Generator Grounding

[SilverArc]

Hi,
I am sure this topic must have been discussed many times here. I tried to search by using different key words but did not make it. You guys might be able to help.

The situation is, I came across a single line diagram where my job was to attach an emergency generator 2 MW,4160 V. Now, I looked at rest of the system and 4160/600 V system was high resistane grounded. I am not sure, what were the considerations. When I read these highly sophisticated discussions on this website about considerations while selecting type of grounding, It is just so exhaustive. Anyways, As per my understanding, these days, Low volatge systems are high resistance grouding systems to reduce arc flash energy. I am not sure, how they accomodate issues like when a ground fault happens in a small motor and the breaker does not trip because the system is high resistance grounded.
My question, what would should be the grounding type of this medium voltge generator feeding a transformer, a VFD and then a AC motor.
Here is what I know, Med. Voltage systems are low resistane grounded but I dont know why. low volatge systems are solidly grounded because it is made sure that there is enough ground fault current to trip low voltage breakers.
Could you please drop a word about this generator grounding.

 

[davidbeach]

Generator (at 4160V?) connected to 4160V loads and a stepdown transformer to 480V? What transformer connections?

The generator should probably be resistance grounded to provide about the same level of ground fault current as would be provided by the utility connection, that way you don't need different ground relaying while on utility or generator. Transformer is probably Dyn1 providing a complete break between the 4160V grounding system and the 480V grounding system.

 

[SilverArc]

Thanks David beach.
I guess I should have been more clear in my post.
The generator is 4160 V and connected to only one transformer 4160/850 V feeding a VFD, which is feeding an AC motor.
You mentioned that It should be grounded to maintain same ground fault level as during utility. I agree It will be better, if there is same ground fault relaying while on generator and utility.
So, I guess it can be either low resistance or high resistance grounded depending upon the utility ground fault level ?
I had an impression that there is a basic thumb rule that all medium voltage generators are low resistance grounded.
Is this correct ?
Thanks

 

[alehman]

How is the utility connected to this system?

 

[SilverArc]

Utility is connected through a 15 MVA, 69KV/4160 V Delta/Star Configuration.
I hope this helps in getting some tips for my question.
Is there any straiht answer Low resistance or High resistance ?

Thanks

 

[rbulsara]

Resistance grounding of neutral does not affect or reduce arc flash energy. In fact arc flash analysis per IEEE 1584 and/or NFPA 70 is only for 3 phase ac systems, considering 3 phase (phase to phase) arcing faults.

Merits of resitance or solid grounding were well established in IEEE Green book, long before the current arc flash energy calculaiton methods were developed.

 

[davidbeach]

There are many different considerations as to the selection of a grounding system, and it is as much art as it is science. What do you want to accomplish, what to you want to spend, and what year are you making those decisions? Practices change with time, but systems need to continue with what they have to a great extent. Ground faults that can't be detected by current can be detected by voltage, but not located. Low resistance means it is easier to detect and locate ground faults but they do more damage. High resistance limits ground fault damage while making them harder to detect or locate. What is important to your or your system?

 

[00123456]

Grounding Practice for Generator is depends on your fault level current, but normally generators are always grounded through Neutral grounding resistor or NGT through NGR. My point of view ground fault current should be limit up to full load current of machine
(Say Ifl = (2*1000)/(1.7325*4160) = 278A)

So, your grounding resistor value should be

R = 4160 / 1.7325*278 = 8.7Ohms

Now, if you are using Dyn1 TR then earth fault current on load side flow throgh secondary winding of TR & zero-sequence current will not refelcetd on Primiry side & generator termionals, but same current will flow as a phase to phase current in Primary winding. Earth fault relays on Primary side will not operate on down steram earth faults, but same will be detected by motor protection relays or stand by earth fault relays on LV winding of TR.

When the earth fault on generator terminals or Primary winding of TR then earth fault current passes thrrough NGR circuit & it will isolate breaker. This fault cuuent will be detected by earth fault relay because fault current is 278A & say CTR = 300/1A & relay settings = 10% then

relay pick up current = 300*0.1 = 30A & fault current is 278A that is almost 8 times more then relay settings which gives correct tripping during HV faults.

