Disadvantages of AC coupling - poor battery charging?

[zanydroid]

What is the AC/DC ratio of your setup? Did you add any panels via DC or are you merely using the 18kpv as an AC coupled battery?

For off grid testing, did you get testing how responsive the 18kpv is to changes in AC output? Like maybe hard cutting some of the AC PV breakers.

Trying to get a feel for how stable it is with 0% DC, which is my theoretical retrofit use case.

 

[y0bailey]

What is the AC/DC ratio of your setup? Did you add any panels via DC or are you merely using the 18kpv as an AC coupled battery?

For off grid testing, did you get testing how responsive the 18kpv is to changes in AC output? Like maybe hard cutting some of the AC PV breakers.

Trying to get a feel for how stable it is with 0% DC, which is my theoretical retrofit use case.

I do have some DC coupled panels 3.6kw.

8kw AC coupled.

Can't really cut breakers on my SolarEdge because I don't have any that I know of.

 

[zanydroid]

I do have some DC coupled panels 3.6kw.

8kw AC coupled.

Can't really cut breakers on my SolarEdge because I don't have any that I know of.

You do not have the SE on a dedicated AC breaker?

 

[1201]

Seems to work quite well in this example

 

[y0bailey]

You do not have the SE on a dedicated AC breaker?

Sorry I read your post wrong...I do have a breaker between the eg4 and the SE.

You let me know what you want me to test and I can test it for you this weekend.

I'm already testing 8s vs 4s2p and the results are underwhelming... So I need a new thing to play with

 

[zanydroid]

Sorry I read your post wrong...I do have a breaker between the eg4 and the SE.

You let me know what you want me to test and I can test it for you this weekend.

I'm already testing 8s vs 4s2p and the results are underwhelming... So I need a new thing to play with

What was underwhelming about it?

Hmm, I'd like to see how stable the AC output is when used solely as an AC battery. IE, when the 18kpv is grid forming and has no choice but to alternate between absorbing AC output from the SE and inverting from the battery.

So:
- DC solar disconnected (remember to do this with zero load, not sure how to do this on this inverter). If the solar is connected then it may give help to the high voltage DC bus, I think this is why SolArk requires some DC.
- Voltage monitoring on AC output (critical loads panel voltage)
- Power monitoring on SE circuit and 18kpv AC output. To see who is sending power when.
- Disconnected from grid

Experiment one:
- Start with zero load. Have a load ready to go that is high enough to exceed AC from microinverters.
- Flip loads on and off, with the loads just high enough to exceed the AC output from the microinverters. This will require the inverter to either switch back and forth between charging and inverting, OR turn off the AC coupled power.
- One way to do this automatically would be to set a dryer on the appropriate cycle, and toss some moist stuff in it. As the stuff dries, it will reduce the duty cycle. On mine it just goes full blast into 5 kW. However this will only test 240V

Experiment two:
- Start with load just below AC output of microinverter
- Cut microinverter when load is on. This will require the inverter to kick in
- Turn microinverter on, see how well inverter manages the rampup while off grid (probably fine and not interesting, it's slow)

It would be interesting to see the behavior for 120V vs 240V loads, since 120V loads still need some help from the inverter to handle neutral current, but that would be getting greedy :laugh:

 

[y0bailey]

What was underwhelming about it?

Solidly in the "didn't get the benefits I hoped" category. Slightly underperformed compared to the day before in 8s, with a perfect day two days in a row and my other array performing identically on both days.

My 4s2p voltage also runs low enough I think it's going to get even worse in the heat of the NC summer...maybe even to the point of dropping out of the MPPT range.

I might try again, but it wasn't a miracle and I'm moving on. I will probably try it again in the summer.

So:
- DC solar disconnected (remember to do this with zero load, not sure how to do this on this inverter). If the solar is connected then it may give help to the high voltage DC bus, I think this is why SolArk requires some DC.

This may be the dumbest question ever...but I also don't fully understand how to do this. Is it as simple as just turning the PV disconnect in the morning before I am getting any significant solar production?

Are there scenarios that just cutting the PV disconnect outside while they are producing causes harm? I haven't had to do this yet, but I'm sure I'll want to tinker in the daylight hours someday and I want to keep things safe for the equipment/myself.



I'll run some experiments this weekend if things go as planned. I'll report in!

 

[Hedges]

My 4s2p voltage also runs low enough I think it's going to get even worse in the heat of the NC summer...maybe even to the point of dropping out of the MPPT range.

If 4s voltage is too low, consider 6s2p. Consider half (6s) facing SE, other 6s facing SW. Depending on relative angles, could be similar peak wattage. But more hours production.

 

[zanydroid]

Solidly in the "didn't get the benefits I hoped" category. Slightly underperformed compared to the day before in 8s, with a perfect day two days in a row and my other array performing identically on both days.

Hmm, I think the 4S2P would give advantage if they were different orientation or there was shading. If there is no shading and all pointing the same way you'd probably get lower loss (voltage drop) out of 8S.

This may be the dumbest question ever...but I also don't fully understand how to do this. Is it as simple as just turning the PV disconnect in the morning before I am getting any significant solar production?

Yes if you turn off the PV disconnect in the morning, that would do it. Doesn't feel like the best way though b/c there might be conditions where you need to pull the plug in the middle of the day.

I think you can also activate RSD or turn off the MPPT.

I don't have a DC system so it's best to get a runbook from someone that knows this.

Are there scenarios that just cutting the PV disconnect outside while they are producing causes harm? I haven't had to do this yet, but I'm sure I'll want to tinker in the daylight hours someday and I want to keep things safe for the equipment/myself.

Yeah this is why I think you want to know the best way of doing this, that doesn't involve planning ahead to when it's dark the day before.

DC is hard to break and can cause arc damage when breaking. I don't think the standard tier IMO-style isolators are switch rated, and a 300V+ switch rated DC disconnect is going to be pricey.

Cutting via RSD will do it via the transistors in the RSD so no arcing.

Turning off the MPPT is effectively also disconnecting via transistors, again no arcing.