Transformer Calculations

[laketime]

The customer has purchased a 112.5kva transformer. We have 200 amps 3phase 240v available on the primary and we want to feed a 100 amp 480v 3 phase secondary panel. It seems to me the 112.5kva is a little large for the 200 amp circuit breaker. I am rusty on calculations. Will this set up work? :?

 

[augie47]

You are correct (112,500/(240 X 1.732) = 270 amp
It's possible you could get by with the 200 amp breaker depending on the actual 480v load (It is also possible inrush might cause the breaker to trip)
Normally that transformer would have a 240v breaker in the 400 amp range.

A couple words of caution: 240 3 phase is becoming more rare. You might confirm that you don't have 208.
Secondly, if you under the '17 Code, there is a provision that a transformer can not be back fed unless shown to be so by the manufacturer so make sure your primary rating matches the primary voltage.

 

[laketime]

It is 240v 3 phase with a high leg. Load will be low and intermittent. There will be no load when the transfomer is turned on...:angel:

 

[augie47]

The devil may be in the details (transformer impedance, breaker trip curves, etc.). An engineer can likely give you some assurance but IMHO, it will be risky or, at the least, a potential future problem but I've seen similar installs when a larger transformer was available from surplus.

 

[laketime]

What is the generation of this? Just labeling issues? It is ok to run the transfomer "backwards" isnt it?

.
Secondly, if you under the '17 Code, there is a provision that a transformer can not be back fed unless shown to be so by the manufacturer so make sure your primary rating matches the primary voltage.

 

[augie47]

What is the generation of this? Just labeling issues? It is ok to run the transfomer "backwards" isnt it?

Ive heard the explanation and there are guys here who can provide you with the technical details but its way beyond my level of comprehension

 

[dmsv10]

There is always a load when you turn on any Transformer especially large ones depends on the level of saturation the Curve and the last time it was started up or energized so when connecting new Transformers specifically 75 KVA 480 volt input 120 208 3-phase output I always Energizer Transformers with a noncritical line utility feed just to pre energized the Transformer before placing it on a critical bus or UPS system output just FYI the in Rush could be a killer

Sent from my LGMP260 using Tapatalk

 

[LarryFine]

:?

I don't understand how "pre-energizing" a transformer could have any effect on inrush with subsequent generalization. With AC, there is no residual core magnetism, or so I've always thought.

 

[GoldDigger]

What is the generation of this? Just labeling issues? It is ok to run the transfomer "backwards" isnt it?

The three potential problems that I can see:
1. Outside the US and for control transformers the effective voltage ratio may not be the same in reverse
2. To avoid core saturation and other problems, the voltage adjusting taps on a transformer should be on the primary side, i.e. the side from which the energizing current is supplied.
3. Probably most importantly, the maximum connection surge will generally be a known multiple of the full load current for the energized winding. But for many types of transformer construction, that multiplier will be much greater (by more than a factor of 2) for the intended secondary winding than for the primary. This discrepancy is usually also much greater for "high efficiency" transformers. A high multiplier can make it difficult or impossible to avoid nuisance tripping of the required size breaker or fuse.

Sent from my XT1585 using Tapatalk

 

[GoldDigger]

:?

I don't understand how "pre-energizing" a transformer could have any effect on inrush with subsequent generalization. With AC, there is no residual core magnetism, or so I've always thought.

Because of hysteresis, there can and will be residual magnetism even with AC excitation. You can minimze the residual flux by tapering the voltage slowly to zero instead of sharply cutting it off at any point in the cycle

Sent from my XT1585 using Tapatalk

 

[LarryFine]

You can minimze the residual flux by tapering the voltage slowly to zero instead of sharply cutting it off at any point in the cycle

Ah, that kind of sudden decay can leave a residual magnetization, but if it clashes with the polarity of the first half-wave of the new inrush, isn't there a 50/50 chance of even greater inrush current?

In other words, aren't both events random?

 

[GoldDigger]

Ah, that kind of sudden decay can leave a residual magnetization, but if it clashes with the polarity of the first half-wave of the new inrush, isn't there a 50/50 chance of even greater inrush current?

