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Calculating fault current on secondaries of Grounding Bank

Calculating fault current on secondaries of Grounding Bank

Calculating fault current on secondaries of Grounding Bank

(OP)
Was wondering if anyone has experience calculating available fault current on the secondaries of a grounding bank utilizing distribution transformers (3-50kVA, 13.86kV/240V, Z=2%) which are wired in a wye/broken delta?
Also any tips on sizing the 13.8kV fuse to protect these transformers or is a fuse even necessary.  The secondaries will be wired into a digital relay.

Thanks  in advance for any assistance.

RE: Calculating fault current on secondaries of Grounding Bank

The impedance of the resistor you insert into the broken delta will be so high in relation to the transformer bank impedance that the resistor will limit the ground current.
I believe that if you close the delta the current will be limited by three times the 2% transformer impedance (6%Z). Will someone confirm this? Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Calculating fault current on secondaries of Grounding Bank

The three phase % impedance of a transformer formed from three single phase transformers is the same as the single phase % impedance, but on the three phase base.  So, 3 50kVA 2% single phase transformers form one 150kVA 2% three phase transformer.

RE: Calculating fault current on secondaries of Grounding Bank

(OP)
Thanks davidbeach and waross for you're responses, though does anybody think its necessary to fuse the high side of the transformers?  I ask since the grounding bank is contacted in wye and the source is delta therefore if a transformer shorts to ground technically very little zero sequence current should flow.  Therefore is there a contingency where the fuses would provide protection for the transformers?
 

RE: Calculating fault current on secondaries of Grounding Bank

if you protect by fuse , after its operaton ,you can keep in operation but  there is no more guarantee the kind of grounding is your system.
I suggest relaying protection to trip substation operation.  

RE: Calculating fault current on secondaries of Grounding Bank

(OP)
Thanks odlanor for your response though to be a little more clear on my question above, given the above parameters would the fuse ever trip since the grounding bank is connected to a delta (ungrounded) system.  Just looking for possible contingencies.

RE: Calculating fault current on secondaries of Grounding Bank

The grounding is through the primary wye. The impedance connected across the open delta will limit the ground fault current.
Fusing. The last thing you want is for a fuse to open on your grounding device during a fault.
It is quite possible that the circuit may be energized with one or more ground faults. For example a crew fails to remove one or more grounding jumpers after a maintenance outage.
This will energize one or more of the grounding ttransformers with full rated voltage. The inrush may be up to 25 times rated full load current.
I would not fuse at less than 50 times full load rated current.
I would use a disconnect switch or fused cutouts fused as described or with wire jumpers in place of fuse links.
You may monitor the secondary voltages to detect transformer failures.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Calculating fault current on secondaries of Grounding Bank

I wouldn't ever fuse a ground bank.  Only a three-phase interrupting device with relays, regardless of how small the ground bank.  Fuses don't know squat about zero-sequence current, and that makes them totally unusable for protection of a ground source.

RE: Calculating fault current on secondaries of Grounding Bank

I agree with you David. For a grounding bank connected through fused cut-outs I would either use wire jumpers in place of the fuse links or use fuse links sized at about 5000% of full load current. Essentially the same as wire jumpers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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