Generator High Res. Grounding

[sobeys81]

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

 

[ScottyUK]

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


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

 

[sobeys81]

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 ?

 

[davidbeach]

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.

 

[sobeys81]

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 ?

 

[davidbeach]

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.

 

[sobeys81]

thanks davibeach. It helped.

 

[odlanor]

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.

 

[cranky108]

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.

 

[sobeys81]

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

 

[slavag]

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

 

[odlanor]

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.

 

[sobeys81]

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.

 

[cranky108]

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 $).

 

[sobeys81]

Appreciate the response cranky,

Kindly eloabroate on alarm functionality during a ground fault

 

[cranky108]

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.

 

[slavag]

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

 

[sobeys81]

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.

 

[cranky108]

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

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

 

[odlanor]

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