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Generator High Res. Grounding

Generator High Res. Grounding

Generator High Res. Grounding

(OP)
Hi,

IEEE C62.92.2 part 2 for neutral grounding talks about using a disttribution transformer in the neutral for high resistance grounding of generator.

Recently, I saw an application of a 3 MVA 4.16 kV Generator where there was a NGR directly connected to the generator neutral rated at 15 A. I was wondering, what are the pros and cons of either methods. I would appreciate a word.

Do we ever use a Low resistance grounding on a medium voltage generator ?

Thanks

RE: Generator High Res. Grounding

A single phase distribution transformer is used to allow low resistance resistor to create a high resistance ground on the generator neutral.  The transformer primary is connected in series between the generator neutral and ground. A resistor is connected to the secondary.  The resistance on the secondary is reflected through to the primary by the square of the transformer turns ratio. This method also helps harmonic mitigation. As for the pros and cons; IEEE green book addresses these very well.

RE: Generator High Res. Grounding

Low impedance earthing is frequently found when the generator is connected directly to an industrial network (as distinct from a public utility) when the generator voltage is the same as the distribution voltage. It saves a transformer, at the risk of writing off the stator core in the event of a stator earth fault. The accountants think this is a good idea because it saves on first cost. Most rotating machines engineers would prefer a high impedance earth and a generator transformer to protect the machine.
  

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

RE: Generator High Res. Grounding

(OP)
Thanks all.  while I am writing this post, I have probably all papers ever published in IEEE on low and high resistance grounding on my table.  here is the dilemna,
I  have a  69 kV/4.16 kV transformer rated at 10 MVA and grounded with 25 A NGR.

If I ask you this question, Is it a low resistance or High resistance grounded system. IEEE says all the systems with NGR rated between 100-400 A are LRG and between 2-10 A are HRG.

What do I say about a 25 A ? I have made sure that it is higher than charging current.
I would appreciate a word. Thanks

 

RE: Generator High Res. Grounding

Do you alarm or trip on ground fault? You typically alarm in HRG systems and trip in LRG systems. If you alarm, you need to be certain all your system phase to ground insulation levels, especially underground cable, are rated for the system line to line voltage.
 

RE: Generator High Res. Grounding

(OP)

we will trip.  Cables at 4.16 kV are by default rated for line to line voltage. What other issue i should be concerned about.

So you would call it a LRG system or a HRG system ?

Thanks

RE: Generator High Res. Grounding

what trosepe said.

Afterall high and low are relative terms. Key is to trip when GF fault current is not sustainable and alarm only if the system is designed to withstand permitted GF fault current indefinitely.

Rafiq Bulsara
http://www.srengineersct.com

RE: Generator High Res. Grounding

I don't understand what you mean by "by default the cables are rated for line to line voltage". Cables rated 5KV, 100% insulation should only be used on solidly grounded systems. Cables with 133%insulation or 8KV rated are needed for resistance grounded systems.  Your system (15 amp ground current)is more in the range of HRG systems. What is the max time rating of your resistor and how fast do you trip on ground fault?















 

RE: Generator High Res. Grounding

(OP)
Thanks trospe, Cables at 4.16 kV are unshielded and rated for 100% insulation. Does 100% and 133 % apply to unshielded cables as well ?

 

RE: Generator High Res. Grounding

No it doesn't; only shielded cable.  Sorry for making the assumption of shielded cables.

RE: Generator High Res. Grounding


1- By Westinghouse - transmission and distribution handbook, a system not effective grounding should have grounding by above 50 ohms at neutral generator primary. So, 15A or 25A are high impedance grounding systems.
2- We usually use medium -resistance(100-600A) for generator less then 40MA. It is good for protection sensibility that should trip fast.
3-We use high impedance for generator more then 40MVA.It is good to protect against damage to stator core. There is no security for fast protection , so neutral current should be more then 30A , to avoid damage in stator core.
In test results showing effects of arc burning on stator core and laminations during earth faults in generator :
Less then   15A  - negligible arc burning
15-30 A – slight burning
40A and above – severe  burning.
 

RE: Generator High Res. Grounding

(OP)

Please dont mind, if my question is stupid.

I have heard this terminology stator core damage so many times. A fair size  generator on 24 Kv carries 20,000 A full load current, so what damage 200 A ground fault current can do to a stator.  

And if it does, I am sure I am missing some thing big time. I know that a ground fault would depending upon LRG or HRG system would bring over voltage during fault. So, if stator winding insulation is rated for certain 1.5 P.u., so we are ok. So why all this fuss is all about.

what is about this small small ground fault current that can do so damage to a stator which is capable of handling
so high full load current.

