GSU Ground Fault CT Size

[deedo1124]

Brothers,

I have been asked by an equipment supplier to size a CT for ground fault protection on the primary side of a 1500KW 480-34.5KV land fill gas generator. The available fault current at the transformer primary is 2000A. I assume that they will want to detect much smaller magnitude currents. I am stumped. Any guidance would be appreciated. System one line is attached

JD

 

[slavag]

Hi.
Your ground fault is more smaller then 2000A. it's high impedance grounfing. 480/(1.73x55.4)=5A
in your drw CT 5/5A. Why? Isn't good solution
what is a current input of relay 1A or 5A or 0.2A?
I recommend use CT 75-100/1A and connect to very sensetive current input.
Regards.
Slava

 

[HVmariner]

Hi Slavag!

If ground fault is limited to say 5A, why use a core CT with primary current so high (75-100A)?

 

[cranky108]

Measuring current on a high impedance ground. Humm.

Why not just measure voltage across the impedance?

 

[HVmariner]

Measuring the voltage across the impedance will tell you there is a ground but not where, whereas measuring the current on feeders will make it possible to say on which feeder the ground fault is located.

 

[slavag]

Hi .
Cranky108, you are right, of course more better voltage principle!!!!
HVmariner.
Your Q is very logical Q.
It's CT performance issue.
5/5A Ct is more accuracy in range from 1A up to 5A.
if you have setting lower, for example 0.5A ( BTW it will be lower setting) 75-100/1A more accuracy.
I sent once graph of CT performance, will try fornd and will attached.
Regards.
Slava

 

[slavag]

Found.
Please attached
thread238-212599
Regards.
Slava

 

[deedo1124]

Gentlemen,

I appreciate your input but my question is about the ct on the primary transformer ground not the ct on the neutral grounding resistor.

Thanks again,

JD

 

[slavag]

OOPS, my bad, sorry.
With CT on the main trafo ground, you don't have any problem. you can use 2000/5A, for example 5% .
Regards.
Slava

 

[REM76]

For ground fault protection on the primary, set the long time pick up to minimum, instantenous greater that four times the self cooled rating of the transformer. Curve shape and time dial to coordiantate with downstream/upstream devices. The intersection point between the curve and the instantenous vertical line should be greater than 0.1 seconds to avoide nuisance tripping during inrush.