A
We have a transformer differential relay protection for a start-up transformer at a Nuclear Plant. The differential scheme for this transformer is achieved using GEH-2057 with BDD16 relays. The transformer is a 345 kV/13.8 kV wye-wye with a tertiary delta. We have a concern that we may in future have an open phase fault on the primary of the transformer with and without ground (Conductor falling to ground from transformer primary side). With regards to that I have a couple of questions.
i) Under what fault conditions (line to ground or line fault without ground) on the transformer primary side not within the zone of the differential scheme, would you expect the differential protection relay to trip. Is it possible for the differential scheme to cause the trip during line to ground fault on the primary 345 kV side of the transformer (not within the differential zone) ?
ii) Due to tertiary delta, on a ground fault on the primary side, the primary of the transformer see extremely high zero sequence currents but the secondary of the transformer almost sees none. Therefore, phase currents on the primary and secondary side of the transformer are much different (after normalizing with turns ratio). Would this trip the differential scheme.
iii) Further during the line to ground fault on the transformer primary the current direction will reverse so as to flow from transformer primary into ground. Would the differential relay consider the change in direction and result in tripping?
The relay is set with 25% slope tap setting.
TH=4.2 and TL=8.7
i) Under what fault conditions (line to ground or line fault without ground) on the transformer primary side not within the zone of the differential scheme, would you expect the differential protection relay to trip. Is it possible for the differential scheme to cause the trip during line to ground fault on the primary 345 kV side of the transformer (not within the differential zone) ?
ii) Due to tertiary delta, on a ground fault on the primary side, the primary of the transformer see extremely high zero sequence currents but the secondary of the transformer almost sees none. Therefore, phase currents on the primary and secondary side of the transformer are much different (after normalizing with turns ratio). Would this trip the differential scheme.
iii) Further during the line to ground fault on the transformer primary the current direction will reverse so as to flow from transformer primary into ground. Would the differential relay consider the change in direction and result in tripping?
The relay is set with 25% slope tap setting.
TH=4.2 and TL=8.7