Transformer inrush current 3

[npngiri]

Dear Friends,

Could anyone please clarify that the transformer inrush current (6 to 12/14 times) normally specified based on Transformer ONAN rating or ONAF rating?

 

[bigamp]

The inrush current is based on the ONAN rating normally.

Regards

 

[jbartos]

Suggestion: The indicated current inrushes may be on higher side. E.g. visit

http://www.qte.com/Main Pages/Technical Papers Page/Inrush Page/qte_tech_papers_inrush.htm

for smaller justified current inrushes.

 

[npngiri]

Thank you Bigamp and jbartos

 

[jghrist]

From oscillography records during energization of a 15 MVA 100kV-12.47kV transformer (unloaded):

Peak inrush (around 2 cycles) Phase A 1.16 x full load amps, Phase B 2.76 x full load amps, Phase C 2.56 x full load amps.

Phase A inrush dropped to 1.13 x full load in 25 cycles.

Peak differential relay operating current was 1.77 x full load. Peak second harmonic current was 1.07 x full load.

 

[npngiri]

Dear All,

The rating of the transformer, I am referring is 15/22.5 MVA, 13.8/34.5 kV, ONAN/ONAF, Z = 10 % step up transformer.
In absense of the manufacturer's data for magnetising inrush current, I have the following doubt:

The magnetising inrush current should be considered as K times the current rating on ONAN rating i.e (K times 627 Amps) or K times the current rating on ONAF rating i.e (K times 941 Amps)?

Please advise.

 

[jghrist]

I agree with bigamp that the typical values normally used are based on the ONAN rating. Keep in mind that there is no standard for what the inrush will be, and it is not something that is normally "specified". Values typically assumed for distribution transformer protection (from Cooper Power Systems Fusing Manual) are:

2 times full load @ 100 sec
3 times full load @ 10 sec
6 times full load @ 1 sec
12 times full load @ 0.1 sec
25 times full load @ 0.01 sec

 

[jghrist]

Correction to my previous post on energizing a 15 MVA 100-12.47 kV transformer:

Inrush at 25 cycles: Phase A 0.55 x full load, Phase B 1.25 x full load, Phase C 1.13 x full load.

 

[Bung]

It also depends on exactly which point on the wave you happen to be at when you switch on. It is alos affected (particularly duration) by the amount of load on the transformer. I think the rule of thumb values normally quoted are extreme worst case - it is pretty difficult to choose a reasonable fuse rating based on the ransformer size when it has also got to be able to handle 6x for 1 sec, 3x for 10 sec when it is standing outside in the full sun on a 40C day!
Bung
Life is non-linear...