Transformer differential, CT in the delta
Transformer differential, CT in the delta
(OP)
Hi,
We have a transformer delta-delta configuration with differential protection.
The CT on the secondary side of the transformer are in the delta of the transformer, so the current measured isn't the same as the line current.
I've made a sketch if it's not clear.
So i want to know to do accurate differential protection with these ct, since the current in the delta is not the line current. We know that current in delta will be the line current divide by 1.73 with a angle of +30 deg vs the line current.
Is it better to add CT somewhere else? We can put CT in the switchgear that is 500 ft away, so the differential protection will aso take the cables.
Thanks
We have a transformer delta-delta configuration with differential protection.
The CT on the secondary side of the transformer are in the delta of the transformer, so the current measured isn't the same as the line current.
I've made a sketch if it's not clear.
So i want to know to do accurate differential protection with these ct, since the current in the delta is not the line current. We know that current in delta will be the line current divide by 1.73 with a angle of +30 deg vs the line current.
Is it better to add CT somewhere else? We can put CT in the switchgear that is 500 ft away, so the differential protection will aso take the cables.
Thanks






RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
It's better to put the CTs in the remote switchgear. If the differential trips for a cable fault, this is a good thing.
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
The CT are connected in wye on the primary and secondary since it's a microprocessor relay. We only put the winding configuration in the relay (Dd0). We can connect the ct in delta if we want, or change the configuration in the relay, but i'm not sure if it will give good result for every unbalance situation!
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
The CT are placed as the drawing attached (red circle represent a CT).
The line current isn't the same as the current in the delta. So i was wondering if there's anything to do with it, or use other CT outside the delta is the only solution? Maybe for 3 phases fault it's possible to make it work, but for 2 phases fault i'm not sure???
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
potteryshard, maybe this will work, we will have to verify.
Thanks
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
Unless these are three single phase transformer connected in delta, I wonder where the CTs are located. It would be difficult to install CTs in the delta of a three-phase transformer.
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
It' a 3 phase transformer, but with 2 possible connection at the secondary, wye or delta, with different voltage.
If you put the jumper to have the delta connection, the CT are inside the delta, so a strange connection.
thanks for your help
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
RE: Transformer differential, CT in the delta
Many modern multiprocessor relays will compensate for the phase shift automaticaly. If your relay does not do this, you will have to compensate for the phase shift by CT secondary connection.
Also agree that hiring a protection engineer would help.
Finally - agree that you can read operate current on modern relays or measure it in ancient ones. This will validate your configuration once you have it installed.
Regards,
JB