 

[davidbeach]

Given a generator-transformer unit connection with nothing else connected to the generator terminals and a delta winding at the transformer, I'd size the resistor to limit fault current to somewhere in the 10-20A range. Ground faults can be detected and cleared based on the voltage across the NGR. With the right relaying you can achieve 100% stator ground protection. It is not possible to achieve 100% stator ground protection if the resistor is sized to allow much more current than that.

 

[ScottyUK]

Does the generator operate in parallel with the utility? To me it has become less and less clear whether this is the case as this thread has evolved. If the generator neutral is the only point on that 4160V system where a connection is made to ground then David's suggestion is right on the money. If there is also a neutral from the utility feed then things are a little more complicated.


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Sometimes I only open my mouth to swap feet...

 

[waross]

The frame of the generator must be directly grounded for safety.
If you are connecting directly to the utility, including a neutral connection, you must use the system grounding scheme to ground the generator to avoid more than one ground point on the system.
If you are connecting directly to the utility without a neutral connection, and you use a monitored high resistance ground on the generator neutral, then as well as providing stator protection, the protection will also "see" the generator contribution to any system ground faults. I am not saying that this is good or bad, but you should be aware of it.
If you are connecting to the utility via a generator step-up unit then your grounding options are open on the generator grounding scheme.
respectfully

 

[dpc]

Let's take a step back. Medium-voltage industrial systems should be resistance grounded to reduce damage caused by ground faults. With a solidly-grounded system, a ground fault inside your generator will likely cause serious core damage, required restacking of the core. In addition, ground fault current magnitudes can exceed three-phase fault magnitudes and many generators are not braced for this.

High-resistance grounding is better than low-resistance grounding in providing this equipment protection. The only downside to hish-res grounding is that selective coordination is very difficult, if not impossible. So a ground fault anywhere on the same voltage system is going to probably require tripping the generator. With low-resistance grounding, ground fault current is high enough (50 to 1000A) to allow selective coordination of ground fault using overcurrent relays.

But you also have to consider how the other ground sources (including delta-wye transformers) are grounded. It doesn't help much to high-resistance ground your generator if it is connected to a solidly-grounded wye winding on a delta-wye transformer.

Also, if this is truly an emergency generator, it may be more important to have selective coordination of ground faults to keep the generator on than to protect the generator core.

You need to work out the system one-line, including all of the ground sources. Then someone has to decide what the priorities are.

 

[SilverArc]

Thanks Everybody for your contribution and time spent. This is my first medium voltage generator project and your contribution helped.

I have done few projects up to 2 MW, 600 V systems for Emergency generators without any transformer using a three pole soild neutral ATS. The generator has always been connected to utility neutral and the generator body is is grounded. I am sure utility transformers feeding commercial buildings are solidly grounded, that means in this situation mentioned above this low volatge generators also by default gets solidly grounded.

So, why this stator fault protection is not considered in this situation. I was never asked by my sr. engineer or neither the contractor asked what will be the generator grounding.
So, how come low voltage generators are solidly grounded while connected to utility neutral ?
How this issue ground fault being more than three phase fault is accomodated.

I know, when a switched neutral ATS is used, in that situation, Generator's neutral is grounded seprately.

 

[davidbeach]

Low voltage generators are normally solidly ground for several reasons:

Line-to-neutral connected loads.

Except for high resistance grounded MV generators with 100% stator ground protection, the last 10% - 15% of the stator is left with no ground fault protection. That would be 240V worth in a 4160V generator, 720V in a 12470V generator. The whole LV generator stator has less voltage to ground than the unprotected portion of the MV generator.

LV units are less expensive, less is spent to protect them. A unit transformer becomes a significant cost adder.

Tradition.

 

[waross]

Also there are code issues.
Canadian Electric Code:
10-106 Alternating current systems
(1) Except as otherwise provided for in this Code, alternating-current systems shall be grounded if
(a)....
(b)the system incorporates a neutral conductor.

I believe that the NEC has similar provisions.
respectfully