Location of Neutral Ground Resistor (NGR) at Main Switchgear?

[philly]

I am looking at a project that is considering converting a plants 4.16kV solidly grounded system to a Low Resistance Grounded (LRG) system. Usually the NGR is inserted between the transformer neutral and ground at the transformer however in this case the utility wye-wye transformer is located in a vault and they will not allow the installation of any outside equipment in their vault. The utility transformer in the vault provides a 4.16kV service into a main 4.16kV Switchgar lineup.

In this case is it possible to locate the NGR outside of the vault, at the customers switchgear if the existing N-G bond at the transformer in the vault is removed? If the neutrals are brought into the 4.16kV Switchgear is it possible to insert the NGR between the 4.16kV Switchgear neutral bus and ground? In theory this seems like it would work as long as the N-G bond at the transformer is removed since any ground current would travel through ground, through the NRG back to the neutral bus and back to the neutral of the transformer.

This only potential issue I see with this is NEC code compliance regarding the required N-G bond at the service entrance switchgear (4.16kv Switchgear). Can this NGR be inserted between the Neutral and Ground bus and still count as the Main Bonding jumper per NEC?

 

[jim dungar]

In this case is it possible to locate the NGR outside of the vault, at the customers switchgear if the existing N-G bond at the transformer in the vault is removed?

Yes.

Be very careful with the Wye-Wye connection from the utility, it must be removed if you plan to install a NGR system. If the utility connection is not broken your NGR is likely to be negatively impacted by the utility grid.

 

[Tony S]

All MV installations I’ve worked on the NGR has been as close as possible to the transformer. The reason as I understand, it is the transformer outgoing conductors are protected from full line → ground short circuit currents.

Moving the NGR away from the transformer will introduce additional impedance due to the cable. The possibility of the transformer X0 → NGR cable developing a direct short to ground would negate the NGR.

Not something I’d be happy about.

 

[philly]

Be very careful with the Wye-Wye connection from the utility, it must be removed if you plan to install a NGR system. If the utility connection is not broken your NGR is likely to be negatively impacted by the utility grid.

Are you saying that the entire wye-wye transformer must be removed and replaced with Delta-Wye or are you just saying the utility N-G bond must be removed at the transformer?

With a wye gnd - wye gnd transformer the transformer will not source zero sequence current but rather will pass zero sequence current from primary depending on the utility zero sequence impedance. Even with the secondary N-G bond removed is it still possible to shunt out the NGR with any N-G bonding on the primary of any N-N bonding across the transformer?

All MV installations I’ve worked on the NGR has been as close as possible to the transformer. The reason as I understand, it is the transformer outgoing conductors are protected from full line → ground short circuit currents. .

Yes I agree with this, but in this case that does not seem to be feasible.

Moving the NGR away from the transformer will introduce additional impedance due to the cable. The possibility of the transformer X0 → NGR cable developing a direct short to ground would negate the NGR.

Not something I’d be happy about.

Yes additional impedance would be introduced by the neutral cables between the transformer secondary and the main switchgear, which would act as additional impedance to further reduce the L-G fault current. I don't know that we'd install an additional X0 --> NGR cable but rather we'd just have the existing neutral cables come into the Switchgear and insert the NGR between the neutral and ground bus.

You are correct that if the existing neutrals developed a short to ground ahead of the NGR this would bypass the NGR but perhaps this is a low probability risk in this case.

 

[jim dungar]

.. just saying the utility N-G bond must be removed at the transformer?

There is nothing wrong with the wye-wye winding configuration.
A problem comes when the utility solidly grounds their neutral point (MGN) and then you install a NRG in parallel with it.
Another problem is when the utility opens their neutral connection at the substation, then runs several miles of neutral conductor to the grounded wye-wye connection in the transformer and your NRG. Did you plan on that much charging current/capacitive coupling?

 

[philly]

There is nothing wrong with the wye-wye winding configuration.
A problem comes when the utility solidly grounds their neutral point (MGN) and then you install a NRG in parallel with it.?

Are you referring to the neutral on the primary or secondary of the transformer here?

Another problem is when the utility opens their neutral connection at the substation, then runs several miles of neutral conductor to the grounded wye-wye connection in the transformer and your NRG. Did you plan on that much charging current/capacitive coupling?

I understand the concept of charging current but don't follow how it would be present in the situation that you are suggesting?

 

[jim dungar]

Are you referring to the neutral on the primary or secondary of the transformer here?

Utilities regularly supply Yg-Yg (grounded wye to grounded wye) transformers where there is a physical connection between the H0 and X0 terminals. This bond often internal to the transformer tank under the oil.

I understand the concept of charging current but don't follow how it would be present in the situation that you are suggesting?

Consider if the utility were to provide you with feeders that use a concentric neutral conductor which is open on their source end, miles away, but is bonded at your Yg-Yg transformer and thus to your NGR.

 

[Ingenieur]

what value of NGR? 10 A, 50A, etc.
if remotely mounted the N must be isolated (from earth) and insulated from the X0 to the NGR
what protection will be on the NGR?

