Short circuit withstand capacity of power transformer 1

[HiSet]

Folks,
What would be typical short circuit withstand time of modern day 110kV/22kV, 50MVA, Z=23% power transformer. Is it safe to assume a that this transformer can supply rated short circuit current for 3 seconds?

 

[RalphChristie]

From IEEE C37.91-1985
Through-fault current duration
Class IV transformers (above 30 MVA three-phase)

For fault current above 50% of maximum possible current use K = I²t (worst-case mechanical duty) with:

t = 2 seconds
I = symetrical fault current in times normal base current
K = constant determined at max I

RCC

 

[RajT]

Based on IEC for ratg 3.15 - 200MVA, the max permissible short circuit duration is 5 secs.

 

[Modula2]

That is a large difference between the IEC and ANSI numbers.

 

[scottf]

Just my 2 cents worth, but it seems that there would have to be a serious, multi-stage protection failure in order for a power transformer to see a fault for 3 seconds!

 

[RRaghunath]

RajT,

Are you sure. I think IEC says 2 seconds as the short circuit with stand duration for transformers (irrespective of rating).

 

[jbartos]

Question: Please how is this 50MVA transformer cooled?

 

[HiSet]

The transformer in question is 35/50 MVA ONAN/ONAF

 

[jbartos]

Suggestion: Transformer Z% is important. Visit

http://www.dromeydesign.com/articles/Power_Transformers.htm

etc. for more info
It is not safe to say for 3 seconds. The maximum withstand time indicated in industry standards charts is 2 seconds.

 

[RajT]

Faulty,
You are correct (about IEC 60076 -5) quoting a figure of 2 sec irrespective of rating, though it does go on to say that "unless a different time is specified" meaning a period exceeding 2 secs.
It is not unusual to specify a duration higher then 2 sec as it all depends on the application of the transformer duty. Indeed in the application of arc furnaces, where higher short circuit currents and times are demanded, a longer time would be specified. Also worth noting is that a transf manufacturer would have no objection (albeit at higher cost) in designing a transf of higher time withstand rating which involves allowing for higher temp rise and thermal bracing.
The figure of 5 sec as I had quoted in the earlier posts is based on German standard DIN 57 532 VDE part 5.
The standard lists max withstand durationas follows
Upto 630kVA 2 secs
from 630 - 1250kVA 3 secs
from 1250- 3150kVA 4 secs
from 3150- 200 000kVA 5 secs

In principle what must be understood is that short time withstand rating should be specified based on transfoprmer application and not to think that exceeding 2 secs ( which being a min default withstand rating) is either dangerous or incorrect.

 

[mahbob]

If you are referencing ANSI standard, the damage curve for transformer rated at 30MVA is given as 2 seconds. For smaller transformer the withstand time is at 1 second for capacity rating not exceeding 0.5MVA. As in the case of IEC 76, the compliance short circuit duration is also 2 seconds. However, for longer duration short circuit withstand time, the requirement should be specified to the manufacturer.

For your information ANSI has provided short circuit withstand time against the symmetrical short circuit current. Based on the impedance, the higher the impedance value the higher it will be the withstand time as the fault current is reduced by the impedance. However the time is limited to 5 seconds.