May I tie a micro inverter to grid via an UPS?

stephen · 2021-06-26T14:23:47.744Z

many UPS are a modified sine (a blockly stepped sine wave ). and you will hear your inverter hammer and most likely die within a minute … even having a ups connected with in house can turn our bad for grid ties as the UPS a lot of them leak back into the grid when operating . but since they do not leak back into the grid at 2KW threshold of a transform start up and they are considered “safe”. but they can leak back enough to destroy a grid tie inverter especial say the UPS is on a same circuit as the GTI that has nothing on that circuit to draw down the leak voltage to zero – I had that happen to me my GTI melted and blow a huge hole in the circuit board–. what happens is the leaks enough voltage that the GTI tried to start but it can not sync it hammers and then blows as it melted away all the solder on the AC side of the circuit board

Bill.Thomson · 2021-06-26T17:26:07.016Z

stephen:

many UPS are a modified sine…

Even with a pure sine inverter, the main thing that’s likely be a show-stopper is the UPS frequency and output voltage stability. (or lack therof) As you mentioned, modified sine output is definitely a show stopper.

paulosch · 2021-06-26T17:29:52.684Z

This is a concern to GTI owners. After installing it the only alternative is to use a pure sine wave UPS, usually much pricier. I intend to use something like this https://www.amazon.com/AIMS-Power-1500-watt-Inverter/dp/B004AAGP1O/ref=sr_1_6?dchild=1&keywords=pure+sine+wave+1500w+ups&qid=1624728457&sr=8-6
What do you think?

Bill.Thomson · 2021-06-26T17:32:46.687Z

Even so, unless the inverter is very forgiving of frequency and voltage drift, (especially frequency)
it’s not likely to work. This question has been asked many times on some of the larger solar and alt energy
forums. The answer I’ve seen is essentially it’s an excercise in futility. i.e. it doesn’t work.

paulosch · 2021-06-26T17:39:46.706Z

Ok, nice to know before spending! Thanks for that. Since I have a two phase mains, maybe better to leave the GTI alone in its phase and to install a standard battery inverter to the other via a transfer switch. Then connect critical loads to that phase.

Bill.Thomson · 2021-06-26T17:42:14.400Z

paulosch:

maybe better to leave the GTI alone in its phase and to install a standard battery inverter to the other via a transfer switch

stephen · 2021-06-26T18:03:43.295Z

if you still want to use UPS the easiest way
get a server one that allows DC input from secondary source. then just put a Y at your solar panel and couple couple isolation diodes another set of batteries and charge controller and plug the DC source into the UPS - one side of the Y goes to the batteries and the other side goes to GTI . place the isolation diodes on the battery side to prevent power from the battery going to the GTI… the inverter will pull down what ever not being used by the battery. what I actually find works the best is is using a large boost/buck converter ( bigger then the panel output) and set it at the float charge for the battery over using a charge controller as it does not interfere with MPPT of the GTI

paulosch · 2021-06-26T18:10:20.651Z

I’m afraid to have not understand your scheme. Perhaps because I’ve read this one: DIY Grid Tied Inverter (using UPS) : 4 Steps (with Pictures) - Instructables

I’ll try to sketch your idea, thanks, @stephen

stephen · 2021-06-26T18:19:07.765Z

Y connector —

Blocking diode —

Panel -> Y -> blocking  diode -> charge controller ( or buck/boost) -> battery -> UPS
          \___ GTI

paulosch · 2021-06-26T18:28:49.349Z

I think this way it will connect both GTI and UPS outputs together. Would this expose GTI to UPS frequency instability issues? Unless I connect each device to its proper separate phase it may not work as @Bill.Thomson said.

stephen · 2021-06-26T18:49:30.241Z

you do not have the GTI connect to the UPS, it is connected at the DC source of both big difference
as you asked Can a GTI be connected VIA a UPS which implies connected to the AC output of the UPS

panel -> GTI  -> UPS > wall outlet - will not work

but
can you have GTI and UPS connect to the same DC charge source is different question

panel  -> blocking diode -> charge controller -> battery -> UPS 
          \__ GTI

Bill.Thomson · 2021-06-26T18:51:26.387Z

I read his question as asking if the UPS could substitute for the grid when the grid goes down.
IOW, make the GTI perform like a hybrid inverter.

paulosch · 2021-06-26T18:56:52.167Z

Sure, that was my intended goal (I am not sure how a hybrid inverter works). @stephen 's unifilar scheme appears to have two AC outputs. Either we have them connected to same grid or not (different phases). Am I wrong (probably so)?

Bill.Thomson · 2021-06-26T19:00:55.464Z

paulosch:

I am not sure how a hybrid inverter works

A hybrid inverter consists of a GTI and a battery charge controller all in one package.
It operates as a normal GTI when the grid is present. When the grid drops, it automatically switches to
battery operation. Usually, operating capacity when powered by the battery is significantly less than when
powered by the grid. e.g. SMA makes one that is rated at 6 kW when the grid is present, but only 1.5 kW
when the grid is not present.

paulosch · 2021-06-26T19:04:09.578Z

Yeah, that’s precisely the description of my goal. Is there any micro inverter with this capability? Anyway, now I have two WVC700 in operating conditions. Both were damaged shortly after installation but I managed to repair them.

stephen · 2021-06-26T19:12:37.944Z

yes because the GTI (WVC700 micro inverter) needs to be connected to grid an UPS connects to grid and then provides back up to devices after it.


                    grid (AC) -> UPS -> to  device that need power 
              GTI _/(AC)        / (DC)
Solar panel _/(DC)_____________/

it a hybrid inverter function

paulosch · 2021-06-26T19:15:16.933Z

Just to be sure, both GTIs failed when connected strictly as described in manual, no UPS at all.
I am very careful now exactly due to that early bad experience, and I don’t want to burn them again.

So, by what I know now, it should work only if GTI and UPS are DC coupled, right?

stephen · 2021-06-26T19:24:39.999Z

yes DC coupled and diode blocked from each other - the WVC700 micro inverter will not like being connect to a battery. so you have to prevent it from drawing power from the UPS battery and only pull power from the solar panel

paulosch · 2021-06-26T19:29:05.620Z

I see. But I am still struggling to understand the AC connection of both units. Can they go to the same AC grid (same phase)? I know an UPS input and output can be the same, but placing GTI output on it?

Bill.Thomson · 2021-06-26T19:32:35.671Z

You might want to have a look at this:

Solar Panels - Solar Panels Forum

How to 'trick' PV into staying on during a grid outage - Solar Panels -...

For people who have experience with solar panels and/or work in the industry. Discuss installation questions here.