Building a PV Shedding Controller for AC Coupled Solar

[toms]

What grid profiles are you using? I am using IEEE 1547 default 2015.

I have heard the California Rule 21 profile might be better for AC coupling when operating without the grid.

My system isn’t grid connected.

 

[sunshine_eggo]

My system isn’t grid connected.

AND... your panels are tilted North!

 

[summit]

AND... your panels are tilted North!

it's good if the panels are in the southern hemi

 

[solar8484]

I am using IEEE 1547 default 2015.

This is likely a cause of the problems you have encountered. That profile does not support proportional freq-watt curtailment. The graph in https://diysolarforum.com/threads/b...oller-for-ac-coupled-solar.80666/post-1042422 shows large sudden battery current changes that are consistent with the micros only turning fully on and off.

You should try the default IEEE 1547 2018 profile.

 

[wheisenburg]

The graph in https://diysolarforum.com/threads/b...oller-for-ac-coupled-solar.80666/post-1042422 shows large sudden battery current changes that are consistent with the micros only turning fully on and off.

The was my PV load shedder working. If I just turn off the "AC coupling" on my Schneider Inverters, this brute force on/off method is actually working fine. Maybe I should just consider the problem solved? The problem is that I was hoping that getting rid of the excess solar might help the AC Coupling work better so it could fine tune production to match consumption. I can run string 3 with AC coupling enabled. Unfortunately, it seems that once I turn on strings 1 and 2 production drops to zero and stays there until I hard boot the micro inverters. I personally don't have any type of frequency logger. It might be easier to solve this issue if I could see what was going on instead of just guessing. It appears that the Schneiders are causing the micro inverters to shut down. Turning off AC Coupling makes everything work, but if my load shedder glitches, I have no fail safe.

You should try the default IEEE 1547 2018 profile.

I am going to try rule 21. I am also going to also try turning on the SOC control. I am happy with my voltage driven charging cycle, but maybe I can get SOC to work properly too.

 

[solar8484]

I am going to try rule 21. I am also going to also try turning on the SOC control. I am happy with my voltage driven charging cycle, but maybe I can get SOC to work properly too.

CA Rule 21 profiles can also work but there are many different versions now, so it may take time to find the best for your system. The IEEE 1547 2018 default profile is newer superset. It would be best to get freq-watt working with SOC control for smoother operations. Frankly, with the appropriate SOC settings for freq-watt, your system should be able to handle large load turn off without significant issues (e.g. bat ovp) even without the PV shedding relays. BMS can typically handle 1X charging current for at least several seconds (enough time for freq-watt response) even when the SOC is high.

 

[summit]

The was my PV load shedder working. If I just turn off the "AC coupling" on my Schneider Inverters, this brute force on/off method is actually working fine.

Is it possible that the iQ8 are communicating with each other ? there is that 5-min grid integrity check upon turning on. Are the iQ8s shutting off within this 5-min or immediately ?

 

[wheisenburg]

They literally ALL shut down as soon as the second string turns on. It is near instantaneous.

 

[wheisenburg]

The IEEE 1547 2018 default profile is newer superset. It would be best to get freq-watt working with SOC control for smoother operations.

I changed over to SOC control. My latest recharge cycle looks fine. It was cloudy here today, so I couldn't do much else.

 

[wheisenburg]

I also have this little guy coming in a few weeks. I couldn't see paying big bucks to get an $8.00 part faster.

https://www.amazon.com/dp/B08TRMG9PP?ref=ppx_yo2ov_dt_b_product_details&th=1

I think the Chinese Photographer didn't know the Hz was supposed to be right side up, not zH. If I were looking at a gauge with Chinese markings, I would probably get it wrong too.

 

[wheisenburg]

Some new Information

I did some testing today and was able to sort out several issues.

About a week ago I changed my settings to use the SOC control. At one point I was having problems with SOC not being reliable. This could have been due to a bad connector cable. My SOC does not seem to have errors anymore after I worked on the cable connection.

I was having a problem when I allowed more than one string to operate at the same time. I thought this might have something to do with the "Max Charge Current" allowed. So I increased this setting from 15% to 60%. Mid summer in the worst case I have 9000 watts and the inverters will need to push about 170 amps into the batteries. With that change I was able to get all three strings to operate at the same time. The only down side is that the charging rate will increase after a power failure and the batteries will also charge quickly (around .5 or .6 C max depending on conditions) while using AC Coupling to recharge the batteries. The charging current will still get limited some by hitting the absorption or bulk voltage limit. It appears that the inverter is attempting to use those settings and Frequency / Watts to charge the battery as closely as possible to what it does during a normal AC charge. So next I turned the AC coupled function back on.

