William,
My whole email is in regards to the point you made when you said
"This is not how battery inverters were originally designed to operate. These
systems require careful consideration to avoid battery overcharging."
I both agree and disagree with these statements... depending on what you mean.
I don't think it's super helpful to say that careful considerations are
required without mentioning what those considerations are, and I'll get to
those below.
When you say that the battery inverters "weren't originally designed for this"
I'd agree to an extent.. The "original" battery based inverters 25 years ago
were certainly not designed for AC coupling. They weren't "originally" designed
for it. But that was then and this is now.
Now William is correct that the Gen input becomes an AC output, and you can't
wire the gen into that "input" if you are using it as a PV input... that being
said, Jay didn't think otherwise, he was just critiquing the wiring diagram in
the original post to say that it wires up differently, so I'd say that you are
both right on this one. That said, I don't think anyone says you have to wire
it into the gen port, I think that even with the hybrid inverters, the way in
William's original post is still a fully acceptable way to do it.
I am not a huge fan of AC coupling, however, I have done it a fair amount and I
have a fair amount of experience with it off-grid with Lithium and with Lead
Acid. Most of my experience is with Outback and Victron, but I have some
experience with Schneider as well. These systems have been working really well
for a good while, but that being said, Schneider had some real problems at
first (major headache).
Now when it comes to hybrid inverters, I'm still not a huge fan... but I'm
starting to try them out. I have not tested AC coupling with any of them yet. I
was at a conference less than a year ago and one of the major brands was saying
that there is no hybrid inverter that supports both AC coupled PV and a
generator at the same time for a grid tied system. They said "you can't have it
all." There may be an exception to that, I've never looked into it, but with
the Victron and Outback (and probably Scheider... IDK) it's easy to have it
all. I've done it and it works well in every mode. The frequency changes super
fast and assuming you have a properly programmed rule 21 compliant Grid tied
inverter, it responds very quickly, and in the systems I've monitored, the
frequency never had to go above 62 Hz, which is good enough for most sensitive
loads. The inverters are totally designed for this these days, but if you have
highly sensitive loads, then it may not work well.
Considerations...
First of all, I will be assuming that you are using quality rule 21 grid-direct
inverters and a good battery inverter that works as well as the Outback Radian
or the Victron. The early Schneiders were too slow in changing their frequency,
and special considerations had to be accounted for because of that, but I heard
that was fixed a while ago. You had to reduce the charging voltages to account
for the delayed throttling, but I can attest that even in the early days,
Outback never had this problem, and Victron works smoothly as well.
The primary problem for AC coupling is in off-grid scenarios or prolonged power
outages, and that's the black start issue (or dark start). Simply put, if your
battery gets low and the battery inverter turns off, then your PV can't charge
the battery. This problem is lessened when you connect the PV Grid Direct
inverters to a dedicated output on the inverter so that the loads shut down
while the battery is still a few percent above the inverter shutdown level.
That is a nice advantage of the Victron inverters. I don't know if the Hybrid
inverters do this or not. In any case, even with this little safety net, the
inverter can still discharge the battery to the point where the inverter turns
off and you have a problem. My solution is that you should have at least a
little bit of the PV DC coupled so that when the sun comes out, the voltage
will rise and the inverter will turn on. This is also recommended in Victron's
AC Coupling manual.
The second consideration is the minimum inverter size... Your battery based
inverter needs to be capable of handling and controlling the full PV power.
Victron recommends that the maximum PV power (DC or AC, whichever is lower)
does not exceed 100% of the rating of the inverter (no more than 10kW of power
from the PV on a 10kW inverter). Outback recommends no more than 6 kW of
Grid-Tied inverter per 8 kW of Radian (so that's 75% of the battery inverter's
power). In some circumstances, this could demand upsizing the battery inverter,
which is another great reason to DC couple half of the PV and AC couple the
other half... because then you could effectively have more PV with less battery
based inverter, depending on your loads.
As was ment
