Is a Ground Detector/Bank Needed or Not?

[George556]

Hi everyone, I'm a little confused on this topic and hoping for some help.

We are going to be putting a synchronous generator on 480V to temporarily carry a 12kV circuit via a step-up transformer.
So the set up will be: Generator @ 480V -> Delta-Wye Grounded XFMR -> 12kV circuit.
The Transformer will be Delta on the 480V and Wye Grounded on the 12kV side. We will be doing this set-up at different circuits. Some circuits are 3 wire while some are 4 wire.

I'm assuming the Generator itself is configured as a wye grounded generator.

My questions:
1. If the generator is wye grounded, it supplies ground current on the 480V. If the high side of the XFMR is wye grounded, it supplies ground current to the 12kV. So to me, that means no additional ground detectors are needed, right?
2. Does it matter whether the circuit is 3 wire or 4 wire if the high side XFMR winding is Wye Grounded to determine if I need a ground detector on the 12kV?
3. If the Generator is configured Delta, do I need a ground detector on the 480V?

 

[Kiribanda]

1) So to me, that means no ground detectors are needed, right?
What do you mean by this statement?
2) If the 12kV side is solidly or effectively grounded you can have both 3-wire & 4-wire circuits.
3) If your generator is DELTA connected then your 480V system is ungrounded which means
you need to provide at least a ground fault alarm system.

 

[FreddyNurk]

1. You'll have a source of ground fault current, but you'll need to be able to detect when that happens and disconnect your generator so you're not feeding a ground fault. Earth Fault CT on the grounded star point and an appropriate relay is one such method.
2. I don't believe it does, although 12kV tends to indicate that its not in an area I'm familiar with. Others will have more experience in distribution in this area than I.
3. If you operate an ungrounded system then you still really need a means to detect inadvertent connections. For an IT System this would likely be a continuous insulation monitoring system and a means to allow trained personnel to locate and remove the fault.

You haven't mentioned whether the 12kV network is part of a larger network or whether you're feeding your own network. This may play a part in additional protection requirements.
There are also likely location specific code requirements, although I am not aware of the specifics of 480V systems that may be relevant.

EDMS Australia

 

[George556]

The 12kV will only be fed from the generator. These are temporary configurations to pick up a portion of a circuit only while work is being done isolated on another part of the circuit.

 

[waross]

The 12 kV circuit is fed from a grounded wye transformer winding.
No ground fault fault detector is needed, however I advise using some device to monitor and trip on a ground fault.
The generator is a grounded wye configuration.
No ground fault fault detector is needed, however I advise using some device to monitor and trip on a ground fault.
A generator does not supply nearly as much fault current as the grid.
A generator may not always supply enough current to trip instantaneously on a fault, line to line or line to ground.

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

 

[George556]

The generator has its own protection and there is a recloser protective device on the high side of the transformer to protect the line.
The generator has an AVR, and can sustain short circuit current at about 300% of its full load for about 10 seconds per the spec sheet.

 

[FreddyNurk]

If you can configure the recloser to trip for an earth fault on the 12kV network I would do so, there is obviously a residual risk of having an earth fault upstream of the recloser that won't get disconnected in such an arrangement.

Its quite likely that fault current from the generator will be unable to do any conductor damage on the 12kV side, so the intent of providing automatic disconnection for a fault is generally for safety reasons, less so for step potential, and more so for inadvertent contact with lines. I'm assuming based on the use of a recloser that you're feeding an overhead network. Use of ABC or private networks reduces the risk somewhat compared to utility bare conductors but is still a risk.

The same goes for an ungrounded LV, they're generally accepted for service but require monitoring, identification and removal of insulation faults in a timely manner.

EDMS Australia