Forums » Electrical Engineering

Grounding generator.

    • 43 posts
    December 27, 2019 9:11 AM PST

    We are looking at a project (conceptual design) which will include four 4160V, 2KW synch generators tied to grid and to in-house loads.

    Regarding grounding, various references, including Blackburn's book, discuss options for grounding, including grounding each generator or installing a common grounding xfmr or NGR at the main paralleling switchgear.

    I would be leaning towards an individual NGR for each generator. However, I'm not certain of grounding of bus when all generators are off-line, and utility is still supplying house-loads.

    Is there a configuration that is more typical than any other?

    • 103 posts
    December 27, 2019 6:48 PM PST

    Do not know if any particular way is more or less typical.

    With an NGR for each of the four generators the fault current will be four times greater when paralleled.

    You could switch out the NGR for each one as it connects to the bus, leaving only one connected.  If that generator suddenly went off line the system would become ungrounded.

    I think the simplest way would be to treat all sources as non-separately derived, a common NGR would serve all generators & the transformer.  

    I am hardly an expert on this subject.  I only offer my thoughts.

     


    This post was edited by Steve Ward at December 27, 2019 6:49 PM PST
    • 75 posts
    January 5, 2020 11:03 AM PST

    Assuming you meant 2000kw (2500kva) generators.

    Wild guess:  The utility transformer is in the 10MVA range

    Highly recommend purchasing a copy of IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems

    Following Steve's post:

    I do like NGR.  It limits the damage one gets from a ground fault.

    One common NGR:

    If one NGR is used, it is necessary to switch the individual neutral connections.  The neutral switches are required to pull a unit off-line for maintenance.  One just uses a standard three pole disconnect.  One pole opens the neutral circuit.  A second pole opens the gen start/run circuit.  Or in the case of the utility transformer, the second pole opens the transformer primary CB

    However, there is an issue with having all the sources neutrals tied together.  If one of the gens develops an internal fault, the alternator has failed and a rebuild (rewind) is required.  The trick is to keep the internal fault from damaging the iron core, which would leave the alternator scrap - not rebuildable.  The problem is with the sources neutrals all tied together,  the other paralleled sources can deliver enough fault current to severely damage the core.

    Multiple NGRs - one per unit:

    At 4160V this is pretty good choice.  Size the resistors for maybe 50A.  Set the neutral current trips to 1 sec.  That will eliminate switching transients issues.

    Total current for the five sources is 250A.  Ground Fault Protective relays will open (1 sec) - no damage issues.

    Generator (and utility transformer) internals are protected with differential relays.  The alternator is still toast in the event of an internal fault, but it will be rebuildable.

    Utility Transformer

    It gets its own grounding resistor, just like the other sources.

    Sometimes utilities don't like to provide anything other than a solidly grounded system.  The project is in multiple $10M range.  If the utility balks, the project buys the transformer and gets what they want.