PV System Design

Photo-voltaic Power Generation System

Design Considerations - Frequency Drooping

During "on grid" operation the interactive inverters will push all of the power being generated from the PV panels into the local grid. In full sunlight that will be about 15KW, far more than can be consumed locally (the peak consumption is about 12KW, and that is only for brief periods). When the backfed power exceeds local consumption the excess is pushed into the utility grid, so in effect the grid is used as a battery of infinite capacity.

The situation is very different for "off grid" operation. The hybrid inverter will maintain the 60Hz waveform for the local grid, which will allow the interactive inverter to continue operating. When the power generated by that interactive inverter exceeds local consumption something has to give, as the local battery bank has a very limited capacity. Once that battery bank is fully charged the interactive inverter has to be signaled to reduce its power output. That is done by means of "frequency drooping" or "frequency shift power control": the hybrid inverter will shift the frequency so that the interactive inverter shuts down.

Note that frequency drooping with the Xantrex hybrid inverter and the SMA interactive inverter is an all-or-none proposition; the interactive inverter either operates at full capacity or not at all. The SMA "Sunny Island" hybrid inverters can modulate the output of the SMA interactive inverters at intermediate levels, a very desirable feature but unfortunately cost prohibitive (an additional $5,000 for this system).

Frequency drooping is a relatively new technique. It is not well documented and none of the PV system installers we contacted were familiar with it. Fortunately the installer we selected, Aurora Energy took the initiative in directly confirming with the manufacturers that this technique was appropriate for our specific system.

The best single written discussion appear to be the Schneider "Application Note 976-0240-01-01 Rev A" entitled "AC Coupling of Inverters". That document appears to no longer be available on the Schneider web site but can be found at multiple other locations.