chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Is there a formula to calculate inrush current and duration for transformers?
Thanks
Thanks
Tranny inrush formula?
- Thread starter chris kennedy
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gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
150403-9840 EST
Not really. However, there may be some emperical ratios that are used relative to transformer kVA, and the type of transformer.
In a sense the real question is what is the likely maximum peak inrush. Roughly speaking the peak inrush is gone after the first 1/2 cycle. See my waveforms at P6 and P7 at http://beta-a2.com/EE-photos.html . P6 and P7 are the same transformer just different residual flux levels and turn on points.
The maximum peak inrush will occur for a transformer that was turned off at a high residual flux density, and is then turned on at a voltage point that forces the core further into saturation.
Cores with a substantial air gap and a soft hysteresis loop (standard transformer iron and EI laminations) will have a lower peak inrush than transdormers with a tape wound no air gap core and square type hysteresis material (high efficiency transformer).
.
Not really. However, there may be some emperical ratios that are used relative to transformer kVA, and the type of transformer.
In a sense the real question is what is the likely maximum peak inrush. Roughly speaking the peak inrush is gone after the first 1/2 cycle. See my waveforms at P6 and P7 at http://beta-a2.com/EE-photos.html . P6 and P7 are the same transformer just different residual flux levels and turn on points.
The maximum peak inrush will occur for a transformer that was turned off at a high residual flux density, and is then turned on at a voltage point that forces the core further into saturation.
Cores with a substantial air gap and a soft hysteresis loop (standard transformer iron and EI laminations) will have a lower peak inrush than transdormers with a tape wound no air gap core and square type hysteresis material (high efficiency transformer).
.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Not really.
Even the transformer designers use simulations that are based on information not available to us outsiders. Thing like the core construction, air gaps between windings and the core, and winding construction.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
ChrisK.
Of course there is!
I'd present it, but unfortunately, posters on this forum appear to abhor (oops) dislike "formulas!" So, if you want the "formula" contact me off-forum!
Regards, Phil Coso
Of course there is!
I'd present it, but unfortunately, posters on this forum appear to abhor (oops) dislike "formulas!" So, if you want the "formula" contact me off-forum!
Regards, Phil Coso
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
What formula are you going to recommend, that only uses commonly available transformer nameplate data?
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Jim,
I agree that accuracy is dependent on the transformer's parameters, but there is a formula!
Phil
I agree that accuracy is dependent on the transformer's parameters, but there is a formula!
Phil
Inrush is dependent upon the KVA where lower KVAs will have a greater inrush, higher efficiencies will result in higher a higher inrush, a lower temperature rise will increase the inrush as well as transformer that are designed with a 'K' factor.
Unrushed may be as low as 8x the FLA for a 150kva and larger for the common 150deg c trasnsformer and larger to 13x for those around 5kva and going up from there for 80 and 115deg c, K4, and K13.
As such the primary OCPD selected must be considered for if they will nuisance trip on inrush when the transformer is energized. When secondary protection is provided I always recommend that the highest rated primary protective device as allowed by NEC 450 be used to avoid the possibility of nuisance trips.
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
What I'm trying to determine is if there is any chance I could energize a 300kva 480delta 208y/120 tranny with a 100A Seimens BQD breaker.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Then that answer is most likely NO!
#1: The primary (assuming step-down) current of a 300kVA transformer is going to be 360A, without even BEGINNING to discuss inrush!
#2: If, by chance, you have a typo and this is a 30kVA transformer, but the application is in a true 480V delta system, then you cannot use the BQD at all, it is a "slash rated" breaker and can only be used in 480/277V systems. That's not about the primary circuit of this transformer, it's about the SYSTEM feeding primary power, or actually the SECONDARY of the utility transformer.
#3: If it is a typo and it is 30kVA, not 300kVA, where the primary current is only 36A and your system is 480/277V, then 100A is too large anyway. 250% of 36A = 90A
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Customer has the 300 and wants to feed a 200A 208y/120 panel with it. Lets say the load on that panel is 167A. That give me a primary current of 72A. (Tranny losses extra)
System is Y, tranny primary is delta.
You may have missed the point that I made in my previous post.
A common 100a breaker will have a mag trip of 10x 100 or 100a +-20%. It is common for them to be calibrated on the high side or up to 1200a. As such how does that fit in with my previous post?.
In addition you omitted what the temp was is, if it was energy efficient, and if it had a K factor.
What is the FLA of a 300kva transformer at 480v? 360a? If the inrush were to be even as low as 10x the FLA it would be 3600? Now how does this we work out with a breaker with a 1200a instantaneous trip?
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chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
If there is no load on the secondary the inrush is the same???
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
I have seen 75kVA 480V-208Y/120V units blow fuses when unloaded, and the same fuses hold when there was a load.
chris kennedy
Senior Member
- Location
- Miami Fla.
- Occupation
- 60 yr old tool twisting electrician
Got a theory behind that?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Yes, but no proof.
Load changes the effective impedance of the secondary and there the equivalent transformer.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Having a load on the secondary changes the current flow at the time that the primary winding is opened and could therefore change the residual flux level seen when the primary is reenergized?
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
150405-1929 EDT
chris kennedy:
My suggestion is that you contact the transformer manufacturer and ask for a recommended breaker.
On your theory question:
As an approximation a switch, whether mechanical or solid-state, when turning off an inductive load stops current flow at about a current zero crossing. In a highly inductive circuit the current zero crossing is near a voltage peak. This leaves the residual flux in the inductor's core near saturation. When voltage is again applied, if the voltage happens to be going in a direction to increase flux, then the core is further saturated and a high magnrtizing current flows.
If a heavy resistive load is on the transformer, then the turn off current zero crossing is closer to a voltqge zero and thus, closer to zero residual flux. When voltage is re-applied a greater volt-time integral is required to get to saturation, and therefore the core will not be driven as far into saturation as when the transformer is unloaded, and thus, lower peak magnetizing current.
Always, the peak input current is going to be the instantaneous sum of the magnetizing current and the load current.
High efficiency transformers have a more square hysteresis curve than standard transformer iron, and therefore, a greater peak inrush current.
What the peak inrush for a particular turn on is is a function of the circuit, past electrical history, turn on time, and random conditions. It is not one single value every turn on time.
.
chris kennedy:
My suggestion is that you contact the transformer manufacturer and ask for a recommended breaker.
On your theory question:
As an approximation a switch, whether mechanical or solid-state, when turning off an inductive load stops current flow at about a current zero crossing. In a highly inductive circuit the current zero crossing is near a voltage peak. This leaves the residual flux in the inductor's core near saturation. When voltage is again applied, if the voltage happens to be going in a direction to increase flux, then the core is further saturated and a high magnrtizing current flows.
If a heavy resistive load is on the transformer, then the turn off current zero crossing is closer to a voltqge zero and thus, closer to zero residual flux. When voltage is re-applied a greater volt-time integral is required to get to saturation, and therefore the core will not be driven as far into saturation as when the transformer is unloaded, and thus, lower peak magnetizing current.
Always, the peak input current is going to be the instantaneous sum of the magnetizing current and the load current.
High efficiency transformers have a more square hysteresis curve than standard transformer iron, and therefore, a greater peak inrush current.
What the peak inrush for a particular turn on is is a function of the circuit, past electrical history, turn on time, and random conditions. It is not one single value every turn on time.
.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Zero current in the core corresponds to zero flux (except for hysteresis), not maximum flux.
It is not applied voltage that produces a magnetic field, it is current.
It is not applied voltage that produces a magnetic field, it is current.
Basically yes. But, if the core is already magnetized and the transformer is deenergized and reenergized again it may be less. But to what degree? A good guess.
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