Reducing short circuit current at an panel with a line reactor vs. an isolation transformer ?

[bdn2004]

We're trying to reduce the available short circuit energy at the incoming terminals of an industrial control panel to be NEC Article 409 compliant.

A line reactor's manufacturer's literature said if you are looking to reduce the short circuit current like we are, to use an isolation transformer, not line reactors.
Line reactors work safely on VFD's, and they definitely reduce the current to acceptable levels, are easier to install, and a fraction of the cost of an isolation transformer - What is the issue?

 

[dpc]

How much do you need to reduce the current? Transformer will provide more fault current reduction than a reactor.

 

[bdn2004]

You're right, see the attached.

No transformer or reactor - Isc = 31.105kA.
With a 225 kVA transformer, 5.2% FLA 270A: Isc = 4.445 kA
with a 289A, 3-phase line reactor: Isc = 5.968 kA

We need to reduce the Isc to under 18kA, which is what the panel is rated for.

 

[waross]

You have to do the math. Smaller current reductions tend to favor reactors. Larger current reductions may favor transformers.
Consider the voltage drop with a transformer versus a reactor.
You are showing reductions from 31 kA to 4.4 kA or 6 kA when you only need a reduction to 18 kA.
A transformer with p.u impedance of 0.015 would limit the available Short Circuit current to 18 kA.
Unfortunately transformers are generally not available with that low a p.u. impedance.
You may have a greater range with reactors.
Does anyone know if "Wireless Reactors" are still available? Wireless reactors were use many years ago to match the impedance of mismatched transformers operating in parallel. A typical wirelsee reactor was a stack of transformer laminations several inches high with a window, similar to a small transformer core. The wireless reactor would be installed on a bracket in front of the transformer with the lower impedance and one of the load conductors would be passes though the window.
If a 5% imp transformer was to be parallel with a 7% transformer a wireless reactor may be used to balance the impedances so that the load sharing would be more equal.
I sounds as if you could use an impedance increase in this range.
I did once install a set of wireless reactors on a trio of generators about 5 years ago. I haven't seen any since.
You may be able to get the required reduction by passing each feeder several times through the window of a large CT,
The existing CT windings may be stripped. You need only the iron core.

As well it may be worthwhile to do the calculations to see if you can get the required reduction with a set of field wound air core reactors.

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

 

[bdn2004]

I know we don't need this much reduction - but I assume we do need a transformer or reactor that is at least rated for the FLA going through it, and that is readily available.
It's a 300A fuse, and a varied load which I am assuming is 80% of the 300A ~ 240A minimum FLA. The next standard size transformer is 300kVA - very large, and likely arc flash issues, OC protection issues etc.

I guess you answering my question - that line reactors can be used.

Interesting proposal with the open window CT.

 

[bdn2004]

I started to look at the CT proposal....but these conductors are 2-3/0 in parallel. Yikes trying to wrap them around a CT. We have considered running the circuit another 100' or so.

 

[waross]

The conductors don't have to be wrapped around. A wrap doubles the impedance but halves the saturation point.

We have considered running the circuit another 100' or so.

That may well be the cheapest both in material and labour. I have seen that done successfully.
A technique for passing two large cables through a window transformer:
Put the CT on the end of the cable and form a loop with the end of the cable passing through the window a second time.
Roll the loop down the cable until the CT is in the desired position.
Give the job to your two strongest electricians.
Been there, done that, when others said that it couldn't be done.
I have also fabricated "Cable Hickeys" to handle large cables in tight spaces.

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

 

[mvcjr]

"No transformer or reactor - Isc = 31.105kA.
With a 225 kVA transformer, 5.2% FLA 270A: Isc = 4.445 kA
with a 289A, 3-phase line reactor: Isc = 5.968 kA

We need to reduce the Isc to under 18kA, which is what the panel is rated for."

Pardon me if I did not get the picture but fault level with reactor (5.968 kA) or fault level with transformer (4.445 kA) is much less than the 18-kA panel Isc rating. Why the need to reduce?

 

[bdn2004]

Without the reactor or transformer it’s 31kA at the panel. The panel is labeled by the manufacturer 18kA. This is very large facility, with lots of design/build contracts . They’ve not adopted a procedure yet to provide this info to their builders prior to building the panels. Considering Article 409 of the NEC is the law, you'd think the panel builders would also seek this info from the plant - but that’s not the case.

 

[bdn2004]

Just as a follow up to this..what I’ve learned....the current limiting reactor will work fine for this application - but it has to be an air core type, not an iron core. An iron core will saturate at fault current levels and become like a solid wire.

The air core reactor has to be made special for your application ($$$). I would think this a reason isolation transformers are a more common solution.