Transformer inrush current feed by low sc-capacity utility.

[Morquea]

Hello

The question : Those anyone had found reference that review, confirmed, explore, challenge, anylize further the statements of that S&C paper about transformer inrush current?

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

The case :
In one of my last project lately, I do the analysis of the distribution grid for a mining operation site. The utility feed the site with a lenghtly 25 kV line. The mining site is about 30 km away from nearest populated area. The utility substation that feed the mining site was design to mainly feed residential and little commercial loads.

So we end with the utility able the provide a 750 A to 900 A short-circuit capacity at 25 kV for around 32 MVA to 39 MVA.

The main step down transformer of the mining operations site is a 5 MVA 25kV/4160V for In = 115 A. By using the common rule of thumb for Inrush current being 12xIn = 1386 A. Thats over the max short-circuit capacity the utility can provide. So the max Inrush current possible is (real actual data) 785A. Thats about 785/115 = 6.8 time the transformer In.

Later I find this paper on S&C website aboute sizing overcurrent primary protection to withstand inrush current of protection.

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

That paper suggest a graphical estimation method of the inrush current with transformer powet rating and utility short-circuit capacity. Using this method for my case, I end up with inrush current would be 3xIn.

Since it was the first time that we applied limitations to inrush current for transformer overcurrent protection, we choose to size the protection rating according to an inrush current equal to utility capacity (6.8 x In).

Applying this method bring a lot more flexibility for coordination of protection device. But I would like to find more solid study about it.

 

[dpc]

The approach in the S&C article seems entirely consistent with standard practices. The inrush current will be limited by the source impedance. A weaker system will have a higher impedance and will limit the inrush current more than a stiff system.

But if you are going to rely on this, it is important that the source impedance data used be accurate.

I've never tried to take advantage of this in sizing primary fuses, but I'd be willing to try it if the situation required it.

It is difficult to provide full range through-fault protection to delta-wye transformers using primary fuses, so this is probably why S&C is detailing this possible "fudge factor" that would allow smaller fuses to be used in some situations.

 

[wareagle]

I hope you don't have any large motors. If so, you are going to have one heck of a flicker problem.

 

[apowerengr]

And when the utility improves its source you'll be recoordinating the entire system. I'd make sure I had coordination at 12x Ifl even though the system can't currently provide it. Recall also that inrush current is comprised of roughly 50% fundamental frequency with the remaining 50% higher harmonics vs. utility short circuit data representative of just the fundamental frequency component. I'd expect the utility has some fairly small overcurrent devices in the circuit which may be harder to coordinate with than this inrush point between the primary transformer protective device and the transformer itself.

 

[Morquea]

wareagle : We already working with the mining company about this. One thing I learn about mining operations : expect very large motors.

apowerengr : I can't coordinate for 12x. Utility protection device for their 25 kV line is Kearney 65T and they can't go over without major modifications of their substation. I would need a 100T for the transformer for a 12x coordination.
The mining operations are expect to last only 5 years.So the utility doesn't plan to improve its source unless an other major project is to share the same line. Unfortunately, there is rumors about an other mining operation to come in the same region but it's still a rumor.

Some precision : Distribution on the mining site is carry overhead lines. The transformer i'm talking about is connected at like 500 meter away from the point of connection of the utility with 5 overhead transformer for offices, workshop, etc. connected in between.
In the end I have that string of protection device :
Utility substation fuse - Mining site main entry fuse - 5MVa transformer fuse - 5 MVA transformer Breaker.

Yes, I have a pretty good challenge with weak utility source, flicker, harmonics (VFD on 450 HP fan) and the mining company wanting to go to fast for their own good with the buying & building process. But everythings ok for now.