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dafo74, one we are using for under frequency load shedding is ABB's SPAF relay. It is having flexibility of combining freq and df/dt in around 3 stages. i.e u can have 3 stages for, say alarm at 49 Hz+ 0.2 Hz df/dt...and so on for other stages.

Are u talking about df/dt, by any chance ??

hello sky72, you have elaborated your question very well. But instead of looking for conventional AVC's, have you thought about DIGITAL AVC's. GE has EX2000, which is digital software operated AVC, with DIs, DOs, AIs and Serial communication facility...you can integrate it with your DCS / MARK V...

I agree with MAIHAM, we have the same system, but from GUTOR,switzerland..Mechanical switch you are talking about is "Make before Break" switch.While operating mechanical switch, both lines ( from static bypass and direct) are in sync (on utility),so no question of sparking...

Nikhil, The relay you are reffering seems to be similiar to siemens rotor e/f relay which works, basically on resistance measurment between winding and ground. In your case ,At what resistance, 1st alarm will come and at what 2nd ? You can go for decade resistance test, right at the terminals...

In cpp connected with grid, Off-LTC are used for distribution trafo's, including generator unit transformers ,,whereas Trasformers meant for grid connection are On-LTC, specifically for voltage regulation. Atleast that is what i have seen.