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Tripping coil inrush current

Jam
Hi all,


I have been asked to re-specify a UPS for an existing switchboard with 230Vac shunt tripping coils and spring motors. The installation was designed and built by others but not commissioned so I can't assume that the existing UPS is up to the job in all realistic scenarios. While we can stagger the motor operations, my concern is that, in island mode, the inrush current to the tripping coils will exceed the overload capacity of the UPS and cause the inverter to panic and shut down before the tripping operation is reliably complete.


Does anyone here know whether a 7-10x inrush assumption is reasonable on this scale (200-700 VA)? Coil operation is fast - 2 to 4 cycles, although presumably it's energised for a bit longer while everything clears - so might this already be considered in the device datasheet?


Alternatively, do small(ish = 10-20kVA) UPSs not react quickly enough to short overloads? After all, it's kind of like a downstream fault being cleared by an external device.


(I would normally use a DC battery supply and the IEEE-485 method for sizing, if it's on any relevance)


Thanks,


Jam
  • 0
    UPS makers that specify overload at all (many do not), generally are a lot less generous than say a transformer or a rotating generator would be


    a typical 10kw Eaton unit has the following spec
    10 min > 100 – 110% load

    1 min > 110 – 125% load

    5 sec > 125 – 150% load

    300 ms  150% load

    so an inrush of 150% could be enough to cause trouble.


  • 0
    Thanks Mike,


    From this old thread I had been basing my first draft numbers on a 200% device... but haven't found one yet, and some are even less. Useful to see that Eaton specify limits for 300ms as that puts paid to my musings that perhaps it'll be too slow to react.


    Any thoughts on coil inrush?
  • Former Community Member
    0 Former Community Member
    Inrush current to shunt trips could easily be in the order of 100 x the duty VA rating if operating at 230V (for typical 25millisecond breaker operation)


    Whilst that peak isn't present over the full tripping cycle, it can be there long enough to seriously disturb the UPS


    If you are going to do this with a UPS then the VA rating needs to be seriously larger than the total operating VA - you would probably be looking at an industrialised or ruggedized UPS with 12 pulse rectifier and a minimum of 300% overload capacity - something like the AEG Protect 8 series derated, to give you a 300% overload capability for say 5 seconds and a short circuit capability of 3 x design rating. (typically 3 phase in and 1 phase out at 230V  - smallest is around 10kVA rating)


    Regards


    OMS


  • 0
    I presume the shunt trips and rewind motors are not the major load on the UPS, and that there is also some proper load as well  ?


    (after all 10kVA is quite a lot of shunt trip !) If there are not many it may be worth considering re configuring for a DC trip if they really are the dominant load.

    Do you have part no.s  or data?
  • Former Community Member
    0 Former Community Member
    I read it as the tripping relays and the motor recharge mechanisms are the only load on the UPS


    I'm surprised a little that the motor mechs aren't recharging from the available mains or generator supply  - probably not that much point in opening and closing CB's on a dead system (and if you had to, then hand pumping the spring reserve is always an option)


    From there swapping the shunts to DC and providing a modest battery charger switch trip and close unit would make maybe more sense


    Regards


    OMS
  • 0

    OMS:

    Inrush current to shunt trips could easily be in the order of 100 x the duty VA rating if operating at 230V (for typical 25millisecond breaker operation)




    Really 100x? That seems huge, but, since I've never measured it, I'm prepared to be advised! Just checking not a typo really




    Whilst that peak isn't present over the full tripping cycle, it can be there long enough to seriously disturb the UPS


    If you are going to do this with a UPS then the VA rating needs to be seriously larger than the total operating VA - you would probably be looking at an industrialised or ruggedized UPS with 12 pulse rectifier and a minimum of 300% overload capacity - something like the AEG Protect 8 series derated, to give you a 300% overload capability for say 5 seconds and a short circuit capability of 3 x design rating. (typically 3 phase in and 1 phase out at 230V  - smallest is around 10kVA rating)




    Thanks for the recommendation OMS; definitely taken on board.




    mapj1:





    I presume the shunt trips and rewind motors are not the major load on the UPS, and that there is also some proper load as well  ?




    The UPS is dedicated to a switchboard, so while there are standing loads such as protection relays and indicator lamps that affect - indeed are the main factor in - the standby time, when the protection operates it gets quite busy.