Once I had the strings staying on, I was surprised that they would charge all the way up to 100% even though I had set the Max AC Coupled value to 90%. I'm not sure the setting of the AC Coupled charging limit actually does anything. It appears that the frequency / watt function doesn't actually get triggered until the battery voltage exceeds the absorption volts.

Next I noticed that when the Schneider started to ramp up the frequency, the PV inverters did not back off until they all went to zero all at once. I updated my IEEE 1547 2015 profile to IEEE 1547 2018 instead. I heard the 2015 is all or nothing and 2018 does frequency / watts. It says this is the right profile to use in PA, if your system was installed after Jan 1, 2022 anyway so I really should be using that one. With that I actually get the Frequency / Watts to work. I did not yet update the Schneider with the 2018 profile. I probably should so the tow systems have the best possible compatibility. I tested this with a single string running. It appears that with one string the Schneider can easily manage the output with the Frequency / watts function. It appears that it actually looks at the charge voltage in the battery section of the configuration and will back the current off to keep the volts from exceeded the absorption value. Once the battery it totally full, if the frequency climbs high enough the PV shuts off completely.

Finally, I retested with all three strings running. At this point it was late afternoon and the array was no longer producing anywhere near the max power. I found that it would get the batteries full and then the taper back when Frequency / watts was applied. Eventually the voltage would get too high and all the strings would turn off.

So knowing what I now know, this my plan to make some adjustments. First, it appears there is now no reason to shed string three. I plan to wire this string to always stay on. Then I will shed string 2 based on the aux relay from inverter 1, and string 1 from the aux relay from inverter 2. The reason I give priority to string 3 is that it has panels that face both east and west. The other string are only west facing. I don't have the ideal roof for solar, but I do get good afternoon sun when i use the A/C in the summer. My winter production sucks.

I did notice that the ramp rate on the new profile is much more gradual. It takes about a minute for the panels to fully ramp up now (plus the 5 minutes before they even start to produce anything). It seemed like that ramp rate was removed from the old profile when Enphase did their firmware upgrade. So with the old profile everything turned on all at once. I don't think it used to do that. The current would spike and then everything shut down. Starting any of the panels up again then required power cycling the strings.

So now I would say that I have the AC coupling fully working. Even without my shedding box I don't think anything really bad would happen, but at mid-day I did notice that when all three strings turned on it would quickly push the voltage up to the point where all the panels would then quit. At least with the other changes I made, this was not a permanent shut down. After 5 minutes they would come on and run for 20-30 seconds and shut down again. Not very efficient for getting the batteries recharged. More batteries would probably solve this problem. My plan for now it to do some more testing to see at what point I should shed strings 1 and 2 to make sure the inverter can push the incoming current into the batteries without an overvoltage condition causing the frequency watts to turn all the PV off.

It might be a week or so before I have time to look at this again. For now I am happy. This was major progress.

 

[solar8484]

Once I had the strings staying on, I was surprised that they would charge all the way up to 100% even though I had set the Max AC Coupled value to 90%. I'm not sure the setting of the AC Coupled charging limit actually does anything. It appears that the frequency / watt function doesn't actually get triggered until the battery voltage exceeds the absorption volts.

That seems odd. Perhaps there is another overall (i.e. not just for AC coupled charging) setting for max SOC charging limit?

I did notice that the ramp rate on the new profile is much more gradual. It takes about a minute for the panels to fully ramp up now (plus the 5 minutes before they even start to produce anything). It seemed like that ramp rate was removed from the old profile when Enphase did their firmware upgrade. So with the old profile everything turned on all at once. I don't think it used to do that.

Slow ramp up is another enhancement (along with others like like freq-watt and voltage/frequency ride-through) in the default 2018 profile based on newer standards. I doubt you had slow ramp up with default 2015 profile based on older standards that did not require slow ramp up. The slow ramp up and ride-through requirements were added to the newer standards after POCO's found that large number of home solar systems suddenly turning on and off actually made the grid less stable.

So now I would say that I have the AC coupling fully working.

Great progress! Good to hear the new grid profile and freq-watt working. Hopefully you can get the SOC based AC coupling figured out. It would be great if you could post your inverter settings at some point.

 

[wheisenburg]

It would be great if you could post your inverter settings at some point.

I will do that. I will also go through any information I have on how some settings seem to affect the operation of AC coupling.

 

[wheisenburg]

I doubt you had slow ramp up with default 2015 profile based on older standards that did not require slow ramp up. The slow ramp up and ride-through requirements were added to the newer standards after POCO's found that large number of home solar systems suddenly turning on and off actually made the grid less stable.

I sure seem to remember that the IQ8 inverters were ramping up slowly before the recent firmware upgrade. Possibly, the ramp up had been over ridden in the firmware. A part of the firmware "fix", may have been to remove the override and respect the ramp rate (or lack of one) in the grid profile.

 

[wheisenburg]

My Settings:

Nothing Special here.



AC Coupling On
Recharge SOC 95 - Doesn't seem to make any difference



Battery Type - Custom
Charge Cycle - 2
SOC Control - Enabled works better with SOC
Battery Bank Capacity - 300 Ah
Maximum Charge Rate - 60% If the actual charge rate when AC coupling exceeds this, the inverters shut down. Hard reboot is then required.
Absorption Set Point - It appears that they will attempt to keep the voltage under this value when charging by using Frequency / Watts.

 

[1201]

My Settings:

Nothing Special here.

View attachment 206597

AC Coupling On
Recharge SOC 95 - Doesn't seem to make any difference

View attachment 206598

Battery Type - Custom
Charge Cycle - 2
SOC Control - Enabled works better with SOC
Battery Bank Capacity - 300 Ah
Maximum Charge Rate - 60% If the actual charge rate when AC coupling exceeds this, the inverters shut down. Hard reboot is then required.
Absorption Set Point - It appears that they will attempt to keep the voltage under this value when charging by using Frequency / Watts.

View attachment 206601

Did you try the 200% setting on the iq8?

 

[wheisenburg]

Did you try the 200% setting on the iq8?

I need to know if there is a 200% / Hz profile that is IQ8 compatible. I found an "Off Grid" profile that is supposed to AC Couple with Schneider, but it says it is compatible with the IQ7 family only.

 

[solar8484]

My Settings:

Nothing Special here.

View attachment 206597

AC Coupling On
Recharge SOC 95 - Doesn't seem to make any difference

View attachment 206598

Battery Type - Custom
Charge Cycle - 2
SOC Control - Enabled works better with SOC
Battery Bank Capacity - 300 Ah
Maximum Charge Rate - 60% If the actual charge rate when AC coupling exceeds this, the inverters shut down. Hard reboot is then required.
Absorption Set Point - It appears that they will attempt to keep the voltage under this value when charging by using Frequency / Watts.

View attachment 206601

Can you also show the grid codes setting? The freq-watt (and volt-watt, etc.) curtail curve (including the frequency shift limits and resolution) is dependent on the grid code setting. It could help explain why the AC Coupling SOC limit didn't work as expected.

 

[wheisenburg]

Well I did more testing today. Based on prior testing I adjusted the load shedding so that string 1 sheds at 92% SOC and reattaches at 90%. String 2 sheds at 97% and reattaches at 94%. String 3 is now always connected.

The AC Coupled Frequency Shifting can manage the 3 KW output of string 3 with no issues. Before I disconnected the first two strings in one phase and the final string in the other. That is not needed.

I ran my drier, dishwasher, and oven and got the SOC down to 80%. I actually had to turn off the PV circuit breakers in the Envoy or it would not drain the battery due to all the PV. Then I turned the PV breakers back on. My dryer cycles the element on and off and so does my oven. Anyway it charged the batteries up to 92% pretty quickly. Then the charging rate backed down until 97%. At that point string 2 shed and the system was nearly balanced with some battery drain. The oven wasn't on anymore. So string 2 cycled on a couple of times. Once the drier and dishwasher were done, it actually got to 100% and killed string 3 through frequency shifting. It came back on in 5 minutes.

So I am very happy. This system handled everything I threw at it. I was pulling 6k each from the inverters for a while. They didn't even get warm that I could tell. I was charging at 160 amps. I believe I now have things working good. I am ready for an outage if we ever get one. The last piece is a generator for the winter time. I have an energy gap un Dec, Jan, and Feb. Three more batteries would help, but my system just doesn't produce enough in the winter.

Finally, there were some questions as to whether AC Coupled charging was "stackable". Some people said Schneider was still working on that. I ran into no issues. The inverters seem to switch seamlessly from inverting to AC coupled charging and sharing the loads across both inverters with no issues.

Frankly, based on my testing, if I needed to run my dryer on a sunny day during a grid outage, I would be fine doing that as long as I waited till the batteries were full. I ran 4 loads of laundry today.

 

[wheisenburg]

This is a trace from this weekend's test. From 10:00 to 1:30 the graph shows string 3 running and string 2 cycling on and off to supply the load from my dryer and other heavy loads. Even with this 5000 watt load running, it can keep the battery between 94 and 100% SOC. The dryer is not a steady load. It cycles on and off to maintain a certain temperature. I wanted to be able to test what would happen with all three strings running. The PV was keeping up even with my large loads, so I turned all the PV breakers off. From 1:30 to 2:15 of the trace here shows running the dryer and oven with all the PV circuits turned off. This was so I could deplete the battery below 90%. This is required to get the third string to turn on. Then at 2:15 to 2:45 all 3 strings are turned on when I turned the PV breakers back on. All three strings were running and the battery charged back up quickly. From 2:45 to 6:00 the graph again shows string 3 running and string 2 cycling on and off to maintain a more steady state condition after all the big loads were finished. The last current spike around 6:00 PM is from turning the grid back on and the resulting charge cycle.

Pushing large amounts of current in and out of the battery does cause them to heat up a little, but the voltage and the SOC stay well controlled despite large swings in current. Once the batteries were charged up, the PV shedding occurred. Then when the system was maintaining a more steady state and not running huge loads, the batteries were no longer heating up. Also, the SOC is maintained at a fairly high level. Sol Arc recommends setting their system up so AC coupling will turn on at 80% and off at 90%. This means that you could go into the night time at only 80% charge if your timing is bad. If you reserve 10% to prevent an unrecoverable "black start" during an outage, you would have only 70% usable charge or 10.5 KWHs going into the night.

Ideally, I would still like to be at 6 batteries rather than 3, but I am calling this a success. Running over night assuming conservative power usage should be fine with this setup. If an outage occurs in summer, I should be fine to run the A/C until around 7:00 PM. I might program the thermostat to "Pre-cool" the house the last 3-4 hours of the day and then turn the A/C off between 7 PM and 7 AM. The running watts on my A/C is around 2000 Watts so it is no problem running it while sunny. My house should stay cool enough overnight in PA given our climate. With 6 batteries, I probably wouldn't need to change anything.

In the winter, with only 8 hours of generation, I might use 8 KWHs overnight just for idle consumption. So useable capacity should be around 15K * 0.85 = 12.75 KWH. I should be able to make it through the 16 hour night, but I will need to supplement once a day with a generator when it is cloudy. If I charge the batteries up every evening when they are low, I should be fine. Being able to keep the battery more fully charged is almost like getting another 1/2 of a battery usable. If I had 6 batteries, the Sol Arc charging model would mean I could lose pretty much an entire battery of usable charge each night.

 

[Gubman]

Back Ground

I have an Enphase system with 35*350 watt panels. It uses the IQ8+ micro inverters at 295 watts each. So the total possible output is 10,325 watts. The actual maximum output is about 9000 watts. I have 7 panels that face east and 28 that face west. The slope of my roof is about 40 degrees so I will never have both set of panels producing the maximum amount of power at the same time...

... Because of these issues manufactures make recommendations to limit the ratio of PV to inverter power, and PV to battery capacity. In some cases they even suggest that the maximum AC coupled PV be less than the DC coupled PV.

I admire what you have done so far! I also had the same dilemma as you with Enphase + XW Pro and off-grid - sudden power demand drop/rise - the bane of an off-grid(er). I've decided to use the Enphase batts like one would a cap (albeit a massive one) in the simplest sense of the word. They have the power control & production-to-storage ratios tweaked quite well, especially with the IQ8s. Now that I was able to sleep comfortably with no imminent smoke in my future, I focused on solving the next challenge - AC coupling. Ideally I wanted the Enphase side to be in sync with the XW side, the way you sync a generator at a power plant before letting it output amperage to an existing grid. The Enphase micro-grid is my "grid" in this sense and the XW with its batts would be the power plant "generator" when supplying and a "pump" (like uphill hydro) when AC charging rack batts. (I also happen to have the Orient Power batts in the lab)

 

[summit]

Frankly, based on my testing, if I needed to run my dryer on a sunny day during a grid outage, I would be fine doing that as long as I waited till the batteries were full. I ran 4 loads of laundry today.

slightly OT; got a new LG heatpump dryer ~800-w 240vac