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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
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    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.
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  • 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.
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