IET logo
 
IET
Decrease font size
Increase font size
Topic Title: SHaring Neutrals Between Sources
Topic Summary:
Created On: 14 June 2013 03:18 PM
Status: Post and Reply
Linear : Threading : Single : Branch
1 2 Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 14 June 2013 03:18 PM
User is offline View Users Profile Print this message



clarky280

Posts: 84
Joined: 10 June 2008

Hi

Im after a bit of feedback regarding interconnected neutrals. Would appreciate some discussion on this.

There is a scheme where a generator and a transformer are connected to a main LV switchboard with three-pole devices, and there is a single neutral-earth link on the main switchboard, load side.

The generator and transformer are not intended to operate in parallel - i.e. they will be a break before make on return to mains.

Also, there is a circuit breaker connected to the generator for connection of a load bank to test the generator. With this scheme it is conceivable that both sources will be energized, supplying different loads and sharing a neutral-earth link on the main switchboard. The neutrals of both sources will be interconnected by virtue of the three-pole devices.

One problem I could envisage in this scenario is that an earth fault in the generator system could circulate to the main switchboard and possibly cause tripping of earth fault protection if present.

Is there any other practical considerations anyone can think of, or is there anything within one of the relevent standards (7671, 7430) that prohibits the neutrals being connected?

Thanks
 14 June 2013 03:33 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11552
Joined: 13 August 2003

One problem I could envisage in this scenario is that an earth fault in the generator system could circulate to the main switchboard and possibly cause tripping of earth fault protection if present.

Provided there wasn't a simultaneous earth fault in the installation and earth fault protection was downstream of the L-N link and monitored currents in live conductors only (i.e. no old VOELCBs etc), how would that happen?

- Andy.
 14 June 2013 03:42 PM
User is offline View Users Profile Print this message



clarky280

Posts: 84
Joined: 10 June 2008

By simultaneous earth fault do you mean earth faults in both systems. I take your point that it depends on the location of the protection CTs. Apart from the earth fault consideration, does anyone see any issues with the arrangement?
 14 June 2013 04:10 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11552
Joined: 13 August 2003

By simultaneous earth fault do you mean earth faults in both systems.

Sorry, it wasn't very clear. Yes, unless you had at least on earth fault in the installation as well as one in the generator, I was wondering could the current circulate in such a way that earth protection would see it?
- Andy.
 14 June 2013 05:22 PM
User is offline View Users Profile Print this message



clarky280

Posts: 84
Joined: 10 June 2008

Well I was only thinking about an earth fault on generator. This would have to flow back to the neutral-earth link and then back to the generator neutral. You are right though, unless the protection CTS were around the busbars on the main switchboard between the breaker and the N-E link, the CTs would not see it.

So that is probably not an issue, but is there anything else? Surely there will be some circulating currents between the two star points due to voltage imbalance? Is this is an issue at all?
 17 June 2013 01:37 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11552
Joined: 13 August 2003

I'm hoping that some of the regulars with experience with 'big systems' chip in on this one. I've a recollection of a document that someone probably posted a link to which covered this kind of thing is some depth. I thought it was one of the "cahier technique" papers (it was certainly that kind of style) and dealt with earthing & N-PE linking with multiple sources (including the link being made by contactor), but I've failed to find it now

- Andy.

Edited: 17 June 2013 at 02:09 PM by AJJewsbury
 17 June 2013 08:51 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 521
Joined: 14 September 2010

Neutral and Earth of generator to neutral bar of the network (bolted connection)
If the neutral/earth link is out/missing/unavailable on the local LV network, then generator neutral to network neutral and generator earth to network earth

Generator neutral/earth can remain connected to local network when generator not in use.

Phases of generator to network phase busbars, with a 3-pole C/B protecting the generator and generator cables.

Generator chasis earthed with a local earth pin.
 18 June 2013 10:10 AM
User is offline View Users Profile Print this message



clarky280

Posts: 84
Joined: 10 June 2008

Originally posted by: Zuiko

Generator neutral/earth can remain connected to local network when generator not in use.



but the scenario I have is that the generator will in use - supplying a load bank for testing while the transformer is happily supplying the load. is there a problem with this.
 18 June 2013 03:26 PM
User is offline View Users Profile Print this message



statter

Posts: 126
Joined: 06 February 2013

I dont think there is anything specific in the regs on this but IMO this would be a pretty unusual setup.

There is a risk of additional protection operating e.g. most forms of standby earth fault protection use a CT on the earth. This would probably sense a proportion of the fault current but would probably have a IDMT type delay on it so it may discriminate OK.

How sensitive is the installation, how reliable does it have to be? To be completely safe you'd really want 4 pole isolation on the generator.
 18 June 2013 04:52 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11552
Joined: 13 August 2003

I've been trying to picture this again - with the generator connected to the load bank and the mains supplying the general installation, is what you propose any different to having two "separate" TN-S systems that share an earth electrode? (or have two separate electrodes but have shared bonded extraneous-conductive-parts).
- Andy.
 18 June 2013 09:20 PM
User is offline View Users Profile Print this message



statter

Posts: 126
Joined: 06 February 2013

.... and thinking some more isn't this PME - 'the' neutral is earthed at two points......- if you think about having one neutral instead of two - that would be a good reason why you shouldn't do it that way
 18 June 2013 10:07 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 521
Joined: 14 September 2010

Neutrals are shared between "sources" as a matter of course on LV networks.

