Neutral Connection For Balanced Load 2

[341968]

Could you please guide me that
I have LV transformer Star connected.
Only 3-phase are going to panel.
Star point(Neutral bus) of TR is only solidly grounded.
Panel has neutral bus BUT empty (No cables), No Ground.
Reason not being using Neutral is completely balanced load (4-Motors DOL and 8-Motors through VFD Each 90KW).
My Question: Can I make connection (Cabling) between TR neutral and Main panel Neutral, as I need 230V for ACB motor. That I want to turn it into TNS system. Doing so any adverse effects? Any recommendation?

 

[7anoter4]

TNS system that means IEC 60364-1 or BS 7671

 

[7anoter4]

BS 7671 recommends [as attached]

 

[waross]

Size of conductor?
Our code requires the neutral to be sized to carry the unbalanced load.
The code is a minimum.
Good engineering may suggest that the conductor be sized to withstand ground fault current until the protection clears.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[341968]

Okay our actual problem is we do not have 230Vac in main panel to charge spring charging motor of ACB.
From Transformer no Neutral is coming to main panel. So what are doing manually charging the spring. And we have faced some problem doing so. So I decided to bring Neutral from transformer to Main Panel and get 230Vac just to charge the spring charging motor. Rest system will stay same. Neutral will not be distributed as all load connected is balanced. Motors 4-DOL and 8-VFD driven 90Kw each.

please assist if my understanding is not violating any code or standard practice.

 

[davidbeach]

Maybe a single phase 400/230 transformer would be better than pulling in a bunch of new wires.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[xnuke]

Normally a spring charging motor is supplied by a control power transformer located in the gear, so I second davidbeach's suggestion.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[waross]

I am surprised that a control transformer was not included in the breaker accessories.
Was the charging motor added to an existing breaker?
I agree with David.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

Mr 341968 (Electrical)(OP)20 May 23 20:17
"... our actual problem is we do not have 230Vac in main panel to charge spring charging motor of ACB...".
1. The more economical way is to have a 400/230V single-phase step-down transformer say about 600VA would do the job, instead of installation of a neutral cable from Trafo to the board neutral bar; as advised by learned Mr Davidbeach.
Note: only a single trafo is needed for the whole board that may have numerous ACBs, as they are unlikely required to be cranking at the same time. Each ACB cranking time is say <8s.
2. No problem to have a Neutral cable from earthed Trafo neutral to the Neutral bar at the board. See above 1 for a much economical solution.
Note: European IEC practice that the Neutral bar at the board shall be isolated from the earth/frame of the board. The board/frame shall be earthed locally, connected by earthing conductor to the local earthing electrode, which shall be located say > 15m from the utility Trafo earthing electrode.
Che Kuan Yau (Singapore)

 

[341968]

Yes thanks David, I can install one Control/Auxiliary Transformer 400/230Vac on the incoming of the ACB. On the Name Plate of Panel it is mention it as external supply from ups. But all these external supplies are far apart and not so feasible to arrange.

But What are the other potential solutions to address the issue of not having 230 Volts Alternating Current in the main panel to charge the spring charging motor of the Air Circuit Breakers when no neutral is coming from the transformer? One proposed solution is to bring a neutral from the transformer to the main panel, allowing for the provision of 230 Volts Alternating Current specifically for charging the spring charging motor. This solution would ensure that the motor can be charged automatically, eliminating the need for manual charging and potentially resolving any issues faced during that process. It is important to note that the rest of the system will remain unchanged, with the neutral not being distributed as all the connected loads are balanced. Additionally, the system includes 4-DOL motors and 8-VFD driven motors, each rated at 90 kilowatts.

Please see attached Panel Tag and ACB Pics.

 

[341968]

Thanks Che Kuan Yau for valuable input.

 

[che12345]

Mr 341968 (Electrical)(OP)20 May 23 22:07
"....What are the other potential solutions to address the issue of not having 230 Volts Alternating Current in the main panel to charge the spring charging motor of the Air Circuit Breakers when no neutral is coming from the transformer?"
1. a) The proposal by learned Mr David is very cost effective. The cost of a single 600VA trafo is nothing for the whole board having numerous ACBs.
b) There are various motor ratings 24V ACDC, 48V ACDC, 100V ACDC and 220V ACDC. If you have any of these voltages in near by boards, you may use them. Info: Inrush power DC 500W, AC500VA; rated power DC 200W AC 200VA approx.
2. You may connect R1 (20 ohm) in series with R2 (24.4 ohm) across 400V. The voltage across R2 would be 220V approx. Switch off the voltage divider <10s after each cranking.
Che Kuan Yau (Singapore)

 

[7anoter4]

If you have not any connection between the two earthing points and no neutral you are in IT System.
The problem is the earthing current it is not controlled, so, if the source is solid earthed the fault current may damage the equipment. Usually, this system works on isolated Y point or connected to earth through an earthing impedence.

 

[che12345]

Mr 7anoter4 (Electrical)21 May 23 06:10
"...If you have not any connection between the two earthing points and no neutral you are in IT System..."
I am of the opinion (in this case) where the utility Trafo Neutral is earthed:
a) It is not compulsory to run a Neutral cable to the Board where there is no single-phase load. Where all three-phase loads are balanced e.g. motor,
b) However, the Board SHALL be earthed, through local earthing electrodes,
c) All equipment exposed conductive surfaces including motor frame etc... SHALL be earthed to the earthing bar which SHALL have good connection with the Board,
d) Strongly recommend that the earthing bar SHALL be earthed by two separate earthing conductors connecting it to two separate local earthing electrodes, separated >8m apart.
Che Kuan Yau (Singapore)
,

 

[Kiribanda]

There is nothing wrong of tapping the neutral and use it for spring charging motor or any other single phase load because
your system is solidly grounded.

1) Since your are going to connect directly to the secondary of the transformer and therefore due to high
single phase to earth fault current, you have to make sure that you insert a current limiting fuse/ current limiting mcb
(not normal fuse or a normal mcb) so that if there is a earth fault on the circuit it will not create an "ELCTRICAL DISATER".
2) The PE conductor of the spring charging motor should be ADEQUATELY SIZED to the maximum 1-ph to earth fault current at the location
and should be properly bonded to the main earthing terminal of the system so that the TNS system is effective.

 

[341968]

Thanks Kiribanda for your important highlights, Thanks to all Gents who benefited me with their professional expertise.The concept is very clear and i will implement accordingly. Thanks a lot. Stay blessed

 

[waross]

1) Since your are going to connect directly to the secondary of the transformer and therefore due to high
single phase to earth fault current, you have to make sure that you insert a current limiting fuse/ current limiting mcb
(not normal fuse or a normal mcb) so that if there is a earth fault on the circuit it will not create an "ELCTRICAL DISATER".

Many codes prohibit fusing of a neutral.
If a neutral conductor is installed it may be well to size it to withstand the possible line to neutral fault current for the time that it will take for the line protection to clear.
There are conditions that may cause increased impedance in the grounding path so that a much larger than anticipated share of line to neutral fault current flows in the neutral conductor.
The time honoured and much implemented solution is to add a control transformer as originally suggested by David Beach.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[davidbeach]

The addition of a neutral conductor, to carry load current, does not increase the prospective ground fault current. A ground fault would not be using that conductor and could happen today. That ground fault would be from a phase conductor to a grounded surface. Since the neutral is only grounded at the transformer neutral there's no way for the ground fault to get back onto the neutral conductor.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[che12345]

@ Mr Kiribanda (Electrical)21 May 23 16:09
" .... There is nothing wrong of tapping the neutral and use it for spring charging motor or any other single phase load because your system is solidly grounded".
No, you should NOT connect any load to the Board Neutral bar, even the utility Neutral is earthed. Take note that (in this case) there is No connection between the utility Neutral to the Board Neutral, which is "floating" and is isolated from earth.

" 1) Since your are going to connect directly to the secondary of the transformer and therefore due to high single phase to earth fault current, you have to make sure that you insert a current limiting fuse/ current limiting mcb (not normal fuse or a normal mcb) so that if there is a earth fault on the circuit it will not create an "ELCTRICAL DISATER".
A normal fuse rated for 100kA will be fine.

" 2) The PE conductor of the spring charging motor should be ADEQUATELY SIZED to the maximum 1-ph to earth fault current at the location and should be properly bonded to the main earthing terminal of the system so that the TNS system is effective".
The PE conductor of the spring charging motor need NOT exceed 2.5 mm2. The motor power conductor is likely < 2.5 mm2.
Che Kuan Yau (Singapore)

 

[waross]

It's been a long time since I saw a bus bar arc in equipment but it was spectacular.
Such an arc could involve the neutral bus.
With a conductor between the panel bus and the transformer neutral, and arcing bus bars, ??????
Over the years there have been a few things that I knew could never happen.... until it happened.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!