SC Ratingf of Power Transfomer

[CKent]

I'd like to know why is it not usual for power transformers to be specified with short circuit rating. Our specification includes only the short circuit withstand capability which is normally the 2s duration of the short circuit symmetrical current available from the transformer which is equivalent to the base current divided by the impedance at OA rating. This is based on ANSI C57.12.00. Assymetrical value is higher depending on the K factor and could be as high as 2.824 times...

However, short circuit simulations will show that short circuit currents passing through the transformers (either towards secondary or towards another fault location) is very much bigger even than the assymetrical value which may be obtained from the above calculation based on C57.

 

[AMBMI]

Interesting topic... so putting the primary side F50 element threshold at 130% of the max secondary side short circuit current (calculated as In/Vcc) could create problems?!?!?!
Usually I set the time delay at 50 ms so I think that with the asymmetrical component the short circuit current could still be greater than this value 50 ms after the short circuit....
Well probably it isn't a problem in the distribution, so far away from the transformers, but what about protecting an unit transformer?

 

[Bronzeado]

Hi folks,

This is really a very interesting topic!

I have seen many engineers worried about transformer loss of life due to overload but, in general, they "forget" that the short circuit currents passin through the transformers will make electromechanic stress on the windings which may quickly reduce the life of a transformer.

We (a big transmission utility in Brazil) just start a R&D Project to develop a methodology to evaluate the transformer loss of live due to these currents.

Our ideia is to monitor the level and duration of short circuit currents passing through the transformers and try to diagnose how the transformer deteriorates by making some tests like FRA, oil analisys, etc. I hope you all can participate virtualy in this R&D Project.

Please, do not hesitate to send to us your ideias. We are open to discuss and, also, to share the results of this Project.

Best regards,

H. Bronzeado

 

[dpc]

CKent,

I'm not sure what version of ANSI C57 you are referring to, but for many years, the ANSI damage curve calculation include both the system impedance as well as the transformer impedance. There is a mechanical and a thermal damage curve, often referred to as the "Z" curve when combined. There is quite a bit more to it than the old "2 second point".

Also, ANSI C57 has "Frequent" and "Infrequent" damage curve definitions, depending on the expected number of through-faults over the transformer life.

Also, if you are trying to protect a delta-wye transformer, the damage curve must be adjusted to account for the reduced phase current seen by the primary protection for a line-to-ground fault on the secondary side.

 

[prc]

The worst scenario for short circuit duty for a transformer is with infinite capacity for the incoming grid and fault occuring when voltage sine wave passes through zero.The maximum assymetry occurs when voltage is zero and becomes negiligible or zero at 90 degree angle.

In realty we know that some amount of system impedance will limit the SC current and voltage angle at which fault happens also vary.So in realty, most onerous fault is very rarely seen by transformers in service during its life span.Even severe faults also will be limited to a very few in the life span.Of course there are exceptions-traction supply transformers or furnace transformers.

But when a transformer is subjected to the special test of short circuit test,most onerous condition is created in the lab to see that the transformer with stand the severe condition.But it is accepted that transformer cannot withstand infinite number of such shots and standards normally limit to a maximum of threeper winding.

 

[CKent]

I understand that damage of over current to the transformer depends on both the magnitude and extent of time. However, what I am thinking is why most specifications do not implicitly state the magnitude of the fault current that the transformer could withstand. I understand also that the C57 establishes both the symmetrical and assymetrical short circuit currents as well as duration depending on the voltage and capacity of the transformer. Does it follow that given the size and voltage ratings of the power transfomer, the short circuit withstand is automatically set and that we only need to refer to the standards? Isn't it possible to diverge from this and specify a different SC rating, maybe higher or lower (for purposes of discussion)? My point is that, why given this, manufacturers still submit different magnitude of SC currents at primary/secondary/tertiary windings (2 secs in our requirement) than what should be the value when based on the C57 requirements.

What we have is the C57.12.00.1987 and C57.12.90.1987 and I can't seem to locate the damage curve you are referring to, dpc. Could you kindly provide details where we can refer to it?

 

[bacon4life]

The curve is in C57.109-1993.pdf.

C57.12.00-2006 recommends that the user define the external impedance and the x/r ratio. If the user does not define it, the manufacturer uses default values from the standard. In my limited experience, the default of 126 kA at 121 kV and 242 kV is much higher than our fault levels.