Reference for Backfeed Transformer In-rush 1

[nightfox1925]

I am into a discussion wherein it was proposed to use a step down transformer as a step up transformer (backfeeding). I cautioned the proposal requiring an in-rush investigation to verify whether the system and protective devices can tolerate such higher than normal in-rush. There was a transformer committee: IEEE/PES which provided a tabulated multiples of FLA for backfeeding.

Can someone quote me the section and IEEE/PES document that defines this in-rush phenomena when transformers are deliberately backfed?

 

[slavag]

Hi.
We have few threads on the topic.
Please try search on the Forum.
Best Regards.
Slava

 

[nightfox1925]

I actually got the threads, it was very interesting and informative.

In thread reference 238-101496, cuky2000 indicated a IEEE/PES reference. I would like to ask what IEEE/PES document number for my further reference.

 

[prc]

In the above mentioned thread,the reason for the change in excitation current for backfeed was asked.The reason for this is the air core inductance of the enegised winding.In rush current is inversely proportional to the air core recatance of the winding excited.Air core reactance is directly proportional to square of number of turns and the mean dia of winding.So if excited from inner winding(usually LV) air core reactance will be less and inrush current will be more.

Cuky has mentioned a diffrence with delta connection and star connection.This is not a universal situation.This change happens due to coupling factor.ie for a YN/d unit excitation current will be same as YN/yn unit when excited from LV.But current will come down by 40 % in a D/yn unit with LV excitation.

 

[nightfox1925]

Thanks prc.

Do you have any link or literature that discuss transformer backfeeding?

 

[prc]

nightfox1925- I dont have any literature on back feeding to guide you. To me,there are two things to look in to while considering back feeding.1) Whether OLTC is suitable for back feeding.Some models are suitable for one direction power flow only.2) In case of Generator transformers,LA and capacitor shall be retained on LV side for limiting the transfereed surges

 

[nightfox1925]

prc, with regards to the air core reactance, are you refering to the Leakage Reactance which is inversely proportional to the Flux in the core?

The winding current is proportional to the mmf, the mmf is proportional to the flux, the flux is inversely proportional to the leakage reactance. And the leakage reactance is proportional to the axial mean length of the winding, ampere turns of the energized winding.

Now since the LV winding is closer to the core, the leakage reactance is lower and if the voltage period during switching-in is zero, then the generated flux will be twice normal in its attempt to integrate with voltage for displacement and there goes the inrush scnenario. If the voltage period during switching-in happened at zero and there is residual magnetism in the core, then it will go even higher than twice...is my understanding now correct?

If the step down transformer LV winding (used as primary)is switched with at voltage zero, with residual magnetism and is connected with load, is the rated load current reflected at the primary going to add up to the in-rush current?

 

[nightfox1925]

In addition with the above query, is this phenomena going to be similar if we backfeed a step down THREE winding transformer (assuming we do not use the tertiary)?

 

[prc]

Sorry for the delay.

Nightfox,I dont think leakage reactance has anything to do with flux in the core.

Yes.Load current will add up with the inrush current.

Yes.Phenomenon same with the three winding transformer.

 

[umrpwr]

See Attached file from the ABB T&D Book which discusses estimates for transformer inrush current. The T&D book indicates engerization of a core-form transformer L.V. side can result in currents as high as twice those listed in the table provided. Shell-form transformers are approximately same for the H.V. and L.V. sides. Hope this is helpful.

 

[rob46]

Please see the attached article.