CT circuit
CT circuit
(OP)
We have 2 CTs (A and C phase) on the 3-ph motor circuit and the CTs are wired as follows:
1) 2 CTs feed motor protection relays followed by the summated CT (2 CT currents summed before) feeding the motor excitation panel.
We are replacing the motor excitation panel that now requires 2 separate CT inputs. So we have been planning to revise the CT circuit as follows:
2a) 2 CTs feed motor excitation panel (add 1200 ft round trip) followed by the motor protection relays
2b) Alternatively, the 2 CTs can feed the motor protection relays first followed by the motor excitation panel (placement of the panel is reverse of option 2a)
Preliminary calculation shows that the increased CT burden is acceptable. What would be your recommendation 2a or 2b? For option 2a, would this have any negative impact to motor protection?
Thanks for your input.
1) 2 CTs feed motor protection relays followed by the summated CT (2 CT currents summed before) feeding the motor excitation panel.
We are replacing the motor excitation panel that now requires 2 separate CT inputs. So we have been planning to revise the CT circuit as follows:
2a) 2 CTs feed motor excitation panel (add 1200 ft round trip) followed by the motor protection relays
2b) Alternatively, the 2 CTs can feed the motor protection relays first followed by the motor excitation panel (placement of the panel is reverse of option 2a)
Preliminary calculation shows that the increased CT burden is acceptable. What would be your recommendation 2a or 2b? For option 2a, would this have any negative impact to motor protection?
Thanks for your input.
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RE: CT circuit
RE: CT circuit
RE: CT circuit
Is there no third CT (in Y-phase)? How is earth fault protection ensured if there is no third CT - Is there Zero sequence (Core Balance) CT in the panel?
Motor protection relay is in switchgear panel and the Excitation panel is near motor I guess.
1) If there is ZSCT, both options (2a & 2b) mentioned by you are possible and either has no specific advantage.
2) If you don't have a ZS CT but have third CT then, I would prefer option-2a.
RE: CT circuit
1) Yes, CTs are located in the 4kV switchgear panel.
2) Yes, excitation panel is near the motor (600 ft away from the swgr; hence 1200 ft of additional CT wiring)
3) No 3rd CT or zero sequence CT.
Hope this clarifies.
RE: CT circuit
RE: CT circuit
You said there is no CT in Y-phase and also no ZS CT.
In such a case, I expected you to explain how is the installation protected against single phase to ground faults?
RE: CT circuit
I went through the drawings, there is a separate CT on A phase for 51N relay. This CT is completely different from the excitation panel CTs on A and C phase.
RE: CT circuit
"there is a separate CT on A phase for 51N relay" - I think you are wrong here.
You may like to check again.
RE: CT circuit
I’ll see your silver lining and raise you two black clouds. - Protection Operations
RE: CT circuit
Nothing to check. See picture below. Highlighted is the excitation panel that we would like to move before the summation point - either before the protection circuit or after the protection circuit like I explained in my OP
RE: CT circuit
The CT connected to 51N is CBCT (also called ZS CT).
RE: CT circuit
I’ll see your silver lining and raise you two black clouds. - Protection Operations
RE: CT circuit
RE: CT circuit
I’ll see your silver lining and raise you two black clouds. - Protection Operations
RE: CT circuit
Also, what would be the reasoning (if I have to explain someone it helps)
RE: CT circuit