Sizing Core Balance CT's

1. Determine Primary Operating Current,POC, (earth fault tripping current.)
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.

If you have a true high-resistance ground system, the core balance CT may not do much good. Ground fault currents with high-res grounding will be less than 5 amps. Traditionally, these faults are detected using voltage relays, not current.

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.

It's not entirely clear how you are planning on using the CBCT for motor differential, but the means of doing so that comes to mind is that you would use three CBCTs, one per phase, with the lead to the terminal end of one phase going through the CBCT and then the lead from the neutral end of the same winding going back out through the same CBCT. (Figure 11.3 in Blackburn's Protective Relaying, 2nd edition.)

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.

Thank you for your help.
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.

Sorry guys - missed the reference to differential protection - I saw "core balance" and my mind just went to the ground fault place.

It is possible to securely detect faults down to 0.1A using a machine differential relay such as an ABB 87M or GRD. You need not worry about the ratios, the sensors are dedicated, and are available from ABB in multiple dimensions.

bigmantony,

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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