Inrush Currents in STEP UP Transformers 8

[ThePunisher]

HI all. I understand that the inrush would be greater if a step-down transformer is energized at the LV side.

However, if we have a STEP-UP transformer, is the inrush "seen" at its LV terminals and no inrush at its HV terminals. The reason I ask is that our contractor has set the relay of the HV side high enough to bypass the transformer inrush current.

If the inrush is at the HV side, is it going to be 12X as well? If not, how much inrush I would get at the LV side?

Thanks

 

[Power0020]

Generator step up transformers are usually energized from HV side to avoide hindering the generator will heavy inrush currents.

Inrush current will not be huge on HV side as the nominal current is not that big on GSU HV side.

In this case, and since no flux has built up in the yoke, no inrush current will be transferred to the LV side when energized from HV side.

 

[davidbeach]

If it is a unit connected transformer, connected to the generator at all times, there is no inrush as the voltage builds up.

 

[waross]

The inrush is limited mostly by the resistance of the winding being energized. To even out the I²R losses between the high voltage side and the low voltage side, the resistance of the low voltage side may be less proportionally than the high voltage side.
If that is the case, then the inrush may be a higher percentage when the transformer is energized from the low voltage side.
Are you following this Electricpete?
Is it possible to estimate the high side resistance by looking at the transformer losses at different load levels? Once you have the high side resistance, can you use the impedance voltage the X/R ratio and the high side resistance to determine the low side resistance?
With the resistance of both the high side and the low side, can you see if the resistance ratio is similar to the turns ratio or if the low side has proportionally lower resistance?
Thanks friend.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

If this is a new transformer, I would just ask the manufacturer for an estimate of the maximum inrush current. But your source impedance will limit the current as well.

 

[ThePunisher]

This is a new step up transformer 4.16-7.2 kV (Delta 4.16 kV, Wye 7.2 kV). The 4.16 kV is the winding that is being energized from an external source. The 7.2 kV feeds an MCC that is consequently feeding couple of 7.2 kV VFDs.

My question is that, if we energize the 4.16 kV side, the inrush would be seen at the 4.16 KV side and not on the 7.2 kV side. Our contractor was thinking that the inrush is also seen at the 7.2 kV but I argue and explain to him that it is not the 7.2 kV winding that is energized first but the 4.16 kV side. Since he insists on it..I want to make it sure I am not missing anything.

I will try asking the manufacturer the inrush data of this step-up transformer.

 

[waross]

You are correct. The side that is energized is the side that sees the inrush. If a transformer is energized with the secondary open circuited, there will be no current flow in the secondary. There will still be an inrush on the primary winding. The primary winding is the winding that is supplied with energy. The secondary winding is the winding that supplies the load. In your case the primary winding is the low voltage winding.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Parchie]

My question is that, if we energize the 4.16 kV side, the inrush would be seen at the 4.16 KV side and not on the 7.2 kV side. Our contractor was thinking that the inrush is also seen at the 7.2 kV but I argue and explain to him that it is not the 7.2 kV winding that is energized first but the 4.16 kV side. Since he insists on it..I want to make it sure I am not missing anything.

You got it right. Inrush current occurs at the side where the transformer is energized. A high initial current happens because the transformer magnetic circuit still holds some remanent flux and could add on to the flux by the impressed voltage->saturating the core--> a higher current than normal.

 

[ThePunisher]

Thank you very much for all your informative responses.

 

[prc]

Transformer inrush current varies inversely as the impedance, consisting of winding resistance plus winding air core inductance. Resistance is negligible, compared to inductance.Air core inductance varies as the square of number of turns and diameter of winding plus inversely as the height of the winding.Hence when energised from outer winding (usually HV),inrush current (as times of full load current) will be small compared to inner winding(LV)

Even if residual flux is nil in the core,there will be inrush current and will be maximum when switching is done at the zero point on voltage wave.Then the flux in the core will be two times the rated flux density(normally 1.7 T),much more than the saturation flux density of core ie 2.0 T.Then air core inductance of winding decides the inrush current required to maintain such flux level inside the winding.

The latest tutorial on the subject is CIGRE technical brochure no.568 (125 pages),issued in Feb 2014,titled ' Transformer Energisation in Power systems: A study Guide' available at e-cigre.org

 

[Bronzeado]

Hi folks,

prc give us a very good explanation on the difference between LV and HV transformer inrush currents.

Best Regards,

Herivelto S. Bronzeado
Brasflia, Brazil

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

 

[ukmike8]

Large VFDS use premagnetising to limit inrush also this is sometimes used on weak systems but in your case it is not applicable.
Do a voltage dip calculation if you want to energize from the LV side.