Run/Update short circuit current-ETAP

[kssschsekhar]

Dear Folks,
Greetings...
In a small Isolated network with two generators are connected to a bus directly (without transformers) and operating in parallel. Each generator's ground (Earth) fault current is limited to 100A due to neutral ground resistor. Each generator has a neutral CT and connected to sensitive Earth fault relay. During update TCC clipping current in ETAP, the ground fault current in each generator neutral circuit is showing 200A rather 100A as shown in the attachment.
Please suggest me that how to update with 100A (calculated)as clipping current. Thank you so much...

 

[DanDel]

Is it simply the sum of both 100A GF current from both gens? I'm not sure how you modeled the system, so I don't know. Not sure what you mean by 'TCC clipping current'; in ETAP, the TCC graph for GF will show you the GF current as a limitation for the end of a protective device's curve.

 

[kssschsekhar]

Though total ground fault current is 200A, the current through each neutral is 100A only.... Then the maximum current through the sensitive earth fault relay is 100A... but ETAP is showing 200A in the senstive earth fault relay clipping current page... Based on this current,TCC graph shall clip at max. earth fault current...100A

 

[DanDel]

What is the value of the neutral ground resistor?

 

[kssschsekhar]

System voltage: 3.3kV (L-L)
Neutral Resistance: 19.0526 Ohms
Ground fault current:100A (through each neutral circuit)
If two units are in operations, total ground fault current: 200A, however the ground fault current in each neutral circuit is 100A

 

[jghrist]

Have you tried ETAP support? If it is possible to change the clipping current, they should be able to tell you how. If they won't help, it would be useful for the rest of us to know that.

 

[DanDel]

I've modeled your system as close as I could, and I have gotten the same results. The only thing I can think of is that ETAP is using the total GF current instead of what is being seen through the individual CT as listed on the TCC kA page.
Modeling the GF relay and (zero sequence) CT on the gen connection to the bus provides the same value. As a matter of fact, it shows the total bus 3-phase fault current as well, not just the contribution from the one gen.
You may want to ask ETAP about it, as suggested.
You can always put in the User Defined value instead of Calculated to get 100A in the TCC graph.

 

[kssschsekhar]

Thank you Dandel & Jghrist... Let me check with ETAP and get back to you if I get any solution...Cheers...