Thanks

RE: Generator High Res. Grounding

Ground faults start off doing damage to the winding insulation.  If the current is kept low enough, that's all that will happen.  If the ground fault current is high enough, and lasts long enough, you start burning both copper and iron.  Once you start burning iron you have a partial to complete restack to perform.  If you can limit the damage to the insulation you can often minimize the necessary repair work.  Burning copper pretty much guarantees at least a partial rewind if not a complete rewind.  Rewinds are vastly less expensive than restacks.  

RE: Generator High Res. Grounding

(OP)
Thanks davidbeach

 "Ground faults start off doing damage to the winding insulation.  If the current is kept low enough, that's all that will happen."

So this will happen because of overvolatge ? not because small 50 A ground fault current ?
Thanks
 

RE: Generator High Res. Grounding

No ground fault current unless there is an insulation failure.  Insulation failures can come from many causes; overvoltage is one of those causes.

RE: Generator High Res. Grounding

(OP)
Thanks Odlanor.

Is it also a standard practice to shut down the generator excitation during a ground fault as well, if it is a stator grounding fault ?

Why I thought so because if gen. excitation is still on it will keep feeding the fault. Is this correct.

RE: Generator High Res. Grounding

Yes , this is the recomendation of IEEE-Std C37.102-1995 see fig.6-1a (59GN-50/51GN)

RE: Generator High Res. Grounding

Hi sobeys81.
You must shut down exicitation in case of stator ground fault. For my pinion, isn't recommendation, it's MUST.

Best Regards.
Slava

 

RE: Generator High Res. Grounding

I agree with Slava, a stator earth fault should trip the main breaker and the field breaker.
  

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

RE: Generator High Res. Grounding

(OP)

Thanks all again.
This is a again a question of how it is done in traditional practice.

If a breaker has a differential protection 87,  then having a low resistance or a high resistance groundded system make a difference in terms of stator damage during an earth fault ?

Would it make a difference, if the generator is high resistance grounded directlt through NGR or with a distribution transformer ?

RE: Generator High Res. Grounding

How much resistance in the neutral-ground path has a significant affect on the amount of damage during a stator ground fault.  For a given amount of ground current, it doesn't matter whether the resister is directly in the neutral-ground connection or if it is on the secondary of a distribution transformer.

RE: Generator High Res. Grounding

(OP)
Thanks Davidbeach,

I did not phrase my question properly. What Iwas trying to get to was that, if I have a differential protection on generator and if a ground fault occurs with in generator winding  than it will shut down the system right away, so neutral grounding resistor does not play any role anyways.

Is the above statement correct ?

Next point, so that means, NGR current detection philosophy
based on high and low value current only plays a role in transient over voltage concept. So, if a generator does not have a differential protection in place, NGR being low and high resistance will be more significant issue.

Is this correct as well ?

RE: Generator High Res. Grounding

Differential just means that you can detect the fault faster.  Fault currents, damage, and other fault related phenomena are not affected by the type of protection.  The rotor speed does not go to zero when the prime mover is tripped, the field current does not go to zero when the field breaker is tripped, you have current into the stator ground fault all the way down to rotor zero speed and that current is limited by the NGR throughout the entire process.

RE: Generator High Res. Grounding

(OP)
thanks davibeach. It helped.

RE: Generator High Res. Grounding

phase differential relay -87 only is only effective for phase-to-ground fault  , for generator grounding with low impedance and probabilly covers partial winding.

ground differential protection 87N is only effective for generator grounding with midlle impedance(100-600A) or low impedance.
 

RE: Generator High Res. Grounding

For high impedance generators, protection of near neutral faults can be achieved by the use of a under third-harmonic voltage detection relay. But it needs to be guarded to shut off before plant shut down.

RE: Generator High Res. Grounding

(OP)
Thanks all. Cranky108, I would appreciate a little more explanation on your statement regarding 27 N3 function and why it should be shut down.

After probably reading everything on the planet on stator ground fault issue, I came across certain basic rules and I will share them with you. Please let me know, if I missed any thing.

Based on my situation, I have a med. vol. 3 MVA generators operating in parallel which High res. grounded for 15 A each. They all are connected to a common bus so that means and they have four med. voltage feeders exiting out of swgr feeding 4160/480 V transformer(HRG -10 Am as well).

Now I cannot have just a resistor in the neutral rated at 4.16 kV with rest of the generators operating in parallel because this will lead to a difficult selective relaying between feeder ground fault protection and NGR relay.

So I will have to either use a PT in Neutral or a distribution transformer  with a sec. resistor and then have a 59 N attached to it. This arrangements will be applicable to all three gen. in parallel connected to common bus.
59 N may not operate due to a ground fault near generator neutral so we need 27 N3.

I have gone this far till now in this parallel operation story. Some body also told me some information avilable in blackburn regarding 67 application and a zero seq. CT requirement between generator and bus in all breakers.

I have yet to understand what is that one good for.

Anyways you guys have helped all along. I would appreciate a word on this one.

Thanks



 

RE: Generator High Res. Grounding

Hi Sobeys81.
Please attach SLD of your object and try help to you.

Yep, maybe we will recommend use 67N ( directional earth/ground fault) in case of several generators in parallel. And of course with dedicated zero. seq. CT and broken delta VT connection.

Good Luck.
Slava
 

RE: Generator High Res. Grounding

sobeys81,
1- It is a bad idea to use 3nd harmonic relays in generator of 3MVA!. That generator do not have enough 3nd harmonic in overvoltage neutral with sufficient sensibility for this protection. European protection books (Warrington  and Ungrad) suggest to use this relay for  50MVA generator and above.
 2- I guess the best solution would be provide by all generator with neutral ungrounded ; generator busbar should be grounded by a zig-zag or Ynd transformer, with low resistance at neutral of transformer (400A).
50N should be installed on each CT neutral of line terminals and 51N should be installed at neutral of grounding transformer.
see  IEEE Std C37.102-1995 item 4.3.3.4 or pag.46.
 

RE: Generator High Res. Grounding

(OP)

I was away gentleman. But I have some more dilemans to get over with :

There is so much concern about stator ground fault protection during a ground fault.

When we issue a Alarm on a gen ground fault, what is the next course of action  by the operator.

I am sure a lot of you see this everday, Could you kindly explain the process. For the time being gen. is seeing a ground fault, Is it not causing continous damage.

RE: Generator High Res. Grounding

Simply the third harmonic under voltage must be guarded, because it will pickup during the generation off line condition (No generation, no 3rd harmonic).

As stated above this may not apply to smaller generation units (I don't know I haven't worked with anything that small).

To guard the 3rd harmonic relay use either a 59 relay, on the main PT's, or you can use a 52a on the main breaker.

Usually the setting for the 3rd harmonic is by test, and measurment, as each unit is different.

Over all the protection should be based on the value of what you are protecting (thousands or millions of $).

RE: Generator High Res. Grounding

(OP)
Appreciate the response cranky,

Kindly eloabroate on alarm functionality during a ground fault

RE: Generator High Res. Grounding

A ground fault on a high impedance grounded unit, dosen't involve much current, so the damage may not be very much. The risk is that a second ground fault on another phase could cause a great deal of current.

If a ground fault is detected, and a unit must run until a time as it is not needed, there is an option to run the unit with the risk.
This is an economic trade off that must be planned in advanced.
However, it is rare that anyone outside the protection people will have any understanding of this risk or trade off.

Usually power plant people see power going out as money, and down time as lost money. Not runing a unit they see as a problem. They don't see the risk because they don't usually understand the electrical system.

I've tried to explain this risk to power plant people, and have only gotten the speech about down time is money lost.

RE: Generator High Res. Grounding

Hi sobeys81.
what crany108 saied, is true.
But, for my pinion, we don't need explain something to someone.
stator ground fault----TRIP, it's all ( OK for big generators with 100% injection principe protection it's something other, you have alarm stage). But for this small... 3MVA, is usually 90-95% stator ground fault protection, w/o alarm stage.
Use 59N or 51N or 67N, don't think about alarms, think about generator life.

Just my opinion.
Good Luck.
Slava

RE: Generator High Res. Grounding

(OP)
Thanks Gentleman,

So here is what I understood :

Generator will be allowed to operate with a ground fault which even might lead to catching fire. I had an impression that to run a HRG system the requirements are that there should be a electrician to detect and isolate the fault.

Is this all true ?

So when these people actually let it go. Till it becomes a  double line to ground fault or some things.

So Do utilities operate on ground fault as well or not.

 

RE: Generator High Res. Grounding

Good question. No we trip our smaller units for ground faults.

Some companies don't trip there units until replacment power can be arranged.

RE: Generator High Res. Grounding

Everything a told about trip refers to 64G-100% ,
not 64G-95%.
 

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