Sizing Core Balance CT's
Sizing Core Balance CT's
(OP)
I am looking for some advice or a reference on how to size a Core Balance CT for differential protection for a 2500HP 6900 volt motor. The motor is 190 FLA. I have been told I should use a 300:5 CT by one person and 50:5 by another. The system is high resistance grounded.






RE: Sizing Core Balance CT's
2. Select an appropriate earth fault sensing relay, either 1A or 5A.
3. From above info, the CT ratio can be defined
eg. assume 1A relay and assume POC 2A
if relay set for 10% tripping (of 1A) then CT ratio to be 20:1 therefore when there is a POC of 2A the secondary output will be 0.1A and should trip relay.
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RE: Sizing Core Balance CT's
Any ground current sensing will have to be very sensitive. I would check with your relay manufacturer to get their recommendations and to see if the relay will be capable of sensing the ground current.
If your ground fault current is more like 100A to 400A, then you have low-resistance grounding. A 50/5 zero sequence CT will work for nearly any low-resistance grounding system, but again, you really need to match the CT to the relay that you are using.
RE: Sizing Core Balance CT's
Choose the ratio of the CBCT to to provide lots of current on the maximum fault current. For instance if your maximum ground fault current is 20A and your relay can take 20A continuously, you could go as low as a 5:5 CBCT. The lower the ratio, the lower the minimum fault current that can be detected. A 50:5 and a relay with a minimum setting of 0.5A would detect 5A of primary current, while a 5:5 CBCT would allow the same relay to operate for 0.5A of primary current.
This is assuming that any internal fault is detected and cleared while it is only a SLG fault. If the fault becomes a LLG fault or a 3-phase fault that low ratio could cause trouble.
On the other hand, if you are actually talking about using one CBCT around all three phases coming into the motor, you aren't really talking about differential protection, but sizing considerations would be similar.
Using a 300:5 CBCT to detect ground faults on a high resistance grounded system (say 20A, since you didn't say) would produce 0.33A secondary for that 20A fault, and if the relay needs at least 0.5A secondary to pick up, you might as well not bother with the CBCT at all since it will not provide enough current to the relay to detect a bolted ground fault, never mind a fault that includes some fault impedance.
RE: Sizing Core Balance CT's
The motor will be wired similar to Fig 11.3A of Blackburns book (I have the first edition). It will also look like Fig. 2-14 in the GE Multilin 469 book. I am not yet sure of the ground fault current because of sketchy information(the substation was installed in the late 50's). The neutral of the transformer is connected to a 75KVA single phase transformer and the secondary of the single phase transformer is connected to a resistor, but I currently do not know the size of the resistor.
RE: Sizing Core Balance CT's
RE: Sizing Core Balance CT's
RE: Sizing Core Balance CT's
I have sent today a RFQ to your sales at itl-uk dot com for some CT requirements.
I would appreciate if you could follow it up.
Thanks.
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