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Topic Title Transformer & Generator Earthing


Date Posted: 25 October 2011 04:10 PM
Posted By: Lee85

I'm working on the design of a distribution network where 2No. 11kV/415V 1600kVA Transformers operate in parallel to supply a common Switchboard.

Each transformer is backed-up by a 415V standby generator (Therefore 2 standby generators in total), whereby if a transformer is disconnected, 1No. generator will start via an automatic changeover arrangement. A Bus coupler within the switchboard will also open which results in half the switchboard being supplied by the standby generator and the other half being supplied by the remaining transformer.

If the other transformer is disconnected at the same time, then in effect we would have 1 generator supplying 1 side of the switchboard and the other generator supplying the other side of the switchboard with the buscoupler in the centre open.

My question is with regard to the earthing arrangements of the generators and transformer.

BS 7430 states that an independant earth electrode system is required for generators. However, these generators are located within the same plantroom as the transformers. Would it therefore be ok to use a common earth electrode system for both the generator and transformer earthing? with a seperate earth bar and connection to the electrode system for the generators? This way a fault on the transformers wouldn't effect the path to earth as there is a seperate connection to the electrodes via the generators earth bar?

Would there be a problem connecting the neutral points of both the generating sets and the transformers to the common earth electrode system?

Hope that makes sense and thanks in advance.

Edited: 25 October 2011 at 04:30 PM by Lee85



Date Posted: 25 October 2011 10:26 PM
Posted By: MickeyB

Lee hi and welcome to the IET forum.
The arrangement you have is pretty standard, save the 'paralleling' of the 400V transformers under normal use.
Circulating current..... this has been addressed elsewhere on this forum, a search for circulating current and restricted earth fault protection should help.

The earth electrode/ reference network can be shared in so far as the 'earth nest/ reference' is common to the earth bars associated with each source, whether generator or transformer. I was told many years ago.... 'there is only one earth on this planet'.

Where you have a shared 'reference' you will have multiple earth paths/ tapes/ cables from the 'reference' to each earth bar to allow for disconnection/ isolation and testing etc.. Each generator will have its own 'earth' via a link to the earth bar on the wall and then from the earth bar to the 'reference' nest of electrodes or rebar/ buried tapes etc...

People worry about touch voltages when you link 400V and 11000V earth bars together.... 'hot' and 'cold' substations etc... This is down to 'poor earthing' in some areas. At 400V with a common earth 'nest' you won't have a problem.

The main thing is to ensure that your NE links are connected correctly.

The neutrals from the LV generators should be brought out and connected to the neutral bar of their switchboard with the NE link 'before' the ACB/ MCCB incomer device. This will ensure that the NE remains in service/ closed when the 4 pole MCCB/ ACB is opened. This can create problems downstream for devices, UPS's that are looking for the NE reference. When you open the 4 pole ACB the 'reference' will be lost for downstream loads, not the Tx or generator, just the downstream. Again, this subject has been addressed in this forum.

The neutral bar will be connected to 'earth' via the NE link. The NE link will connect to the earth bar that is typically on top of the switchboard (or can be internal). You can then take an earth cable or 2 from the switchboard earth bar to the earth bar on the wall and then a cable or 2 or 3 etc.. to the earth references, depends how many pits/ connections you have.

Typically for LV generators the NE links are connected at the set end for a single unit. If you have long cable runs the earth fault current required to 'trip' the upstream MCCB/ ACB can be a problem. It is best to use a Restricted Earth Fault relay, NE link at the switchboard, to protect the downstream cable zone and the generator LV windings. The ACB/ MCCB can then be used for overload protection/ short circuit only.



Date Posted: 27 October 2011 01:42 PM
Posted By: Lee85

Thanks Mickey,

That should prove to be very helpful. I'll digest this when I get a chance. Thanks again.



Date Posted: 01 November 2011 01:13 PM
Posted By: mikenoppen

Originally posted by: Lee85

BS 7430 states that an independant earth electrode system is required for generators.


It's easier to understand when you remember that independent does not necessarily mean separate. Here, it should be read as meaning 'not dependent' , and only has to be a separate installation when there are good engineering reasons.