Medium Voltage Neutral Connection

[WiringBoy]

Hello All,

My application involves a 5 MVA 27.6 kv/4.16 kV Primary Delta and Secondary is star solidly grounded transformer. A 4.16 kV switchgear is fed from the secondary and I have been told by my utility standard guys that the practice of connecting outgoing feeder cables is as following:

1. X0(neutral) bushing connection of solidly grounded tranformer is not brough in to the switchgear building. It is connected throught a bare copper wire to through ground grid and a 600 V insulated cable is taken outside the street pole for the outgoing three phase 4 wire feeders.

2. 4 kv switchgear has a 1/2"x2" ground bus running bus within. Switchgear has three outgoing feeders, each cable is a single conductor 500 MCM with concentric neutral(3X 1C, 500 MCM).Swgr is 2000 A with each outgoing feeder with a 1200 A breaker.

Here is my question, Is it acceptable to connect concentric neutral for each feeder to the ground bus with in the switchgear. I have attached a picture. COuld you please suggest, if what is scientfically correct way to do things or I have it right.

 

[waross]

Be aware that you will not be code compliant in North America if you connect 2400 Volt loads from line to neutral with this arrangement.
There is no problem with three phase 4160 Volt loads and single phase 4160 Volt loads. Consider your concentric neutrals as grounding conductors and connect them to the ground bus.
You may want to insulate the far ends of the concentric neutrals to prevent circulating currents. In this case you may need to run a separate equipment grounding conductor. If you use the concentric neutrals for equipment grounding you will have to derate the cables due to the heating from circulating currents in the concentric neutrals. I expect circulating current heating to be an issue that must be addressed with 1200 amp circuits. It may be better to use a separate equipment grounding conductor and isolate/insulate the field ends of the neutrals.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[WiringBoy]

Thanks Waross. XFMR output based on 5 MVA is approx. 700 A. There are three outgoing feeders each fed from a 1200 Amp breaker.

So in nutshell this is how it works. Please let me know if I got it right.

Feeder #1: Three single conductor 4.16 kV 3X 1C, 500 MCM URD concentric neutral have their concentric bonded to-gather at the grounding bus inside the switch gear. this grounding bus is sized at 1/4'X2".

A dedicated neutral is taken to the pole from the X0 bushing directly. What happens to the pole side of the concentric neutral of cables ? are they left un-bonded to the neutral.

 

[WiringBoy]

Waross, Does the size of grounding bus would make a difference of the switchgear if concentric neutral are to be bonded, Do we have to size grounding bus like we size neutral.

Appreciate your input.

 

[MKFPE]

For more than 30 years of doing similar medium voltage installations I have always carried the neutral with the phase wires.
I also use power cable with a separate 600V insulated neutral conductor rather than URD cable for substation applications.
So the neutral goes from X0 to the switchgear ground bus and the distribution feeder neutrals are tied to the switchgear ground bus as well.
The power cable shields are grounded at the source end only and do not carry any continuous current.
The ground grid is a safety ground system only and shouldn't carry any unbalance current.
Routing the neutral away from the phases increases the current seeking other paths since the mutual coupling between the phases and neutral is now much lower.
Thus I would expect undesirable current in the safety ground grid with the arrangement shown in the sketch.
See Powell Technical Brief #58 (

http://www.powellind.com)

for switchgear ground bus information.
Anderson Electrical Connector Technical Data Reference Book gives ampacity of 1/4 x 2' copper bus as 647 amps, almost the full rating of the transformer.
The unbalanced continuous current in the neutral bus would hopefully be no more than 10%, so there should be no issue with bonding everything to the ground bus and carrying the phase and neutral conductors in close proximity to reduce unwanted circulating currents in the safety ground grid.
I have lifted safety grounds for testing in older substations where the neutrals were not properly handled or have deteriorated and have drawn a nice arc.
Kind of unsettling for what is supposed to be a safety ground.

 

[MKFPE]

Regarding my earlier post I should also add that that I work in the utility field on installations not covered by the National Electrical Code.
If under NEC jurisdiction in NA, I would specify a separate neutral bus in the switchgear and install separate neutral and grounding/bonding conductors for L-N connected loads.

 

[WiringBoy]

Thanks MKFPE. Please see the attached schematic. What do you think, would this work.