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Diesel Generator Fault Levels

MG
Hello all,


I am looking to specify a temporary mobile diesel generator and as such looking into the disconnection times and the maximum fault current provided by the genset.

I have contacted a provider and got the specification of the alternator in their genset, images below of the fault current graph and the alternator per unit values.


I am looking into understanding what values to use. Am I correct in getting the sustained fault level and for an earth fault to apply the multiplication factor of 2.5 for sustained? I will make sure my disconnection times are less than the sustained maximum duration in order to not trip the generator breaker and loose all of the supply.


Alternator is 50Hz 415V

fc5681c638e7bada3947e466b96400fd-huge-newage-hci534e_1.jpg

3106a5e7994134e881a5487cbbe73194-huge-newage-hci534e.jpg


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    I'd like to think that you are not contemplating overloads or shorts of such duration that the quasi-steady state condition is ever reached before before the supply is automatically removed from the faulty section.


    Do you need to use some very long time delay breaker as the first device ?

    ADS needs to be sensibly fast, and I'd suggest that a few hundred mS would be enough to allow discrimination with final circuits on B and C type breakers.

    How many tiers of fuses and breakers do you have between the windings and the final loads, and how big is the system you are generating for ?

    In the absence of detailed design, a time factor of 3:1 between tiers is usually reliable for a first stab.( .i.e. 1sec/300msec/100msec/ near-instant  sort of thing for 4 layers)

    For normal operation stay below the rated KVA,  say 800A /phase or less, for sizing the main breaker I'd be assuming you'd be fast enough to use either X'd or X''d to estimate a PSSC, and the 2 figures are both close enough to being a multiply by ten for arm waving purposes.

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  • 0
    I'd like to think that you are not contemplating overloads or shorts of such duration that the quasi-steady state condition is ever reached before before the supply is automatically removed from the faulty section.


    Do you need to use some very long time delay breaker as the first device ?

    ADS needs to be sensibly fast, and I'd suggest that a few hundred mS would be enough to allow discrimination with final circuits on B and C type breakers.

    How many tiers of fuses and breakers do you have between the windings and the final loads, and how big is the system you are generating for ?

    In the absence of detailed design, a time factor of 3:1 between tiers is usually reliable for a first stab.( .i.e. 1sec/300msec/100msec/ near-instant  sort of thing for 4 layers)

    For normal operation stay below the rated KVA,  say 800A /phase or less, for sizing the main breaker I'd be assuming you'd be fast enough to use either X'd or X''d to estimate a PSSC, and the 2 figures are both close enough to being a multiply by ten for arm waving purposes.

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