Transformer residual flux remains or decays with time? 1

[Bronzeado]

Hi folks,

When we de-energize a transformer, some residual flux remains in its core.

I wonder if somebody knows if the residual flux remains forever or decays with time?

Best regards,

Herivelto Bronzeado

 

[davidbeach]

Decays very, very, slowly. More so when warm, less so when cool.

 

[Bronzeado]

Thank you David,

How to explain this decay, if it happens?

How long do you think it decays, for example, 10% of its value?

Regards,

Herivelto Bronzeado

 

[itsmoked]

Years.

You have essentially ordered the magnetic domains. There is little to change their oreintation without power. The Earth's field is pretty much DC and weak to boot.

Thermal motion is about all there is to slowly randomize the domains.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Bronzeado]

Keith,

Your answer is based on your feeling or you have some experience on that?

Regards,

Herivelto Bronzeado

 

[prc]

Bronzeado,

Hope you are aware of the method of removing residual magnetism from core by means of sending a DC current at site?

 

[itsmoked]

Hello Bronzeado.

There is no mechanism for 'natural' removal, dissipation, or, 'decay' of residual magnetic fields in ferromagnetic materials. That is the material science and physics of the situation.

The only known methods are:
1) Intense vibration.
2) A slowly diminishing alternating magnetic field to quasi randomize the magnetic domains.
3) Heating the material to its Curie point.

This same magnetism is the basis of dating the Earth's regular magnetic pole reversals. Residual magnetism left in iron rich materials lasts for many millennia allowing dating.(approximately every 75,000 years)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Bronzeado]

Thanks Keith,you deserve a star.

Just for your information, I have done a work on transformer comtrolled switching and I used the residual flux "measured" as a parameter to calculate the point on wave of circuit breaker closing.

Based on this residual flux, I energized a three-phase three-limbed 100MVA, 230/138kV transformer (Yn-Yn)and got practically no inrush current. I energized the transformer just 10-20 minutes after de-enegize it.

My doubt is if a transformer remains de-energized for a long period, saying 6 months, if the residual flux still in the same value. My feeling is like yours, Keith, but I have not prove that, only theoretically, as you explained.

I have talked to some transformer designer and the majority (60%) said that the residual remains, 30% said it decays and 20% did not know.

Reagards,

Herivelto

 

[itsmoked]

Ah Bronzeado. Most interesting!

Pretty much what I'd expect if I poled a bunch of engineers too.

We had this discussion about, as you call it, "point on wave". I thought it should work well and am delighted to hear you actually did it and it does work! Nice.

Thanks for the info.

Hey, how did you figure out where to energize the beast to be at the right place on the wave? Did you do this by just keeping track of where you disconnected it?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Bronzeado]

Keith,

Please, find attached a paper that shows the test we did with transformer controlled switching.

As far I know, few companies use this strategy (measurement of residual flux) to mitigate inrush current when energizing a transformer.

Best regards,

Herivelto bronzeado

 

[prc]

Bronzeado,

Controlled switching is a known technology and Companies like ABB are making such devices to reduce the in rush current in large transformers and to reduce stress in reactor breakers.
Coming to itsmoked's assertions, Iam not sure will it take decades for electromagnetically induced magnetism to die down.Any way all over the world, transformer engineers do demagnetise the transformer core after a severe line fault to conduct low voltage excitation test.They will not get an adjustable AC high voltage at site.Using car battery,Dc current equal to normal Ac trf excitation current will be sent through one of the windings and then suddenly change the direction of current.This cycle will be repeated using lower and lower current values.Doble has been recommending this for decades.

 

[slavag]

Hi.
In lot of cases is problem used this type of devices.
Lot of CB's have only one close coil for CB and not per phase. In lot of cases is single mech. of CB ( one trip coil too).
Special CB will be request. Such devices used also for HV capacitor bunks.
As I wrote once on the Forum, trafo;s mnf start used new type of steel for decrease level of inrush current.
Regards.
Slava

 

[Bronzeado]

prc and Slava,

As you may note in the paper I attached in my last message, I did this experiment (transformer controlled switching) with a device from ABB, which, at that time, it was under development.

Now, after more that 5 years. I think that technology is already acceptable. It is known but not very well known.

Of course, the controlled switching depends fundamentally on the "repeatebility" (I think this is a new word. Please, help me the English speakers) of the circuit breaker. I mean, if it repeats its operation (closin or openning) aproximately at the same time (difference of around 1ms). The most important is to know the behavior of the equipment to be switched.

prc, if there is any intervention in the tranbsformer it can be demagnetized. You are right. But the issue is, if nobody touch the transformer, what hapens with the residual flux? I think it will remains there forever, as Keith explained. Anyway, I am not 100% sure.

Best regards,

Herivelto Bronzeado

 

[itsmoked]

repeatability

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Bronzeado]

Thanks Keith.

Regards,

Herivelto Bronzeado

 

[slavag]

Hi.
Yes Herivilto, you are right:
"Now, after more that 5 years. I think that technology is already acceptable. It is known but not very well known."
For expend Herivelto's explanation please see attached

http://www.abb.com/product/db0003db002618/c12573e7003302adc1256e9e00443c21.aspx