High Resistance Ground on 480V M-T-M SWGR

[philly]

I'm looking at a proposed system that has 480V M-T-M switchgear fed from two separate transformers with each of the transformers being high resistance grounded. I'm familiar with an HRG system on a single transformer/switchgear arrangement but am trying to figure out if there are any special requirements with a M-T-M system.

I would assume that you connect the ground buses between the two Switchgear lineups as you would do on a solidly grounded system however the potential issue I see is that when operating under normal conditions with the tie breaker open the ground bus will be connected to both the HRG's and perhaps cause circulating current? Or will the fault simply follow the least resistant path back to the source transformer through its HRG? Is there any particular considerations here? I'm thinking maybe I'm missing something?

When only one transformer is in operation and being operated through the tie, you will only have one HRG source to which the fault current will return.

 

[JoeStillman]

I've seen arrangements like this where there is only one bonding jumper and it's common to both ends. The ground fault relays are switched in or out according to the M-T-M breaker positions.

 

[Ingenieur]

Can both transformers supply the buses with the tie closed?
ie xfmrs in parallel?

 

[GoldDigger]

Can both transformers supply the buses with the tie closed?
ie xfmrs in parallel?

I would be concerned whether one transformer could drive two ground resistors, or vice versa. That could affect resistor sizing and trip current settings.

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[Ingenieur]

I would be concerned whether one transformer could drive two ground resistors, or vice versa. That could affect resistor sizing and trip current settings.

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that is why I asked

a 2 xfmr mtm arrangement is common in mining
only 1 ngr is used

if both transformers are online in parallel both will supply a gnd fault on either bus
the rtn current will seek its x0 but will see 2 ngr in parallel
so fault i will double (ngr is halved)

imo he only needs 1 bonded on the x0 side and split to each bus on the grounded side

 

[philly]

that is why I asked

a 2 xfmr mtm arrangement is common in mining
only 1 ngr is used

if both transformers are online in parallel both will supply a gnd fault on either bus
the rtn current will seek its x0 but will see 2 ngr in parallel
so fault i will double (ngr is halved)

imo he only needs 1 bonded on the x0 side and split to each bus on the grounded side

The tie breaker will be interlocked so that with the tie closed one of the main breakers will be open. Each section of Switchgear will have the HRG located in the Switchgear enclosure with the neutral brought from the transformer.

With the tie breaker open and both mains closed the NGR from each transformer will be connected to a common ground between the two Switchgear sections. For a ground fault on one of the buses however the current will try to return to its source X0 through the source NGR. Although the ground bus will be connected to the NGR on the other transformer it will not flow through the other NGR because there is no patch there to return it to its source X0.

Even if the sources are paralleled with the tie breaker closed as you suggested I don't think there will be a path through the opposite NRG for the same reason but instead in this case you will have half the source impedance (two sources in parallel). So its not a function of the NGR's being in parallel but rather the lower source impedance that will create double the L-G fault current?

 

[publicgood]

High Resistance Ground on 480V M-T-M SWGR

Assumed: The neutral and ground bus are common on both sides of the tie.

To my knowledge this is no different then a solidly grounded system bonded at both mains of a M-T-M. Will there be circulating currents: Yes. Within the double ended gear there will likely be circulating currents due to the parallel paths. Is this a safety issue? Maybe.

So far as the code is concerned, is the installation is made and objectionable currents are found on the ground bus, preventative measures may be taken. Not until the objectionable currents are observed may we take action.

Sometimes the utility requires the bond be made at both mains. The industry standard of care for a multi grounded situation such as this is to implement a partial differential ground fault protection scheme. This will ensure the ground fault is not defeated due to the parallel paths and also prevents nuisance trips. Two tier ground fault is best (add GF add feeder breakers).

Other options for multi point grounding: Limit the neutral bus to the main sections only and derive neutrals as needed with transformers for say lighting loads. Utilize a 4-pole tie: Caution - if there is breaker failure both neutrals could be tied accidentally.

Alternatively, if the utility allows, the code let's you provide a single point ground at the tie. This is not appropriate if paralleling of the sources is considered. Nor is it if the M-T-M is made up of multiple gear sections physically separated, due to added impedance.

 

[Ingenieur]

The tie breaker will be interlocked so that with the tie closed one of the main breakers will be open. Each section of Switchgear will have the HRG located in the Switchgear enclosure with the neutral brought from the transformer.

With the tie breaker open and both mains closed the NGR from each transformer will be connected to a common ground between the two Switchgear sections. For a ground fault on one of the buses however the current will try to return to its source X0 through the source NGR. Although the ground bus will be connected to the NGR on the other transformer it will not flow through the other NGR because there is no patch there to return it to its source X0.

Even if the sources are paralleled with the tie breaker closed as you suggested I don't think there will be a path through the opposite NRG for the same reason but instead in this case you will have half the source impedance (two sources in parallel). So its not a function of the NGR's being in parallel but rather the lower source impedance that will create double the L-G fault current?

the gnd path impedance is lower because they are in parallel

another way to look at it
if the tie is closed and both xfmrs are online
BOTH xfmrs will supply gf current (just like both would share any load)
a gf will put phase-n voltage across each ngr
so each will flow rated current
The sum being twice 1 ngr rating
if the ngr is 100 A
the gf will be 200
100 from each xfmr
200 pn the common ngr feed
divided 100 to each ngr/x0

you still only need 1 ngr for any scenario
but if they want 2 fine
only an issue if tie is closed and both xfmrs are on line

 

[publicgood]

the gnd path impedance is lower because they are in parallel

Good point. The would need to size the resistors twice as large if they use both due to the paralleling of them. Better to single point ground at tie with one resistor.

 

[philly]

the gnd path impedance is lower because they are in parallel

another way to look at it
if the tie is closed and both xfmrs are online
BOTH xfmrs will supply gf current (just like both would share any load)
a gf will put phase-n voltage across each ngr
so each will flow rated current
The sum being twice 1 ngr rating
if the ngr is 100 A
the gf will be 200
100 from each xfmr
200 pn the common ngr feed
divided 100 to each ngr/x0

Ok I see it now thank you for explanation. This would not apply to situation with tie closed and one main open correct?

you still only need 1 ngr for any scenario
but if they want 2 fine
only an issue if tie is closed and both xfmrs are on line

So are you saying that you can have both transformer neutrals brought to a common point before it hits the NGR with the ground side of the NGR then splitting off to each bus? Customer is actually retrofitting two single ended switchgear to be M-T-M so individual NGR already exists in each section.

 

[Ingenieur]

Ok I see it now thank you for explanation. This would not apply to situation with tie closed and one main open correct?



So are you saying that you can have both transformer neutrals brought to a common point before it hits the NGR with the ground side of the NGR then splitting off to each bus? Customer is actually retrofitting two single ended switchgear to be M-T-M so individual NGR already exists in each section.

Correct
only an issue tie closed both on line

yes
X0's paralleled to ngr
Then thru ngr to ground grid and to each bus

 

[Ingenieur]

Good point. The would need to size the resistors twice as large if they use both due to the paralleling of them.


Better to single point ground at tie with one resistor.

bingo

and monitor ngr
current
voltage across it
open ckt (very bad if it burns open)
startco/littelfuse, bender, etc