AC Coupling

[inetdog]

It doesn't. It pulls power based on the power that its charging profile wants, and based on the sell and AC input limit settings. In no case will it back off charging because the GTI is supplying less power than expected (or vice versa.) Thus the requirement for the crude "bang-bang" relay to stop GTI inverter operation. Beyond that the system remains "in balance" because the batteries source/sink power based on the demand at the AC output. It does this by measuring voltage; it uses the inverter/charger to regulate the voltage to around 120 volts.

But if the Outback draws only what the batteries need, the DC voltage on the batteries will not rise just because the GTI is providing more power than the combination of the loads and charging.
Assuming that the Outback can both provide a reference AC voltage, contributing power, and suck AC for charging at the same time, it would also have to have some way of sensing that the GTI output is greater than the sum of load and charging. I suppose it could do that by monitoring for negative power transfer at its own AC output, but that has its own set of vulnerabilities and was never mentioned in any of the descriptions.

[billvon]

It is also not clear to me how the Outback magically manages to choose how much power to pull from the AC bus for battery charging based on the excess of GTI output over load consumption.

It doesn't. It pulls power based on the power that its charging profile wants, and based on the sell and AC input limit settings. In no case will it back off charging because the GTI is supplying less power than expected (or vice versa.) Thus the requirement for the crude "bang-bang" relay to stop GTI inverter operation. Beyond that the system remains "in balance" because the batteries source/sink power based on the demand at the AC output. It does this by measuring voltage; it uses the inverter/charger to regulate the voltage to around 120 volts.

[inetdog]

I did a couple searches with no finds, so I guess I will ask the question... I was looking at the Enphase website and came across a document regarding "AC Coupling" using their 250 micro inverters and power grid power failures.

I was wondering if anyone has implemented this in their systems. I know that some of the inverter mfr have incorporated small AC systems in their inverters with very limited output. I was under the impression that you could not fool the micro inverters by using an inverter's output to simulate the power grid allowing the micro inverters AC output to be used in your private system. I understand that there is a lot of design that needs to be included to protect the grid from the micro inverters output and limiting what can be powered by the micro inverters, overcharging.....

The link to the document is below.

Just wondering...

Don

AC Coupling of Enphase Micro inverters to Battery Based Systems

[URL="http://enphase.com/global/files/Enphase_Application-Note_AC-Coupled-Battery-Based-Systems.pdf"]

I find the Enphase PDF dangerously incomplete and potentially misleading.
There are a limited number of off-grid inverters which are bidirectional and can either provide AC from the battery bank or tap the AC for battery charging with no additional external hardware (such as an external CC).
The PDF gives the impression that all off-grid inverters work this way.

I also find that turning the GTI micros on and off based on the battery voltage is a crude and unreliable way to avoid back feeding the off grid inverter AC output.

The mention of frequency shift regulation (by the off-grid inverter) as done within the SMA SunnyBoy/SunnyIsland family leaves out what I believe to be a critical feature of the system, namely that the AC frequency shift provides a proportional analog control of the SB output rather than simply taking the frequency out of the acceptance limits of the SB to shut them off entirely.

I see that there may be some situations in which AC coupling of Enphase micros may be practical and desirable, but I think that the PDF is just too general to be useful in evaluating a particular proposed installation.

Looking at the Outback documents that ButchDeal linked, I wonder just what the effect of high frequency micro-cycling (by their estimate as often as every six minutes) will have on battery life and health.) The Outback document does at least make clear that the GTI modulation is a strictly on-off rather than proportional control method.

It is also not clear to me how the Outback magically manages to choose how much power to pull from the AC bus for battery charging based on the excess of GTI output over load consumption.

[ButchDeal]

I did a couple searches with no finds, so I guess I will ask the question... I was looking at the Enphase website and came across a document regarding "AC Coupling" using their 250 micro inverters and power grid power failures.

I was wondering if anyone has implemented this in their systems. I know that some of the inverter mfr have incorporated small AC systems in their inverters with very limited output. I was under the impression that you could not fool the micro inverters by using an inverter's output to simulate the power grid allowing the micro inverters AC output to be used in your private system. I understand that there is a lot of design that needs to be included to protect the grid from the micro inverters output and limiting what can be powered by the micro inverters, overcharging.....

The link to the document is below.

Just wondering...

Don

AC Coupling of Enphase Micro inverters to Battery Based Systems

[URL="http://enphase.com/global/files/Enphase_Application-Note_AC-Coupled-Battery-Based-Systems.pdf"]

you do AC coupling but not with a small system and it needs to have support for back feeding the power and know how to control the micros like the OutBack systems http://www.outbackpower.com/index.ph...pled-solutions

[Mustang65]

I did a couple searches with no finds, so I guess I will ask the question... I was looking at the Enphase website and came across a document regarding "AC Coupling" using their 250 micro inverters and power grid power failures.

I was wondering if anyone has implemented this in their systems. I know that some of the inverter mfr have incorporated small AC systems in their inverters with very limited output. I was under the impression that you could not fool the micro inverters by using an inverter's output to simulate the power grid allowing the micro inverters AC output to be used in your private system. I understand that there is a lot of design that needs to be included to protect the grid from the micro inverters output and limiting what can be powered by the micro inverters, overcharging.....

The link to the document is below.

Just wondering...

Don

AC Coupling of Enphase Micro inverters to Battery Based Systems

[URL="http://enphase.com/global/files/Enphase_Application-Note_AC-Coupled-Battery-Based-Systems.pdf"]