Tranny inrush formula?

[gar]

150405-2121 EDT

GoldDigger:

I should have been more clear. Hysteresis can not be ignored. The magnetizing current drives the ferromagnetic material to some flux level. When the magnetizing current returns to zero the core material flux does not return to zero (hysteresis), but to some substantial value, the residual flux density.

If we next apply magnetizing current in the direction of the previous excitation, then we start from the last residual flux level, and increase to a next peak and higher residual flux level. Because of the characteristic of saturation of the core the magnetizing current has to increase to a greater level than before. This is because there has to be a balance between the counter-emf from the core with the excitation voltage.

As an aside, by this technique I can build a counter with square loop material that does not go into saturation until X number of current pulses have occurred.

.

 

[gar]

140405-2153 EDT

chris:

If there is no load on the secondary the inrush is the same???

An unloaded transformer has an inrush current that is not the same every time power is applied to the transformer. The inrush can be all over a large range. The value is dependent upon:
1. The residual flux and polarity (direction) at the time voltage is applied.
2. The point in the applied ac voltage relative to the residual flux when AC voltage is applied.
3. The peak value of the applied AC voltage.
4. Characteristics of the transformer.

Your approximate unloaded steady-state current at the primary for the transformer fully loaded is (300,000/3)/(480/1.732) = 100,000/277 = 361 A. This is the basis upon which to estimate inrush current. Try a 15 times ratio that results in a peak inrush of about 5400 A. Even cutting the ratio in half the peak inrush is 2700 A. And possibly these two values need to be increased by 1.414 depending upon the real meaning of the breaker magnetic trip level.

Note feeder line impedance will reduce the peak values somewhat.

I believe your goal should be selection of a breaker that in combination with the transformer and load has a low probability of falsely tripping. I still recommend getting advice from the transformer manufacturer. They may have better advice than anyone else. Certainly the breaker you suggested is not adequate.

.

 

[templdl]

I provided my input on a previous post based upon my experience as a breaker applications engineeo determiner as well as a DTDT csales and applications engineer for a major electrical equipment manufacturer.
Because of my knowledge of both products my marketing manager for the DTDT product line requested that I do a study of our transformer products along with a seasoned transformer design engineer to determine where there may be nuisance tripping issues. Since there are no tests allowed b the NEC that would provide the highest instantaneous value regarding inrush across the product line we used so sample data from those where information was available to establish a trend for the rest of the product line.
As stated in my previous post the higher the kva the inrush decreases, 115 and 80 deg c rise will increase inrush as well as energy efficient and K factor transformers.
Basically, there is no established values across the product line except for a few which lead to only ball park values. The safest thing to do is to use the largest PRI OCPD as allowed by the NEC to provided the highest instantaneous breaker to reduce the chance of nuisance tripping.
We would all like this to be and exact science and simply go to a table to select a breaker to go with a specific transformer but it isn't. We must apply our technical knowledge in doing so. With secondary OCP provided simply use the largest PRI. OCPD as allowed by the NEC that way you get the highest instantaneous trip rating.The real goal is not transformer protection as that is already provided by a properly sized sec. OCPD. Should the PRI. device trip it would be as a result of a trasnsformer failure. By reducing the instantaneous value of the PRI. OCPD one can start to play Russian roulette with nuisance tripping.
But I must say that I am impressed with the theorys expressed in this string of posts.

 

[Phil Corso]

Gentlepeople,

Causes of Magnetizing-current inrush phenomena... more , a distortion of its wave-shape... has been collectively mentioned! Following are the most salient causes:

1) Core-material, Core-construction (EI, Circular, toroidal, etc, for single-ph; 3-, 4-, 5-leg, etc, for 3-ph) and its B/H curve) !

2) Residual-flux and saturation!

3) The phenomenon is quite random depending upon the instant of energization along the supply-voltage wave. Maximum magnitude occurs when voltage is near-zero for single-phase xfmrs, or single-phase xfmr banks. Of course, in 3-ph Xfmrs, one-line will be max, the other two, less! (For those experiencing fuse-action , I'd be interested in kowning how may of the three operated!)

4) The idea that the the inrush-magnitude is less with secondary-load connected is only valid when paralleling Xfmrs! However, there is merit to the assertion because, technically, the secondary-load impedance, as well as the secondary-winding impedance can be "reflected" into the primary-winding circuit! Recall, however, the secondary-circuit impedances are divided by the square of the turns-ratio! Hence, their influence on magnitude is virtuall nil!

5) There's also data that suggests smaller kVA Xfmrs have larger inrush-current magnitudes, but, larger ones are much longer in duration!

6) In my experience I always recommended the instantaneous element of primary over-current protective device be set to twice that of a bolted secondary-fault current! There were, of course, a few times when it didn't work!

Contact me if interested in mathematical proof,!

Phil Corso

 

[GoldDigger]

Just to clarify your point #6:
You are saying to use the primary current corresponding to a bolted fault on the secondary, yes?

 

[Besoeker]

Gentlepeople,

Causes of Magnetizing-current inrush phenomena... more , a distortion of its wave-shape... has been collectively mentioned! Following are the most salient causes:

1) Core-material, Core-construction (EI, Circular, toroidal, etc, for single-ph; 3-, 4-, 5-leg, etc, for 3-ph) and its B/H curve) !

2) Residual-flux and saturation!

3) The phenomenon is quite random depending upon the instant of energization along the supply-voltage wave. Maximum magnitude occurs when voltage is near-zero for single-phase xfmrs, or single-phase xfmr banks. Of course, in 3-ph Xfmrs, one-line will be max, the other two, less! (For those experiencing fuse-action , I'd be interested in kowning how may of the three operated!)

4) The idea that the the inrush-magnitude is less with secondary-load connected is only valid when paralleling Xfmrs! However, there is merit to the assertion because, technically, the secondary-load impedance, as well as the secondary-winding impedance can be "reflected" into the primary-winding circuit! Recall, however, the secondary-circuit impedances are divided by the square of the turns-ratio! Hence, their influence on magnitude is virtuall nil!

5) There's also data that suggests smaller kVA Xfmrs have larger inrush-current magnitudes, but, larger ones are much longer in duration!

6) In my experience I always recommended the instantaneous element of primary over-current protective device be set to twice that of a bolted secondary-fault current! There were, of course, a few times when it didn't work!

Contact me if interested in mathematical proof,!

Phil Corso

Consider yourself contacted.

 

[chris kennedy]

I'd present it, but unfortunately, posters on this forum appear to abhor (oops) dislike "formulas!"

Contact me if interested in mathematical proof,!

I really appreciate everyone's input here.

I don't understand why Phil can't throw his math out there for all to see. If you don't want to publicly post these I would be very interested if you PM them to me.

Thanks all!

 

[Phil Corso]

Another point:

7) Peak Flux Density of Xfmr should be in saturation area!

Besoeker,
Yu gotta be kidding!

Goldigger,
Yes!

Phil

 

[Besoeker]

Besoeker,
Yu gotta be kidding!

Just responding to your invitation.
If you wish to decline, that's fine.

 

[iwire]

I don't understand why Phil can't throw his math out there for all to see.

The only thing keeping Phil from posting it is Phil.

 

[topgone]

I agree on choosing the highest Inst setting of the breaker allowed for the primary CB and rely on the seconary breaker for transformer overload protection.

On some cases where energization of transformer becomes a problem (main breaker trips on inrush), we use de-sensitizing of the primary instantaneous trip setting-->meaning, a delay in tripping(for breakers with relay protection sensing).

Another method I once used (in my other life before) was closing the secondary breaker before closing the primary breaker-->that way, some resistance/load connected at the secondary side help in tempering the highly inductive current of the transformer.

Still, bigger transformers are energized using closure of one pole at a time, sometimes using insertion of series resistance to each switch pole before shorting the employed resistors at a specified time delay - costlier but effective.

 

[templdl]

I agree on choosing the highest Inst setting of the breaker allowed for the primary CB and rely on the seconary breaker for transformer overload protection.

On some cases where energization of transformer becomes a problem (main breaker trips on inrush), we use de-sensitizing of the primary instantaneous trip setting-->meaning, a delay in tripping(for breakers with relay protection sensing).

Another method I once used (in my other life before) was closing the secondary breaker before closing the primary breaker-->that way, some resistance/load connected at the secondary side help in tempering the highly inductive current of the transformer.

Still, bigger transformers are energized using closure of one pole at a time, sometimes using insertion of series resistance to each switch pole before shorting the employed resistors at a specified time delay - costlier but effective.

I appreciate that you pointed out the field experiences that you used in energizing a transformer. I have addressed countless complaints where the installer misinterpreted the tripping of a PRI. OCPD as being the result of a defective transformer and requested engineering services and or a warranty replacement.

 

[hamgunner]

Is there a formula to calculate inrush current and duration for transformers?

Thanks

Let's attempt to quantify question a bit more from a practical perspective without mfg's data, for coordination purposes :

Dry type xfmr 115 or 150 degrees C rise

</= 75 kVA ~ 8.0 PU of primary FLA
> 75 kVA ~ 12 PU of primary FLA

for 80 degrees C rise 1.5 x aforementioned rating

I personally have seen 26 PU on some models/ratings

Please remember that inrush begins decay as CEMF rises and XFMR is magnetized. Essentially zero after 5 cycles.

Setting primary CB to maximum trip (without engineering analysis) will increase AF energy on secondary terminals of xfmr.

Kindest Regards