HRG or NRG Resistance grounding

[iceworm]

... The 50A NGR system does seem to fall between HRG & LRG. Maybe it should be called MRG for medium resistance grounding.
How bout it ??

Ok - that's pretty funny. Do we flip a coin to see who gets to send in the change to IEEE 100?:roll:

OR

Nope. My comm tallents are screwed up enough with out using a term only you and I know the definition. You sparked me to read the standards (pun intended) - I'll stick with their definitions.

ice

 

[iceworm]

A high ground fault can be more evil than a phase to phase fault inside a generator and in such cases high resistance grounding of the neutral of the generator may help. ...

Sahib -
Give some thought to what you are saying. Maybe even draw out a schematic of the gen, grounding, fault. If the fault is inside of the gen then grounded, resistance grounded, ungrounded makes no difference. That gen is toast. The only way I know of to limit the damage to where the gen might be rewindable is to use a differential protective relay. The type of system grounding make no difference to differential protective relay.

ice

 

[iceworm]

And in the meantime you have 10 amps flowing through unintended current paths.

You can change the settings so that the generator is tripped instantaneously on ground fault. ...

Sahib -
Your comments are way to general. You really need to narrow down the systems you are including. I'm going to assume you are refering to 480Y HRG systems. If this isn't true, cancel the rest of my comment.

Of all the HRG systems I've seen:

• The NGR is continuous rated.
• The design spec is to provide continuity of service.
• A single ground fault causes an alarm. There is no trip programmed.
• A second ground fault is phase to phase. Overcurrent trips are as set by coordination. System grounding plays no part.
• The 5A-10A of a single GF is not considered damaging or a safety hazard.
• An instantaneous trip for GF is not even in the picture. If you mentioned this to an even minimally competent design engineer, the best you would get is a blank look. They would not know what you are talking about.

I respectfully request for you to narrow the scope of your comments to where they makesense.

ice

 

[mbrooke]

Other than ungrounded generators all that I have ever seen is solidly grounded systems. Doesn't matter if it is big or small single or three phase, and the units that are ungrounded are usually portable units and they become solidly grounded if connected to a premises wiring system.

I guess maybe all those generator manufacturers maybe need to have a talk with you and change their ways of doing things.

If you are primarily talking about medium voltage here then maybe there is some value in HRG, I certainly am not much of an expert in medium voltage applications, but I am talking 600 volts or less with my replies.

It does matter a huge deal, and for good reason. Nearly all large scale generators (those above 5MVA) have the neutral grounded via a low or medium resistance impedance. A few high but its for special apps only. A phase to ground fault can be highly destructive and draw more current than a symmetrical 3 phase fault, causing stator damage, even rotor damage at worst. However after the GSU (generator step up transformer) the neutral is usually solidly grounded. As for 600volt and lower Standby generators, yes, most of them below 2,500kw are solidly grounded. But those that are above this range regardless of voltage are usually grounded via impedance.

http://ieeexplore.ieee.org/xpl/logi...re.ieee.org/xpls/abs_all.jsp?arnumber=1216904

 

[iceworm]

... It is not a good idea to operate a generator with its neutral solidly grounded either.

Again - Too general a statement. I went back and re-read the posts looking for the context - still too general.
Here are several of the different scenerios I see covered by your statement:

• Are you discussing an off-grid 480Y industrial installation? Then yes I would agree. No N-G bond in the gen, premisis system is HGR.
• Are you saying that in general you would rather not see an N-G bond in the generator? I'd agree with that. For solidly grounded systems, I tend to put the N-G bond in the switchgear, not the gen. Mandatory to put the bond in the switchgear if there is multiple gens, or a utility connection.
• Are you saying that for non-industrial, small generation (<750KW) you don't want to see any N-G bond in either the gen or in the premisis switchboard? I don't agree with that at all. This is perfect case for a solidly grounded Y. My preference would be to put the bond in the premisis switchgear.

I don't expect you to answer these. But I would hope you would consider limiting your statemenst to where the appliction is narrow enough to make sense.

ice

 

[Sahib]

Sahib -
Give some thought to what you are saying. Maybe even draw out a schematic of the gen, grounding, fault. If the fault is inside of the gen then grounded, resistance grounded, ungrounded makes no difference. That gen is toast. The only way I know of to limit the damage to where the gen might be rewindable is to use a differential protective relay. The type of system grounding make no difference to differential protective relay.

ice

Phase to phase fault does not involve neutral grounding resistance and it should not be so mistaken just because I mentioned it for comparison with ground fault which involves neutral grounding when no NG bonding is on the generator.

 

[Sahib]

Sahib -
Your comments are way to general. You really need to narrow down the systems you are including. I'm going to assume you are refering to 480Y HRG systems. If this isn't true, cancel the rest of my comment.

Of all the HRG systems I've seen:

• The NGR is continuous rated.
• The design spec is to provide continuity of service.
• A single ground fault causes an alarm. There is no trip programmed.
• A second ground fault is phase to phase. Overcurrent trips are as set by coordination. System grounding plays no part.
• The 5A-10A of a single GF is not considered damaging or a safety hazard.
• An instantaneous trip for GF is not even in the picture. If you mentioned this to an even minimally competent design engineer, the best you would get is a blank look. They would not know what you are talking about.

I respectfully request for you to narrow the scope of your comments to where they makesense.

ice

In earlier times the low voltage systems were only solidly grounded because of too low sensitivity to operate protective devices if resistance grounding were employed. But with introduction of solid state relays, resistance grounded low voltage system is possible now and I see no problem in programming it to trip the generator on high internal ground fault even though the industrial practice is to just to give an alarm for any one ground fault no matter how high its magnitude may be. I just tried to improve this industrial practice.

 

[Sahib]

Again - Too general a statement. I went back and re-read the posts looking for the context - still too general.
Here are several of the different scenerios I see covered by your statement:

• Are you discussing an off-grid 480Y industrial installation? Then yes I would agree. No N-G bond in the gen, premisis system is HGR.
• Are you saying that in general you would rather not see an N-G bond in the generator? I'd agree with that. For solidly grounded systems, I tend to put the N-G bond in the switchgear, not the gen. Mandatory to put the bond in the switchgear if there is multiple gens, or a utility connection.
• Are you saying that for non-industrial, small generation (<750KW) you don't want to see any N-G bond in either the gen or in the premisis switchboard? I don't agree with that at all. This is perfect case for a solidly grounded Y. My preference would be to put the bond in the premisis switchgear.

I don't expect you to answer these. But I would hope you would consider limiting your statemenst to where the appliction is narrow enough to make sense.

ice

Where there is a generator with four wires and to serve phase to neutral loads also, its neutral must be solidly grounded per NEC and I do not see how NG bond plays a role in deciding whether a generator can be resistance grounded or not, specifically when it is not on the generator .

 

[kwired]

Where there is a generator with four wires and to serve phase to neutral loads also, its neutral must be solidly grounded per NEC and I do not see how NG bond plays a role in deciding whether a generator can be resistance grounded or not, specifically when it is not on the generator .

Give us an example of a solidly grounded neutral, and resistance grounding scheme all derived from the same power source:?:? Or clarifiy what you intended to say.

NEC is also going to require a bonding jumper from that solidly grounded neutral to the equipment grounding conductor which will bypass any impedance you attempt to install in the grounding system.

 

[Sahib]

Give us an example of a solidly grounded neutral, and resistance grounding scheme all derived from the same power source:?:? Or clarifiy what you intended to say.

Pay a visit to a place such as a paper mill where a HRG retrofit install is planned or is done to get an idea.

NEC is also going to require a bonding jumper from that solidly grounded neutral to the equipment grounding conductor which will bypass any impedance you attempt to install in the grounding system.

The best option is to provide an isolation transformer of suitable capacity to serve phase to neutral loads in a high resistance grounded system.

 

[kwired]

Pay a visit to a place such as a paper mill where a HRG retrofit install is planned or is done to get an idea.

The best option is to provide an isolation transformer of suitable capacity to serve phase to neutral loads in a high resistance grounded system.

If you have an isolation transformer you have a separately derived system and your grounding and bonding of the second system has nothing to do with the first system other than they both have a reference to ground if both are grounded.

You do not have both high resistance and solid grounding on the same system.

