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
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
RE: Generator High Res. Grounding
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If we learn from our mistakes I'm getting a great education!
RE: Generator High Res. Grounding
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
RE: Generator High Res. Grounding
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
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
RE: Generator High Res. Grounding
RE: Generator High Res. Grounding
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
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
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."
So this will happen because of overvolatge ? not because small 50 A ground fault current ?
Thanks
RE: Generator High Res. Grounding
RE: Generator High Res. Grounding
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RE: Generator High Res. Grounding
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 LESS then 30A , to avoid damage in stator core
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ITEM 5.2.3 CORE DAMAGE CAUSE BY GROUND FAULT
RE: Generator High Res. Grounding
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
RE: Generator High Res. Grounding
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
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If we learn from our mistakes I'm getting a great education!
RE: Generator High Res. Grounding
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
RE: Generator High Res. Grounding
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
RE: Generator High Res. Grounding
RE: Generator High Res. Grounding
ground differential protection 87N is only effective for generator grounding with midlle impedance(100-600A) or low impedance.
RE: Generator High Res. Grounding
RE: Generator High Res. Grounding
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
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
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
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
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
Kindly eloabroate on alarm functionality during a ground fault
RE: Generator High Res. Grounding
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
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
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
Some companies don't trip there units until replacment power can be arranged.
RE: Generator High Res. Grounding
not 64G-95%.