Backfeeding from isolated inverters

[mauzilla]

I have two Deye inverters, both connected to the same grid line (as per the diagram). Each inverter has its own battery bank and PV array. The inverters are not in parallel because the battery banks are different (different batteries with different BMSs, so they cannot be connected in parallel).

My concern is with backfeeding. Both inverters allow for either not backfeeding, backfeeding according to the CT reading, or dumping all excess PV directly into the grid. For a single inverter setup, this is simple (I presume), but my concern here is that if I have two inverters on the same grid input (both of which have direct access to the non-essential/main DB/circuits that are not isolated to each inverter’s essential load), backfeeding from both inverters could cause problems. Specifically, I’m worried that this could fry:

• a) one of the inverters,
• b) appliances on the non-essential DB board, or
• c) the grid output.

Here are the scenarios I believe could happen:

1. We are drawing 2 kWh from the grid (to power the non-essential grid and possibly power consumption for each inverter). Both inverter CTs will read 2 kWh, and if both have excess electricity (let’s assume each has 2 kWh of excess PV power), they will both attempt to push out 2 kWh, essentially pushing 4 kWh onto the grid.

2. If the inverters are both set to backfeed regardless of the CT reading, they will push excess electricity through the line, doubling the amount of electricity pushed back. As both inverters are also connected to the same line, I’m not sure if this poses any electrical issues for the other inverter or the non-essential grid.

3. I could set one inverter to backfeed and disable the other inverter's backfeeding altogether. Although this makes sense in principle, the concern is that if someone later resets the inverter (if we sell the house, for example) or enables backfeeding on the other inverter, we might encounter the risks associated with scenarios 1 or 2.

4. Lastly, we could disable backfeeding altogether, essentially leaving the non-essential main DB without an additional mechanism to power these devices (while the grid is online). My only logical conclusion, if this is the only option, is to move the non-essential DB to be part of the essential DB board of one of the two inverters. However, this poses an issue, as there is an obvious reason for having two inverters: we do encounter situations where the electricity usage may exceed 8 kWh. For example, if the grid is offline, the batteries themselves may not be able to sustain the draw from the entire non-essential and essential grid.

Other considerations:

• Both inverters are set to stop backfeeding when they detect the grid is offline. However, if I am backfeeding from both inverters, this could lead to a situation where the inverters do not recognize that the grid is offline because they are both still receiving power from each other.

Ultimately, the best-case scenario would be to have these inverters connected in parallel. However, this would require me to replace one of the batteries (and lithium batteries are expensive) so that it matches the other battery. This brings me to a couple of questions:

1. If the inverters are set up in parallel, they would still be connected to the same grid line and would have a combined "essential grid," as both would need to be able to power both the office and home essential DBs. If this is the case, then both inverters should be able to backfeed, whether they are isolated or in parallel. So, I’m not sure if my concerns are valid. What “magic” occurs if both are in parallel and I have them both set to backfeed excess electricity beyond the CTs back into the grid? It seems like the result would be the same, as all excess electricity would be pushed back.

2. I think my understanding of the flow of power is the main concern. Are the inverters simply "pushing" electricity, or are they just feeding electricity to the appliances? For example, if I’m only drawing 2 kWh from an appliance, the inverter isn’t so much "pushing" electricity as it is feeding the electricity required by the appliance (essentially opening the circuit and providing the necessary power).

I’m hoping someone can clear this up for me, as we’re installing the second inverter this weekend (yes, a qualified electrician is doing it, but my scenario is not common). I would not want to have a setup that could literally blow up in my face.

[Scorp007]

Without going into detail your 2 inverters cannot feed each other a grid connection when the grid is down. Each one will sense the grid is down. You also don't set them up to cut the grid connection where the non essential is connected to they do it by default and is one of the requirements before these inverters are approved to be connected to the grid. 

Part of what I mentioned is like your neighbour could be connected to the same phase and so some other houses. When the grid goes down they cannot make all the inverters think the grid is still up and running. 

A bit technical to explain how they all see the grid is down. The CTs plays no part in telling the inverters the grid is down. 

Sorry I did not actually respond to your concerns.

Edited August 22 by Scorp007

[Gerrie]

9 hours ago, mauzilla said:

What “magic” occurs if both are in parallel and I have them both set to backfeed excess electricity beyond the CTs back into the grid?

If you have them in parallel one will be the master and one the slave inverter, you only set settings on the master inverter and they will automatically work as one inverter. No need to worry about the slave backfeed as the master will control the setpoint. You also don't need a CT coil for the slave inverter.

[TaliaB]

11 hours ago, mauzilla said:

If the inverters are set up in parallel, they would still be connected to the same grid line and would have a combined "essential grid," as both would need to be able to power both the office and home essential DBs. If this is the case, then both inverters should be able to backfeed, whether they are isolated or in parallel. So, I’m not sure if my concerns are valid. What “magic” occurs if both are in parallel and I have them both set to backfeed excess electricity beyond the CTs back into the grid? It seems like the result would be the same, as all excess electricity would be pushed back.

Frequency matching refers to the necessity of a grid tied inverter to match the speed (frequency) of the AC wave that it generates to that of the mains power  that it is trying to connect to. Not only does it have to be exactly matched in terms of frequency, but it also has to be matched in terms of where the 2 different wave forms peak, this is called synchronisation. When the inverter matches the frequency and synchronises with the mains, it’s then electrically “locked” with the mains and is forced to follow it exactly since the inverter power is small compared with the mains supply.

If you don’t match the frequencies, the inverter will never get the chance to synchronise with the mains, and you have no grid tie. If the inverter attempts to connect with the mains when the synchronisation is out, then you have a short curcuit situation, and your inverter gets fried. The same principle needs to happen between 2 inverters in parallel or rather"stacked" hence the reason that 2 inverters needs to be identical even down to identical firmware versions. Also never switch the load on if you are not 100% certain the 2 inverters are indeed in parallel mode and synchronized.

Edited August 22 by TaliaB