Using Grid Tied Solar with Off Grid Inverter for Off-Grid System!?!?!

[BB.]

Over at Solar Guppy's board, user IBOC has been looking at a SkyStream Wind Turbine system. And through SkyStream's documentation and discussions with the vendor, it turns out that they use their grid tie wind turbine to back drive standard off grid 120/240 VAC inverters (like Outback and Xantrex) and charge battery banks. Skystream uses a small battery monitor to control the wind turbine to turn off its grid tie feed once the battery bank is charged.

Also, IBOC has been looking at the Xantrex documentation for the new XW series and it too appears to use this back feed charging of a battery bank with feed back to the grid tied inverter to control battery state of charge.

Here is the link to the SG thread, and IBOC's last post:

Well, the reason the skystream works is because it has its own voltage sensor for the batteries that you buy separate. When the batteries reach full charge, the windmill receives signal to power down and put on the brakes. Thats why the skystream system works.

When researching this concept I found 2 other things that may interest you guys.
#1) the Sunny Island system works exactly like this through AC Coupling, in essence allowing you to use Grid Tied 600v inverters instead of 120v charge controllers. The sunny island ties in directly to SunnyBoy WindyBoy grid tie inverters using them as charge controllers when the grid is not functioning.
#2) I also found an AC Coupling diagram for Xantrex XW inverters that showed a feature I was not able to find any info on. Apparently you can take a Xantrex GT inverter, connect it to an XW Inverter on the AC out side. And connect the two together with a communication cable allowing the XW system to shut down the GT in the event that the Batteries reach a full state of charge.

Both concepts are probably worth further investigation. It would seem to me that this method of setting up a solar power system, grid tied with backup, is likely much simpler than using charge controllers and would effectively eliminate charge controllers from the setup. Even though, apparently, you can still use them if you want to. Also I think I can use this setup to have a negative ground battery based backup connected to a positive ground solar array and not lose any efficiency in panels manufactured like my "Sunpower" solar panels.

Sounds like a great way for those of us with Grid Tie system to add battery backed off grid systems that can still use the grid tie solar array without rewiring/adding a DC charge controller (bypassing the old problem of high voltage DC grid tie inverter inputs vs the low voltage inputs that DC charge controllers require).

-Bill

[crewzer]

Bill,

I’ve done some work on this “back-fed AC Coupled” configuration. For example, it is to some degree an undocumented feature of non-grid-tie OutBack inverters.

The Sunny Island approach does look promising. However, I urge caution to anyone considering this technique with other inverters. While a U-I inverter may synch up to the output of the non U-I inverter, the internal battery charger will operate in an unregulated mode. Accordingly, some alternate way of dealing with the U-I’s full power must be available for when the batteries reach some target voltage. One technique is to switch in a diversion load, and another is to turn off the U-I inverter.

The technique I evaluated with a client in my area was two-fold. One step was to use the OutBack inverter’s AUX connection to drive contactors (heavy-duty relays) connecting the U-I inverter’s outputs to the local “mini-grid”. When the battery voltage reaches some target and the grid is down, the U-I inverter is disconnected from the OutBack’s AC output. The second step was to use an automatic transfer switch as a sort of anti-islanding scheme to disconnect the grid from the OutBack inverter’s AC inputs when the grid fails.

My thinking is that even this approach is not perfect, and I recommended against it to my client. Specifically, if the U-I inverter is operating when the grid is restored, the two may be out of sync. At best, the U-I inverter would drop off for the UL-required five minute safety interval and than reconnect. At worst, the OutBack inverter might be damaged. I believe I may have lost this client over this issue.

I’ve sought to explore this matter further with OutBack. Although I enjoy a generally very good although informal working relationship with OutBack, and they did provide me with unofficial application schematics, they’ve not been particularly responsive to me on this issue.

I just can't get behing this application until I fully understand it. In sum, should you proceed, please do so with great caution.

Regards,
Jim / crewzer