CT sizing for transformer overcurrent application
CT sizing for transformer overcurrent application
(OP)
Good day,
I'm in an IEC world.
I have a 11kV/0.4kV, 200kVA xfr. This means 10.5A high side nominal current, and 282A low side nominal current.
How do I size the 11kV CT for overcurrent protection? The fault levels on 11kV is 25kA.
b.
I'm in an IEC world.
I have a 11kV/0.4kV, 200kVA xfr. This means 10.5A high side nominal current, and 282A low side nominal current.
How do I size the 11kV CT for overcurrent protection? The fault levels on 11kV is 25kA.
b.






RE: CT sizing for transformer overcurrent application
I meant to say, please provide me with technical papers or bulletins to find this information.
b.
RE: CT sizing for transformer overcurrent application
However in IEC world IEC-185 and IEC-44-6 are the standards for CTs.
Regards
RE: CT sizing for transformer overcurrent application
The xfr is indoor type. This was my thought as well, the use of a microprocessor relay to protect a small xfr like this is not justified.
This I cannot change becos it was requested, priced and accepted before I had to have a look at it.
Thanx,
b.
RE: CT sizing for transformer overcurrent application
RE: CT sizing for transformer overcurrent application
RE: CT sizing for transformer overcurrent application
RE: CT sizing for transformer overcurrent application
The relay will trip an 11kV circuit breaker that is provided for as well.
EPEngineer,
It surprised me too...
b.
RE: CT sizing for transformer overcurrent application
Using fuses as standard practise on a plant creates discrimination problems for the larger (3-4MVA) transformers especially in high impedance earthed systems.
RE: CT sizing for transformer overcurrent application
Thanks for this invaluable input!
I'm new at Distribution type work, more familiar with 220kV and above. I just need clarification on busbar configurations for 11kV indoor networks:
i) What are typical busbar layouts? Or reference document (IEC).
ii) If a double bus layout; would incomers have isolators to switch them between the two buses? Would you then need two bus-sections?
Thanks.
b.
RE: CT sizing for transformer overcurrent application
Regarding your orriginal question on CTs - most common problem in protecting a small transformer using CTs is the large physical size of a low ratio CT - you should not be surprised when your design came back from the switchboard manufacturer because the CT can not be fitted in the switchboard. You should accept heavy satration - just make sure the instantanuous setting is well below saturation. With modern low burden relays the saturation is not as bad as in the past.