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Thread: 3 generators one supply

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    3 generators one supply

    Need clarity of thought - am I missing something.

    An LV panel currently has 6 MCCB feeding 6 different submains.

    Client wants to run 2 sub mains of one generator, another 2 of another generator and a final 2 off another generator.

    What am I missing. I have installed plenty of generator in many different industrial and commercial installs, one at a time, but never 3 to one LV room.

    The pairs will have to be separated into individual panels and each will have its own neutral bar.

    What is confusing me is that the neutral and earth on the start point of the generator is connected (fine) but now there is 3 points where neutral and earth are connected (at each star point of each generator). I know that the panel feeding the generator supplied circuits has a separate neutral to the main neutral.

    It may be because I haven't had a day off for a while for a while but if at the star points, neutral and earth are linked in 3 generators (with the common earth) then essentially the neutrals from the 3 generators are linked as well.

    That doesn't sit right for some reason in my tired mind. What can go wrong there...

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    Diamond Member AndyD's Avatar
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    What's the earthing arrangement of the main Eskom supply?
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    Interesting topic, I'm not an expert in this, so hope you guys don't mind me joining in on the thread.

    Doesn't the SANS regulations say that you should not bond the neutral and earth?
    I guess this depends which earthing system the location uses?

    And if you break both live and neutral with a switch over to generator it should be safe to bond the neutral and earth I assume?

    Bonding multiple generator earth/neutrals should be the same as bonding multiple transformer neutrals (which in turn may be bound to earth if not isolation type). Given that the generators are not referenced to each other at all before bonding, is there really any reason it would matter how many you bond?

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    Quote Originally Posted by skatingsparks View Post
    Need clarity of thought - am I missing something.

    An LV panel currently has 6 MCCB feeding 6 different submains.

    Client wants to run 2 sub mains of one generator, another 2 of another generator and a final 2 off another generator.

    What am I missing. I have installed plenty of generator in many different industrial and commercial installs, one at a time, but never 3 to one LV room.

    The pairs will have to be separated into individual panels and each will have its own neutral bar.

    What is confusing me is that the neutral and earth on the start point of the generator is connected (fine) but now there is 3 points where neutral and earth are connected (at each star point of each generator). I know that the panel feeding the generator supplied circuits has a separate neutral to the main neutral.

    It may be because I haven't had a day off for a while for a while but if at the star points, neutral and earth are linked in 3 generators (with the common earth) then essentially the neutrals from the 3 generators are linked as well.

    That doesn't sit right for some reason in my tired mind. What can go wrong there...
    Recently installed 3 Gensets for one supply, they can sync and act as one automatically. If you look in SANS you will see that at any given moment you can have only one PEN, if you have more than one, loop currents will form, you can remove the earth to neutral bridge at the generator and put the common starpoint as close to the gensets as possable, there are other ways which automatically does this if the gensets are not in the same area

    It gets quite complex and will advise copying an existing system or else appoint an electrical consultant

    Regards

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    Yep, its the loop currents thing that was the thing that was bugging me.... Then I got into... what happening only one area goes down not just a complete power failure from the municipality side. If there was an overload (for what ever reason) and the Circuit breaker covering the 2 circuit breakers for the 2 sub mains.... then that one generator would start up or would it have to be a case that all generators would have to come on if one did (then you could do the common star point as mentioned above).

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    Quote Originally Posted by AndyD View Post
    What's the earthing arrangement of the main Eskom supply?
    TN-S. Need to double check as at the moment I'm taking that info from a drawing but TN-S.

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    It's about getting the arrangement right.

    There needs to be a separate crossover switch for each submain.
    Each crossover switch needs to switch the phases and the neutral. (Remember the neutral must connect before the phases connect, and neutral must disconnect after the phases disconnect).
    The earth neutral bridge must be on the generator supply side of the crossover switch (and as close to the generator as possible).

