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Topic Title: Neutral Earth linking with Generators
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Created On: 03 August 2011 01:52 PM
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 03 August 2011 01:52 PM
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Adam1V

Posts: 2
Joined: 03 August 2011

I'm in the process of upgrading some existing generator control equipment and wanted to ensure I have a good understanding of the requirements and correct way of controlling the new equipment.

We have a single (existing) 600kva transformer which will eventually be used to paralleling for periods of time (customer dependant).

Am I correct in saying that according to G59/2 we're required to link the Neutral to Earth only when running the site equipment from the generator and not paralleling?

Lastly, how should the neutral/earth contactor be sized? I understand by linking all three poles in parallel, we could get away with an 185A AC1 contactor.

Any further information would be a great help,
 03 August 2011 04:23 PM
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ArthurHall

Posts: 736
Joined: 25 July 2008

You are correct in your thinking, you should only earth your generator neutral when in island mode.
The DNO may want to see the interlocking demonstrated.
The neutral should be sized to carry the maximum earth fault current.
 15 August 2011 09:24 PM
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Nedryerson

Posts: 39
Joined: 12 December 2009

Adam,

The DNO will require that only one point of earthing is in service at any time however ...

It may be advisable to prevent spiking to ensure when going from grid to island that the NEB is closed before the transformer ACB opens. Also, when going from island to grid the NEB should open after the transformer ACB closes. In addition, When bringing the generator on to the bars the NEB should closed and shed immediately following synchronisation.
 15 August 2011 09:46 PM
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Nedryerson

Posts: 39
Joined: 12 December 2009

Adam, I engineered a similar scheme some time ago and experienced phase voltage instability on switching from grid to island and vice versa because there was momentarily no system neutral in service. Certainly the DNO will require that only one point of earthing is in service at any time however ...If you experience spiking you may wish to consider when going from grid to island that the generator NEB is closed before the transformer ACB opens. Also, when going from island to grid the NEB should open after the transformer ACB closes. In addition, when bringing the generator on to the bars the NEB should be closed first and shed immediately following synchronisation.
I recall the overlap was in the order of milliseconds.

Hope this helps
 15 August 2011 09:48 PM
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Nedryerson

Posts: 39
Joined: 12 December 2009

Adam, I seem to have sent my reply half way through writing it !
 23 August 2011 08:28 PM
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bob1963

Posts: 6
Joined: 23 August 2011

Am I right in thinking NER,s will reduce this effect as it is not a direct link?
 09 September 2011 11:29 AM
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Adam1V

Posts: 2
Joined: 03 August 2011

Im slightly confused as ive found a set of drawings showing a neutral earth breaker disconnecting the neutral feed to the site, rather than the neutral-earth link.

Can anyone confirm which is correct?
 15 September 2011 12:00 PM
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mikenoppen

Posts: 24
Joined: 17 June 2004

There is some confusion here because the system hasn't been described fully. Your transformer will be isolating you from the HV network. If you are not introducing an earth onto the DNO's network, they won't be too concerned. If it is your transformer, I presume your generator is on the LV, same as your site. The LV system neutral must be earthed at all times, for safety reasons. This is normally done at the transformer or main switchboard, and should be left. The generator should have the neutral earthed as it is run up, but this is disconnected in parallel operation, to prevent circulating currents. If the main neutral-earth link is at the switchboard, and the transformer can be disconnected without breaking this, there is no difference between island and synchronised mode (as far as the earthing of the system is concerned, anyway). If the DNO have provided your earth point, they will insist that your site is isolated in island mode, to prevent neutral voltages being fed back onto their terminals.
If your transformer is only there to connect the generator onto the network, that is a different matter.
Hope this helps.
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