Grid interactive: what benefits make it worthwhile?

Re: Grid interactive: what benefits make it worthwhile?

danielh wrote: »

So ...
AC coupling capable inverter/chargers go into standby mode when useable AC power is detected from the grid. In this state the inverter/charger is acting like a simple wire:
power can flow FROM the GT inverter, through a sub panel, through the inverter/charger, into to the main cb box and hence the grid.

When AC-from-the-grid is not detected, the inverter/charger switches to invert mode, AND disconnects from the grid.

The nightmare part, of what to do with excess PV power during invert mode, seems to be where AC coupling isn't quite ready for prime time.

Not so. In the SMA system (Sunny Island and Sunny Boy), the Sunny Island monitors the batteries' state of charge and the current into the loads. At the point where the batteries are full and the load is consuming less than the output of the Sunny Boy, it shifts the frequency of the AC waveform. The Sunny Boy responds to this by reducing its output. The farther the SI shifts the frequency, the less the SB produces, and the SI uses this to match the SB's output to the current being drawn by the loads. This process can take the system to the point where the Sunny Boy's output is zero when the batteries are full and the loads are off. It's a pretty slick system.

Something I learned in a SMA workshop last week is that the SB responds the same way whether the frequency shift is positive or negative. The SI is smart enough to balance the duration and shift amount between positive and negative shifts so that clocks and timers cued to 60Hz don't drift.