrid setup would do for you the best, but you can have the enphase setup and a separate backups with inverter and batteries should you want to backup only 1 or 2 circuits. it would be cheaper and better to have the hybrid and would cover more of the power needs in the house automatically in an outage. the sizing of the battery bank would need to be such that it would not go below 50% soc in a worst case scenario before you are able to shut it down and possibly switch to a generator.

aleman83

Re: Micro Inverter at power outage

One more thing,

This all seems to work out for me so far. I have inquired about some prices and was told that the company does not recommend micro inverters for over seas. Why would that be? Could there be a problem with the local power grid that they might not work? As far as I know the local grid is 120VAC split phase ( 2 120VAC phases) and at 60Hz.

Cariboocoot

Re: Micro Inverter at power outage

Possibly they don't recommend them overseas for two reasons:

1). The large number of locations that use single phase 230 VAC 50 Hz which the inverters are not compatible with;

2). The large number of locations that have inconsistent, unreliable grid power that would make GT of any type a wasted investment.

For a specific answer you'd have to ask the company "why not?"

aleman83

Re: Micro Inverter at power outage

I did ask them and they just replied that they don´t because they don´t know the power companies. Talking about inconsistent grid power,... do you mean power with fluctuations in voltage? How big of a problem would that be and how much tolerance is there so it would work with grid tie?

Cariboocoot

Re: Micro Inverter at power outage

Depend on how 'tolerant' the inverter is.
They 'test' the grid periodically to see if the Voltage is too high, too low, or too far off frequency. If it is the inverter shuts down (anti-islanding). Should there be too much fluctuation of the grid power, the inverter would be turning on/off a lot and be a wasted investment.
Some people have had this problem due to too much resistance in the wiring to the grid: the inverter's output Voltage can go high (reverse of the usual V-drop problem) and so the inverter thinks the grid is down and turns off. The Voltage then drops back to normal and the inverter comes on again. Lather, rinse, repeat.

The statement about not knowing the power companies pretty much confirms my reasoning; they don;t want to have to guarantee it will work when the power specs could be changing moment to moment.

BB.

Re: Micro Inverter at power outage

If you can justify the monitoring hardware--You could try one or two MicroInverters and see what happens (assuming the basic voltage/frequency requirements are within range).

There may be an issue with not having local support... Say your power sags in summer to 103 VAC, or that +/-2 Hz is OK. Some of these GT Inverters are programmable by the installer to let them work with relaxed specifications (if "OK" with the utility).

See what you can do can lead to a business opportunity as in distributor/installer (if you are interested)--It has happened here before.

-Bill

aleman83

Re: Micro Inverter at power outage

Thank yous so much for your quick help. This really helps me a lot. I will monitor the frequency and voltage of the local grid for some time and see how good or bad it is. What about "cleaning" the power from the grid? Isn´t there some sort of a buffer that can be installed between the grid and the house?

Again thank you so much for the great help!

BB.

Re: Micro Inverter at power outage

About the only thing that may be useful is a transformer will multiple taps (usually computer controlled for highly varying voltages).

There are ferroresonant transformers. They tend to work well for voltage but are really poor if the line frequency is not stable (they "resonate" at 50/60 Hz--off frequency is not a good thing for them). I think they are bi-directional for power flow (?).

It is very difficult and not cheap (equipment costs, losses in efficiency, etc.) to stabilize an unstable power source. If this was "critical" application (i.e., server computers), you would just take an AC to DC battery charger and an AC Inverter for server power. Then there is no variable in AC input power that will affect the output.

But, of course, you are looking what to do with GT Inverters. Your best bet would be to find out what is "acceptable" to your utility and get the GT inverters programmed to those limits (if the moon and stars are all in alignment).

-Bill

solar_dave

Re: Micro Inverter at power outage

Enphase specs have a pretty wide range of operating voltages
http://enphase.com/wp-uploads/enphase.com/2011/10/Enphase-Datasheet-M215-Microinverter.pdf

240V/211-264V

inetdog

Re: Micro Inverter at power outage

BB. wrote: »

There are ferroresonant transformers. They tend to work well for voltage but are really poor if the line frequency is not stable (they "resonate" at 50/60 Hz--off frequency is not a good thing for them). I think they are bi-directional for power flow (?).

Correct.
They are, to some extent bi-directional for power flow, but the ferro-resonant circuit is optimized to regulate just one of the two windings. I have used a CVT I had lying around as a 120 to 240 autotransformer in a pinch, but it is not efficient.
The exact frequency problem is that for frequencies close to the design frequency a 1% change in frequency will produce a 2% change in output voltage. Beyond a certain point they are just useless.
Combined with a cheap voltmeter, it does make a very expensive and inaccurate analog frequency meter.:-)
(And it can be used to regulate generator speed by increasing the voltage and therefore the load when the generator speeds up. (I am not seriously recommending that, of course.))