The current goes to a load, which is also serviced by a second 120 VAC source in parallel. When the load decreases the second AC source begins to feed power back into the inverter, which could destroy it. How can I sense when the current begins flowing backward into the inverter so that i can activate a relay to open that circuit and thus protect the inverter? A current comparator circuit? What would that look like? Thanks. --M

 

Putting an inverter in parallel with line power is incredibly dangerous unless the inverter is capable of "grid tie", to get in phase with the line power. Since your's cannot tolerate reverse current, I would assume you do not have a grid tie inverter. Stop!

 

You are trying to think of an AC system in DC terms. AC -- alternating current. The current does not "flow back" into the inverter only when the second source "feeds power back into it". The current is going in and out of the inverter even if it is the only source powering the load (assuming the load doesn't have a diode at the front end). The issues are far more complicated than that, as is dealing with them safely.

As wayneh said: Stop!

 

You may hire someone to do that for you to keep you and the device safe.
This thread will be keep locked because safety reason.