Transformer Inrush Nuisance Tripping with current limiting protection

[rnsd]

I have a 75KVA 208:480/277V transformer that is only about half loaded so the primary conductors are downsized to 3/0 and protected by a 200A breaker. I am using ETAP to model this and the protective device coordination analysis shows that there may be some nuisance tripping of this circuit breaker as the transformer inrush current on the TCC is being calculated at 8X the FLA of the rating of the transformer (208.2A). Is this ETAP defaulted 8x of the FLA of the transformer a standard calculation or can this be less than 8X?

Thanks

 

[jim dungar]

I have a 75KVA 208:480/277V transformer that is only about half loaded so the primary conductors are downsized to 3/0 and protected by a 200A breaker. I am using ETAP to model this and the protective device coordination analysis shows that there may be some nuisance tripping of this circuit breaker as the transformer inrush current on the TCC is being calculated at 8X the FLA of the rating of the transformer (208.2A). Is this ETAP defaulted 8x of the FLA of the transformer a standard calculation or can this be less than 8X?

Thanks

The load on the transformer does not significantly impact the inrush of a transformer. There is a fair amount of anecdotal evidence that fully loaded transformers have less inrush than lightly loaded ones.

 

[beanland]

In rush

In rush

Agree, the inrush is not really affected by the secondary load. The magnitude and duration of inrush depends on the available fault current; the higher the fault current, the higher the inrush and shorter duration. If you are sizing the transformer 208V OCPD to load, you may have tripping. You can size it up to 225% of nameplate; that will get you out of the chances of inrush nuisance tripping. Or, you can use a MCB with thermal-only element.

 

[gar]

130626-1954 EDT

jim:

More than anecdotal I think there is some valid theory for why.

In a practical circuit it is likely that even with a mechanical switch that the arc between the switch contacts when the switch is opened will continue until near a current zero crossing.

When an unloaded transformer is switched off it can be considered a moderately good inductor. Thus, the current lags the applied voltage, and the residual core flux will be near a maximum at current zero. If the next turn on tries to force the flux in the same direction, maybe a 50% probability, then the core goes more into saturation producing a very large inrush current.

On the other hand, if the transformer has a large resistive load, then current and applied voltage are in phase, and the residual flux will be more near zero. Thus, the next turn on will have less of a peak current.

"rnsd" you can see the difference in inrush current to a transformer for two different initial flux states in photos P6 thru P8 at http://beta-a2.com/EE-photos.html . All the plots shown are unloaded. The peak full load current would be about 2 A. So the inrush peak is about 20 times the normal full load peak.

.

 

[jim dungar]

More than anecdotal I think ...

Anecdotal, in that I have never seen a 'white paper' or article specifically on this topic.
But my personal experience is 'loaded is absolutely better' for starting.

 

[GoldDigger]

Anecdotal, in that I have never seen a 'white paper' or article specifically on this topic.
But my personal experience is 'loaded is absolutely better' for starting.

One way of looking at it would be that the unloaded winding is a pretty pure inductor, which can ring and resonate with any capacitance present in the system. But when you load the secondary you add damping to the resonance. Whether it leads to greater instantaneous current values or not, the unloaded system will definitely be harder on bouncing switch contacts and make a much bigger noise.

 

[gar]

130627-0036 EDT

GoldDigger:

Resonance is not the issue. The issue is residual flux in the core after turn off, and the non-linear saturation characteristics of ferromagnetic materials.

Look at the hysteresis curve for a typical transformer iron. Put the flux state at a point near saturation. Now apply a voltage of a polarity that tries to drive the flux state higher on the saturation curve. The inductance decreases and the current increases a very great amount as you go further into saturation. The more square is the hysteresis curve the greater is the result with a small change in flux near saturation.

.

 

[GoldDigger]

130627-0036 EDT

GoldDigger:

Resonance is not the issue. The issue is residual flux in the core after turn off, and the non-linear saturation characteristics of ferromagnetic materials.

Look at the hysteresis curve for a typical transformer iron. Put the flux state at a point near saturation. Now apply a voltage of a polarity that tries to drive the flux state higher on the saturation curve. The inductance decreases and the current increases a very great amount as you go further into saturation. The more square is the hysteresis curve the greater is the result with a small change in flux near saturation.
.

The original question seemed to me to involve inrush to a de-energized transformer, not inrush on switching, for example, from POCO to generator.
I can certainly see core saturation, among other things, when an alternate source comes in somewhere between 90 and 180 degrees lagging and therefore imposes a second half cycle of applied voltage on top of a just ended half cycle in the same direction. And the main thing I can see there that would be made better by the presence of a secondary resistive load would be that the initial flux in the core might have died down under load before the next, delayed, voltage peak hits it.

 

[gar]

130627-0814 EDT

GoldDigger:

If you have a transformer in active service, then over time it will be turned on and off. In other words switched. A switch can be anything --- two wires connected and disconnected, a mechanical switch, an SCR, Triac or other solid state device.

When a transformer with a ferromagnetic core is turned off (switched) the circuit conditions at the time that the switch actually becomes a non-conductor determines the residual flux state of the core material.

That residual flux state and the conditions of the circuit at the time power is applied will determine how the flux changes. If the residual state of the core is near saturation, and the application of voltage to the transformer causes a further increase in flux then turn on current becomes very large.

For the photos I referred to on my web site the transformer primary DC resistance is about 1.7 ohms. The absolute maximum peak inrush current with a 120 V sine wave input is therefore 170/1.7 = 100 A, and this is 2.5 times what I show in the photo. There is an additional limitation and that is the inductance of the primary coil with no magnetic core material. I do not know the air core inductance of this primary coil.

