HRG or NRG Resistance grounding

[mbrooke]

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.

Correct. To many issues that could come up. Even if its low impedance you could still have a phase to neutral fault cause a voltage rise on the non faulted phases and the neutral due to the limitation of fault current. P-N loads will be fried. Neutral could become grounded as well like you mentioned. And if its a high impedance type a phase to ground fault can cause the ground to rise well above the neutral in potential. Up to 277 on a 480 volt system, which would be a major hazard.

 

[D!NNy]

Sorry for late response

Yes


Yes


I'm pretty sure you, Jim, and kb already know this - so, just for the rest of us:
Two different design specs between 480V HRG and 13.8kV HRG. OP did not differentiate. I work with both.

Solidly grounded systems generally fail phase to ground first and often don't trip until the fault goes phase to phase. Lots of damage occurs during the phase to ground fault time.
What kind of damage, i thought resistance grounding limits the fault current so there will be no damage and continuous service under fault condition
Ungrounded systems are subject to arcing
In Ungrounded system for single phase to ground faults currents are low and cannot form arc as there is no closed loop for the power or it is dependent on the fault resistance,
re-striking ground faults.
when do these happen?? can you give me more explanation on this one?
These faults subject the system to severe overvoltage transients. I heard this is one of the disadvantages of resistance grounding but how??

So, one selecst an HRG to limit the damage of solidly grounded system GF faults and not be subject to the OV transients of ungrounded system GF faults. This is the normal reasoning for using an HRG on 13.8KV systems. And they are set to trip on GF.
Got it so HRG is used to limit the fault current for limited amount of time, not to damage any equipment using tripping. But why is HRG in 13.8kV can't limit the fault for higher duration!

And, as you said, continuity of service is the reasoning for setting the 480V HRG systems to not trip.

To clarify the for the OP:
480V HRG resistors are continuous. 13.8kV HRG resistors are very short time rated

ice

 

[D!NNy]

hello,

See my highlights below:

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. this could happen only in solidly grounded system, if so why cant we go with un grounded system? in this case you will never know if there is fault correct !
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]

fat fingered the buttons

I'll try again

ice

 

[iceworm]

Solidly grounded systems generally fail phase to ground first and often don't trip until the fault goes phase to phase. Lots of damage occurs during the phase to ground fault time.

What kind of damage, i thought resistance grounding limits the fault current so there will be no damage and continuous service under fault condition

This part of my comment is about solidly grounded systems - not about HRG. It is about why one sould want to pick an HRG over a solidly grounded system

Ungrounded systems are subject to arcing re-striking ground faults. These faults subject the system to severe overvoltage transients.

In Ungrounded system for single phase to ground faults currents are low and cannot form arc as there is no closed loop for the power or it is dependent on the fault resistance,

when do these happen?? can you give me more explanation on this one?

Yes, ungrounded systems can have Arcing, Re-striking, Ground faults. Following from ieee242, p238-239:

8.2.5 Ungrounded systems
Ungrounded low-voltage systems (see Figure 8-5a and Figure 8-5b) employ ground detectors
(e.g., lamps or voltmeters connected from each phase to ground) to indicate a ground fault.
These detectors show the existence of a ground on the system and identify the faulted phase,
but do not locate the ground, which could be anywhere on the entire system. The system
operates with the ground fault acting as the system ground point. The ground-fault current
that flows is the capacitive charging current of the system, generally only a few amperes.

If this ground fault is intermittent or allowed to continue, the system could be subjected to
possible severe overvoltages to ground, which can be as high as six or eight times phase voltage. Such overvoltages can puncture insulation and result in additional ground faults.

These overvoltages are caused by repetitive charging of the system capacitance or by
resonance between the system capacitance and the inductances of equipment in the system.​

I heard this (ice - arcing GF) is one of the disadvantages of resistance grounding but how??

No, the NGR is selected to supply the capacitive charging current so this won't happen.

So, one selecst an HRG to limit the damage of solidly grounded system GF faults and not be subject to the Over Voltage transients of ungrounded system GF faults. This is the normal reasoning for using an HRG on 13.8KV systems. And they are set to trip on GF.

Got it so HRG is used to limit the fault current for limited amount of time, not to damage any equipment using tripping. But why is HRG in 13.8kV can't limit the fault for higher duration!

I'm getting out of my area here. The 13.8KV systems I work with are not considered HRG, as mb pointed out. The 50A of fault current is too high for an HRG and too low for a Low Resistance grounded system. I think the designers picked it because it was as low of a GF current as they could go and still differentiate between true GFs and switching transients.

Why not continuous? Well, again not my area, so I'm guessing:
Safety: Step and touch potentials are higher with 13.8KV faults than 480V faults, they don't want to leave them on
Money: A continuous 400KW resistor is HUGE compared to a 10sec rated resistor

ice

 

[Sahib]

Sahib -
I can't make any sense out of what you are saying.

If this is what you are referring to

They do not have the same ground reference.

consider a HRG system and a solidly grounded system separated by an isolation transformer. Measure the phase to neutral voltage of HRG system and that of solidly grounded system. If you compare the voltage readings of the two systems, you may find that they differ widely. This is due to the capacitive effect of the neutral of HRG system with respect to ground. So the two systems do not have the same ground reference.

 

[kwired]

If this is what you are referring to



consider a HRG system and a solidly grounded system separated by an isolation transformer. Measure the phase to neutral voltage of HRG system and that of solidly grounded system. If you compare the voltage readings of the two systems, you may find that they differ widely. This is due to the capacitive effect of the neutral of HRG system with respect to ground. So the two systems do not have the same ground reference.

Might be just a little nit picking here, but the two systems do not have the same reference to ground. Ground is still ground and is the same to everything bonded to it via a low resistance conductor. The high impedance grounded conductor is ground potential on the ground side of the resistor, but the "neutral" of the transformer is not at ground potential - at least not if there is any current flowing through the resistor.

 

[Sahib]

the "neutral" of the transformer is not at ground potential - at least not if there is any current flowing through the resistor.

The neutral of HRG system may not be at ground potential irrespective of whether any current flowing through the resistor or not.

 

[Sahib]

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

No because, even though

Ground is ground,

the ground of HRG system is not the same as the ground of solidly grounded system because the ground fault current of one system does not pass through the ground of the other system i.e the grounds of the two systems are isolated in spite of the EGC connecting them together.

you could still connect line to neutral loads

Wrong because this can result in equipment malfunctioning.

a line to neutral fault would not have the resistor in the circuit and would be a high current event.

A line to EGC would not be a high current event and that is the purpose of a HRG system.

 

[kwired]

No because, even though

the ground of HRG system is not the same as the ground of solidly grounded system because the ground fault current of one system does not pass through the ground of the other system i.e the grounds of the two systems are isolated in spite of the EGC connecting them together.


Wrong because this can result in equipment malfunctioning.


A line to EGC would not be a high current event and that is the purpose of a HRG system.

I am still sticking with "ground is ground".

Ground can not be two different potentials. It can have unlimited number of different potentials to an unlimited amount of different electrical system "ungrounded" conductors. Remember the "neutral" of the HRG only extends from transformer XO to the resistor. The other side of the resistor is ground. You could connect 277 volt load to the "neutral" and it would work, but would introduce other problems as mentioned in other posts.

We call it a high resistance ground system, not because ground has a high resistance but because there is a resistance between ground and any conductor of the system.

Should L1 fault to ground - then L1 actually becomes ground potential, but there is a resistor between ground and the neutral and you know the rest.

 

[Sahib]

I am still sticking with "ground is ground".

Ground can not be two different potentials.

Remember ground potential gradient as would happen during a lightning strike to ground, for example.

You could connect 277 volt load to the "neutral" and it would work,

It won't. If you do not believe me, try it yourself.

 

[ggunn]

The neutral of HRG system may not be at ground potential irrespective of whether any current flowing through the resistor or not.

How can the voltage at two ends of a resistor be different if current is not flowing through it?