 

[Ingenieur]

we only allow a wye to wye connection with a delta sec tertiary winding
the issue isn't so much with the NGR but voltage instability

all mining power systems have an NGR at every transformation
wye, just the NGR on the X0
delta, a 'zig-zag' or grounding transformers arranged in a wye with NGR

look at Post Glover's website, tons of info on this subject http://www.postglover.com/
basics http://www.postglover.com/download/neutral-grounding-resistors-technical-information/?wpdmdl=11302
charging current, capacitive coupling, leakage, whatever it's called must be considered

 

[philly]

Utilities regularly supply Yg-Yg (grounded wye to grounded wye) transformers where there is a physical connection between the H0 and X0 terminals. This bond often internal to the transformer tank under the oil

Ok I see now. Yes with this internal bond it appears that this would provide a N-G connection (secondary neutral-->primary neutral-->Gnd) which would be in parallel with the NGR and thus shunt the NGR and make it useless. So if the secondary N-G bond is removed and the neutral bond across the transformer is removed then will the NGR operate as intended assuming that only remaining N-G bod would be at the transformer primary

Consider if the utility were to provide you with feeders that use a concentric neutral conductor which is open on their source end, miles away, but is bonded at your Yg-Yg transformer and thus to your NGR.

Are you saying that in this case the charging current from the transformer primary cables (miles of cables) will flow back on the concentric neutral to the transformer through the internal bond and thus through the NGR? Wouldn't this be eliminated with the removal of the internal bonding jumper?

Also it appears that the transfoemer secondary conductors may be 3-wire with tape shield between the transformer secondary and switchgear. I'm assuming in this case the tape shield would be landed on the switchgear neutral and a main bonding jumper would be installed between N and G bus?

 

[philly]

what value of NGR? 10 A, 50A, etc.
if remotely mounted the N must be isolated (from earth) and insulated from the X0 to the NGR
what protection will be on the NGR?

I was thinking about a 400A resistor but this would obviously be subject to charging current calculations. Overcurrent protection at the NGR.

we only allow a wye to wye connection with a delta sec tertiary winding
the issue isn't so much with the NGR but voltage instability

Can you elaborate a bit more on the voltage stability issue?

 

[Ingenieur]

I was thinking about a 400A resistor but this would obviously be subject to charging current calculations. Overcurrent protection at the NGR.

Can you elaborate a bit more on the voltage stability issue?

at your voltage perhaps 1 A cc per xfmr MVA
400 A is big
100 is typ for that v
we use 15 or 25 up to 12.46 kv

are you going to trip or run with a fault?
we put a 59G across the ngr and a 51 on the gnd conductor

we want complete electrical isolation from the utility
a fault on their end can raise our ground/frame potential
if prim wye must not be grounded (we usually get delta)
if wye-wye the tertiary winding traps any ground loop currents

I have some data on wye-wye voltage issues at work
I'll try to find it

 

[jim dungar]

Wouldn't this be eliminated with the removal of the internal bonding jumper?

Yes.

The problem is that unground Wye - ungrounded Wye is not likely to be a standards transformer from your utility, so Trust but Verify.

 

[Ingenieur]

FYI

WYE-WYE: (No Isolation, least secure design) the primary and secondary coils are normally both grounded and some are supplied with an integral ground common to the primary and secondary. Environmental noise and background noise will enter the users facility unimpeded to every machine, drive and control as it is directly coupled from input to output.

The WYE-WYE transformer connection has no filtering and all the noise on your power system is shared. The WYE secondary is very susceptible to harmonics. WYE-WYE systems generate considerable interference with communications systems and some jurisdictions prohibit their use. With the neutral not used or not connected to ground in a WYE-WYE system, the phase voltages become unstable and the entire system is susceptible to damage from transient voltage events and the harmonics now travel in the phase conductors.



you must derive a ground on the low side of the NGR
X0 - NGR hi - NGR lo - ground bed||gnd bus bar-EGC's to equipment

H0-X0 no bond
H0 no ground

 

[philly]

Ok so that way I see it for the NGR to work on the secondary of a wye-wye transformer there are two options.

1) Remove the N-G bond on the secondary of the transformer as well as remove he neutral bonding jumper which is internal to the transformer. With this bonding jumper removed the transformers primary neutral can still be bonded to ground since there is no connection to the neutral on the secondary.

2) If the transformers neutral bonding jumper cannot be removed then the N-G bond must be removed on both the primary and secondary of the transformer. This would leave the transformers primary neutral ungrounded (which may cause other issues) but would allow the NGR to function as intended because there would be no bond from primary to secondary.

Am I seeing this correctly or am I missing something? It appears that the transformers secondary feeders are Bus Duct with the three phases and ground while a separate neutral is run from the neutral collector bus into the Switchgear. I'm supposing that the N-G bonding jumper inside of the Switchgear can be removed and the NGR can be cabled between the switchgear ground and neutral buses?

 

[Ingenieur]

imo the only proper way
no bond prim to sec
prim ungrounded, if grounded a seperate station bed

sec X0 to ngr using an insulated/isolated conductor
other end of ngr to another ground bed, call it the neutral gnd bed
this bed is bonded to your gnd bus

you can't serve l-n loads off this system unless transformation and new neut derived