AC Coupling

[inetdog]

The above inverter doesn't solve the problem of the MPPT though. I don't know of any MPPT's that work HV to HV (i.e. a 500 volt input, 400 volt output.)

Your raise a very important point. What you leave out is that the same DC to DC inverter hardware can be controlled equally well by an MPPT algorithm or by a constant voltage input algorithm. Typical grid tie inverters only include the former, but when designed for use with the PowerWall or other battery storage, it is not particularly expensive to add the second control circuit option. That is apparently what SolarEdge has done with their storage compatible systems.

[jflorey2]

It will be interesting to see how this pans out. And at what cost.
http://www.solaredge.us/files/pdfs/p...tasheet_na.pdf

I think there's a future in HV DC inverter systems, especially as load power distribution moves to DC. Wire gauges go way down and protection devices become easier to source.

The above inverter doesn't solve the problem of the MPPT though. I don't know of any MPPT's that work HV to HV (i.e. a 500 volt input, 400 volt output.)

[ButchDeal]

IMO adding an auto-transformer to help balance the system output is just another failure point.

It is only required for backup capability.

[SunEagle]

It will be interesting to see how this pans out. And at what cost.

http://www.solaredge.us/files/pdfs/p...tasheet_na.pdf

IMO adding an auto-transformer to help balance the system output is just another failure point.

[Ben25]

It will be interesting to see how this pans out. And at what cost.

http://www.solaredge.us/files/pdfs/products/StorEdge/se_storedge_inverter_datasheet_na.pdf

[jflorey2]

So . . . is anyone building a unit like that Outback system that will work with the Tesla Powerwall so that Enphase system owners can easily add a Tesla Powerwall and critical load panel their systems?

I don't get the whole Tesla Powerwall thing. It's expensive and not all that useful, since it operates at ~400 volts. It is shiny though.

If you want battery backup, you can do that with an Outback of your choice and batteries of your choice. Like lead-acid? No problem. Like LiFePO4? They are available too.

If you want to do AC-coupled, the options I know of are the SMA Sunny Island system (fairly well integrated) or a Rube Goldberg-ish system with relays that turns banks of microinverters on and off to match battery state. There are a few app notes out on how to do this, but nothing very clean.

[Naptown]

I am not a fan of A/C coupled systems
Particularly in a back up situation.
Batteries get too low and the entire system shurs down and will not restart.
In this instance i prefer a system like the Schneider Context system. In this case the inverter may not start but the batteries will charge.

[Dag Johansen]

So . . . is anyone building a unit like that Outback system that will work with the Tesla Powerwall so that Enphase system owners can easily add a Tesla Powerwall and critical load panel their systems?

[billvon]

Key words are there

Yes. The limits are:
-The inverter's power rating. GT generation must be less than the inverter's rating.
-Ability of batteries to accept charge. A large bank at 50% will be able to support the inverter's full power rating. As the battery moves from bulk (constant current) to absorb (constant voltage) then its ability to absorb power will decline. Hence the additional relay needed, to disconnect GT generation before the battery reaches the point at which it can no longer accept full power.

[russ]

the inverter will be able to do this to some extent.

Key words are there

[billvon]

Which does not necessarily work if the Outback has to maintain the AC output voltage so that the SB or other GTI will think that it is syncing to the grid.

Well, sure it does. Think of the Outback as a bidirectional voltage regulator. Voltage too high due to GT operation? It draws power. Voltage too low due to loads? It supplies power. The other GT inverters like this just fine since they see a valid AC signal. As long as the batteries have headroom to accept or source power (i.e. they are neither fully charged nor fully discharged) the inverter will be able to do this to some extent.

[ButchDeal]

Which does not necessarily work if the Outback has to maintain the AC output voltage so that the SB or other GTI will think that it is syncing to the grid.
The mind boggles, and intuition may fail to give a correct answer.

stumbled upon this which gives some more details

https://www.youtube.com/watch?v=gOy2oZMA_gw

[inetdog]

Right. The inverter won't take into account the status of external GT generation. All it senses is the AC output voltage, and will keep that within regulation when operating independently. (When grid is present, of course, it just passes power to the grid and this isn't an issue.)

Thus, in independent operation, if sources balance loads then nominal voltage is maintained and the Outback will do very little. If sources are significantly lower than loads then the Outback will use battery power to maintain the voltage (in other words, be a regular inverter.) If sources are higher than loads then the inverter will draw AC power and send that power to the batteries to regulate the (now too high) AC voltage. This is the "unusual thing" that a bidirectional inverter will do.

Which does not necessarily work if the Outback has to maintain the AC output voltage so that the SB or other GTI will think that it is syncing to the grid.
The mind boggles, and intuition may fail to give a correct answer.

[billvon]

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.

Right. The inverter won't take into account the status of external GT generation. All it senses is the AC output voltage, and will keep that within regulation when operating independently. (When grid is present, of course, it just passes power to the grid and this isn't an issue.)

Thus, in independent operation, if sources balance loads then nominal voltage is maintained and the Outback will do very little. If sources are significantly lower than loads then the Outback will use battery power to maintain the voltage (in other words, be a regular inverter.) If sources are higher than loads then the inverter will draw AC power and send that power to the batteries to regulate the (now too high) AC voltage. This is the "unusual thing" that a bidirectional inverter will do.

The relay is needed because batteries are not infinite sinks. At some point the battery absorbs all the power it can. At that point the battery voltage rises and the relay opens, ending the grid tie operation.

An interesting question that I don't know the answer to is - what if the relay fails to open and excess GT power is still available? I suspect the result would be that the Outback limits voltage to the batteries to prevent battery damage, the AC voltage rises out of range, the Outback trips off-line due to an overvoltage fault followed quickly by the GT inverters. However I have never seen this described explicitly.

[ButchDeal]

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.

This document seems to have more detail: http://sunwize.com/documents/OutBack...ril%202013.pdf

and a webinar: https://www.youtube.com/watch?v=gOy2oZMA_gw