Help to demystify neutral grounding on large motors 2

[cuky2000]

It is recognized that grounding the neutral of a motor in industrial and commercial applications is not advisable. However, the situation appears to be different for a large system where stability is a concern.
Although large generators are usually resistance grounded there is no clear guidance about how neutral grounding is applicable to large motors.

Can anyone elaborate on this subject?

 

[ScottyUK]

Where does the quote originate? And in what context?

 

[edison123]

I have done multiple MV motors rewinding from 100 KW to 10,000 KW. Never seen the motor neutral grounded in any of them. Mostly the neutral - with 3 leads with donut CT's and then terminated to a bushing - is brought out for differential protection of the winding.

Muthu

http://www.edison.co.in

 

[electricpete]

I agree, I've never seen or heard of neutral grounding of motors (my experience is limited to grid-connected applications in power plant or industrial plant environment).

There was a thread on similar subject before
thread237-430539



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(2B)+(2B)' ?

 

[dpc]

Never seen separate neutral grounding for a motor. If the sources are properly resistance-grounded, I don't see any reason the motor needs separate grounding.

 

[cuky2000]

Hi ScottyUK,

Enclosed is the requested reference and additional illustrations on motor neutral grounding found in the net.

I hope you or someone else bring some light to this subject.

>>>>>>>>>>

 

[xnuke]

The graphics you're showing are for academic exercise only. They are not how power systems are typically designed. If I remember right, the first is used in a chapter teaching per-unit analysis, the second is used in a chapter teaching asymmetrical fault analysis using Thevenin-equivalent networks. Even though I used those diagrams while teaching because they were in the book(s) I used, I always pointed out the issues in them to my class when we went over those problems. I cringe to think about how many professors don't know there are significant problems in the diagrams and use them as-is. Just because a textbook shows something it doesn't mean it is correct. Where did you find these?

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ScottyUK]

I'll be honest - I've seen some really big motors but nothing in the 100MVA power range.

The only reason I can see for grounding it is because the transformer winding is delta, but I could flip that logic around and say the transformer is a delta to suit the motor. At that size the motor will have many similarities with a generator - it would pretty much have to be a synchronous machine - and earthing the star point makes sense to protect the stator core. I can't help with the reasons why you'd choose a reactor over a resistor (or 1-phase transformer with a resistor burden on the secondary). As xnuke says, that looks like an academic exercise - any motor of that size would be noteworthy.

 

[cuky2000]

To separate this issue from an academic exercise, below is an excerpt from the IEEE Press regarding a classical stability publication on "Effect of Grounding on Stability". The author, Mr. Kimbark is a worldwide subject matter expert in this area.

Should be noted that system stability is not a major concern in most industrial application and even for the bulk electrical system, there are several options more popular than ground the neutral of large rotating machines.

Fill free to challenge the merit of this publication with technical arguments if there is no benefit of increasing system stability following the advice of grounding the neutral using resistance at the generation end and reactance at the motor end?.

>>>>>>

 

[edison123]

I think the author himself had pretty much made the arguments in his last two lines.

You asked a question about MV motors neutral grounding and it has been answered by people who have actually seen and handled those MV motors in real life.

Muthu

http://www.edison.co.in

 

[dpc]

I don't see anything in the Kimbark excerpt about neutral grounding of motors.

 

[prc]

1) When we consider text book Vs actual practice, let me point out some thing related to transformers. In text books transformer is always shown as a two limb core with primary winding on one limb and secondary on other limb. Generations of students blindly believe this. We know that in actual practice this will not work. We will get voltage on secondary but current cannot be delivered as due to lack of coupling (ie high leakage impedance) secondary voltage will drop when we start loading. In real transformers primary and secondary are always concentrically placed to get mutual magnetic coupling.

2) In pumped storage hydro plants, generator is running as motor during pumping mode. Do we change the neutral grounding during such operation?

 

[cuky2000]

dpc, the reference of motor grounding is on page 241.
It is recognized that motor neutral grounding is not a common practice.
What Is not know if is technically feasible to run a motor with the neutral grounded and if there are any advantages from the stability point of view.

 

[cuky2000]

prc,

Below is a link for generator/motor operation In typical pumped storage hydro plants.
The schemes 1 to 6 in this publication maintains the neutral grounding in generator and motor mode as well

https://prorelay.tamu.edu/wp-conten...lated-to-Pumped-Storage-Hydro-PSH-Units-1.pdf

 

[dpc]

If the motor is designed to be wye connected, I don't see any issues with a neutral grounding resistor. I just don't see a purpose.

 

[dpc]

And the leads are brought out.

 

[prc]

Thank you Cucky for the info !

 

[waross]

Considering the motor alone:
Grounded, allows graded insulation.
Ungrounded, possible greater core damage during an internal fault.

Considering the system, in the event of a single line to ground fault far out on the system with the motor also far out on the same circuit.
With the motor ungrounded, the far out voltage to ground of the unfaulted phases will approach the line to line voltage.
With the motor neutral grounded the motor contribution to line to ground fault currents will be greater.


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

 

[cuky2000]

Thanks all for your help associated with neutral grounding for motor application.
Below is a historical background associated with motor neutral grounding recommendations in a response of an incident that burned out many motors in an industrial plant in the West Coast back in the early '50s.

 

[fouadf]

For very high leakage impedance I think even the voltage will not be delivered at secondary side