Cogenerator Grounding

[living2learn]

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?

 

[living2learn]

wow, just re-read that - the cogens are grouned through a 200A, 10S resistor not solidy grounded.

 

[davidbeach]

Once the generator breaker opens, the utility will no longer be feeding the fault. Remember, a generator internal fault exists until the generator stops spinning (or below some low speed) whether the breaker is open or not. Getting the breaker open removes the rest of the sources and the current is then limited by the resistor for the remaining duration of the fault.

 

[ScottyUK]

200A is way too high to offer much protection to the stator core. Given the option to choose you should be aiming for 5A - 10A to limit the effect of a stator earth fault to a value which would normally not cause irrecoverable damage. A Cogen often has to compromise on a lot of things that in a large plant wouldn't be tolerated.

 

[dpc]

Ideally, the entire 5 kV system would be resistance-grounded. If the utility transformer is a solidly grounded wye on the 5 kV side, then it will source a lot of ground fault current until the generator breaker trips. But the resistance ground of the generator is still preferred over solid grounding for the reasons that David has already mentioned.

 

[living2learn]

Thanks guys, your insite is greatly appreciated. The delta wye solidly grounded transformer is oversized and I was thinking about breaking the delta to prevent zero sequence current from flowing unrestricted. I am going to draw out the zero sequence diagrams, but not sure if this is a solution or not.

 

[waross]

Consider increasing the grounding resistance value rather than "breaking" the transformer. An option may be to switch in more resistance in the event of a fault so as to limit the current and subsequent damage.
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

 

[living2learn]

The cogen plant is grounded through a 200A resistor and each utilize their own resistors.
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.

 

[dpc]

There will only be 200 A (max) flowing through each resistor for a ground fault if these are 200 A resistors. I think there is a problem with your model. Check the transformer grounding resistor. Also, you're looking basically at the bus fault currents, not the individual contributions.

 

[odlanor]

aximum short-circuit at terminal line(beggining of generator windings) of one cogenerator should be 399A.

 

[odlanor]

by SKM-SLD maximum short-circuit at phase terminal(beggining of generator windings) of one cogenerator should be 399A.
I guess 400A as very common as grounding current of generator with this voltage and power at industry area.

 

[living2learn]

Thanks. I doubled checked the model and impedances for the transformers and they are in fact grounded through a 200A, 34.6 ohm resistor for each one, while the fuel cell transformer is solidly grounded.

 

[odlanor]

living2learn ,
BUS COGEN1: MVA and kV base?

 

[mls1]

Are you certain the 200A resistors are directly on the neutral and not on the secondary side of a neutral transformer? If they are on the secondary side of a neutral transformer then your primary ground current is likely less than 10A per source.

 

[living2learn]

odlandor, I believe the base is 100MVA, 12000V.

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

 

[odlanor]

living2learn,
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
????