Why doesn't YNd1 transformer biased diff protection operate from Aux. ETx bleedoff current?

[electrical429]

As per below SLD it is normal practice within UK to provide 33 kV earth reference + substation LVAC supplies from ZNyn11 earthing auxiliary transformer connected on 33 kV side of YNd1 132/33 kV transformer. Logically this creates a bleedoff current for biased differential protection zone. Why doesn't protection trip during normal site operation? Is it because biased diff. protection settings take into account this bleedoff current?

Also, wouldn't any fault on substation LV network fed from this earthing aux. transformer operate main transformer biased diff. protection?

 

[Mbrooke]

I'm guessing the load on the aux trafo, plus its high impedance (what is the kva? 150?) doesn't contrbute much current to the differential relaying.

 

[cranky108]

Also filter out zero sequence on secondary.
Just set the threshold a little higher for tripping.

 

[stevenal]

87T minimum pickup and slope settings likely put station service load into the no trip zone. Unless you've had secure LVAC faults, the effect of such a fault would need to be calculated.
Are you sure of the one-line? Transformer bushing CTs would be my choice, which would exclude station service from the differential zone.

 

[sushilksk]

What is the MVA rating of Main transformer and Aux. ETx?

 

[Palletjack]

For the same reason it doesn’t trip on CT ratio differences.
We use slope for a differential protection to take in account CT ratio differences, CT saturation and tolerances, LTC, Harmonics and that 15-25kVA station service transformer.

 

[Kiribanda]

Since there are so many standard protection schemes available to protect the earthing transformer
from O/C & E/F currents, I cannot understand why the earthing transformer was included into the
differential zone of the main transformer. Because, if the main 132-33kV transformer is
tripped on Diff. protection, it means that there is a serious trouble in the main transformer which
should be investigated thoroughly before it is restored and therefore, it should not be mixed
with other. The earthing transformer could have easily installed after the transformer secondary CT set.

 

[RRaghunath]

Zero sequence impedance of ZNyn0 transformer is of the order of 50% for faults on LV side. So, the reflected current on to 33kV side for earth faults in LV network will be negligible and will not be able to cause operation of 87T.
For phase faults in LV network, the current is decided by the kVA rating & %Z of the Aux Tx. I guess this also will not be that much to cause maloperation of 87T.
Nevertheless, it is the duty of protection engineer to check the stability of 87T for load as well as fault currents as part of 87T setting study and confirm during testing.
It is also advised not to set Id> setting sensitive. Typical setting could be 30%.

 

[electrical429]

Are you sure of the one-line? - Yes its standard practice

What is the MVA rating of Main transformer and Aux. ETx? - For example 60 MVA Main and 150 kVA Aux.

 

[sushilksk]

Aux ETX has rating 0.25% of main transformer. Fault on LV side of this transformer will cause very small current in main transformer (can be calculated from % impedance). Normally differential setting has setting of 20% which will prevent operation during faults on LV side of Aux ETX.