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Anonymous Poster #1

Three Parallel Connected Alternators Have one NGR

04/21/2015 9:54 PM

Dear expert,

We have 6 Generators (15KV, 17.9MW, 50HZ) which are star connected. Out of 6 three alternator are running in parallel with common busbar and neutral point of every alternator is going to Neutral grounding busbar where we have load break switch. During parallel connection of alternators; Out of three load break switch only one is connecting with neutral busbar which is further connected with NGR.

I have problem with NGR, during operation only one alternator make connection with NGR are the rest two alternator's neutral load break switch remain open. Out three if any one run 1st it's Neutral connecting with NGR via load break switch.

Please advise if it is good technique to have only one alternator connected with NGR in Parallel operation? If so then would you please explain, how it protect the rest alternators in case of Earth fault?

Actually we already face a problem of alternator, which caught fire it seems that it was the earth fault, but alternator tripped on diffrential protection instead on Earth fault.

Thanks in Advance for your help.
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jack of all trades
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#1

Re: 3 prallel connected alternator have one NGR

04/21/2015 10:19 PM

Can you provide a diagram of your circuit, based on what you have said it sounds like something is fundamentally wrong or missing with your existing design.

Have you discussed this with the consultants that provided and/or approved your (not small at all) power station design?

Actually we already face a problem of alternator, which caught fire it seems that it was the earth fault, but alternator tripped on diffrential protection instead on Earth fault.

Is a free online forum really your best bet for finding an answer. We are talking about a power station here aren't we? What do the other site Engineers and senior plant operators have to say on the matter?

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lyn
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#2
In reply to #1

Re: 3 prallel connected alternator have one NGR

04/22/2015 1:18 AM

"something is fundamentally wrong or missing with your existing design."

Or, with your existing personnel.
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Muhammad Imran
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#3
In reply to #1

Re: 3 prallel connected alternator have one NGR

04/22/2015 1:45 AM

Dear Jack of All trades,

Thank you so much for your quick response.

Here you will find the SLD. If Generator 1 is the first generator connected to MV bus bar then Another generator's neutral earthing switch (LBS) on MV bus bar will switched off.
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RAMConsult
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#4

Re: 3 prallel connected alternator have one NGR

04/22/2015 10:27 AM

You do not have a problem with the NGR, what's missing is understanding the connections to the NGR. For simplicity's sake we'll only consider a SLG (Single Line to Ground) fault. The current path is from the generator terminal through the bus, fault, ground, the NGR, then back into the machine through the bonding point and machine neutral. So the impedance of the path is the machine's reactance plus the NGR's.

Similarly if all three machines had their neutrals tied together then each of them would contribute to the fault because there would be a path from each machine's terminal back to the neutral, and the impedance would be the parallel combination of the three machine's reactances plus the NGR.

But if we don't attach the remaining machines' neutrals to the NGR, there is no path for those machines to provide any current, and the fault current can't get higher than what the single grounded machine can provide. It wouldn't matter if you had ten machines or more connected to the same bus, you still have the same manageable fault levels.

Regarding your earth fault, without more information I would say that it was a race between the two relays, so check the sensitivity and pickup times to verify if everything operated as intended. Usually differential is instantaneous, while earth fault has some minimum level before actuation. Please let us know what you find.

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#5
In reply to #4

Re: 3 prallel connected alternator have one NGR

04/22/2015 10:52 AM

Dear Mr. RAMConsult,

Thank you very much for your quick and qualified help.

Regarding the earth fault I will get back to you soon.
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Wal
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#6
In reply to #5

Re: 3 prallel connected alternator have one NGR

04/24/2015 2:30 AM

Stop posting anonymously. Please.

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iqbalkhan
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#7
In reply to #4

Re: 3 prallel connected alternator have one NGR

04/24/2015 6:04 AM

If only one machine is connected with NGR, AND IF GROUND FAULT happens on another machine whose neutral is not grounded then what will happen.

In case of fault earth fault current will flow or not and how the earth fault will be detected.
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RAMConsult
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#10
In reply to #7

Re: 3 prallel connected alternator have one NGR

04/24/2015 10:11 AM

That would be a second contigency fault and would be instantly detected by the differential protection. That is why professionally designed, engineered, and operated power stations have redundant and separately instrumented and controlled protective relaying systems.

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1capybara
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#8

Re: Three Parallel Connected Alternators Have one NGR

04/24/2015 9:27 AM

altho I cant read you diagram, its blurry when enlarged, switched neutrals is a NEC code violation( im not sure about BS 7671) unless its a 4-pole switch which opens the phases when the neutral is open

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RAMConsult
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#9
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Re: Three Parallel Connected Alternators Have one NGR

04/24/2015 10:01 AM

Please provide a specific NEC code reference for your statement.

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1capybara
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#11

Re: Three Parallel Connected Alternators Have one NGR

04/25/2015 2:43 AM

NEC 2011, section 240.22

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RAMConsult
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#13
In reply to #11

Re: Three Parallel Connected Alternators Have one NGR

04/25/2015 5:23 PM

Donkey,

I suggest that you reread your old copy of the NEC, paying particular attention to:

"...Article 90, Section 90.2 Scope, (B.) Not Covered,...(5) Installations under the exclusive control of an electric utility where such installations a. Consist of service drops or service laterals, and associated metering, or b. Are on property owned or leased by the electric utility for the purpose of communications, metering, generation, control, transformation, transmission, or distribution of electric energy or c. Are located on legally established easements or rights-of-way..." (emphasis mine).

Electric Power Engineering is not an exercise in handbook engineering, it requires highly specialized advanced training and techniques that are exclusive to utility installations. This special knowledge is codified in a standards library formerly known as the IEEE Color Books, now renamed IEEE- 3000 Standards Collection; wherein you will find a section on Neutral Grounding in Generating Stations and other locations which is different than the NEC.

RAM

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1capybara
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#12

Re: Three Parallel Connected Alternators Have one NGR

04/25/2015 2:47 AM

http://www.ecmag.com/section/codes-standards/switching-neutral-ok

lists several other code sections which say this

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#14

Re: Three Parallel Connected Alternators Have one NGR

04/25/2015 11:31 PM

RAMconsult, understood and agreed.

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