Parallel Generator Grounding

[Gizmo1985]

Here is the problem (NEC 2012):

Generator normally operates in parallel with the power grid. Grid is at 12.47 kV with a wye-wye transformer going down to 480V.

The customer owns the transformer.

The original design had the N & G bonded at the wye wye transformer, the 480V breaker, and at the 480V MCC. The high and low side neutrals of the wye-wye are also connected

I should note there is no Neutral load at the plant

**The problem is the high neutral current, which is about 7-800A** The attached sketch was how the unit was installed. Granted, the wye-wye xfmr may exasperate this problem, but I am thinking of the following:

Option 1
-> Disconnect all of the N-G Bonds except at the xfmr
Option 2
->Add in a low resistance ground at the generator
->Disconnect the neutral except at the transformer & generator...........removing it at the panel.


Any suggestions?

 

[waross]

How well are the primary phase to phase and phase to neutral voltages balanced?
Does this transformer have a three legged core?
A three legged core develops a "phantom" delta.
A wye:delta transformer with a four wire primary connection must have a balanced primary system. Any primary voltage unbalance (unequal line to neutral voltages) and/or phase angle errors (shown by unequal phase to phase voltages) will cause heavy neutral currents.
If that is not an issue and if the issue is only present when the generator is connected, then again look for phase to phase and/or phase to neutral unbalances. You may have a primary unbalance or a failing generator.
Other possible issues:
Dissimilar wave forms. A generator seldom produces a perfect sine wave. This causes small circulating currents and neutral currents at no load. As soon as the load current exceeds the circulating current, the circulating current may cease. The initial neutral current is replaced by offsetting ripple currents on both the generator and the grid.
Harmonics.
Do your loads generate a lot of harmonics? Harmonic currents may not cancel but may instead sum on the neutral conductor.
Given that your neutral current is almost 50% of load current, I suspect primary unbalances. These are limited by three times the transformer % imp. voltage.
Example: Transformer impedance = 3%
Zero sequence currents limited by 3% x 3 + 9%. A 4% primary voltage unbalance may cause 800 Amps on the neutral.
This is a rough first order estimate to illustrate the serious effects of voltage unbalance. There are several other effects that may reduce the neutral current such as the grid impedance and the generator impedance.

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

 

[squeeky]

You must never have two earths at the same time. This will cause a circulating earth current. If your transformer has an earth disconnect your alternator earth but leave the star point.

 

[odlanor]

Option 1-
No good! You will increase potential ground , reducing personal security

Option 2-
Good!Add in a high resistance ground (<10A) at the generator frame.

 

[ScottyUK]

Option 1 is OK if done properly - the system neutral and system earth are bonded at one place only, but local metalwork can (and should) be earthed and bonded to avoid touch and transfer potentials.

 

[dpc]

Option 1 is OK as long the generator will never supply islanded loads when the transformer is out of service.

Option 2 is out of the question. LOW resistance grounding is never acceptable at 480 V. High resistance grounding is OK, but only limits generator fault contributions, not from the transformer. But it cannot be done if you have any L-N (277 V) loads. It is also probably an NEC violation - I'd have to think about it and have more data on how you'd connect things.