Phasing question

[Michael Merrigan]

Regarding solar PV: Can anyone tell me if a grid tie inverter and an off grid inverter can be tied together on the AC side when the Utility power is disconnected from the main electric panel? i.e. A Grid tie PV system is being used to power part of the building and also feed excess generation back into the grid through the main panel. At the same time an Off grid PV system with a battery bank, feeds a separate part of the same building from a separate electric panel. There is an open disconnect between the 2 panels. The utility power goes off, or the disconnect between the utility and the grid tie inverter is opened. The grid tie inverter then stops sending PV generation, as required, while the off grid inverter continues to supply it's load. IF the disconnect between AC side of the grid tie inverter and the AC side of the off grid inverter is closed, while the disconnect between the grid tie inverters and the electric utility remains open, will the grid tie inverter see this as a source of AC and come back on line in sync with the off grid inverter?

[Rob Beckers]

Michael, yes, grid-tie inverters generally work fine on the local 'grid' created by an off-grid inverter. This is often used to charge from sources that require a grid-tie inverter, such as wind turbines meant for an MPPT (grid-tie) inverter.

All the usual off-grid inverters from Xantrex, Outback, Magnum will work in this setup, and any excess power that is not used by the local grid will funnel back over the inverter to charge the batteries. This is called "AC coupling", and Xantrex wrote a nice white paper about it.

The downside is that the off-grid inverters will not provide any charge control in this mode, so it is possible to overcharge the batteries. To prevent that from happening the usual solution is to put a contactor in line with the grid-tie inverter (on its grid side), and control that from the Aux output of the off-grid inverter, programmed to disconnect the grid-tie inverter when the batteries reach a certain voltage. The Morningstar relay controller will do this job too.

The other issue to keep in mind is that worst-case all the power from the grid-tie inverter will funnel over the off-grid inverter to the batteries. That means the off-grid inverter needs to be able to handle at least the same amount of power as the grid-tie inverter.

-RoB-

[Michael Merrigan]

Perfect! You not only answered my question, but also explained it in a manner I could understand. Thank you very much for your prompt reply.

[Joe Blake]

Yeah, Rob is pretty good at that.:blink1:

Joe

[Marvin jordan]

Rob is one of the most helpful member here.