 

[Sahib]

How can the voltage at two ends of a resistor be different if current is not flowing through it?

Capacitive effect.

 

[kwired]

Capacitive effect.

Ever heard of discharge resistors installed on capacitors? Eventually they drain the capacitor. If it is never drained there is constant current flow and a voltage across the resistor. I did say " at least not if there is any current flowing through the resistor" in the earlier post.

Don't even bring lightning into this, that is like grasping for anything to keep you from falling. We all know lightning introduces problems into any system and can be fairly unpredictible. Even in a system designed for withstanding lightning -things happen.

As far as introducing a 277 volt load, why will it not work? I am talking about connecting it from one of the ungrounded conductors to X0, not to the grounded side of the resistor. I am not saying it will not introduce other potential problems with the system, just that there is still 277 volts from each ungrounded conductor to X0, and applying a 277 volt load there will operate the load. X0 is nothing more than a point in the windings. You could center tap each of the three phase windings and have 138 volts to either end of each winding and connect a load rated for 138 volts and it would work. It would introduce more complication to the system but it will operate the load.

 

[Sahib]

Ever heard of discharge resistors installed on capacitors? Eventually they drain the capacitor. If it is never drained there is constant current flow and a voltage across the resistor. I did say " at least not if there is any current flowing through the resistor" in the earlier post.

My point is it is not necessary that a current flow be maintained for the neutral potential of a HRG system to be other than ground potential.

Don't even bring lightning into this

I did it to emphasize the ground need not be always the ground we occasionally stand on i.e safe.

 

[kwired]

My point is it is not necessary that a current flow be maintained for the neutral potential of a HRG system to be other than ground potential.

I did it to emphasize the ground need not be always the ground we occasionally stand on i.e safe.

What ground is safe to stand on in the vicinity of a lightning storm? Please don't tell us you know the answer.

 

[Sahib]

As far as introducing a 277 volt load, why will it not work? I am talking about connecting it from one of the ungrounded conductors to X0, not to the grounded side of the resistor. I am not saying it will not introduce other potential problems with the system, just that there is still 277 volts from each ungrounded conductor to X0, and applying a 277 volt load there will operate the load. X0 is nothing more than a point in the windings. You could center tap each of the three phase windings and have 138 volts to either end of each winding and connect a load rated for 138 volts and it would work. It would introduce more complication to the system but it will operate the load.

The capacitive coupling of each line conductor of a three phase supply system with ground remains balanced when the neutral is solidly grounded so that the phase voltages remain balanced. When the neutral is ungrounded or high resistance grounded, the phase voltages become unbalanced.

 

[Sahib]

What ground is safe to stand on in the vicinity of a lightning storm?

Please don't tell us you know the answer.

That is why the ground is not always the ground as you used to believe.

 

[kwired]

The capacitive coupling of each line conductor of a three phase supply system with ground remains balanced when the neutral is solidly grounded so that the phase voltages remain balanced. When the neutral is ungrounded or high resistance grounded, the phase voltages become unbalanced.

The voltage between the three phase conductors and X0 will remain 277 volts, and the 277 volt load will see 277 volts. What you are mentioning very well may happen, but is not going to effect the voltage between phase conductors and X0. This is part of what I said may introduce other complications to the system. I did not say we are connecting this 277 volt from phase to ground, we are connecting it from phase to X0. The resistor is between XO and ground.

 

[Sahib]

The voltage between the three phase conductors and X0 will remain 277 volts, and the 277 volt load will see 277 volts. What you are mentioning very well may happen, but is not going to effect the voltage between phase conductors and X0. This is part of what I said may introduce other complications to the system. I did not say we are connecting this 277 volt from phase to ground, we are connecting it from phase to X0. The resistor is between XO and ground.

We connect single phase loads between line and neutral (connected to X0), which are at a considerable distance from a HRG transformer. So even though the phase to neutral voltages at the transformer terminals are the same, they are not the same at the single phase load terminals which are at considerable distance from the supply transformer due to unbalanced ground capacitive coupling as mentioned in my earlier post.