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Transformer protection

Jam
Hi all,


I am working on the LV side of a scheme with a private HV transformer (>1MVA) and have been told by the HV contractor that since it is fed / protected by a fuse switch, it is not appropriate to make use of the transformer overtemp / tank pressure / oil level relay to disconnect at HV, reason being that it can't break a fault and that's what the fuse is for. I do get the principle that RMU switches are not fault-breaking devices and you shouldn't, for example, use an external overcurrent relay on the HV terminals to operate one, but should these functions not be considered to prevent the fault developing sufficiently to fire the fuses in the first place? For example if the oil level alarm triggers via SCADA, or the oil temp trip triggers despite LV load shedding, is there really time to wander up to site and manually operate?


Also I would have thought that you'd be pretty unlucky for the fault to strike at precisely the same time as the transformer relay, even if both happen in a single event, due to thermal inertia etc?


Please correct me before I make a fool of myself questioning it in public!


Ultimately it's not our scope but we'll be picking up the multicore wiring one way or the other; also for future reference.


TIA,

Jam
  • Former Community Member
    0 Former Community Member
    If they are protecting the HV side with fuses, then basically you have no real protection until the fuse operates, and the damage at that point could well be catastrophic and the transformer is lost (albeit safely lost).


    There is no reason that you couldn't put the tank oil temperature into the LV circuit breaker (high temperature on the oil is basically an overload condition.


    High oil pressure is usually indicative of an interwinding fault so is pointless as an input to the LV - it typically goes to the HV side, but in this case, you can't


    Low oil, will lead to a high temperature condition, so again, if you put that into the LV side, you kill off the load on the transformer.


    What the contactor is suggesting is basically a very simple scheme with the rare occurrence of an internal TX fault making the transformer effectively "sacrificial"


    Just be sure they aren't actually saying they have CT operated time limit fuses acting on a circuit breaker (acting as a decontactor) - if that's the case, then they could easily integrate to the HV side to provide a unit protection scheme from HV, through the transformer to the LV breaker (usually including restricted and unrestricted earth fault)


    Regards


    OMS
  • 0
    Thanks OMS, I agree with what you've said (and it's definitely not a TLF arrangement) ... But what then is the point of the transformer protection relay (e.g. RIS2) that they / the HV consultant have specified in this situation if it's not to remove the transformer before said sacrifice? Is it really just for alarms?


    To clarify I wasn't intending to wire up the trip contacts to the LV breaker (though overtemp alarm will shed load); my question was whether they really can't operate the HV. The comment from the HV contractor was a surprise as I've seen quite a number of sites with that exact arrangement.


    (I am assuming that there is is a facility to open the switch disconnector remotely, e.g. for E/stop)


    The upshot is likely to be that I'll be bringing out the contacts to a marshalling box and sending a multicore from there to the RMU "for future use", picking up anything not claimed by the HV contractor for SCADA alarms.


    Jam
  • Former Community Member
    0 Former Community Member
    What's upstream of the HV switchfuse - ie have you got HV circuit breakers feeding RMU's with switchfuses between the input and output switches feeding the transformer ?


    There must be a CB somewhere I would have thought


    In this case the TX protection relay is pointless, although it may well have an alarm function - which unless it goes somewhere is also pointless


    HV switch fuses don't normally have a trip function - they are just manually on or off, and faults just blow one or more fuses, but you will almost certainly lose the transformer first


    Regards


    OMS


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