Residential Power Distribution

[Sean.Day72]

Was looking through some design and had a questions.

Supposed there is a 3 Wire (7200V) medium voltage feed going into a Outdoor 3 Way pad mounted switchgear (S&C 321). 3 Phases come out of the gear and head north into a 3 phase primary junction Primary Junction Cabinet (PJC). Phase A heads west out of the PJC to feed residential transformers (3 wire, 120/240), Phase B head East to do the same, Phase C continuous north to do the same. First question, the transformers feeding the houses out of each phase should be connected in parallel correct? There are four transformers servicing 12 houses on each road, each road fed by one phase.

The primary side wire is 1/0 aluminum with a full concentric neutral.The transformers on the rode are looped together. When the cable comes into the transformer they braid together the neutral and connect into a dead break connector right? Where should the neutral be grounded on the primary side? At the last transformer in the line? At the PJC? Neither it somehow makes it way all the way back through multiple switches to the transformer (source). This is a large subdivision so I'm having trouble visualizing the circuit. I attached photo for reference.

 

[electrofelon]

I am not a lineman or MV guy, but I'll make a few comments. First is, answers may vary depending on if this is a utility system or a system fallin under the NEC.

It sounds like it is a MGN sytem, in which case typically there would be a grounding electrode at every transformer. These electrodes would be connected to the concentric and the transformer tank.

I can't really comment much on the use of a dead break elbow vs a load break elbow, other than to say I see load breaks use much more frequently

 

[Sean.Day72]

I am not a lineman or MV guy, but I'll make a few comments. First is, answers may vary depending on if this is a utility system or a system fallin under the NEC.

It sounds like it is a MGN sytem, in which case typically there would be a grounding electrode at every transformer. These electrodes would be connected to the concentric and the transformer tank.

I can't really comment much on the use of a dead break elbow vs a load break elbow, other than to say I see load breaks use much more frequently

This is a utility system. I did some more digging and found a name plate photos and kinda answered my own question. You would ground one leg of the primary side to the electrode to give your 0V earth reference. So in this case the concentric neutral is just a means to carry transient voltages if the ground was to get hit my lighting and protect the phase conductor correct? It is not used to carry any unbalanced load. So you would connect your primary side ground, both concentric neutrals, and the center tap neutral on the secondary side to the common ground.

I'm used to wiring up buildings, first time looking at campus wide distribution. :happyno:

 

[mbrooke]

This is a utility system. I did some more digging and found a name plate photos and kinda answered my own question. You would ground one leg of the primary side to the electrode to give your 0V earth reference. So in this case the concentric neutral is just a means to carry transient voltages if the ground was to get hit my lighting and protect the phase conductor correct? It is not used to carry any unbalanced load. So you would connect your primary side ground, both concentric neutrals, and the center tap neutral on the secondary side to the common ground.

I'm used to wiring up buildings, first time looking at campus wide distribution. :happyno:

The concentric neutral on the MV is the MV neutral. The ground electrode is just one of many on the system used to keep the MGN near earth voltage and to help the lightning.

BTW, the ground symbol on the trafo diagram is one end of the 7,200 volt lead connected to the metal frame or shell of the unit. The concentric neutral connects to the shell as does the LV neutral bushing via a strap.

In short the LV and MV neutrals are all tied together, and the grounding electrode(s) at the pad are connected to the neutral ties as well.

 

[mbrooke]

I am not a lineman or MV guy, but I'll make a few comments. First is, answers may vary depending on if this is a utility system or a system fallin under the NEC.

It sounds like it is a MGN sytem, in which case typically there would be a grounding electrode at every transformer. These electrodes would be connected to the concentric and the transformer tank.

I can't really comment much on the use of a dead break elbow vs a load break elbow, other than to say I see load breaks use much more frequently

The NEC allows MGNs also on MV and exterior power system. Just pointing it out in case the OP is wondering

 

[electrofelon]

The NEC allows MGNs also on MV and exterior power system. Just pointing it out in case the OP is wondering

Yeah, that has come up now and then. I've never really read the wording with any detail and attention to know if "utility style" is nec compliant. Do you think it is?

To the OP, just thought I would mention: there are lots of non MGN utility systems out there. In that case you would have a two bushing transformer, one for each phase, and the concentrics would still be connected to dirt and the transformer enclosure, but wouldn't carry any current. We run into these here and have to pull TWO cables to the transformer (utility let's us supply and pull the primary here).

 

[Ingenieur]

this may help pg 27

http://www.lipower.org/pdfs/commercial/trade/1phase.pdf

the n is bonded to gnd for both prim and sec, a gnd rod is required (maybe 2 depending on
your utility requirements)
the primary is fed thru
the fed thru prim neut is NOT grounded
connection material and quality are important
check with your utility, they likely have a similar manual

 

[mbrooke]

Yeah, that has come up now and then. I've never really read the wording with any detail and attention to know if "utility style" is nec compliant. Do you think it is?

