Transformer Inrush - theoretical or real - and sacrifices

[buddy91082]

I just reviewed a coordination study with a 750kVA transformer (5kv primary - 208Y/120v secondary. The primary protection is a 125E fuse. The coordination curves shows the transformer inrush trippng the 125E fuses. The trip is just on the border line of the fuse melting band.

If i use a 150E fuse the inrush is no longer a problem but the transforer damage curve is worse and arc flash hazard increases a level.

My thought is that the inrush will not occur all the time and is only a theortical value that has the potential to occur. Is there any industry standard i can reference to make a decison on which to sacrifice (nusiance inrush tripping or arc flash and transformer damage). Replacing the 5kv fuses with power breakers is not an option.

Also, is there a good article on how to read transformer damage curves?

Thanks.
c

 

[prc]

Transformer through fault duration curves are given in IEEE Std C57.12.109-1993(R2008) for liquid immersed transformers and in C57.12.59-2001 for dry type transformers. These are basically I2t=k curves, assuming that all heat generated from over currents is absorbed by copper and limiting conductor temperature to less than 250C for copper.Inrush current is not theoretical but of course peak magnitude will vary depending on the switching point on the voltage wave (max at zero point) and amount of residua flux in the core.Inrush current, anything 10-15 times full load current for this rating, will decay rapidly to no-load current with in 1-3 seconds.

 

[Skogsgurra]

It is very, very real and has been discussed at some length for many years. One of the more complete threads is here: thread238-205768

There is also the thread about sympathetic inrush by Bronzeado. It is here: thread238-231422


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[dpc]

You're right, it will probably never happen - the fact that it's been well-studied and documented for the past 100 years means nothing. Inrush is a complex event - and dependent on a lot of variables. So nine times you get away with it and the 10th time a fuse melts.

What inrush was assumed in the coordination study?

 

[prc]

Dear Gunnar, the thread for sympathetic inrush current was
thread238-317135 : sympathetic inrush. The thread you mentioned was for neutral inversion raised by Bronzeado.At that time he wanted from me reference to literature on Neutral inversion. I finally got the paper where I jotted down the reference in 1986. I dont know whether Bronzeado is following this site and whether he is still interested.Any way I am attaching it here.Six years have just gone like that!Neutral inversion was first reported in 1905 and these 1931 &1941 AIEE papers helped me in resolving a serious neutral inversion problem at a project in 1986.

 

[Skogsgurra]

Thanks! prc. I found no match when I searched for "Sympathetic Inrush" - so I searched for Bronzeado instead. Didn't check if the thread was right. Sorry.

You brought the thread back to order...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Bronzeado]

Hi folks,

I still alive!
However, I am now working for in a big project to supply water to the Northeast of Brasil and I have no time to enjoy your nice discussions.
Thank you prc for posting the references on neutral instability after "sleeping" 6 years!

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[Skogsgurra]

Nice to see you around! Herivelto.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Bronzeado]

Thanks Gunnar for the "welcoming"!

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[LionelHutz]

I am now working for in a big project to supply water to the Northeast of Brasil

Interesting. We are building a couple of big motor starters (12660kW, 6900V) for an irrigation project in Brazil.

 

[Bronzeado]

Hi LionelHutz,

Those are ours, but they are not for irrigation.
We will do their inspection them soon. Be prepared!

The world is round and it turns!



Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[LionelHutz]

Irrigation was a wrong word choice. Supplying water to towns and cities. I won't be there for the inspection because I don't work in that office. We just built the 2 larger units.

 

[prc]

Bronzeado,I cannot resist the temptation to ask you some details of the irrigation project that you are involved in,say MW of the motor pump set etc.Let me give the background and beg the pardon of the elite who may think I am polluting this forum with non-engineering trivia.

I was born in an Indian village before India became a free nation and my earliest memory of life is that of the inauguration of an irrigation project in my village.It was more than 65 years back and the king of the country had come to switch on the 10 HP motor pump set imported from England ! Water was pumped out from the river to irrigate paddy(rice) fields. My mother lifting me up so that I could see the king among the crowd is still alive in my mind.Paddy cultivation, three times an year became possible with irrigation, alleviating poverty to a great extent.The same land now remain barren as farming is no more remunerative and all young people went for IT programming instead of planting paddy saplings in mud.Later I was involved in supplying 35 MVA 220 kV dedicated transformers to power 30MW motor pump sets for some large irrigation projects in the country.

 

[Skogsgurra]

THAT is a very interesting remark!

I think we are about the same age (me 1940) and I also have strong memories of how technology has been introduced, lived and died in northern Europe. Technology as well as industries. I think it is about time to start a thread about that. We already have "where is industry going?" and I think that a thread "from where did we go?" could be a very valuable source of information.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Sorry - we already have"Engineering history". I have never visited that forum. Will do.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Bronzeado]

prc,

As I said before, the project I work for IS NOT a irrigation project.

The Project official name is "Projeto de Integração do Rio São Francisco com Bacias Hidrográfica do Nordeste Setentrional" or in short "Projeto da Integração do Rio São Francisco - PISF". It is a project of the Brazilian Federal Government, under the responsibility of the Ministry of National Integration to ensure water supply in 2025, about 12 million of small, medium and large cities in the semiarid region of the four states of Brazil (Pernambuco, Ceará, Paraíba and Rio Grande do Norte).
The Brazilian Northeast region has 28% of the population and only 3% of water availability, which causes great irregularity in the distribution of water resources.

The PISF is the largest piece of water infrastructure in the country and ranks among the 50 largest buildings infrastructure running in the world. Altogether, the project has an extension of 477 km organized in two axes water transfer - North and East. besides 470 km of canals, the work includes the construction of four tunnels, 14 aqueducts, and 27 reservoirs.

Regarding the electric system the PISF has 300 km of transmission lines (230 kV), 9 substations (230/6.9 kV) to supply 9 pumping stations along two axes. In the North Axis the nominal water flow will be 100 m3/s and there are 3 pumping stations, 8 pumps each. The pumps are from 5.5 MW to 13 MW.
In the East Axis the nominal water flow will be 28 m3/s and there are 6 pumping stations with 4 pumps each. The pumps are from 2.5 MW to 5.5 MW

I hope this information could "kill" the curiosity of you at the moment. I think we could start another thread to discuss this kind of matter, as Gunnar suggested.

Please, find attached some slides that show schematically the PISF. Also I include a 3 minutes video showing a general views of the Project. That´s is time to learn some Portuguese!!!!!


Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[Bronzeado]

Sorry, I forget to send the video.
I will do as soon as possible.

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[prc]

Thank you Herivelto for sharing that wonderful information. In India also we have similar large projects for diverting river water to arid regions.

I am a regular visitor to engineering history, but that forum is almost inactive.Gunnar, it will be great if you can start some threads there so that we can have your experience and memories on technology and social changes.Engineers should not be purely engineering oriented.

 

[mls1]

Back to the question of fuse size, I am skeptical that the larger fuse will encroach on the part of the damage curve it should be protecting. Primary protection of the transformer should be the mechanical portion of the curve only (the area to the right). The left side is thermal damage and should be protected by secondary protection which clears the load that caused it. It is likely you're trying to clear the entire curve when you don't need to.

As for the "theory" of inrush, inrush is a result of saturation of the core caused by asymmetric offset of the core flux on step changes of terminal voltage. It is a statistical event with the worst case occurring when the angle is at 90 degrees. That's why the worst case doesn't always occur but is always possible the next time around.