High Impedance Grouded Generator

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mreeves007

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I have a two concerns with the use of a high impedance grounded generator.

NEC 250.36 states this is premitted where four conditions are met. The one I have a question about is the fouth concern: Line-to-neutral loads are not served. Our current configuration has a 480VAC generator feeding 6 isolation transformers that control various functions of a crane (travel, hoist, rotate, etc.). One of the transformers feeds lighting circuit and computers where the neutral of the load side is grounded (y connected) and provides the return path neutral for the circuit. The generator is also provided with a high impedance neutral.

Based on NEC 250.36, this is not allowed. Our concern is to limit the amount of fault current during a phase to ground fault. There is a ground fault relay across the generator netural impedance so that when a fault is detected, it removes all power from the crane. This is safer than having a solidly grounded generator which would allow up to 1200A of fault current before a breaker trips. The use for the 120V (line-to-neutral) system is to provide power for lights and the drive systems that operate the crane. These are necessary.

Thus my first question is "why are line-to-neutral load not allowed on a high impedance grounded generator?" NEC does not provide any explanation for this.

My second question is "for the other transformers that do not have a neutral, should the load side (y connected) be grounded?" Currently they are ungrounded.

Thanks
Matthew
 
Matt, not sure I quite understand your first question. Your single phase loads are derived from a transformer correct? If so you would ground the Xo side of the transformer and OCPD works normally, or you could use impedance grounding and GFD scheme.

As to your second question if all your loads are 3-phase, there is no need to derive a neutral circuit, in fact you wouldn?t want to as it would only add a point of failure in the event of a P-G fault.
 
The 120V L-N loads are derived form a 3 phase isolation transformer (delta to Y) that is feed from the 3 phase generator that has the high impedance grounded neutral. The line side of the 3 phase transformer is grounded at the netural (X0) point.
 
You may be doing something that is simply not anticipated by the NEC, and which in my opinion is a code violation because of a missing 'Exception'.

The NEC generally requires systems to be grounded. The NEC permits ungrounded and impedance grounded systems with ground fault indication, pretty much for use in situations where you don't want a ground fault to shut the system down. In such situations, during a ground fault the neutral is raised to the line-neutral voltage, something that really is not appropriate given how the neutral conductor is used and protected. For example, during normal operation a ground-neutral fault would go unnoticed. But in the event of a phase to ground fault you would suddenly have high current on your ersatz ground-neutral bond, with no OCPD on that conductor.

What you are doing is something different. You are using a high impedance grounded system to limit fault current, but shutting down the circuit(s) immediately in the event of a ground fault. Depending upon how you are detecting the ground fault, a ground-neutral fault in the system would shut things down as quickly as a phase-ground fault. IMHO this is safer than any of the three approaches outlined in the NEC, and should be permitted. I have used exactly this system in a non-NEC covered experimental setup.

That said, it is not clear to me that you actually have a violation. You say:
Our current configuration has a 480VAC generator feeding 6 isolation transformers that control various functions of a crane (travel, hoist, rotate, etc.). One of the transformers feeds lighting circuit and computers where the neutral of the load side is grounded (y connected) and provides the return path neutral for the circuit.
Click to expand...

An isolation transformer is a separately derived system, meaning that there is no circuit between the primary and secondary side. The neutral on the secondary side of the isolation transformer is _not_ a neutral as far as the primary feeder is concerned, and a line-neutral load on the secondary is a line-line load on the primary side. This means 2 things: 1) line-neutral loads on the secondary side don't cause a violation of 250.36 on the primary side and 2) your high impedance grounding system won't provide any protection at all, and won't reduce fault currents on the secondary side. A ground fault on the secondary side will be seen as a line-line load on the primary side.

-Jon
 
mreeves007 said:
The 120V L-N loads are derived form a 3 phase isolation transformer (delta to Y) that is feed from the 3 phase generator that has the high impedance grounded neutral. The line side of the 3 phase transformer is grounded at the netural (X0) point.
Click to expand...

What is the 'line' side of the transformer. Is it the primary or secondary side?

If you have a delta primary with a center tapped phase, and you ground that center tap, then you have introduced a ground fault.

If you have a wye secondary and you ground the neutral, then you have properly and _solidly_ bonded the secondary. This secondary could serve line-neutral loads.

-Jon
 
No Neutral Loads

No Neutral Loads

You cannot have line-neutral loads on *ANY* high impedance grounded system - transformer-based or generator based. Any neutral current on those systems is a ground fault - resulting in a current flow through the resistance to the trans or gen neutral.
If , indeed you do have iso transformers, then there is not any problem as the seondary of the iso's are seperate systems from the primary.
I put in high-R systems all the time (1-amp on 480 vac, 10 amp at 12,470) & have to convince the POCO they cannot have line-ground utility transformers in their subs on a regular basis. That is why we typically build our own subs.
 
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I believe I understand. Since the L-N is served on the secondary side of the Isolation transformer (Y connected), it is now independent of the generator. The generator still meets 250.36 because it does not serve the L-N loads due to the isolation transformer (retains indepedance).

NOTE: The generator is connected to the primary side (delta connected) and the loads are connected to the secondary side (y connected).


Now, for the other isolation transformers. The generator feeds several 480V to 480V Isolation transformers. The secondary side (Y connected) feeds several loads. The secondary side is ISOLATED from the generator (feeds primary side). Is the secondary XO requried to be grounded or can it remain ungrounded. NO neutral loads are served on the secondary and no ground fault circuits are used.


Thanks
 
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