If Step Up transformer is installed neared to Generator, and generator has already been equiped with overflux (v/f) protection then do we still need to have a separate overflux protection (v/f) for step up transformer? OR generator (v/f) protection is enough for SUT as well..

I believe, as per IEEE C37.91 Guide for Protection relay application to Power Transformer, 8.2.4 Over excitation protection, is it correct to assume that voltage of generator transformer and generator voltage are equal and the generator transformer is located at the generator closely. Additional V/Hz protection for generator transformer is not required.

If it is required, then under what circumstances e.g. in case of grid fluctuations etc...Any thoughts about it?

I think if you dont have GCB, you can set 24 in your generator protection relay by proper coordination of V/F curves of generator & GSU. In case if you have GCB installed and if you are back feeding from grid while generator is OFF, you need separate protection for GSU in it's relay.

V/F excitation relay is provided for power transformer protection at the substations near the generation stations as well as at switchyard to watch excitation voltages increase. I think it has two stages alarm and trip and time set is also high.

Naveed,

Your interpretation is correct, originally V/Hz relays were used to protect GSUs and direct connected auxiliary equipment from off frequency operation of a generator due to governor failure or over-excitation due to AVR, operator error, failure to remove excitation after a turbine trip.

That's the history, fast forward to the present where V/Hz relays form part of the overall grid protection and/or blackout/islanding scheme, independent of what any particular generator is doing. In that case the overall protective relaying/operational philosophy of your particular system would guide you as to how and when to include another layer and type of V/Hz relaying.

Alan

thanks Alan...and what would you say about kamal khan comment..he exactly said the same thing that I meant from my subject query.

Naveed , today I re-visited this setting at my plant. For Generator , we have 24 in Gen Prot. Relay by taking VT input from Gen Terminal (18kV). For Transformer, we have 24 in Transformer Prot Relay by taking VT input from 380kV Side. Transformer LV side will be protected by Gen Prot Relay (18kV base is same fro Gen Terminal & Trafo LV).

Remember in GSU , HV voltage usually follows the grid voltage, and LV side controlled by Gen Excitation (if Gen is connected), so you may have different ratio of V/F for LV & HV at same time. So protection for both windings can be set in this way, one in Gen Prot & other in GSU Prot Relay