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UPS Fault Level Calculation

Semaj

Does anyone know if there is a standardised way of calculating fault levels at the output of a large UPS like IEC60909 does for transformer and generator supplied faults?

I imagine things will change between manufacturers but a good rule of thumb would be very useful as it seems that no matter the installation, downstream of the UPS the breaker settings have been poorly thought out and my concern is that these downstream circuits will not actually acheive ADS due to the fault level being too low to trip the downstream breaker because the trip settings are too high.

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  • 0

    Manufacturer's info, but a rough guide for inverter output is they typically current limit at around 20 % above their nominal output rating, or:

    Ig(max) = 1.2 × Ig

    Where

    Ig(max) is the prospective short-circuit current of the inverter at the output terminals; and

    Ig is the maximum nominal output rating of the inverter.

    (Section 14.2 of the IET Electrical Installation Design Guide.)

    Up to this maximum current limit, the inverter typically acts as a constant current source and internal impedances can often be ignored, although again the manufacturer may be able to provide more information on their product to permit you to take something into account so that the loop impedances for RCD (and if they meet the criteria OCPD) can be verified to comply with BS 7671.

  • 0 in reply to gkenyon

    I think you meant to say constant voltage up to the 120% of I max, and then constant current and falling voltage, or indeed roll-back (both falling voltage and falling current) as the load increases beyond that point.

    Mike

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  • 0 in reply to gkenyon

    I think you meant to say constant voltage up to the 120% of I max, and then constant current and falling voltage, or indeed roll-back (both falling voltage and falling current) as the load increases beyond that point.

    Mike

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  • 0 in reply to mapj1
    mapj1 said:
    voltage

    Sorry yes, in self-commutating mode that;s the case.

    Effective constant current-source if inverter is grid-connected (for a given output power) ... which a UPS inverter would never be.

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