Inrush current in a transformer

SNIP on Magnetic Amplifiers

As part of a study relating magnetising inrush currents to system protection I measured the remanent flux on a selection of British Distribution transformer. These included single phase transformers ranging from 5 to 50 kVA, and three phase transformers from 50 to 500 kVA.

The single phase transformers clearly fell into three groups according to the core construction :-

Strip wound (Torroidal) - 0.8 to 0.9 pu

Stacked core - 0.5 to 0.6 pu

"C" core - 0.05 pu

The three phase ones were all stacked core, and the figure on the highest limb ranged from 0.4 to 0.6 pu

These figures confirm your thoughts

John

Reply to
Eur Ing John Rye
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"Don Kelly" skrev

In the meantime I have found som datasheet over the transformer. It categorizes the two kVA ratings as nominel power and shorttime power.

By the way, the datasheet doesn't say anything about inrush current but proposes a specific Siemens circuit breaker on the primary side. This circuit breaker can handle inrush currents over 208 A with the primary side given 415 V.

Reply to
Cubus

What is the circuit breaker rating in amps?

Reply to
Nam Paik

"Nam Paik" skrev

The proposed circuit breaker is adjustable from 7-10 A. Should be adjusted to 9,2 A.

Don't know if these adjustable types of cicuit breakers have a special name in English?

Reply to
Cubus

If you are supplying 230VAC input, then maximum output power available is about 230VAC x 9.2A x 0.9 (transformer efficiency) = 1900VA (or

1900 watts). If you are trying to power something which can draw more than 1900VA from the transformer, you will trip the breaker. Actually, you should not power anything over 1500 watts so that the circuit breaker is not always operating at its limit.
Reply to
Nam Paik

"Nam Paik" skrev

Well, input was 415 V and the problem about tripping was due to inrush current, not the load.

Reply to
Cubus

No, I think you misunderstood. When Don Kelly (or I if I had responded first) says 'power transformers' he most likely means *POWER* transformers. Ratings from a few kVA to several MVA in our business. You know, things the size of a desk and upwards ;-)

Transformer 'in-rush' with large *power* transformers can exceed the trip ratings of the differential protective relaying. Time delays, or 'harmonic restraint' relaying is used to avoid nuisance tripping when first energizing the primary.

daestrom

Reply to
daestrom

You wrote that the circuit breaker can handle 208 amps inrush current. With 415V primary voltage, the start-up load must be over 87 KVA before the breaker can trip! The tranformer being only about 2.5KVA output capacity, what kind of load is the transformer powering?

Reply to
Nam Paik

They don't really make transformers that size, do they ;-) The core would be to long for the flux to go all the way around and you could never get it on aboard a space shuttle. So what would the need for one be.

transformers.

Reply to
bushbadee

.

responded

transformers.

Something the size of a desk is a lighting transformer, or possibly a very small distribution transformer. In the 50/60Hz power scheme of things these are not considered very large units at all. Some differential relays won't let you set a unit size smaller than 10 MVA because such small units are rarely provided with full differential protection.

I got a free all-expense-paid trip to a remote location to conduct an impromptu hands-on seminar to demonstrate my practical knowledge of harmonic restraints. I'm still a little baffled that a transformer on a line with say 30 MVA short-circuit capacity could have an inrush of more than 60 MVA.

Bill

( who's seen a 60 MVA transformer sailing through the air...on a crane, of course.)

Reply to
Bill Shymanski

Have seen 25kV/4160V load center transformers with full 4160V bus differential relaying as well as differential (with harmonic restraint) on the supply transformer. Open up the metal enclosure and the air-cooled transformer inside has the foot print of my desk, just a bit taller (about 5 ft).

Or try the one recently replaced at a nearby power plant. Two in parallel carry the full 900 MVA (450 MVA a piece when fans and oil pumps running). Pulled it out from underneath related switchyard on skids with two catapiller-style bulldozers. *THEN* lifted with a crane to put on truck (don't remember how many axles, but it was a lot).

Yeah, "it's a puzzlement". But if it happens, it must be possible. Winding resistance of these things is very low and cores are very large. If you could energize each phase just at the zero-crossing, wouldn't be so bad. But since they're energized in 3-phase configuration, at least *one* phase will be at a high voltage when contacts close. No/little mag. field in core

  • nil resistance = MAJOR DC offset inrush. Not long, just a cycle or few, but quite high.

daestrom

Reply to
daestrom

sip

I'll take a shot at this:

  1. After looking at the nameplate a little closer I'm not sure if I agree with the dual KVA rating theory. If you look at the secondary current for either the 115 V or 230 V secondary (ie 28A or 14A, bottom far right of picture) then the transformer size looks like a 3.2KVA (or 2.5 KVA derated). I believe the 15 KVA rating is the short time rating (possible 6 sec) for this transformer and the 2.5 KVA is the full load rating.

