transformer inrush currents

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murli8

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Hello
I have a transformer of 40kva that is tripping the circuit breaker as the inrush current is 1200 amps. Due to costs, I am unable to change the circuit breaker out.( 10000$ ). I was thinking of putting some kind of current limiting reactor in front of the transformer. Say of 50 milliohms. The transformer input voltage is 440. Do you think that by putting the reactor and introducing the impedance of the reactor, I will be able to prevent the breaker from tripping ?
Is it possible to create drop of 25 volts and provide less volts to the transformer say 440 - 25 = 415 will help reduce the inrush current. The reactor will only be in the circuit during the startup of the transformer. After the stratup, it will be bypassed. The freq is 50Hz in Asia where this is to be used,
thanks
murli
 
If the CB cost $10,000.00, would it be safe to assume it has adjustable some or all of the following features long time, short time, instantaneous, ground fault (LSIG) OCP?

When does it trip when turning all the time or occasionally or or during transfers of an ATS?
 
transformer inrush

transformer inrush

10000$ is not the cost of the circuit breaker. The breaker cannot be changed as it is housed in an enclosure that cannot be changed. So th eoption of changing the circuit breaker does not exists.
 
You have omitted a great deal of information.
What is the primary voltage of the transformer?
Is it a standard efficiency, high eff, K-factor, 150, 115, 80degC rize, etc?
What is the circuit breaker frame size and actual breaker rating?
 
transformer inrush

transformer inrush

The circuit breaker cannot be changed due to costs. There is no option for this at all. Else this could be the best option.
No bus taps. CB is fine. If we cannot change the CB, CAN i install a reactor or impedance to bring the inrush down in value?
thanks
 
I don't have any experience with this sort of situation; all of the transformers that I deal with are small enough that they can simply be powered up by a suitably rated contactor, and the feeders are long enough that they limit inrush and the breaker doesn't trip.

However I have had to deal with capacitor pre-charge circuits for mains voltage DC supply systems. These systems have a simple dumb rectifier followed by a capacitor bank, and usually operate at perhaps 480VAC on the input side and 670VDC at the capacitor bank. When the capacitor bank is discharged, it essentially looks like a short circuit to the supply. There are fancy ways to deal with this, involving the use of SCR devices for the rectifier.

However the simple approach is to have a small contactor which is connected to the rectifier via three resistors, to 'precharge' the capacitor bank. The resistor is selected to get the capacitor bank perhaps 80% charged in a few seconds, and then the main contactor closes and connects the rectifier directly to the mains, bypassing the resistor. As long as there is no load on the capacitor bank during the precharge period, this system works perfectly.

I see no reason in theory why a similar system could not be used to 'precharge' the inductance of the transformer. The transformer should have a no load 'magnetizing' current. I would select a resistor which, at primary voltage, would limit short circuit current to about 10x the magnetizing current. In this way, when the circuit via the resistor is closed, the initial current drawn would be limited to a reasonable value. Once the transformer magnetization reaches the correct value and synchronization with the applied voltage, then the main contactor/breaker could be closed with no 'inrush'.

-Jon
 
Murl:

We are trying to HELP, answer a few basic questions regarding the transformer installation. Please!

Why build a Titanic when a canoe will work.
 
winnie said:
P.S. A couple of potentially useful links:

A company that sells solid state relays that essentially eliminate inrush:
http://www.emeko.de/en/index2.htm

Using 'precharge' to eliminate inrush:
http://www.edn.com/article/CA198898.html

Using controlled timing of supply connection to eliminate inrush:
http://www.ewh.ieee.org/soc/pes/switchgear/CIGRECSseminar/Txinrushpart2.pdf

-Jon
Click to expand...

Excellent info,thanks for these links.

In automatic capacitor power factor installations,they will use similar precharge switching systems to limit the voltage transients generated into the electrical system.:)
 
ghostbuster said:
...In automatic capacitor power factor installations,they will use similar precharge switching systems to limit the voltage transients generated into the electrical system.
Click to expand...

ghostbuster -
Would you have any links on this equipment? I'm fighting a similar suitation.

carl
 
Motor starter?

