Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 4 replies
  • Subscribers 32 subscribers
  • Views 899 views
  • Users 0 members are here
Options
You may also be interested in
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

Isolation transformer tripping

Jam
Hi all,


Was undertaking a visual survey of an existing installation the other day and the site electrician mentioned as an aside that the auxiliary isolation transformers for each station (of which there are several) need to be closed in a specific sequence; expecting to see that inrush dictated the secondary side protection must be opened on tx energisation I was surprised to see that in fact the secondary MCB must be closed otherwise the primary trips (with visible arc).


The equipment is built into a panelboard, fed via a private transformer (no upstream protection) earthed IT. The auxiliary arrangement is a 3P C25 MCB (50kA) -> Tx 10kVA Dyn11 -> 3P C25 MCB (16kA). Downstream is the auxiliary panel including SPD, measurement/control circuits, cooling fans and small power/lights.


There are a large number of units all doing the same so it's unlikely to be wiring errors or equipment faults (though this has not been tested and it's possible that the design was in error or there's a systemic installation problem). The earthing is poorly defined generally on the site but the NE link is made at the aux tx secondary N so it's TN.


It's not (currently) within my scope to investigate but since then I've been scratching my head. The best I can offer is inrush current somehow is mitigated due to something downstream. I've suggested the electrican try with different loads disconnected as it might narrow down what's holding the breaker in and perhaps give us some clues, but we didn't have time to investigate further at the time.


I might be missing something obvious but would like some ideas as to what's going on!


Jam
  • 0
    Not inrush on the first half cycle then but a damping of the transient so it takes less time. (a lower Q if you will.)

    Ideally you'd close onto the supply when current was at or near zero crossing, i.e. for a pure inductor near the voltage peak, and already have the core magnetised up in that direction.

    In practice you cannot do that so there are several cycles where the current and voltage are not in the right phase relationship, and the less loss in the circuit the longer and more violent this transient reactive current is.

    Still poor design - either step start with series resistance and 0.1 - 0.5 second time delay or thermal inrush limiting would be better than what is in effect a partial damping with shunt resistance.
  • 0
    Thanks Mike,

    Forgive me but are you saying that the magnitude of the initial inrush would be the same but that it is damped quicker with a load, so the MCB just isn't quick enough to trip, but without it "rings" for long enough to go?

    Jam
  • 0
    yes, and actually I'd expect the initial current to be higher if anything when starting on load, but the ringdowm time to get I and V pulled into the right phase and amplitude relationship to be less.


    Sometimes you can even hear this with toroidal transformers, where there is an initial 'thump' and then a sort of  decaying  buzzing "twang" that can last a good half a second.

    I suspect the breaker is not seeing the fast onset of the inrush, but is seeing the longer tail while the current oscillations die down. Starting on load is not an ideal solution though, and it must be pretty marginal.
  • Former Community Member
    0 Former Community Member
    The system designer may well have been concerned with a ferroresonant effect, so has mandated some load on the secondary side (usually 10 - 15%) to substantially dampen the effect. You shouldn't really be seeing this when you are switching all 3 poles simultaneously, but in can occur with the right (or wrong) combinations of HV capacitive reactance, distance of the switching from the TX  and the significant inductive reactance of the transformer core at switch on


    Regards


    OMS

Community Rules & Guidelines