  1. The inrush is independent of the load ( its and magnetizing current)

  2. Sizing the overcurrent device for this transformer; What you have is a control power transformer that does not require primary protection. If you noticed on the nameplate the secondary ratings list a
3RV10... number. This is telling you the size of the MCP to use on the secondary. This device also protects the primary. (all code legal for a single phase transformer). You still have to protect the primary conductors, and those are sized for the transformer load 2.5KVA The primary current is determined by the primary voltage which looks like a 415 V (based on the picture pins 1&5) or 6 FLA or 36 amps short time (2.5 KVA @ 415V or 15 KVA @ 415V). I would think your 10 circuit breaker should work. What looks screwy is the secondary connection. Looking at the picture I can't tell how the secondary is connected, but the load should be connected on pins 31 & 34 and then you either series or parallel the coils for 230V or 115V (I've seen this screwed up a bunch or times). Just speculating; but in the picture it looks like the ground is on the blue wire (hot?). It could be the transformer has a internal ground on pin 34 (white wire) If you short circuit the secondary you will trip the primary 10 A CB (the primary will draw 36 Amps).

  1. Your real question is more general and the answer is to look at the correct design standard. In this case its EN 61558. Buried in this standard is the design inrush for different KVA ratings. For power transformers (in the US) it would be ANSI C57. I usually don't worry about inrush on small control power transformers. They are short duration and not a problem unless you are using a fast blow fuse (wrong application)

hope this helps and sorry for the long-winded post.

Reply to
deanmk

"deanmk" skrev

When I found some datasheet over this transformer, your believe was confirmed.

(Light)blue is used for the neutral wire in Denmark. Ground is connected to "neutral" on the secondary side and 230 V is the output voltage here.

Yes, nice reply..

Reply to
Cubus

-------- But not with what I call power transformers. -i.e. those 50 or 60 Hz devices from 2KVA to several hundred >

MVA (with very narrow B-H curves and very low residual flux They were built. Take a sheet of transformer steel and roll it up.

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer

Reply to
Don Kelly

----snip----

If you

Zero crossing is near the worst place to turn on. The best place would be at some point in the cycle where corresponding magnetizing current would match the remnant magnetism. Remember that under steady state operation, the flux is at the peak at the voltage zero crossing. For the first 1/4 of the cycle, the flux is decreasing until the peak. At the peak, the flux reverses and increases until the zero crossing. Ignoring residual magnetism, the peak flux will be double when closing in at the zero crossing. Somewhere after the crest the core will saturate and then the high peak currents will occur. Again ignoring residual magnetism, if the transformer is energized at the crest, there won't be a large inrush in magnetizing current. (But still one associated with the winding capacitance.) When residual magnetism is included, the ideal point will shift. With typical power transformers, the crest of the line is pretty dang good.

I have wondered when large power transformers will be energized in a more controlled fashion. The energizing circuit only needs to get the transformer running in a non-loaded state. After it's energized, a mechanical breaker could then close in to carry the load current. The technology could be developed to help those cases when it's a real problem - like the far end of long lines. After development it could be more widely applied.

Matthew

Reply to
Matthew Beasley

Which, for good transformer steel is nearly zero. Transformer steel, ideally, has a B-H curve that is collapsed to a simple line with no hysteresis.

The flux of a deenergized transformer is only residual. The flux is only maximum when current is maximum. In the steady state, with no load, the current lags almost 90 degrees from voltage, so the current (and flux) is maximum when voltage is near zero. The flux 'reverses at the peak' simply because that is when current is crossing zero. So, what you say is true for the steady state.

But you can't apply the steady-state circumstances to the non-steady-state of energization. When first energizing, the flux is *not* maximum when the voltage crosses zero, the flux is still residual. Energizing at voltage-zero-crossing would result in only small current that would rise with the rising voltage. As voltage crests, the current and flux would continue to rise and reach their peak sometime later. This is what establishes the lag between current and voltage.

Energizing with the voltage at maximum means the winding (with very little resistance) is placed across the line. The only significant limit to current flow is a very rapid build up of flux ( a high d(B)/dt) created by a very high dI/dt.

Interesting idea, but don't see a large demand for it. Energizing an unloaded transformer isn't *that* much of a problem. Just a bit of trouble with the primary circuit protection schemes. And simple time-delays or harmonic-restraints or voltage-restraint schemes have solved this problem.

daestrom

Reply to
daestrom

-snip-

non-steady-state

created by a

Please review the document:

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Particularly page 6 heading E.

Matthew

Reply to
Matthew Beasley

OK if you want a very very general rule of thumb to estimate inrush current..

Compare the locked rotor current of an equivalent motor load sized in kW and configured to operate at the same voltage as the primary of your transformer.

There are many other variables that could affect this number, however you will get the approximate range of inrush current if you make this general comparison. Only in situations where there may be extreme differences in transformer design will you get something very different, so keep that in mind. Often when these calculations are made, we are dealing with a typical industry "average" efficiency factor and "average" coil resistance and "average" core mass compared to an "average" motor with similar "average" specifications. You won't be accurate but you will be in the ballpark provided that you are energising both loads from the same source system.

Reply to
Martin Glasband

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