To the OP, just thought I would mention: there are lots of non MGN utility systems out there. In that case you would have a two bushing transformer, one for each phase, and the concentrics would still be connected to dirt and the transformer enclosure, but wouldn't carry any current. We run into these here and have to pull TWO cables to the transformer (utility let's us supply and pull the primary here).

It should be. My understanding is that the NEC's requirements for an MGN system are inline with the NESC. IE, the NESC requires a minimum of 4 times per mile grounding and at every transformer as well as the NEC.


250.184 (C)

(C) Multigrounded Neutral Systems. Where a multigrounded
neutral system is used, the following shall apply:

(1) The neutral conductor of a solidly grounded neutral
system shall be permitted to be grounded at more than
one point. Grounding shall be permitted at one or more
of the following locations:
a. Transformers supplying conductors to a building or
other structure
b. Underground circuits where the neutral conductor is
exposed
c. Overhead circuits installed outdoors

(2) The multigrounded neutral conductor shall be grounded
at each transformer and at other additional locations by
connection to a grounding electrode.

(3) At least one grounding electrode shall be installed and
connected to the multigrounded neutral conductor every
400 m (1300 ft).

(4) The maximum distance between any two adjacent electrodes
shall not be more than 400 m (1300 ft).

(5) In a multigrounded shielded cable system, the shielding
shall be grounded at each cable joint that is exposed to
personnel contact.

 

[electrofelon]

But here is what I'm not clear on: so that says we can ground the neutral, multiple times, ok....I don't see that it let's us bond metal parts to the neutral for fault clearing....the famous question, what does "ground" mean here.

 

[Ingenieur]

But here is what I'm not clear on: so that says we can ground the neutral, multiple times, ok....I don't see that it let's us bond metal parts to the neutral for fault clearing....the famous question, what does "ground" mean here.

look at pg 27
the hv bushings are bonded to the gnd pad (and hv and lv n's, gnd rod and xfmr frame)
they are connected to gnd in the cable

 

[electrofelon]

look at pg 27
the hv bushings are bonded to the gnd pad (and hv and lv n's, gnd rod and xfmr frame)
they are connected to gnd in the cable

My last post was questioning what the NEC says, not utility practice.

 

[Ingenieur]

My last post was questioning what the NEC says, not utility practice.

:thumbsup:

 

[mbrooke]

But here is what I'm not clear on: so that says we can ground the neutral, multiple times, ok....I don't see that it let's us bond metal parts to the neutral for fault clearing....the famous question, what does "ground" mean here.

Nothing says you can't.

 

[electrofelon]

Nothing says you can't.

250.24 (A)(5)

 

[mbrooke]

250.24 (A)(5)

(A) System Grounding Connections. A premises wiring
system supplied by a grounded ac service shall have a
grounding electrode conductor connected to the grounded
service conductor, at each service, in accordance with
250.24(A)(1) through (A)(5).

Key term is premises, IMO.

 

[electrofelon]

Key term is premises, IMO.

I'll get back to you in a bit when I have my code book in front of me and have cogitated on it for a few minutes.

 

[Ingenieur]

I'll get back to you in a bit when I have my code book in front of me and have cogitated on it for a few minutes.

is it really any different than a pole mntd xfmr with the n bonded to a butt coil or rod at
the pole and again on a rod at the house?

 

[electrofelon]

is it really any different than a pole mntd xfmr with the n bonded to a butt coil or rod at
the pole and again on a rod at the house?

No it isn't.

 

[electrofelon]

Key term is premises, IMO.

Ok, yeah I can go along with that. But take a look at 250.30(A) for SDS's. It doesn't say "premises". Then there is 250.142(B). But all that said, part X for over 1KV which "supplements an modifies...."

I still say they need to clarify what they mean by "multigrounded neutral system". I guess one could say that at a transformer bonding everything together and to the neutral and not running a EGC is the only thing it could mean, because any other permutation doesn't make sense.

 

[GoldDigger]

Ok, yeah I can go along with that. But take a look at 250.30(A) for SDS's. It doesn't say "premises". Then there is 250.142(B). But all that said, part X for over 1KV which "supplements an modifies...."

I still say they need to clarify what they mean by "multigrounded neutral system". I guess one could say that at a transformer bonding everything together and to the neutral and not running a EGC is the only thing it could mean, because any other permutation doesn't make sense.

Not really. One transformer by itself is not enough to define a multigrounded neutral system. To have an MGN you need Multiple. In particular multiple transformers on a distribution line, each connected by a common metallic neutral which is grounded at each transformer and often at some intermediate poles.