 

[Sahib]

they both have a reference to ground if both are grounded.

They do not have the same ground reference.

 

[kwired]

They do not have the same ground reference.

In a NEC installation, equipment grounding conductors will be bonded together at some point and they will have a common grounding reference because of that. Only an ungrounded system would not have a ground reference other than equipment grounding conductors or during a fault condition.

 

[Sahib]

In a NEC installation, equipment grounding conductors will be bonded together at some point and they will have a common grounding reference because of that. Only an ungrounded system would not have a ground reference other than equipment grounding conductors or during a fault condition.

Generally, only one ground reference; but the code permits more than one ground reference somewhere in Art.250.( not beyond 250.30, I think).

 

[iceworm]

Sahib -
I can't make any sense out of what you are saying. I'm going to get out of this one.

ice

 

[kwired]

Sahib -
I can't make any sense out of what you are saying. I'm going to get out of this one.

ice

You are not alone.

Generally, only one ground reference; but the code permits more than one ground reference somewhere in Art.250.( not beyond 250.30, I think).

NEC installations - there is no limitation to how many times equipment grounding conductor can be "earthed". It will happen potentially many times even if unintentional. Grounding the neutral or other "grounded conductor" is only done at service equipment, or on non service supplied / separately derived systems, either at the source or first disconnecting means. Beyond that point it needs to be insulated so that it can not unintentionally be earthed again creating undesired parallel current paths.

I don't know what may be common practice where you are or if NEC applies in any way where you are.

250.30 is where system bonding jumper is mentioned, but the system bonding jumper is essentially the point where the grounded conductor and equipment grounding conductors go their separate ways. The grounding electrode conductor is what makes the ground reference, along with any utility owned electrodes on service side of service equipment (but those are outside the scope of the NEC).

 

[Sahib]

kwired:
Where to connect the EGC of the HRG LV system?
It can not be connected directly to the neutral, prior to the high resistance connection to the neutral in the service equipment. It can only be connected to the neutral after the high resistance connection at the bus bar where the GEC is connected in the service equipment. With such a kind of connection, the ground of the HRG system is isolated from the ground of solidly grounded system with an isolation transformer in between so that any grounding problem in one system does not affect the operation of the other. In this sense the two systems do not have the same ground reference.

 

[kwired]

kwired:
Where to connect the EGC of the HRG LV system?
It can not be connected directly to the neutral, prior to the high resistance connection to the neutral in the service equipment. It can only be connected to the neutral after the high resistance connection at the bus bar where the GEC is connected in the service equipment. With such a kind of connection, the ground of the HRG system is isolated from the ground of solidly grounded system with an isolation transformer in between so that any grounding problem in one system does not affect the operation of the other. In this sense the two systems do not have the same ground reference.

That is exactly what I said earlier, maybe worded slightly differently.

Ground is ground, the equipment grounding conductor will still have same impedance to earth as it does in a solidly grounded system. In an impedance grounded system the resistor is inserted between the source and the ground reference, so in a wye system the impedance is between "earth" which is at same potential as the equipment grounding conductor and the transformer "X0" point. I don't know what common practice is, but you could still connect line to neutral loads here, but a line to neutral fault would not have the resistor in the circuit and would be a high current event. To the best of my knowledge most of these systems only supply phase to phase loads.

If you have a ground fault on high impedance ground system the voltage potential is across the resistor, not between the equipment grounding conductor and earth, the voltage between earth and the equipment grounding conductor will for the most part be negligible.

 

[jim dungar]

...To the best of my knowledge most of these systems only supply phase to phase loads...

Correct,
The NEC does not allow any L-N loads when using an impedance grounded system.

 

[kwired]

Correct,
The NEC does not allow any L-N loads when using an impedance grounded system.

Kind of what I suspected. I have no experience with impedance grounded systems, but I do understand the concept of what is being done there. Then my mind got to wandering a little and I figured that one could still connect line to neutral loads and they would work, but if NEC allowed that it would probably want overcurrent protection on the neutral, which introduces other problems if you had a multiwire circuit involved and opened the neutral conductor. You also would need some method of detecting neutral to ground faults otherwise they would just bypass the grounding impedance.