Powering up grid tie array when power is not available from electric company

FormerDilbert

I have a grid tie system.
I want to be able to power it up if there is a loss of power from the power company.
I know that the main breaker in my power panel needs to be shut off to prevent feeding power back on a dead grid.

I am thinking of using a few additional panels, a mppt charger to power a battery bank. Any extra power would be sent through a grid tie inverter. In the event of a power failure, the output of the batteries would feed an off grid inverter to create the 60hz split phase inverter to allow the main array to output power again.

Can anyone confirm my proposed method or tell me how to make it work 

Thanks

BB.

Welcome to the forum FormerDilbert,

There is a way of "activating" GT inverters when utility power is down... Basically, you have a (good quality) (typically) PSW/TSW 240 VAC @ 60 Hz as a "micro grid"---And then connect the output of the GT Inverter(s) to the output of the OG inverter--And the GT inverters will share the local AC loads (of course, usually a "transfer switch is used" to disconnect the Grid from the home, and connect to the OG inverter...

There are some rules... The OG (or hybrid) inverter needs to be PSW/TSW (Pure/True sine wave), have accurate 60 Hz output (ideally, less than 0.1 Hz error), and the OG inverter needs to be rated higher Wattage than the Wattage of the GT Inverters.

If there is more GT Watts vs AC loads, then the "extra" energy from the GT inverter(s) will be pushed backwards through the OG inverter and actually charge the battery bank...

There are older OG Inverters that work--But you have to setup some way to limit battery charging--The older OG inverters had no way of "regulating" the battery bank charge and could over charge/over voltage the battery bank. There are newer OG/Hybrid inverters that will work with GT inverters and "adjust" the AC frequency (for example) and "knock" the GT Inverters offline (i.e., vary AC frequency by +/- 1.0+ Hz and the GT inverters will disconnect until 60.0 Hz is reached and stable for 5 minutes).

There are GT+OG/Hybrid inverters that will "talk" via line frequency... I.e. 60.0 Hz, 100% output from GT Inverters... 60 +/- 0.5 Hz, then 50% GT inverter output, and 60 +/- 1.0 Hz, 0% GT inverter output power (or something like that). You would typically use a 120/240 VAC 60 Hz (for north America) OG inverter to give you 120/240 VAC, and the 240 VAC output for the GT inverters (which are typically 230/240 VAC "everywhere").

This is not a "plug and play" setup... You need to check the GT and OG inverter specifications to make sure they will play nicely with each other (is a relatively "recent" function).

The other option... is to setup a "standard" Hybrid AC inverter with a DC Solar Panel charge controller to charge the battery bank directly. A Hybrid AC inverter can do both GT and OG function, will typically have an internal AC transfer switch, and generally be easier to configure and maintain. And you skip the whole dedicated GT Solar Inverter and programming GT/OG/Hybrid inverters issues.

There are other options too... One is some GT Inverter (such as SMA's) offer a "secure power" option. Basically a second 120 VAC 60 Hz output that can run local AC loads while the sun in in the middle of the sky (AC loads have to be "smaller" than the Solar Power available).

Another option--Depending on the size/panel specifications/configuration of your solar array--You might be able to "get rid" of you central GT inverter and wire directly to one (or more) MPPT Solar controller(s) directly... And then setup the rest of the system (OG or Hybrid AC inverter, transfer switch, battery bank, etc.).

You might also want to look at how often your power goes out and for how long... In you are in a location where you can have "weeks" of utility failures once a year, or more often, then a full solar=>charger=>battery bank=>AC inverter system may make sense...

If you only have a few short outages and/or go years between outages--Then a simple backup genst + AC transfer switch may be a better solution. Propane or Natural Gas is usually a good fuel. Gasoline or Diesel can be an option (fuel stabilizer in gasoline or diesel). Propane and Diesel can be an issue in very cold weather (propane tank may get too cold to pressurize, Diesel can be difficult to start/fuel flow). For gasoline, even with fuel stabilizer, I recycle into my car one a year to keep the gas+stabilizer "fresh".

Keeping your AC loads (particualrly your "emergency loads") relatilvey small and efficient and using a "smaller" genset--You can stretch your fuel usage quite easily...

