11/3.3kv Transformer grounding & switchgear rating - time

[power2020]

Dear Folks,
We have 11kv substation and then 11/3.3KV step down transformers and 3.3kv substation for 3.3kv motors and then to 400v system through 3.3kv/415v system.
Now My questions..
1) what is the best way to design the 11/3.3kv transformer grounding systems, i mean, either with solid grounding or resistive gorunding? I know most of the MV transformers are designed for resistive grounding. However, i want to know, Is there IEC standard which should says resistive grounding is better for MV transformers (3.3kv or 11kv transformers)?
2) If the transformer,11/3.3kv Dyn11, is with solid grounding what could be the 3.3KV switchgear short circuit rating (withstand time)? we have got the maximum calculated sym sc current is around 10kA, and hence we are going for 25kA switcgear. Now the issue switchgear withstand time rating 1sec or 3 sec.
Is some thing 1 sec is ideal for resistive grounding system? and 3 sec is ideal for solid grounding?
Is there IEC/BS standard which tells about switchgear short circuit current/time rating?


Thanks in advacne for all your valuable suggestions.

Thomos.

 

[power2020]

Can someone advice..

 

[rbulsara]

Look up the IEEE Green Book on grounding for relative benefits/drawbacks and limitations due to impedance grounding.

Impedance grounding does not help reduce required short circuit rating of the equipment, as the neutral impedance does not affect the 3 phase or phase to phase short circuit currents.

Impedance gronding limits the damage during a single line to ground faults which happens to be more frequent or likey type of faults.

Also you can not use impedance grounding if you are planning to feed line to neutral loads at 3.3 kV bus.

 

[power2020]

Thank you rbulsara for your comments..
However, we dont have 3.3kv Loads.

 

[davidbeach]

for 3.3kv motors

However, we dont have 3.3kv Loads.

Say what? Motors aren't loads?

 

[power2020]

My apologies..
I mean 3.3kv single phase loads..
we dont have single phase 3.3 Kv loads.

 

[waross]

You can use 3.3 Kv single phase line to line loads.
You can't use 1.9 Kv line to neutral loads with an impedance grounded system.
respectfully