100% Stator Ground Application

[Enuf2bDangerous]

I am upgrading protection on two 2.5MW diesel engine gen sets with groundng reactors. My spec required Schweitzer SEL-300G relays, and also called out ANSI 64G - 100% Stator Ground protection.

We recently met with the protection designer, and he said that a 64G was not appropriate for these generators. I read through the 300G manual, and the manual says that 64G is recommended for high impedance systems.

Who do I believe? Schweitzer's manual, or the protection engineer who by his own admission, is not overly familiar with SEL gear.

Thanks!

 

[itsmoked]

Neither.. Call Schweitzer and ask them..

"Tie Breaker"

Plus they may be able to say yes works for that but you need to do X to allow it to.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

My guess is that you can't use the 64G on that generator. Easy way to tell - do you have access to the field leads? If not, you can't use the 64G. Most gen-sets use a system with diodes on the machine so that the DC never comes out, a self excited machine, and those are not possible to provide with 64G protection as there are no DC leads to connect the 64G module to.

 

[waross]

Did you misread rotor for stator in the original post davidbeach?
If so, RF corrections and RF to delete this post.
Respectfully

 

[burnt2x]

Google SEL-300G!

Verify if you can get neutral current against residual current (CT's current sum at phases).

 

[Enuf2bDangerous]

All CT's and connections needed to provide a 64G are available. One of the A/E's reasons for not using 64G was the size of the generator: he said they were too small to use 64G.

As long as we are on this topic, he also said that 78 (out of step) was also inappropriate for this system. Thoughts?

Note: These two generators are in one of six separate emergnecy backup diesel generator buildings supplying between 5 and 10 MW per building, all feeding onto the same network over a couple mile campus.

 

[slavag]

Hello.
Several points.
David, Waross, OP told about 64G ( stator 100% ground fault) not about 64R ( rotor ground fault).
Burnt2x, you told about REF-- isn't 64S(100%).
We have now mix of terms.

Enuf2bDangerous, your protection eng. is right for this size
of generator 64S (100%) and 78 are not used. In all technical books those functions aren't recommended.( issue it's price and type of relay).

First of all for the 64G ( 100%) not 90% or 95% you must
VT connection, not CT connection. I attached document of Mr.
Mozina from Beckwith ( very good explanation about 64S(100%) functionality). Please check what is a type of old 64S protection. I assume it's something like to 51N or maybe
59G. Are you have ring CT or VT on the neutral point of generator?
Second. 2MW generator with grid, function 78 is out-of-step
protection (also possible use term power swing protection). Grid is more, more strongly then generator ( is not 200MW generator) and unpossible any power swing. But in several case of internal problem of generator possible overpower situation, but we talk about emergency back-up generators, where enough 50,51,50N,51N functionality.
Next point. 64S ( 100%) protection recommended for the high impedance grounding of generator. I assume, this generator have low-impedance grounding (yes, it's reactor, but what is a ground fault current level?).
Next. Please see again Burnt2x's recommendation and check with SEL-- REF option, you have all CT's and very easy add REF to your applycation. BTW, are you use standard 87G protection?
SEL is good relay, but for your applycation????
Regards.
Slava.
Please see attached

 

[davidbeach]

Oops, yep I miss read it, thinking rotor grounds and the separate add-on box that connects to the 300G. 100% stator ground fault is possible in that size of equipment, but probably not if it is using grounding reactors. 100% stator ground fault coverage requires a high resistance grounding scheme, and requires neutral voltage into the relay as already mentioned. No machine is too small for 100% stator ground fault coverage, but it may not be worth the cost and trouble. No mention of voltage level that I can see; for a 480V system the disadvantages of high resistance grounding (no use of the neutral) probably out weigh the limited benefits of 100% stator ground. At 12kV or above the significantly improved protection with 100% stator ground are probably worth the cost if the generator is in a critical application, particularly if it would be difficult to replace the generator. As with so many things, the answer is "it depends".

 

[burnt2x]

On "it depends" comment: follow your protection engineer's recommendation if has done short-circuit/ studies and verified the things he told you as non-issues in your case; or if you already have the SEL 300G relay at hand, choose which element fits your requirements.
Basically, you must do short-circuit and stability studies on your system and take off from the results and specify your protections. 78 (out of step protection) is needed on systems with problems on stability.
Sometimes you have to think about replacement lead times as against the generator size criteria. Your goal should be to "install and forget".
Hope this closes any question in your mind.

