High Impedance 12kV generators
High Impedance 12kV generators
(OP)
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?
Since the generators are grounded through an impedance will this downstream transformer be a problem?






RE: High Impedance 12kV generators
RE: High Impedance 12kV generators
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.
RE: High Impedance 12kV generators
RE: High Impedance 12kV generators
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.
RE: High Impedance 12kV generators
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
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"Why not the best?"
Jimmy Carter
RE: High Impedance 12kV generators
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?
RE: High Impedance 12kV generators
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
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"Why not the best?"
Jimmy Carter
RE: High Impedance 12kV generators
For some reason and I don't know why, but not satisfied with the result.