Cogenerator Grounding
Cogenerator Grounding
(OP)
I am at a facility where the utility transformers are solidly grounded and parallel with a cogen unit. The cogen is 5kV solidly grounded, 10MW. It is grounded through a resistor.
if there is an internal fault the resistor will help protect, but won't the utility transformers feed that fault?
if there is an internal fault the resistor will help protect, but won't the utility transformers feed that fault?






RE: Cogenerator Grounding
RE: Cogenerator Grounding
RE: Cogenerator Grounding
RE: Cogenerator Grounding
RE: Cogenerator Grounding
RE: Cogenerator Grounding
There are a few little details that may have a bearing on your situation.
1> Is the generator wye point grounded, the transformer wye point grounded or both?
2> Is the generator wye point connected to the transformer wye point?
3> In the event of a fault initiated disconnect, will you open the co-gen side of the transformer or the utility side of the transformer?
Fault current is limited by the impedance of the fault circuit.
In a solidly grounded generator, a bolted fault current is limited by the reactances (X, X', and X") of the generator winding and the point on the winding where the fault occurs.
A fault on A phase of a wye connected transformer will be fed by A phase of the generator. However if there is a four wire (Neutral connected) to a wye/delta transformer the transformer will also contribute to the fault.
When looking at a single phase load or fault on the secondary of a four wire wye/delta connection I find it helps to consider the transformer bank as a single phase transformer in parallel with an open delta bank.
An open delta fed from a stiff network will form a virtual transformer on the open side. The virtual transformer will be equivalent to the missing transformer.
So, a generator single phase fault with a four wire wye/delta transformer connection will be fed by the faulted phase, limited by the effective phase reactance and by a back feed from the healthy phases limited by the effective reactances of the healthy phases and the impedance of the transformer windings.
If twice the transformer rated impedance is low in comparison to the generator effective reactances the bolted fault current may approach 200% of the simple single phase calculations.
BUT you are using a grounding resistor. The grounding resistor will limit the fault current.
The back feed will not increase the fault current. The back feed will have the effect of reducing the current in the faulted winding by the amount of the back feed contribution.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cogenerator Grounding
The utility transformers are both grounded through a 200A resistor The fuel cell is solidly grounded.
Q1) both the cogens and the utility have their wye point grounded through a resistor.
Q2) the system is a 3w+g connected system so there wye points are not connected with a neutral. There is an equipment ground conductor bonding both of these systems together.
I put this info into SKM and ran some scenarios.
Scenario #1 - Cogens running by themselves Zero Sequence cogen contribution current = 200A As you can see the cogens each contribute 200A of SLG current. OK
Scenario #2 - Cogens running with utility Zero Sequence cogen contribution current = 228A As you can see the cogens each contribute 228A of SLG current. OK
Scenario #3 - Cogens running with utility and fuel cell Zero Sequence cogen contribution current = 903A As you can see the cogens each contribute 903A of SLG current. ?
Scenario #4 - Cogens running with fuel cell Zero Sequence cogen contribution current = 1311A As you can see the cogens each contribute 1311A of SLG current. ?
I just am having trouble getting my head around this one...I am going to digest your comments
thanks for being patient.
RE: Cogenerator Grounding
RE: Cogenerator Grounding
RE: Cogenerator Grounding
I guess 400A as very common as grounding current of generator with this voltage and power at industry area.
RE: Cogenerator Grounding
RE: Cogenerator Grounding
BUS COGEN1: MVA and kV base?
RE: Cogenerator Grounding
RE: Cogenerator Grounding
MLS1 - The solidly grounded transformer is on the 12kV distribution network along with all the cogens and the two other impedance grounded transformers. I agree that if the nuetral was grounded on the other side of the transformer then we would clearly have no additional zero sequence currents on the 12kV network
RE: Cogenerator Grounding
1phase-to-ground fault at BUS COGEN1(or phase-terminal of cogenerator #1):
1- base 100MVA and 5kV : 22.3 A
2- base 100MVA and 12kV : 9.3 A
????