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GENTLEMAN
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Join Date: Aug 2011
Posts: 6

Why Is Isolated Neutral Preferred In Some Turbines/Generators?

08/21/2011 5:02 AM

Dear engineers,

I need your help in analyzing this query:

Our plant consists of 11x 635 KW, 415V Waukesha gensets, 2 x 3.5MW,11KV wartsila gensets, 11 KV main bus for synchronization & 10 MW,11KV, 50 Hz gas turbine. The neutral of gas turbine is earthed against the manufacturers recommendations.

Now, a 2.7MV, 11KV, 50 Hz steam Turbine is being added to the system whose neutral is recommended to be earthed. but before taking this action the Neutral of existing Gas Turbine is to be un-earthed.

So, Under which conditions the Neutral of a generator should be earthed & what are the conditions when "isolated neutral" is recommended?

THANKING IN ADVANCE :)
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Pathfinder Tags: GENERATOR STAR CONNECTION NEUTRAL EARTHING
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GENTLEMAN
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Join Date: Aug 2011
Posts: 6
#1

Re: WHY IS ISOLATED NEUTRAL PREFERRED IN SOME TURBINES/GENERATORS??

08/21/2011 5:33 AM

I am not asking from manufacturer's point of view but I am simply asking from synchronization point of view. Any questions will be welcomed!
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electricalexpert65
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#2

Re: WHY IS ISOLATED NEUTRAL PREFERRED IN SOME TURBINES/GENERATORS??

08/21/2011 8:22 AM

Do you mean that you are synchronising 11 x635kW (415V), 2 x 35 MW (11kV) 1 x 10MW (11kV) and 1 x 2.7MW (11kV) generators? I am surprised.

Assuming that you are synchonising only the 11kV generators, then it is but natural to keep the highest capacity generator's neutral earthed (in your case, it is the 10MW gas turbine Generator) amnd to keep all other generators neutrals floating (unearthed). This is to avoid the circulation of 'triplen harmonic ' currents between the synchronised generators (which would happen if the neutrals are also 'synchronised'). As the generators are of different prime movers, so also will be the makes & vintages and winding pich factors. This would lead to the circulation of trilen harmonic currents between the synchronised generators, when their neutrals are also tied together.

If all the synchronised generators are wound to 2/3rd Pitch, then their neutrals can be tied together (vide the IEEE Green Book).
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GENTLEMAN
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Join Date: Aug 2011
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#3
In reply to #2

Re: WHY IS ISOLATED NEUTRAL PREFERRED IN SOME TURBINES/GENERATORS??

08/21/2011 8:53 AM

Dear expert,

Thank you very much for trouble shooting which has cleared the picture to a much extent. one thing which is still irritating me is that why the alternator of 2.7 MW steam turbine is being earthed instead of the 10 MW gas turbine alternator!
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electricalexpert65
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#4
In reply to #3

Re: WHY IS ISOLATED NEUTRAL PREFERRED IN SOME TURBINES/GENERATORS??

08/21/2011 9:29 AM

But you have mentioned that the 10MW Neutral is earthed and the 2.7MW neutral is isolated.
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GENTLEMAN
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Join Date: Aug 2011
Posts: 6
#5
In reply to #4

Re: WHY IS ISOLATED NEUTRAL PREFERRED IN SOME TURBINES/GENERATORS??

08/21/2011 10:47 AM

yes, it "was" earthed. but now the neutral of 10 MW gas turbine will be isolated before the installation of 2.7 MW steam turbine.

Now, the neutral of 2.7 MW steam turbine generator will be earthed instead of 10 MW gas turbine neutral !!!
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rsbarve
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Join Date: Aug 2011
Posts: 3
#6

Re: Why Is Isolated Neutral Preferred In Some Turbines/Generators?

08/22/2011 5:03 AM

1) The practice of earthing the neutral of the largest generator may have been due to the fact that any single phase load current will pass thr'o the neutral of the highest rated generator. If the neutral of a smaller generator is earthed, it may not be capable of allowing the unbalanced load to pass thr'o its neutral.

2) The 10 MW gas turbine might be having a full pitch winding on which effect of harmonics is more and the manufacturer might have recommended not to earth its neutral.

3) The new 2.7 MW generator may have insulation based on earthed neutral which can allow lesser insulation thickness due to positively lower voltage to earth ( Phase voltage).
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GENTLEMAN
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Posts: 6
#7
In reply to #6

Re: Why Is Isolated Neutral Preferred In Some Turbines/Generators?

08/22/2011 8:37 PM

@ RSBRAVE: this may be regarded as the right analysis.. Thank u v much.. any additional hypothesis in this regard will be welcomed...
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