Motor starter?

murli8 said:
Hello
I have a transformer of 40kva that is tripping the circuit breaker as the inrush current is 1200 amps. Due to costs, I am unable to change the circuit breaker out.( 10000$ ). I was thinking of putting some kind of current limiting reactor in front of the transformer. Say of 50 milliohms. The transformer input voltage is 440. Do you think that by putting the reactor and introducing the impedance of the reactor, I will be able to prevent the breaker from tripping ?
Is it possible to create drop of 25 volts and provide less volts to the transformer say 440 - 25 = 415 will help reduce the inrush current. The reactor will only be in the circuit during the startup of the transformer. After the stratup, it will be bypassed. The freq is 50Hz in Asia where this is to be used,
thanks
murli
Click to expand...
Long time since I messed with them, but it seems that a motor starter might be applicable here.
 
murli8 said:
...I have a transformer of 40kva that is tripping the circuit breaker as the inrush current is 1200 amps. ...
Click to expand...

murli8 -
As has been said, there is some equipment available. Most is used by utility companies for large xfmrs - say 100MVA plus. Unless one of our utility wizzards comes up with brilliance, likely you have some research to do. But you likely already knew all of this.

I can't help, but since I am fighting a similar problem - capacitors - I am interested in yuour solution.

I am curious about a 40kVA xfm that draws 1200A inrush. FLA for 40kVA at 440V is:
40,000/440/1.732 = 52A
I would expect a maximum inrush of maybe 10X or 500A - not 1200A.

So, why the very high inrush? Is this xfm special low impedance? Are you backfeeding and the 440V winding is the low side? Is this a single phase xfm? Is this a 480V, 60 Hz rated xfm being used at 440V, 50Hz.

I understand you are only looking for an answer for your basic question, however answering the questions to allow us to get a handle on the installation will help us a lot.

carl
 
coulter said:
ghostbuster -
Would you have any links on this equipment? I'm fighting a similar suitation.

carl
Click to expand...

Carl
Generally, the supplier of the auto cap system will custom co-ordinate the specific precharge required for their caps.I suggest contacting your cap supplier for this info.:)
P.S.I live in the frigid north as well
 
A 40KVA transformer is tripping the breaker? What size is the breaker? I think something must be wrong with the breaker, the transformer, or the load on the secondary.

It seems like a few feet of #8 wire should be enough resistance to keep the breaker from tripping.

Steve
 
steve:

Depends on many factors.

First of all the CB size.
Type of CB.
Does it have adjustable instantaneous?
When does the CB trip? During normal closing of the CB or during ATS transfers?
Is the transformer K rated?
Is the CB good has it been tested?
Is the transformer good, has it been tested?
 
brian john said:
steve:

Depends on many factors.

First of all the CB size.
Type of CB.
Does it have adjustable instantaneous?
When does the CB trip? During normal closing of the CB or during ATS transfers?
Is the transformer K rated?
Is the CB good has it been tested?
Is the transformer good, has it been tested?
Click to expand...

Yes, but the first two and the last three items you listed all count as "something being wrong". (At least in my opinion:) )

Nothing was said about a transfer switch, so I would assume that's not the issue.

That leaves the K rating. Does the Krating affect the inrush current?
 
NEVER ASSUME that it is not an ATS...matter of fact never assume... And second it seems something might be something is wrong, in troubling shooting I ALWAYS rule out all possibilities, not necessarily in the correct order that is why somethings it takes 5 minutes and something it takes 5 days, but perseverance pays off!


You'd be surprised how little some facility people or electricians for that matter know about the system they are responsible for. AND IN NO WAY AM I REFLECTING ON THE OP, with this statement. I just never assume anything till I see it and test it.

Rather than reversing two leads move three leads to keep rotation right if this is A-B-C make it C-A-B.
 
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What load is the transformer serving?

If it is a directly connected motor, then you may have a more complex issue, in that the transformer is not large enough to supply the vars required to the load during starting, and the breaker is actually tripping on overload, not on inrush.

With a high voltage side of 440V, @50Hz, it is not apparently obvious as to what the low voltage winding is, since in many IEC designed systems 400, 415, 430, etc. is the utilization voltage.

There are too many pieces missing to determine the cause of the problem.
 
As I had asked previously:
You have omitted a great deal of information.
What is the primary voltage of the transformer?
Is it a standard efficiency, high eff, K-factor, 150, 115, 80degC rize, etc?
What is the circuit breaker frame size and actual breaker rating?
Still no answer!
As a former molded case and DTDT application engineer I was assigned a project to study on transformer inrush a few years back as they are compared to the primary OCPD. All of the above questions greatly affect transfomrer inrush values.
 
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