High Impedance 12kV generators

[living2learn]

I am working on a project where there are three 12kV generators grounded through a low impedance resistor. The system is 3w+g. The normal power service is fed from a (3) 69kVd:12kVy transformers and protected by a network protector. A fuel cell was just installed at 300V and connects to the 12kV system via a delta wye transformer. I feel like this fuel cell transformer has thrown off my previouse coordination study. I now have another zero sequence current path?
Since the generators are grounded through an impedance will this downstream transformer be a problem?

 

[living2learn]

Ignore the "high impedance" title, it is in fact a low impedance 200A system.

 

[rcw retired EE]

A grounded wye 12kV connection will affect your ground fault current levels and coordination. But I can't tell how your system is put together.

A one line would help understanding.

Are all three generators paralleled on the 12 kV bus?
Are there individual or one common grounding resistor.
Is the 69 kV/12 kV transformer's 12 kv wye winding solidly grounded, connected to the grounding resistor, or left floating?
Is the 12 kv winding of the fuel cell transformer delta or wye? If it is delta, it will not be a zero sequence source.

 

[living2learn]

Great point. I have uploaded the SLD. they want to add a load bank on the secondary of the fuel cell xfmr so if the utility drops out they can prevent the fuel cell from shutting down

 

[rcw retired EE]

The grounded wye fuel cell transformer is a source of ground fault current at 12 kV. It should have protection.

Without a grounding impedance to limit fault current from this transformer, damage during a generator fault will be worse. Instead of a rewind, the fault may burn enough to damage the stator iron and the plant gets to buy a new generator.

It also increases your fault level during normal operation.

 

[waross]

Looking from the utility side you have a four wire wye feeding a closed delta. Not good. Any unbalanced voltages or phase angle errors on the utility side will cause circulating currents in the delta. Loss of a primary phase will result in circulating currents in the delta and back fed voltage and current into the unfed phase.
Float the neutral but be aware that you may experience transient over-voltages when energizing the transformer.
By the way this is a good connection to develop a ground for a delta system. For impedance grounded schemes the wye:delta transformer has to be large enough to supply the fault current in the grounding impedance. For a solidly grounded system available fault current and clearing time should be considered when verifying sufficient withstand capability.

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

 

[living2learn]

Great thanks.

So I will float the neutral of the fuel cell transformer. I am adding a load bank connected directly to the seconary of that transformer. During a power outage the normal switchgear opens and the generators come online, and a breaker will seperate the fuel cell from the generators, then the fuel cell will feed a load bank. While feeding that load bank the system will be ungrounded because the neutral was floated. Isn't that a violation of the NEC?

 

[waross]

Be aware; When energizing from the wye side expect transient over-voltages. Consider energizing the transformer before the fuel cell is connected or energizing from the 330 Volt side (fuel cell).
Under the Canadian Electrical Code grounding is optional if by so doing you will have a voltage to ground greater than 150 volts. An ungrounded supply (the fuel cell) must have a suitable ground fault detection device. The NEC is often similar.


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

 

[living2learn]

Well the solution to the problem is to ground the transformer through a 200A, 10 second resistor. They said they would coordinate the transformer circuit breaker secondary ground fault relay to trip before any other ground fault relay in the system.

For some reason and I don't know why, but not satisfied with the result.