The quickest acting trip in order to protect the turbine/generator bearing is best. I’d choose #1. As soon as 27 relay senses the under frequency, according to the diagram it will let the dc travel through it to activate the trip circuit. Depending upon what 27 relay you use, I’d strongly recommend testing this relay in order to maximize response time. It should also have some internal settings to adjust the response time/voltage that should be calibrated. Hope that helps. ~From Technician to Engineer D.Priebe