Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 16 replies
  • Subscribers 32 subscribers
  • Views 4784 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Neutral Earth connection

Former Community Member
Former Community Member
Interrogating an issue with a substation set up - The substation is a split one with two 11Kv to 400v cast resin transformers that supply each substation half. The set up incorporates back up generators that kick in on a power cut, I noticed that we have a UPS system installed that takes two supplies from the substation, one from each half. either one is on and the other supply stays as backup to the UPS system. What was picked up is that when there is a power cut and until the power is restored there is a load shed that disconnects the supply to the UPS. This UPS remains on feeding 400v three phase to the site, but the earth neutral connection is lost until one of the supply breakers closes back up again. means that the output is not referenced to earth for a few minutes till the gens spin up. I believe that through checking manuals and reading up this isnt a good scenario to be in in terms of sensitive equipment. 


I would be looking at providing a transformer in there along with a local earth neutral link so that this is maintained on all eventualities. has anyone come up with this sort of issue before ? it could also be addressed through neutral earth contactors at the appropriate places, Had looked at possible three pole devices on circuit breakers but that does not allow full isolation to be achieved and could also lead to possible paralleling of the neutral earth link at each transformer.
  • 0


    Also would have concerns about if the neutrals linked together between the substations then there could be issues with the sizing of the cables, fault current from two systems would have to be taken into consideration as the system could end up sharing the neutral earth links.



    This is not uncommon with two transformers operating in parallel, it is perfectly OK if there are 2 NE links, if, and only if, both links, and the wiring between them, is up to the maximum combined fault current.
  • Former Community Member
    0 Former Community Member

    mapj1:


     



    Also would have concerns about if the neutrals linked together between the substations then there could be issues with the sizing of the cables, fault current from two systems would have to be taken into consideration as the system could end up sharing the neutral earth links.



    This is not uncommon with two transformers operating in parallel, it is perfectly OK if there are 2 NE links, if, and only if, both links, and the wiring between them, is up to the maximum combined fault current.

     




     

    It's absolutely essential with two transformers if under any circumstance the bus section can open (4P) or the ACB from any transformer can open. You can't leave a system neutral unearthed under any configuration


    For a standard two transformer substation, each half of the LV switchboard would be equipped with a N-E bond and normally 3 pole incoming devices - and in many cases the bus coupler would also be a 3 pole  device with a neutral link


    It would be pretty unusual to be considering the use of N-E contactors in this set up  - this fear of "circulating" currents is often unfounded and as Mike points out, is just usually solved by sufficient CSA of material.


    If you have multiple generators (at LV) then yes, it would be more usual to have N-E contactors (or a N-E Switchboard) so typically, you only have one bond in place (the rest are just pushing the phases and don't need the neutral reference)


    Regards


    OMS
  • 0
    Historically there has been a fear of running transformers in parallel with a neutral/earth link on each transformer. With similar but not identical transformers, substantial circulating currents could result in extra heating of transformers and cables.


    However with a pair of well matched transformers, from the same manufacturer and of identical specifications, I doubt that significant circulating currents would result.
  • 0
    The problem of putting a 230V transformer in parallel with a 231 volt one is not really a function of the earthing, and in any case great care should be taken to avoid a significant mismatch across a low impedance, as a lot of energy is wasted warming up the wiring. Paralleling transformers that are not carefully matched, ideally from the same maker and pattern, is generally  not sensible.
  • 0
    And often gives rise to problems with the maximum fault level so that some installations require break before make changeover, others are a bit more lenient and one is set to trip after the other closes.
  • Former Community Member
    0 Former Community Member
    No possibility of the transformers being run in parallel - two substations each one supplied from 11Kv / 400v transformer and a coupler between the LV sections, coupler can only close if either of the board main LV is open. Interlocking prevents the transformers being paralleled up.


    N-E links on the supply side of the incomer ACB's both incomer ACB's open no N-E link and the UPS is still outputting 400v with no reference between the neutral and earth. looking at this it should be as said three poles for each incoming ACB and the N-E link rated for transformer and UPS power fault currents combined.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines