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Grounding Generators and Wye-Wye Transformers

Grounding Generators and Wye-Wye Transformers

Grounding Generators and Wye-Wye Transformers

(OP)
I have a couple of questions about an existing system that I have encountered:

System consists of qty (3) 480VAC, 1125 KVA, continuous rated generators.  Each generator feeds a piece of paralleling switchgear at the 480VAC level, which then feeds individual 480V:12.47kV wye-wye step-up transformers.  This then feeds the common power distribution network at the 12.47kV level.  Note:  There is no common bus at the 480V level; the common point is at the 12.47kV level.  Voltage is then dropped to 480V at various points in the facility using 12.47kV:480V wye-wye transformers.

My questions center around the grounding of this system and more specifically at the generators.  The case of the generator is grounded (via a 4/0 wire) to a common ground ring.  The neutral (100%) of the generator is brought to the neutral bus bar of the paralleling switchgear.  The neutral bus bar of the paralleling switchgear is grounded (via a 4/0 wire) to the common ground ring through a current transformer (ground fault monitoring).  The neutral was not brought out to the wye-wye step-up transformer.  The star points on the wye-wye step-up transformers are grounded (via a 4/0 wire) to the common ground ring.

1)  Should the neutral have been brought out to the star point of the wye-wye transformer – i.e. directly connected?

2)  Is the only reason that this system works, due to the indirect connection between the paralleling switchgear neutral bus bar and the star point on the wye-wye transformer via the common ground ring?  I am thinking that the zero-source current needs a path back to its source.

3)  If the paralleling switchgear neutral bus bar and star point on the wye-wye transformer were not tied together via the common ground ring, would the entire power distribution system be ungrounded?  I am thinking that the zero-source current would only have the earth to travel through (once it got back to the step-up transformers) to get back to the star point on the generator.

4)  Is it ok to have both the direct connection between the generator and the wye-wye transformer via the neutral and the indirect connection via the ground ring?  Or does this cause any issues?

At the end of the day, we are adding additional generator capacity to this system.  I would like to ensure that it is done correctly and not necessarily match the existing setup.
Thanks in advance.
 

RE: Grounding Generators and Wye-Wye Transformers

The generator neutral should be connected to the transformer wye point to carry unbalanced current. Running the neutral current on the ground connections is bad practice and a code violation in many places.

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

RE: Grounding Generators and Wye-Wye Transformers

I agree with Bill - a neutral conductor is needed from the transformer neutral to the generator neutral.  

Without a one-line, it is hard to be more specific.  Care must be taken with any ground fault relaying to make sure it is really seeing all of the ground current.  If the neutral is tied to ground in multiple place, this can cause problems and may likely be a Code violation.  

 

David Castor
www.cvoes.com

RE: Grounding Generators and Wye-Wye Transformers

There are two things to discriminate:
1- From the viewpoint of short circuit your 480V-distribution system is effectivelly grounded  with path: generator grounding grid + EARTH + transformer grounding grid.
2- From the viewpoint of load your distribution system will have problem of neutral circulating current if you distribution system is a three-phase unbalanced system.
 

RE: Grounding Generators and Wye-Wye Transformers

Bld 5,

I agree a single line would be useful indeed essential if your question is going to be answered.
In the UK for private embedded generators in parallel with the national grid the generator star points would be disconnected after synchronisation and the neutral carried by the duty transformer. When running islanded one neutral breaker for each group of generators would close.
Again a one line would be useful

RE: Grounding Generators and Wye-Wye Transformers

bld5
 There is no common bus at the 480V level;...
I understood there is no parallel between 480V generators.Each generator is connected to step pup transformer.Step up transformer are connected to common 12.47kV busbar.
I guess this kind of configuration -generator system effectivelly grounded - is not applied to 4MVA generator and up or with voltage more than 480V.

 

RE: Grounding Generators and Wye-Wye Transformers

Ia this a grid tie system or are there times when it may be islanded?
I would
1> Ground everything in sight. Transformer enclosure, switch enclosures, Panel frames, generator chassis and conductive raceways.
2> Run the generator neutral to the transformer neutral in the same raceway as the phase conductors.
3> Connect the neutral conductor to the ground grid in one place only. Put your ground current CT on this connection/jumper.

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

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