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WHY IS ISOLATED NEUTRAL PREFERRED IN SOME TURBINES/GENERATORS??
3

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

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

(OP)
Dear engineers,

I am in a fix to decide where should isolated neutral be preferred & where it is not recommended to ground the neutral. There is already a Gas turbine installed which is recommended to run on isolated neutral by manufacturer. but on the other hand, it is recommended to ground the neutral of steam turbine. what may be the logic behind it?

Thanking in advance :)

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

1) Where in the world are you? It makes a difference in terms of customary practice.
2) What do you mean by isolated neutral? I'm guessing that what isolated neutral means to me is not what it means to you. Others may have additional definitions.
3) A better description of the system could help. At least get out to the first delta winding in a transformer from each generator.
4) What sizes of equipment are you talking about?
5) Any thing else that might help someone who's never seen your system.  

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

(OP)
dear david,

by "isolated neutral" I mean that should neutral also be grounded besides the necessary equipment grounding?

In our (10 MW, 11KV,50Hz) gas turbine, the generator neutral is not grounded (i.e isolated neutral) but for the 2.7 MW steam turbine (11KV , 50Hz), we have to ground the neutral. what may be the logic behind it?

Our plant consists of 11 Waukesha gensets (635 KW, 415V), a gas turbine (10 MW) & upcoming steam turbine (2.7 MW). It is recommended to un-ground the neutral conductor of gas turbine before the installation of steam turbine whose neutral will be grounded!      

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

(OP)
waiting for someone's help :)

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

It is the weekend here. You aren't paying for any advice. If you were, you would be paying premium rate.

Posts like the one you just made will not make anyone want to help you. In fact instead of offering some you some advice about your question I'm writing this instead in an effort to get you to show some manners.
  

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If we learn from our mistakes I'm getting a great education!
 

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

(OP)
@ scotty: thanks for teaching manners :p lol...

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

Tell us the whole story.  Why are any of the generators left ungrounded (isolated neutral)?  Who says that you have to unground one and ground a different one?  You never said where you were, but only that it was 50Hz.

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

(OP)
The manufacturers of Gas Turbine & Steam Turbine recommend floating neutral & grounded neutral respectively. And I am from Karachi.

Why are any of the generators left ungrounded (isolated neutral)?
This is what my question is!

 

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

Seems to be an IEC thing.  Never made sense to me.

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

WHY IS ISOLATED NEUTRAL PREFERRED IN SOME TURBINES/GENERATORS??
Upside: Running ungrounded or high resistance grounded greatly reduces the damage to the iron core of the generator in the event of an internal ground fault. An internal ground fault in a grounded generator may do unrepairable arcing damage to the iron core before it is detected. Residual magnetism may cause some arc to continue until the generator is physically brought to a stop or slow speed.
Downside: With a floating neutral or high resistance grounded neutral the voltage to ground impressed on the terminal ends of the windings will rise to line to line voltage from line to neutral voltage. With a floating neutral there is the possibility of winding damage from high frequency, high voltage that may result from a discontinuous ground fault (arcing ground fault).
Solidly grounding more than one neutral may result in neutral circulating currents if the generators are not wound to the same pitch.
Grounding multiple generators is often a compromise. In a small system it may be decided to solidly ground all generators and accept some circulating current and the possibility of internal fault damage.
Resistance grounding may cause challenges with protection settings as the resistance to ground varies with the number of generators on line.
There is no one right answer when it comes to grounding generator neutrals.

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

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

Sure, there's a right answer - high resistance grounded connected to a unit transformer.

winky smile

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

It's not a British thing David - but we're not IEC: we're British! smile I agree that the best overall solution is likely to be a high resistance earthing scheme using a resistor and earthing transformer, unless there are specific factors such as existing transformers on the generator bus with their own earthing arranegements which we aren't aware of.
  

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If we learn from our mistakes I'm getting a great education!
 

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

I agree with you in principle David, but with  11 x 635 KW sets, individual unit transformers may be a significant financial compromise.
Yours
Bill

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

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

Posting a one-line diagram (or better a 3-line) schematic of the existing system and the new additions would help for experts on the forum to better be of help.

rasevskii

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

Well, I said it was the right way, I didn't say it was the most economical way, now did I? winky smile

To me, that's too many units for a single plant anyhow, should be fewer, larger, units.  If a plant of that configuration were dropped in my lap, I'd pair up the 635kW sets two per winding on 3 three winding transformers and give the other two each their own transformer.  Then high resistance ground every one.

All is well with an ungrounded machine, until it has an internal fault.  Then once the breaker opens the internal voltages can escalate significantly; changing it from a rewind to a restack.  On the other hand, who even rewinds a generator that small?

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

(OP)
MY HYPOTHESIS:

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).

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

1) Do you really have single phase 11kV loads connected to the machine terminals? I know they exist on the distribution system, especially in rural areas, but it would be very unusual to have them within a power plant or industrial facility.

2) Possible

3) Very unlikely, especially on such small machines.
  

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If we learn from our mistakes I'm getting a great education!
 

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

I'll agree with ScottyUK.  But, with proper transformer selection 2) doesn't matter.

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