An air core coil has no magnetic saturation like a iron core coil has. I believe that it was Edison and his workers that discovered magnetic saturation back in 1878 or 9.

.

 

[GoldDigger]

GoldDigger:
If you have a transformer in active service, then over time it will be turned on and off. In other words switched. A switch can be anything --- two wires connected and disconnected, a mechanical switch, an SCR, Triac or other solid state device.
When a transformer with a ferromagnetic core is turned off (switched) the circuit conditions at the time that the switch actually becomes a non-conductor determines the residual flux state of the core material.
That residual flux state and the conditions of the circuit at the time power is applied will determine how the flux changes. If the residual state of the core is near saturation, and the application of voltage to the transformer causes a further increase in flux then turn on current becomes very large.
.

I was under the impression that regardless of the current and the magnetic field in the core at the time the circuit was opened, the magnetic field would rapidly decrease to just the magnitude of the hysteresis. And that building a transformer where the hysteresis level was anywhere close to the saturation level would be a really bad idea.
I will have to read up on it some more.

 

[gar]

130627-2215 EDT

GoldDigger:

Power transformers are built with some comprimise on performance and cost, plus government regulations.

What this means is that a suitable ferromagnetic core is used to tightly couple the magnetic field of a primary to a secondary, and at nominal source voltage and frequency to run the core somewhat into saturation at the end of the volt-time integral in one direction. This causes a magnetizing current pulse to appear near the voltage zero crossing of the excitation. In my photos P6 and P7 you can see the pulse coincide with the negative slope voltage zero crossing. Unfortunately the P8 photo where there are both + and - current pulses does not have a reference voltage waveform for comparison.

To demagnetize a material you apply an AC magnetic field, and gradually reduce the excitation to zero. This keeps reducing the hysteresis curve, and approaches a zero residual flux when the excitation reaches zero. But just turning off excitation at a high flux level just leaves the core at some residual flux level.

By using a square loop material you can make a counter. Incrementally pulse the core in one direction. When saturation occurs, a pluse output can be generated.


References:

"Electric and Magnetic Fields", Stephen S. Attwood, 1949, John Wiley, Chapter 13.
"Electrical Circuits and Machinery", Hehre and Harness, 1942, John Wiley, page 262. Very nice plot.

.

 

[gar]

130628-2354 EDT

GoldDigger:

A experiment that is easy to perform that illustrates the saturation of transformer core material follows:

Use a Variac to provide an adjustable voltage from 0 to about 140 V. Use a voltmeter and ammeter to measure voltage and current to a small unloaded transformer. I used my Signal A41-175-16 that has been used in other experiments. Size does not matter except for convenience.

I measured in 10 V steps from 80 to 140 V. Calculate the apparent impedance of this load using Za = V / Irms.

My results were:
Note: problems with the forum. Tab does not do a tab, but saves the post.
Continuing:
Volts Amps Ohms
80 0.026 3076
90 0.367 245
100 0.640 156
110 1.019 107
120 1.587 76
130 2.35 55
140 3.5 40

It is quite obvious that the apparent impedance is decreasing with increasing voltage. This is because the core is being driven further into saturation as the voltage increases. Ultimately the impedance would approach the impedance of the air core coil. But long before that level the primary will burn up.

Depending upon the core material it is possible that at some very low voltages that the impedance would be less than its maximum.

.

 

[gar]

130629-0750 EDT

Correction to last night's data. Part way thru I quit dividing voltage across 10 ohm resistor used for current measurement. Corrected data follows:

Volts Amps Ohms
5 0.002 2500
10 0.0037 2700
20 0.0062 3225
40 0.0108 3700

80 0.026 3076
90 0.0367 2450
100 0.0640 1560
110 0.1019 1070
120 0.1587 760
130 0.235 550
140 0.35 400
146 0.44 330

Still the apparent impedance shows a dramatic change with voltage.

If you look at the magnetization curve for U.S.S. Transformer material it shows a curve that would imply a maximum impedance at some midpoint as seen above.

.

 

[GoldDigger]

130629-0750 EDT

Correction to last night's data. Part way thru I quit dividing voltage across 10 ohm resistor used for current measurement. Corrected data follows:

Volts Amps Ohms
5 0.002 2500
10 0.0037 2700
20 0.0062 3225
40 0.0108 3700

80 0.026 3076
90 0.0367 2450
100 0.0640 1560
110 0.1019 1070
120 0.1587 760
130 0.235 550
140 0.35 400
146 0.44 330

Still the apparent impedance shows a dramatic change with voltage.

If you look at the magnetization curve for U.S.S. Transformer material it shows a curve that would imply a maximum impedance at some midpoint as seen above.

.

Thank you very much for the tutorial, and in return may I suggest that you try the "code" tag (# symbol on format tool bar) and select Courier or Courier New as the font?

Volts    Amps  Ohms
5        0.002 2500
10      0.0037 2700
20      0.0062 3225
40      0.0108 3700

80      0.026  3076
90      0.0367 2450
100     0.0640 1560
110     0.1019 1070
120     0.1587 760
130     0.235  550
140     0.35   400
146     0.44   330

You still cannot use Tab, but at least you can keep the spaces and get consistent spacing per character.
Or you could put the table in an attachment, which would be harder for everyone to get to....

 

[gar]

130629-1930 EDT

GoldDigger:

Thank you. That will make it easier for others to read.

.