Impedance for inrush current
- Thread starter m sleem
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kingpb
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Don't know you can. What are you trying to accomplish?
If, i'm in a project where using 25% of the transformer & i don't want to use the maximum size of the ocpd at the transfomer's secondary say 125%, i'm afraid if i used a breaker which is sized to 25% of the transformer that will trip because of the inrush current. That leading to another question, if the inrush current equals to 30x flc & of course we don't have such breaker, how the ocpd allows the inrush current when the transformer get ignited?
I know this is easy by the LSIG breaker, but what was happened befroe this technology?
kingpb
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First, your breaker on the LV side does not see any of the inrush, only the device on the HV side will need to carry this value. If you want to use a smaller breaker on the LV side, in essence the transformer is oversized; no problem.
Next, if you look at fuses on HV side , they have a much better curve shape that when properly sized ride through the inrush but also fit under the transformer damage curve.
If protective relays are used, it is much simpler, as you can adjust the instantaneous setting to not trip in that range, the short time setting would have enough time delay to avoid trip on inrush.
If you truly have a transformer with that high of inrush, you will probably need to have a protective relay on the HV side that can have the instantaneous turned off.
Next, if you look at fuses on HV side , they have a much better curve shape that when properly sized ride through the inrush but also fit under the transformer damage curve.
If protective relays are used, it is much simpler, as you can adjust the instantaneous setting to not trip in that range, the short time setting would have enough time delay to avoid trip on inrush.
If you truly have a transformer with that high of inrush, you will probably need to have a protective relay on the HV side that can have the instantaneous turned off.
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30x is a really high inrush. I thought 6x was a typical design (but I'm going from memory here, maybe is was 12x)?
I have been told the new energy efficient transformers take more inrush current (21x was the number I was told).
In reality, I think feeder wiring and source impedences usually keeps the number down lower than the max. inrush the transformer manufacturer will list. And I also think the max. inrush the xformer manufacturer lists may last for such a short duration that the circuit breaker doesn't even see it.
Also look at square D mission critical circuit breakers:
http://www.schneider-electric.com/p...kers/61298-mission-critical-circuit-breakers/
They are designed to hold for higher inrush currents, but I doubt any go up to 30x.
I have been told the new energy efficient transformers take more inrush current (21x was the number I was told).
In reality, I think feeder wiring and source impedences usually keeps the number down lower than the max. inrush the transformer manufacturer will list. And I also think the max. inrush the xformer manufacturer lists may last for such a short duration that the circuit breaker doesn't even see it.
Also look at square D mission critical circuit breakers:
http://www.schneider-electric.com/p...kers/61298-mission-critical-circuit-breakers/
They are designed to hold for higher inrush currents, but I doubt any go up to 30x.
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The only practical way to limit the inrush current of a transformer is to limit the voltage, similar to how we use 'soft-starters' on motors.
Adjustable trip units have been available almost since breakers were invented, electronics is what is new.
For the situation you describe, you need a breaker with both low thermal (long time) and a high magnetic (instantaneous) trip characteristics. For example a 100A thermal trip combined with the magnetic portion of a 400A one, might give you a 30X range.
These are easy to find in the electronic trip versions, they are relatively hard to find in thermal-magnetic versions (this is the premise behind the Mission Critical referenced by Steve66).
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If I am interpreting you correctly, the 30x value is the result of the fact that you are using only 25% of the transformer capacity? That would make more sense.
But as kingpb stated, you size the PRIMARY protection for the transformer capacity, then you size the SECONDARY protection for the load. The secondary protection does NOT see the transformer magnetizing inrush, but the primary device does.
If what you are trying to do is to save money by protecting the secondary side load by only installing a primary breaker, therein lies your problem. It is not going to work with such an over sized transformer. The solution is not to attempt to reduce inrush to the transformer, the solution is to add a separate secondary protection scheme that doesn't care, then use the proper primary device for the transformer size.
gar
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m sleem:
In a DC power supply I have made I used a P&B circuit breaker at the input to the transformer as the ON-OFF switch, and overload protection. The transformer was a 170 VA with a large filter capacitor load. You can see the input turn on current transient of the transformer alone at http://beta-a2.com/EE-photos.html photo P6. Note: the approximate full load RMS input current is about 170/120 = 1.4 A. This transformer uses normal good quality core material. Its characteristics are quite similar to most standard industrial transformers.
To allow use of a low current circuit breaker, 2 A, I used a series negative coefficient thermistor to limit peak inrush current. This wastes a small amount of power during normal operation. A more power efficient method is to use a fixed current limiting resistor with a shunting relay contact that closes after a couple cycles of time.
With the thermistor technique if you turn the supply off and immediately turn it on, then there is a good chance the breaker will trip because the thermistor does not have time to cool.
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m sleem:
In a DC power supply I have made I used a P&B circuit breaker at the input to the transformer as the ON-OFF switch, and overload protection. The transformer was a 170 VA with a large filter capacitor load. You can see the input turn on current transient of the transformer alone at http://beta-a2.com/EE-photos.html photo P6. Note: the approximate full load RMS input current is about 170/120 = 1.4 A. This transformer uses normal good quality core material. Its characteristics are quite similar to most standard industrial transformers.
To allow use of a low current circuit breaker, 2 A, I used a series negative coefficient thermistor to limit peak inrush current. This wastes a small amount of power during normal operation. A more power efficient method is to use a fixed current limiting resistor with a shunting relay contact that closes after a couple cycles of time.
With the thermistor technique if you turn the supply off and immediately turn it on, then there is a good chance the breaker will trip because the thermistor does not have time to cool.
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mivey
Senior Member
Depends on duration. Here are the numbers I generally look at if I don't have other info:
Inrush: 0.01s=25X, 0.1s=12X
Cold Load: 1s=6X, 10s=3X, 900s+=2X
mivey
Senior Member
I suppose you could have a resistor in series during energizing then short it out afterwards. Also using a circuit to switch on peak instead of at zero crossing. Not sure if this is used in non-electronic apps like power transformers. I'll look and see if I have any material on it.
mivey
Senior Member
In addition to series resistance and reactors: injection of canceling currents, pre-loading of flux, and sequential phase energization. I'm sure there are more ways.
kingpb
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Yes, there could be some scheme that might be devised, but by the time you incorporate all that, wouldn't it just be easier to have protection on the high side that can ride through the inrush?
I second that.
Either size the breaker for the transfomer, or if the transformer is 4x too large, why not get the right size transformer? It would also waste less power.
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- Electrical Engineer
They make NTC current limiting thermistors that accomplish this for small transformers, very commonly used now. When cold, the resistance is high and it limits the current. After the transformer is energized, the NTC thermistor has heated up and the resistance drops to close to nil. Not sure where the size limit stops now however. Used to be only around 3A.
The OP made a comment about using LSIG breakers, I took that to imply this was not a small transformer, i.e. 400A or more on the primary side.
mivey
Senior Member
Of course, as long as the system is stiff enough and shifting the curve does not cause coordination issues.
mivey
Senior Member
We adjust the relaying to handle the inrush. Can't remember when we had a problem where unmitigated inrush couldn't be handled by the system. That's why I had to go look to see if there were mitigation solutions for power transformers and there are (I did not look at size constraints).
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Put a VFD on the primary side and ramp it in.
Seems cheaper to just use a bigger breaker.
mivey
Senior Member
Various mitigation techniques are applicable for service transformers as well as large transmission-level transformers.
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Put a VFD on the primary side and ramp it in.
Actually a soft-starter, I mentioned in post #6, would be probably be better. VFDs are not very good at supplying short circuit currents which might be beneficial to starting secondary loads and clearing secondary faults.
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