 

[Enuf2bDangerous]

burntx: Our meeting with the protection engineer was prior to the coordination study being performed. No data yet.

davidbeach: Our gens run at 12.47KV, although depending on routing needs, we can step up to 115KV. Also, according to the SEL-300G manual, the 64G is 'recommended' on high impedance systems, but only 'suggested' on resistance grounded systems.

Misc: I think what some of you are calling 64S is what SEL calls 64F. This requires the add-on SEL-2664 field ground module.

I corresponded with SEL today. Verdict: all generators with appropriate outputs wil benefit from incorporating 64G, especially te 3rd harmonic aspects.

 

[slavag]

No problem with your verdict.
If SEL say, we all must do it. For curios, please contact to Beckwith, GE, ABB, Areva, Siemens, NARI, may be you also will add 64G and 64R ( SEL say 64F, also no problem) with injection principle.
But you not answered on Q: What are you have on the generator neutral point VT or CT.
Some intresting document:
The writer of the book went through 6 different references (books) looking for protection recommendations for the generator. Result attached.

From other side, you bay relay with all functionality, add all function what you want, it's multi-function relay, after several unwanted trip you will disconnect those function or will change settings. You are not first and not last.

 

[ScottyUK]

Slava,

Is there any relationship between recommendations and the age of the text or the part of the world where the author is based?


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

 

[slavag]

Hello Scotty.
I don't know this book, author and age of the text.
I have only this page. In one of serious company ( training center) used this page for explain to students : what is a "recommended" protections for generator.
If you want, I can attach several other recommendations from several sources.

Enuf2bDangerous , sorry, if I say something not correct.
It's not against you. Up today I remember my first commissioning of generator-grid protection ( It was before 9 years)where PE puted all possible functions with settings according to manuals.
Regards.
Slava.
SEL, sorry also, but I have other guru in generator protection.

 

[ePOWEReng]

Part of whether the 64G protection can be used depends on the generator producing enough 3rd harmonic voltage during operating conditions. The slot configuration of the generator largely determines the 3rd harmonic voltage that is produced (2/3 pitch winding generators do not produce enough third harmonic voltage to use the 64G element).

 

[slavag]

Hold6448.
Thank you for back-up.
64G (100%) aren't detect ground faults on standstill generator.
Possible continue
Regards.
Slava

 

[ScottyUK]

Hi Slava,

I wasn't questioning the validity of the reference, I was just curious to know if thinking has changed over time, particularly with the advent of the modern integrated relays.


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

 

[davidbeach]

In the SEL-300G, the 100% stator ground fault protection (64G) can be set as a 3rd harmonic voltage differential across the generator. That leaves a 10-25% "hole" in protection in the middle of the winding, and that, along with most of the rest of the winding, is covered by the 59N function. 64G is always voltage based, and is always only applicable to high impedance (high resistance) grounded machines so that the ground fault generates sufficient voltage.

It is true that it won't detect ground faults in a non-rotating generator, but as the generator is spun up and the field applied, any ground fault will be detected prior to the generator breaker being closed.

 

[Enuf2bDangerous]

Thanks to all for the good advice.

In the case of the 64G, all I've read seems to point towards the 3rd harmonic option being a valuable tool to protect your generator, so I will be requesting the A/E either incorporate the 64G as called out in the scope of work, or provide sound engineering support as to why it is not applicable.

After all, better to know that you've got a gorund fault prior to discovering your 2nd ground fault...

 

[ePOWEReng]

Enuf2bDangerous

That sounds like a good plan. The only reason I can see that you wouldn't use the 64G is if the generator is 2/3 pitch. You should be able to find that in the generator data or by asking the generator manufacturer.

 

[slavag]

Hi.
Enuf2bDangerous , it's a good and right way. I'm happy for you.
David, I don't have any problem with 64S/64G (3rd har), I'm also sure that SEL used all new concepts of this protection.
And of course combination of 59N (59G) and 59THD ( also new term., used today in Europe) provide a good protection of whole stator winding.
As disadvanteges, possible write also advantages:
- very cheaper solution.
-no additional eq. needed.
-simple setting calculation.
Scotty. I don't have answer to your Q's. As David saied "it depends". Like to our discussion about reverse power before several day. Your post was right, for big generators (brand name) is.. for commodity generators is...
For my pinion new relay give us option build full duplicated protection system and build flexcible logic.
But if I have generator 0.9MW LV, I don't need used 78,87, REF protection or for generator 650MW I will request 64G injection principle. Or I don't need add inter-turn protection to generator with one rods for same phase or rotor overload ( current image based) for genratore 5MW, etc..
Regards.
Slava