Transformer Inrush

[OdessaEE]

I have never run into this scenario before so I thought I would double check:

I have a 2400V service that feeds a Delta Delta 2000KVA 2400V/12470V step up transformer that is connected in series directly to a Delta Wye 1500KVA 12470/4160V transformer (This was done this way due to transformer availability and is a temporary installation.)

Since I will be energizing both transformers at the same time will I still assume I will only see the inrush of the first transformer only at the primary of 2400V/12470V? I am making sure the fusing is sized properly and it is sized only for the 2000KVA 2400V/12470V.

 

[dpc]

If you energize both at the same time, you will see something quite different than just energizing one. Inrush will be greater, but it is somewhat complicated because the two transformer are in series.

But why would you want to do this? Can't you open the connection between the two transformers?

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[OdessaEE]

The transformers are connected directly togethor. We needed to get from 2400 to 4160 and this was our only option for a one week delivery.

 

[marks1080]

No switch between them?

 

[OdessaEE]

No switch, they are connected directly togethor.

 

[Zogzog]

"The transformers are connected directly togethor. We needed to get from 2400 to 4160 and this was our only option for a one week delivery. "

Wow!, should have called me, could have doen 2400V/4160V in a week easy. I am trying to wrap my mind around the inrush on these in series, Seems the inrush on the 2nd one would cause similar inrush on the first one as energizing one transfomrer with some loads, which I have done many times without a problem.

Any of the smart guys on here have a forumla for calculating this inrush? I am curious to what it would be.

 

[dpc]

So how do you provide primary overcurrent protection for the second transformer?

Is this a temporary installation? The inrush will be difficult to determine - better have some extra fuses on hand - in various sizes.



"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[PHovnanian]

Keep your fingers crossed and try it.

The decay of the asymmetrical dc component of the first transformer's inrush is affected by connected load. The second transformer is that load and will result in a quicker decay. Meanwhile, the voltage at the first transformer's secondary won't rise instantaneously (like closing a disconnect) due to the inrush and impedance of the first transformer. The second transformer will see its input voltage ramp up over a couple of cycles which will help demagnetize its core's residual flux. Back in my utility days, our substations consisted of transformers followed by separate LTC regulators. Nobody ever gave a second thought to picking up both from the transformer high side switch and I've never heard of a problem.

But as dpc suggests, bring extra fuses.

 

[jghrist]

As PHovnanian said, the impedance of the first transformer will reduce the inrush of the second.

http://www.sandc.com/webzine/2003/012003_1.asp

has a chart that shows the effect of source impedance (or reduced available fault current).

 

[OdessaEE]

We only have protection on the feeder to the first transformer and on the secondary of the second transformer.

This is temporary.

 

[rogerj1]

We've had concerns about using digital relays with 50 (instantaneous) elements on the high side of power transformers (for instance, 115-25 kV 12/16/20 MVA). We set this element to clear trouble on the high side inst, to avoid miscoordination with upline transmission breakers. And yes, we use 87's on TX's as well, but this element gives us some backup. There's a lot of good dicussion in the forum about TX magnetizeing inrush, I've thumbed through some of the previous threads on the topic. But I can't seem to find a good number to use that is based only on the fundamental (60 Hz) content of TX inrush. Sure, you'll see figures that recommend 10 to 30X TX FLA, but isn't that based on a combined RMS total of all components, fundamental plus a host of other harmonics? If a digital relay is capable of filtering out the DC and all other harmonic content beyond the fundamental, what is the actual value of 60 Hz current present?
Thanks for the discussion,

 

[dpc]

The problem is that transformer inrush is basically non-deterministic for practical purposes. It will be different every time.

If you already have differential protection, there is not a lot to be gained by dialing down the primary instantaneous to the razor's edge of tripping on inrush.

For what it's worth, we recently had a case of nuisance tripping on a 15 MVA transformer with a GE SR-750 set to 12X rated current. We had to increase it to 15X.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[VTer]

This case is not very suited for numerical discussions or calculations. The magnitude of inrush is very dynamic and is very much dependent on the source voltage and residual flux in the core. In my opinion, the magnitude of inrush will not change drastically if any at all, but I would expect that the duration of inrush would notably increase. If its setup for temporary application, as stated, just temporary oversize the fuses than you typically do and hope for the best =). Good luck!

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla