Hybrid inverter back feed to grid: how does it work?

[WindSoul]

I have a 10kW Easun split phase that can back feed the extra solar to grid. I need help understanding the back feed, since the inverter has both ac input and output ports.

Question is: does it back feed directly via the ac input port? If the grid fails does it know to isolate the input in order to continue to provide power to the house via ac output port? I can’t tell wether the inverter back feeds to the ac input port but I don’t see how else it could work.

Why am I asking: I had an error of bus over volt while back feeding. This error means the internal dc supply went above the limit which hovers around 520v. I have a solar array that supplies 500v max at 4.25kW. My concern is that the inverter must’ve tried to grid tie into an isolated panel (as if the power meter won’t accept back feed) and it increased the voltage internally attempting to deliver until it overloaded. The inverter is still working fine weeks later but I need to understand why the back feed didn’t work. Checked with customer support, still waiting for answer.

I’m asking these questions because I haven’t received answer from manufacturer. The model is 10kW split phase mlv 10u and it seems identical with the powmr 10kW split phase or the srne 10kW split phase non-parallel hybrid inverters. If you have experience with this otherwise excellent inverter, please let me now. Thank you in advance.

In the first picture is the inverter working while in the second picture is yet a second error: battery over volt, which I seem to have overcome by increasing the residual charging current to 8 amp (it stops charging when the absorbed charging current drops under 8A. It has a voltage setup parameter as well but that didn’t work).

 

[42OhmsPA]

Do you have an interconnection agreement that allows you to backfeed?

What option are you using for setting 34?

Are you using battery communications or running open loop?

What is the VOC of your array?

 

[Ampster]

I had an error of bus over volt while back feeding. This error means the internal dc supply went above the limit which hovers around 520v.

That DC overvolt is unrelated to whether your inverter was backfeeding or not. However if it was feeding a panel isolated from the grid then it should only supply the current that the loads are requiring
If you are getting close to the DC input maximum, you should adjust your strings to avoid getting even close to the DC maximum.

 

[WindSoul]

Do you have an interconnection agreement that allows you to backfeed?

What option are you using for setting 34?

Are you using battery communications or running open loop?

What is the VOC of your array?

1. I don’t have any agreement. I don’t run in grid tie mode. Just happen to try it briefly once. I have two uncomissioned inverters with export limiters to address the consumption on the extra panel connected to grid only. The solar setup-excepting for the grid tie inverters feeds a panel that is isolated from the rest of the house-the emergency panel meant to service essential circuits in case of grid failure.
2. Now I run SBU with MIX LO[A]D; it says par 34 is validated by UTI mode only so I guess it should be disabled.
3. No communications. It charges the batteries correctly and they cycle down to some 11% then back up to 100% with no reoccurrence of battery over volt.
4. 450V. Panels are Qcell 425W 50V voc; 9 in series plus the tenth in parallel to keep it all under 500V.

 

[WindSoul]

That DC overvolt is unrelated to whether your inverter was backfeeding or not. However if it was feeding a panel isolated from the grid then it should only supply the current that the loads are requiring
If you are getting close to the DC input maximum, you should adjust your strings to avoid getting even close to the DC maximum.

the solar array doesn’t get above 400v usually but the inverter dc bus gets above 520, as seen on attached charts. I have to say that while the bus voltage fluctuates the ac output is consistently solid; there is no indication in the output that the dc bus misbehaved. However, looking at the charts i don’t understand why the dc bus went over mppt voltage.

 

[42OhmsPA]

1. I don’t have any agreement. I don’t run in grid tie mode. Just happen to try it briefly once. I have two uncomissioned inverters with export limiters to address the consumption on the extra panel connected to grid only. The solar setup-excepting for the grid tie inverters feeds a panel that is isolated from the rest of the house-the emergency panel meant to service essential circuits in case of grid failure.
2. Now I run SBU with MIX LO[A]D; it says par 34 is validated by UTI mode only so I guess it should be disabled.
3. No communications. It charges the batteries correctly and they cycle down to some 11% then back up to 100% with no reoccurrence of battery over volt.

I would leave 34 disabled completely, I don't have any experience with it so I can't offer any more advice.
I'm guessing it's the same inverter as the SRNE ASF...
I'm approaching 4mWh of solar through mine with a few minor issues, it's been extremely reliable; no communications here either. https://diysolarforum.com/threads/3-mwh-srne-asf48100u200-h-10kw.67809/post-1044376
That's a link to my most recent settings I've been running, they may help you but it sounds like it's working well for you.

4. 450V. Panels are Qcell 425W 50V voc; 9 in series plus the tenth in parallel to keep it all under 500V.

Can you expand on the 10th in parallel, I don't understand what you mean.
Are you using both MPPTs?
Note the working voltage range of the MPPTs is 125V-425V with 500V being the max.
What's the record cold temp for your location? You could possibly go over 500VOC in extreme cold.
The MPPTs can handle 22A each, have you considered 5S2P instead of the 9S?
I'm guessing your panels are ~9A?

 

[42OhmsPA]

the solar array doesn’t get above 400v usually but the inverter dc bus gets above 520, as seen on attached charts. I have to say that while the bus voltage fluctuates the ac output is consistently solid; there is no indication in the output that the dc bus misbehaved. However, looking at the charts i don’t understand why the dc bus went over mppt voltage.

I don't quite understand it either.
I attached mine so you have something to compare to.
MPPT1 is ~250VOC, MPPT2 is ~360VOC (9S 315W Q-Cells).

 

[WindSoul]

I don't quite understand it either.
I attached mine so you have something to compare to.
MPPT1 is ~250VOC, MPPT2 is ~360VOC (9S 315W Q-Cells

Thank you for the charts. Good to see your bus voltage is similar to mine. Also the parameters are quite the same, the 57 was 1[A] and most probably that alone led to battery over volt. As for main bus over volt, I won’t fiddle with grid on for now because the inverter works fine otherwise. Question remains on how the backfeed is actually done, time will answer that.