For example, a Honda eu2200i (or similar) inverter-generator can run upwards of 9+ hours on a gallon of fuel (average 25% load)... Or around 2.5 gallons per day. Or almost a week on 15 gallons of gasoline. Enough to run a fridge+freezer+LED lighting during an outage.

Get a "big/cheap genset"... And your 5 kWatt genset will probably run on 0.69 0.44 GPH on gasoline (standard genset) or 16.56 10.56 gallons per day @ 50% load: [updated to correct GPH numbers. -BB]

https://www.centralmainediesel.com/order/Honda-5kw-Gasoline-Generator.asp?page=H04584

A nice 6 kWatt diesel genset with a 100 gallon tank, maybe almost 300 hours (or 12.5 days):

https://www.centralmainediesel.com/order/Super-Quiet-Long-Run-Diesel-Generator.asp?page=3930

Similar issue with a battery based off grid solar power system... Designing a system that will power 3.3 kWH per day or 100 kWH per month (fridge+led lighting+laptop computer+cell phone charging+small water pump) is a much less expensive system then a 10 kWH (300 kWH per month) solar power system.

The major issue with any off grid capable solar power system is the battery bank... Lead acid battery bank lasts around 5-7 years (if used or not) and flooded cell batteries need to check electrolyte/add distilled water every month or so. Or you can go with Lithium Ion which has no maintenance, and can last several times longer than lead acid battery banks... But Li Ion typically are more costly (and need a Battery Management System)--And they must be kept above freezing when in use... (can be stored below freezing, but must be warmed up >freezing to power cycle).

Details matter here in solar/power systems... I highly suggest not to buy any hardware until you have a detailed paper design first. There are lots of dependencies between batteries, chargers, inverters, solar array, etc... It can be very difficult to put "random" hardware together into a working system.

-Bill

FormerDilbert

Thank you Bill for your time to provide such a detailed answer.
I will need to read your response a couple of times to fully understand.

One question.
I have a 16Kw array. You stated that the off grid inverter would need to be rated at 16Kw also.
The array uses 40 IQ7 micro inverters.
Can I disconnect all of the loads and then power the array up with a (say a 5 Kw OG inverter) and then reconnect the loads once the array is fully up?

 Thank you 

BB.

One possible option... If you want to use a smaller OG AC Inverter, I would guess you have around 4x strings of IQ7 inverters--Probably around 4 kWatts per string.

You could setup a "protected panel"--Basically run a 4+ kWatt OG AC Inverter and connect 1x 4 kWatt IQ7 string to the protected panel, and the other 3+ strings to the main AC panel. That way you could use just 1/4 of the array and a 4 kWatt OG AC inverter to power your "emergency" loads (fridge, lights, etc.).

You cannot use a 16 kWatt array to power a (for example) 4 kWatt OG AC inverter--At some time during the day, you will have lots of sun, a small(er) amount of House loads, and the GT Inverters will overload (backfeed) the OG Inverter and cause it to fault or fail (again, the OG AC inverter when running "backwards/backfeeding" does not (I think) have the ability to limit the backfeed current other than faulting (turning off) or blowing internal fusing/external circuit breakers.

Also--I am not an expert in doing this (GT Inverter(s) "backfeeding" OG Inverter. So I cannot tell you that everything will work perfectly.

Ideally you will want an OG Inverter that is rated/designed to be feed from GT Inverters.

And you would need to talk with Enphase engineering to see if their (and your version) of IQ7 inverters will function correctly (either 0% or 100% feed, or proportional feed based in 60 Hz +/- 0.xx Hz).

To check with Enphase, you may need to go through your installer or distributor to get answers to these questions--And what brands/models of OG Inverters they support (if any).

People have done similar setups before--But I don't know the details--Perhaps somebody here can let us know if and how they may have done GT to OG inverter connections successfully.

SMA Sunny Island is designed for this:

https://www.solar-electric.com/lib/wind-sun/Sunny-Island-specs.pdf

And some versions of Schneider Hybrid inverters may support this too:

https://www.solar-electric.com/schneider-electric-xw-pro-6848-21-inverter.html

This is not a typical configuration and there may be some experimentation on your part to do this.

-Bill

FormerDilbert

Thank you.
You gave me a couple of ideas. I am going to try a couple of things.