Wind turbine protection comparator circuit
Since it is a safety system you want to make it fail-safe. 

For this we make sure that in case of no power your relay is in the safe position. There are also fail-safe relays if you're interested in that and all your protection will be electrical, i.e. no vane getting perpendicular to the axis of the turbine.

C1 will get charged to the voltage given by the divider, minus a diode voltage drop. Whatever voltage you get in C1 plus a voltage drop you'll read with the microcontroller.

That is unless the dv/dt of the voltage is so high that the capacitance in the diode is in effect, which I don't think is the case. You have a low pass filter given by the resistors and the series of the diode capacitance and C1. 

Forward voltage is highly dependent on temperature, and capacitors can vary their value with time significantly. I would avoid the use of this to set thresholds. 

I would make a latching circuit that when triggered turns off the relay (should be normally on for safety) and also triggers a timer for 20min that after it's done resets your latch. You can do this easily with a 555.