    (after all 10kVA is quite a lot of shunt trip !) If there are not many it may be worth considering re configuring for a DC trip if they really are the dominant load.

    Do you have part no.s  or data?




    Perhaps I misled, sorry: The 10kVA isn't the shunt trip rating (!) but the order of UPS I was thinking I might need. (I was actually thinking 12-16kVA but perhaps even that's too small by the sound of it.

    For example, the open shunt trips are (mostly) stated as 180VA 45ms; under the existing scheme it's possible that ten of them will operate simultaneously on a loss-of-mains event

    There are other coils with different ratings but keeping it simple for the moment.




    OMS:



    I read it as the tripping relays and the motor recharge mechanisms are the only load on the UPS


    Yes. (Well, as above, other than indicators and protection relays, which do add up but are a bit more understandable)


    I'm surprised a little that the motor mechs aren't recharging from the available mains or generator supply  - probably not that much point in opening and closing CB's on a dead system (and if you had to, then hand pumping the spring reserve is always an option)


    The plan would be to stagger the motors, so to be honest I'm less worried about those. It would be preferred if it were possible to reduce dependencies


    From there swapping the shunts to DC and providing a modest battery charger switch trip and close unit would make maybe more sense



    I agree it's not how I would have done it! It's reassuring to see you're thinkng the same. But  the switchgear manufacturer sold the kit with a 230Vac option so I thought I might be missing a trick.

    I will investigate swapping out though


    Thanks both!


  • 0
    You could also think about using a 1:1 Ferroresonant Transformer on the output of the UPS.  You might be able to get the inrush current that the UPS sees down with this. I have seen these used successfully for riding through voltage dips and although this is the reverse I think the same magnetic energy storage mechanism will help to cut the instantaneous load on the UPS.  I suspect you would need to rig up a test as calculation / modelling would require lots of data which you prob don't have and be beyond most software.  You might need to try a seriously overrated unit to get the effect you need.

    Let us know if you go this route.


    Peter
  • Former Community Member
    0 Former Community Member

    Jam:


     


    OMS:

    Inrush current to shunt trips could easily be in the order of 100 x the duty VA rating if operating at 230V (for typical 25millisecond breaker operation)




    Really 100x? That seems huge, but, since I've never measured it, I'm prepared to be advised! Just checking not a typo really



     

    In an AC coil the first instant you energize it there is very limited impedance, because there is no mutual inductance from the magnetic fields of the shunt trip core. So for that instant, all you have is the resistance of the coil wire itself and that is so low that it is essentially a "short circuit", so all available fault current at the relay terminal is drawn. A cycle later, the core fields begin to interact with mutual inductance and create impedance, bringing the coil current down.


    So at switch on, the inrush is very high but very short - on a mains supply with a very low source impedance that's not usually a problem - on a UPS with very limited source impedance, that short inrush can often be a problem - it's not necessarily a case of increasing the UPS kVA - more a case of selecting a UPS that can handle that high inrush for the required time without fooling itself into thinking there is a major overload or short circuit emerging.


    Get the shunt trip data (both VA and inrush VA over the defined time  - I think you said operating time is 45mS, but you need the energising multiple of the VA (or the actual inrush peak VA)) and put that in front of a reputable industrial UPS supplier  - you may need to consider a transformer based rectifier UPS rather than a transformerless PWM machine.


    It's not impossible to achieve what you want to do, but not necessarily with a generic UPS.


    Regards


    OMS

  • 0
    Hi all,


    Apologies for letting this go cold but I have recieved an update from Schenider (who make the coils and motors). Tables and charts attached for future reference by those who're interested, but the upshot appears to be that the stated trip coil current is indeed the maximum including inrush, since switchgear operates before everything can settle down. The motors only appear to have about 3-4x operating current as inrush. As suggested I'll be taking this data to UPS suppliers to see who can provide a stable supply.


    Thanks all for your help, very much appreciated.


    Jam

    attachments.zip
  • Former Community Member
    0 Former Community Member
    Interesting numbers - Schneider look like they are operating some degree of current limiting on the relay coils. The operating currents look right to me, just surprised that the inrush it totally limited to the operating current


    The motors are easier to control as you probably are far less time dependant on the operation for rewind so they can be softer starting.


    Be interesting to see what a reputable UPS supplier comes back with


    Regards


    OMS

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