In other words, aren't both events random?

Yes, both the turn off and turn on events *may* be random, depending on the nature of the switching.
You will not always have a worst case surge, but since it will predictably take place eventually you need to design for it.

Sent from my XT1585 using Tapatalk

 

[dmsv10]

It's just something I experienced adding new 200 KVA power distribution units to existing UPS systems critical output bus it was a standard that we used to pre energized on a non-critical 10000 day I see standard power breaker rather than placing the unenergized equipment directly onto the critical bus in summary there is a risk and the risks are based on percentages of saturation on the primary or the secondary of the Transformers you can Google it yourself and find many different answers but it's just a precaution that's all

Sent from my LGMP260 using Tapatalk

 

[gar]

181220-2338 EST

Residual core flux and when in a cycle the core is energized determines whether the core is driven into severe saturation just following turn on. This is relatively random, at least in terms of the voltage when the switch is closed. The residual flux state is less random in that it is somewhat dependent upon transformer load at the time of turn off. Plus it is a function of voltage polarity at turn on and whether this tends to force the core toward saturation or away.

You can see this difference in my plots at P6 and P7 at http://beta-a2.com/EE-photos.html . Full load primary sine wave RMS current is about 1.5 A which is a peak of about 2 A. Thus, peak inrush to full load peak is 40/2 = 20 times. This is mostly a 1/2 cycle event.

With opposite phasing, P7, note how much smaller is the peak inrush current.

Pre-energization of a transformer is of no value.

.

 

[gar]

181220-2420 EST

The way to minimize inrush current is to place a resistor in series with the primary, source of power, and have a contactor normally open contact across the resistor. After a time delay of 30 to 60 cycles energize the contactor. You could do this with a manual switch as well. The resistor size in combination with the transformer will determine the maximum inrush current.

You can use a thermistor to do this automatically, but then there is a continuous power loss. On a small transformer this might be OK, nut not on a large one.

Whatever the voltage rating of a transformer secondary is you can apply that voltage to that winding when the transformer is run in reverse.

.

 

[LarryFine]

Pre-energization of a transformer is of no value.

That's what my instincts told me.

 

[LarryFine]

:?

I don't understand how "pre-energizing" a transformer could have any effect on inrush with subsequent generalization. With AC, there is no residual core magnetism, or so I've always thought.

That should have been "subsequent energization," not generalization.

 

[brian john]

One question I have is

1. Did they buy a 240 to 480/277 or 480 to 208/120?
2. I have read that inrush on a back-fed transformer can be significantly higher if not designed to be back-fed.
3. In testing where there were inrush issues tripping the primary CB we saw 4-7 times the rated full load current of the transformer utilizing a high-speed recorder.
4. Where we had an ATS transferring, source to source, returning to the utility without a delay (center off) and no in-phase-monitor we saw inrush over 8 times.
5. I would bet money on inrush issues with a 200 amp CB, maybe not every time, maybe not the first time but more likely on the 3rd of July at 5:15 PM with a momentary power outage and someone gets the call for a tripped CB.

 

[gar]

181221-1616 EST

brian john:

I have never run a specific experiment on a transformer to determine whether I saw a relative difference in inrush current based on whether the transformer was powered from one winding or the other. Theoretically there should be little difference.

Most power transformers are designed to have close coupling between windings and the core. This means high permeability in the core material, and very little or no air gaps in the core.

In running experiments it is necessary to accurately define the experiment and how it is performed. Test conditions need to be setup so that you really have comparable experiments. Whether you wind a core with 1 turn or 1000 turns, if the excitation current is correct for that number of turns, then the flux level should be the same. Most transformers are made with materials that have good coupling between the ferro-magnetic material and the coil. Thus, coil position is not a big factor.

.

 

[Gary11734]

The customer has purchased a 112.5kva transformer. We have 200 amps 3phase 240v available on the primary and we want to feed a 100 amp 480v 3 phase secondary panel. It seems to me the 112.5kva is a little large for the 200 amp circuit breaker. I am rusty on calculations. Will this set up work? :?

What's the load is the question...