fault current 1

[tulum]

Does temperature rise of a dry type transformer have any effect on available Short circuit current on the secondary?

The reason I am asking is many time in industry a 800kVA 80degC transformer is used as a 1000kVA. i.e. at an "extended" FLC.

I guess I am asking if it can continuously supply more current can it too supply more kA?

Regards,
TULUM

 

[davidbeach]

Fault current is a function of impedance, continuous load current is a function of how well the heat can be removed. Transformers with multiple load ratings, depending on different cooling schemes, have a single impedance rating and therefore a single through fault current for a given source.

 

[waross]

davidbeach I wish I'd said that.

The impedance rating of a transformer is rated with the temperature at full load.
I found out the hard way. The first time I taught a lesson on impedance voltage and then sent the students out to the shop to verify the nameplate impedances of some dry type transformers. The results were close but not close enough. I did some fast research and found the explanation. We repeated the test with a hot transformer and got acceptable results.
If the transformer is rerated and operated hotter, there will be a slight increase in impedance and a slight decrease in fault current.
With extra cooling so that the transformer operates at the same temperature, the only change is in the Percent Impedance Voltage which will be greater in the ratio of the new KVA rating to the old KVA rating.

 

[dpc]

There will a slight increase in resistance if the transformer windings are at rated temperature versus a cold winding. But since the X/R ratio of a transformer is very high, this will not have any significant impact on the short circuit current.

But it is important to know what kVA base the transformer impedance given on the nameplate is based on. For ANSI transformers it should always be the lowest kVA (self-cooled) rating. But for IEC-rated transformers, I believe the %Z is given on the highest kVA rating. This doesn't change the fault current, just the kVA base and hence the %Z. Maybe someone in IEC-land can enlighten me.

 

[waross]

Hi dpc
I have seen power transformers with multiple KVA ratings based on cooling method that listed the impedance at each KVA rating.
In the absence of multiple impedance ratings, I agree with you completely.
yours

 

[davidbeach]

My guess is that the multiple impedances are just a convenience to the user, and that if they were all converted to the same base, or to ohms at the same voltage, they would all turn out to be the same thing.

 

[waross]

Hi davidbeach
While I have not been responsible for permit applications on multi KVA rated transformers, I did apply for a few permits were I was required to show on the application the available short circuit current based on the nameplate impedance voltage.
Your observation is correct. They do all convert to the same thing at the same base.
respectfully

 

[amps21]

I agree with David beach on impedence, but as far as the thermal behaviour/capability of the transformer during short circuits, here my thoughts:
During designs, it is assumed that large short circuit currents produce such rapid heating that the heat dissipation is of minor importance & the ohmic heat generated thereof is considered to be contained in the conductor it self.
For oil filled transformers the Std. allows 250Deg C attainable for copper conductor, any thing beyond that might be considered to do severe mechanical damage to the copper like annealing action and then brittleness / loss of mech. strength thereafter also severe insulation damage.

The nature of short circuit is Transient Heating which does not allow the transformer to attain its thermal time constant (due to extremely short duration). Since the rate of rise of temp. in the winding is directly proportional to the rate @ which heat generates and inversely proportional to the thermal capacity of the winding, the amount of copper in the transformer decides the ability of the winding to carry heat (which is essentially a design feature). So if i were u i would only stick to the DOG LEG CURVE for mechanical damage due to heat or otherwise. Also i would not
relate temporary overloading with ability of transformer to supply SC currents in the same proportion.

Hope this helps

Thanks

 

[mykh]

In case of single impedance rating, transformer current doesn't have much effect on available short circuit current on the secondary. The available short circuit current is typicaly calculated with no continuos load current taken into account anyway. As an example secondary short circuit current of 5%Z 100KVA 600V no-load secondary transformer fed from infinite source equals to 1930A (assuming equal source and xtans x/r ratios). If transformer was 100% loaded prior to fault, the available fault current would increase by 5%Z (96A) total of 2026A (assuming no short circuit fed from downstream equipment). If the transformer was 120% loaded feeding 120KVA, the anount of the fault current would increase by 115A comparing to no-load case, or by 19A comparing to the 100% load case. 19A is less than 1 % on 1930A basis. Unless you are doing capacitive calculations where you take into account input data tollerances, you can neglect the difference of 1%.

As far as I'm aware, no commercial short circuit analysis software consider component tollerances. So, in fact, you may get most precise result with number of digits after decimal point, but not necessary accurate result. If you are interested in tracking error propagation while performing short circuit analysis, and it's practical implementation, please visit www.arcadvisor.com

Aslo, transformer is not intended to sustain short circuit current for any prolonged period of time. Transformer primary fuse is expected to clear the fault within quarter cycle as well as limit prospective short circuit current value if properly selected.

 

[davidbeach]

How do you figure that load current would increase fault current? The presence of grounded wye loads can add fault current to ground faults, a factor often ignored in fault calculations, but otherwise load does not add to fault current. In fact, load will tend to slightly reduce fault currents by pulling down the prefault voltage.

 

[mykh]

davidbeach,

Under transient conditions, source internal voltage depends on the amount of load. In fact, it's higher for loaded compared to unloaded source. I used 600V as the reference phasor - that's why in my example short circuit value calculated for loaded system is higher comparing to unloaded case. I should have probably used terminal voltage instead of secondary voltage as the reference - agreed. Difference between no-load / 100% load / 120% load etc. available fault current value will be even less than shown in my example if voltage drop is considered at the transformer secondary. In my example, I tried to show that the amount of transformer load doesn't have significant effect on the fault current. Other factors, such as neglecting x/r ratios, inaccurate / typical / reference input data with inherent margin of error substituted in place of accurate values can impact results in surprising and much more detremental ways ( read more... ).

Also as a clarification, my example shows calculations for 3 phase bolted fault. Fault current values will depend on type of fault indeed, where sequence networks need be considered.

 

[leoliu]

I agree with davidbeach. The short circuit current should be depending on the impedances of the transformer, connectd power line and other devices located between the transformer and the infinte sources. If the fault is not balanced, the configuration of transformer (delta or wye, grounded or not) also affects the current. The load should be out of table since it is shorted.

 

[mykh]

yes, one may think of it in terms of "load doesn't impact fault current value because it is shorted" to keep it clear. Although being somewhat simplistic, the explanation like this helps to understand why fault current doesn't depend in any significant fashion on the amount of load.

 

[amps21]

I think the load on the tranformer will not effect the Short Circuit current (As u know urselves) but a fully loaded or over loaded transformer already has its hot spot temperature close to design values(I might be exagerrating a bit)

So what??

Well, the copper is hot and thermally stable at that time, the heat now generated due to the SC transient shall not dissipate in the short time and hence cause even more damage to the transformer winding than what it would if the unit was colder.

That is the only implication load has on SC behaviour of transformer.

 

[waross]

Hi tulum
I think the consensus of the forum is that the short answer to your question is NO.

 

[tulum]

I believe it is. Thanks.