    This way, the neutral conductor between the generator and the crossover switch acts either as a supply neutral when connection is made at the crossover switch point, or a redundant part of the earth (for the generator) when the connection is broken.
    Last edited by Dave A; 09-Jun-17 at 09:26 AM.

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    You do not mention the supply size or generator size needed to feed the different circuits.
    Assuming the consumer is a bulk user and has MV supply to the premises , owns the transformers , then there is no need for a 4 pole changeover .
    The reason for 4 pole is due to "only the supplier may earth the neutral " - if you own the transformer you are the supplier.

    I have pasted some relevant info I wrote up a while back below.
    Happy reading

    Written 15 Sep 2015

    My feeling is we need to look at the reg’s then come back to the Annexure S pages
    I have listed an abbreviated version of the regulations - double check against SANS.

    6.1.6 – pg 83 – Neutral shall not be connected direct to earth on the load side of point of control.

    7.12.3 - Earthing requirements

    7.12.3.1.1 - pg 258 - Operation of the protection devices shall not rely upon the connection to the earth point of the main supply when the generator is operated as a switched alternative to the main supply

    7.12.3.1.2 - Installation supplied from a combination of transformers and alternative supplies , the neutral points of each of these items shall be connected to a single earthed neutral bar .This earthed neutral shall be the only point at which the neutral of the installation is earthed.

    7.12.3.1.3 - Where alternative supplies are installed remotely from the installation and it is not possible to make use of a single neutral bar which is earthed the neutral of each unit shall be earthed and these points bonded to the consumers earth. The supply from each unit shall be switched to disconnect all lives and the earthed neutral point from the installation neutral when the alternative supply is not connected.

    7.12.3.1.4 - Where only part of the installation is switched to the alternative supply the neutral bar shall be split.


    Now if we look at Annex S.1 – the neutrals of transformer and gen are being earthed at one point only (7.12.3.1.2) .The complete installation is being fed from both supplies and therefore 7.12.3.1.4 does not apply. This would apply where the factory owns the transformer and is supplying his complete installation. The factory has access to the star point.
    If the factory was taking LT supply and the municipality owns the transformer he does not have access to the star point or alternatively to the neutral bar that is earth and will therefore be earthing the neutral at a second point in the installation – goes against 7.12.3.1.2 and therefore to comply you would need to take into account 7.12.3.1.3 *
    Annex S.1 is showing the neutral earthed at the point of control, technically before as it would be the star point earthing bar of the transformer – It would conform to 7.12.3.1.2 - “the earthed neutral shall be the only point at which the installation is earthed” –

    The whole reasoning behind the 4 pole set up, the way I see it
    Logically the cable sizing from transformer and genset would be the same size, if both where say 800Kva, so that if you lost the star point on the transformer and the genset star point become the main star it would not overload the cable.
    *If you have a 800kva transformer and a 10KVA genset *, the cable size would not be equal and if you lost the transformer star point and the genset became the star point for the installation then you run the risk of the neutral conductor becoming a fuse wire and catching fire.
    My feeling is that the regulations are written around this scenario.

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    O.K. Given this some thought.

    The Generators - 2 are 30 - 40 meters away (1 x 165 KVA and 1 x 225KVA) the other is near the Main DB (1 x 165).

    Now as I understand it in this case.

    6.1.6 – pg 83 – Neutral shall not be connected direct to earth on the load side of point of control. But where alternative supplies are installed remotely (as is the case here)
    from the installation and it is not possible to make use of a single neutral bar which is earthed the neutral of each unit shall be earthed and these points bonded to the consumers earth. The supply from each unit shall be switched to disconnect all lives and the earthed neutral point from the installation neutral when the alternative supply is not connected.

    So 7.12.3.1.3 overrides 6.1.6 and they must disconnect all phases and the neutral from the generator.

    But the separate neutrals (7.12.3.1.4 ) will all be be linked as the earths from the star point of the generators goes to installation main earth.

    So now I'm looking at 7.12.4.2. I guess I have to synchronise the generators which is another then it won't matter if the neutrals are linked.

    Then there is harmonics....

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