You might have several 11kv/400V TXs (from different primaries); and occasionaly, generating sets, on the LV network. The Neutral/Earth is continuous between all the sources. The isolation between sources is on the phases only.
 18 June 2013 10:16 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 521
Joined: 14 September 2010

Originally posted by: clarky280
but the scenario I have is that the generator will in use - supplying a load bank for testing while the transformer is happily supplying the load. is there a problem with this.


I can't see a problem.

I've installed many a generator that is feeding a load on one feeder way of a LV board, and a transformer (or more than one transformer) is feeding the other ways; and the generator's neutral earth is bolted to the LV board neutral earth which is commonly bolted to the transformer neutral earth and their respective star points (just ensure the generator is connected to the outgoing side of the feeder so it is not feeding through the bars to the other loads)

It is good practice to do this, because everything is at the same reference potential. The last thing you want is your generator neutral to be lost and floating about. That can cause some serious and irreperable damage.


You have to ensure that you have isolation between the HV and the generator once the generator is running.

Edited: 18 June 2013 at 10:25 PM by Zuiko
 18 June 2013 11:02 PM
User is offline View Users Profile Print this message



jcm256

Posts: 1892
Joined: 01 April 2006

You do not have a 4-pole change over switch, I know costly to fit one now. If the supply was TNS using a 3pole, you would be changing the supply to TN-C-S therefore a four pole is essential. Exception to this could be the mains transformer used for a single consumer. I know what you mean you thinking of two or more generators running in island mode with the problem of circulating neutral earth current in unmatched generators. Just in case a 4-pole changeover with your setup, would be good.

This is American but may help.

http://www.cumminspower.com/ww...6-GroundingAC-2-en.pdf
 21 June 2013 10:00 AM
User is offline View Users Profile Print this message



clarky280

Posts: 84
Joined: 10 June 2008

If we just consider that the neutral is a "live conductor" for a second, then if we have these neutrals interlinked and there is a significant amount of neutral current in both (unsyynchronised) systems - then we could get a non-begligible current flow from the star point of one syustem to the other.

Also, I see no way of installing separate restricted earth fault protection for each source if there is a common neautral-earth link shared between the two systems....
 21 June 2013 10:00 AM
User is offline View Users Profile Print this message



clarky280

Posts: 84
Joined: 10 June 2008

If we just consider that the neutral is a "live conductor" for a second, then if we have these neutrals interlinked and there is a significant amount of neutral current in both (unsyynchronised) systems - then we could get a non-begligible current flow from the star point of one syustem to the other.

Also, I see no way of installing separate restricted earth fault protection for each source if there is a common neautral-earth link shared between the two systems....
 21 June 2013 12:44 PM
User is offline View Users Profile Print this message



Zuiko

Posts: 521
Joined: 14 September 2010

If the system is unsynchronised, then either the generator breaker or TX LV breaker or bus-section between them must be open.

If we consider that these would be 3-pole devices with a shared neutral that is earthed at one point - the TX star point - then you are not going to get current flowing from one-star point to another because there is no circuit. The generator star point is earthed at the TX star point.




Solidy earthed neutrals do not need to be disconnected when isolating transformers. As long as the connection is sound (bolted) then the neutral conductor is considered to be at earth potential.
 21 June 2013 04:18 PM
User is offline View Users Profile Print this message



clarky280

Posts: 84
Joined: 10 June 2008

Originally posted by: Zuiko

If the system is unsynchronised, then either the generator breaker or TX LV breaker or bus-section between them must be open.



If we consider that these would be 3-pole devices with a shared neutral that is earthed at one point - the TX star point - then you are not going to get current flowing from one-star point to another because there is no circuit. The generator star point is earthed at the TX star point.




I agree - there isn't a path for the current to flow between the neutrals in this scenario. So the only possible problem I could imagine with this scenario is that I cannot see a way that restricted earth fault protection could be applied to each source independently of the other...

For this particular installation, I would argue restricted earth fault at the switchboard is a necessity - if just to protect the LV tails. For this job, the LV tails for the TX and gen are about 20m and 60m long respectively...
 21 June 2013 04:20 PM
User is offline View Users Profile Print this message



clarky280

Posts: 84
Joined: 10 June 2008

actually after typing send I see how REF could be applied in this instance by using CTs around the lines and neutral for each source (rather than the ground return sensing)...
 21 June 2013 05:10 PM
User is offline View Users Profile Print this message


Avatar for davezawadi.
davezawadi

Posts: 2693
Joined: 26 June 2002

I can see no problem with the arrangements you describe, the interconnected neutrals do not share one another's currents and the only possible problem is earth leakage monitoring rather than RCD protection. Monitoring the installation earth and neutral currents sat the correct points should get around any difficulties. This is not a shared neutral problem!

-------------------------
David
CEng etc, don't ask, its a result not a question!
IET » Wiring and the regulations » SHaring Neutrals Between Sources

1 2 Next Last unread
Topic Tools Topic Tools
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.