SKM Current Limiting vs. Total Harmonic Distortion Reactor

[ish1284]

Hi all,

I need help modeling a reactor power panel (RPP) in order to perform accurate short circuit calculations. This RPP is equipped with two different kinds of reactors. A line reactor for THD reduction, rated 160uH, and an air core reactor for short circuit current limiting, rated 54.7uH.

The vendor provided drawing shows these two reactors in series with each other. Do I perform my short circuit calculation as if these are two reactors in series? I'm not too familiar with a THD reactor vs. a current limiting one. It throws me off that the THD reactor has a higher reactance than the current limiting reactor because the THD reactor would limit the current more than the current limiting reactor based on their impedance.

I've attached a screenshot of the vendor provided oneline (see L1 and L2). I've also attached a screenshot of the reactor information.

 

[Ingenieur]

for determining downstream current treating them as the series value imo makes sense

 

[ish1284]

for determining downstream current treating them as the series value imo makes sense

Any idea why the reactor data sheet shows that the peak let thru is 10kA when 40kA is applied? The math certainly doesn't work out that way. Is that referring to the peak current vs. the RMS current?

 

[Ingenieur]

what voltage?

 

[ish1284]

what voltage?

480V

 

[Besoeker]

Hi all,

I need help modeling a reactor power panel (RPP) in order to perform accurate short circuit calculations. This RPP is equipped with two different kinds of reactors. A line reactor for THD reduction, rated 160uH, and an air core reactor for short circuit current limiting, rated 54.7uH.

The vendor provided drawing shows these two reactors in series with each other. Do I perform my short circuit calculation as if these are two reactors in series? I'm not too familiar with a THD reactor vs. a current limiting one. It throws me off that the THD reactor has a higher reactance than the current limiting reactor because the THD reactor would limit the current more than the current limiting reactor based on their impedance.

I've attached a screenshot of the vendor provided oneline (see L1 and L2). I've also attached a screenshot of the reactor information.
View attachment 18912 View attachment 18913

Under short conditions the THD reactor, assuming it is an iron cored reactor, will saturate so can no longer be treated as 160uH, just a fraction of that. The air cored reactor should not. As a starting point for short circuit calcs, I would assume that iron cored reactors play no part.

 

[Ingenieur]

480V

first their numbers are rounded
fault va = sqrt3 40000 480 = 33.26 mva
system Z = 480^2 / mva = 0.0069 ohm inductive

reactor Z = 54.7 uH 2 Pi 60 = 0.0206
total Z = 0.0275
gnd fault i = 277 / 0.0275 = 10,060 A

 

[Ingenieur]

Under short conditions the THD reactor, assuming it is an iron cored reactor, will saturate so can no longer be treated as 160uH, just a fraction of that. The air cored reactor should not. As a starting point for short circuit calcs, I would assume that iron cored reactors play no part.

it will not saturate until 15-20 x rated current of 250 A
4-5 kA
assuming no saturation
Z = 160 uH 2 Pi 60 = 0.0603 ohm
gnd fault i = 277 / 0.0603 = 4.6 ka
so it will allow ~ max avail i thru before saturation
even a 2% xfmr at a fault i ~ 50 x rated still limits fault i
and is fairly linear from 1 pu rated i up to 50 pu rated i

check with the mfg to see if they have a saturation i / rated i ratio
you can adjust off that
if ratio is say 10 or 2500 A
and max is 4.6
use 2.5 x 4.6 x 160 mH

 

[ish1284]

first their numbers are rounded
fault va = sqrt3 40000 480 = 33.26 mva
system Z = 480^2 / mva = 0.0069 ohm inductive

reactor Z = 54.7 uH 2 Pi 60 = 0.0206
total Z = 0.0275
gnd fault i = 277 / 0.0275 = 10,060 A

Thank you both of you. That answers my question about the THD reactor.

Regarding your calculation, I agree it works when doing it by hand...for some reason I have ~37KA available at the line side of the reactor in series with a 30ft 400MCM feeder, but SKM is giving me about 30kA at the load side of the feeder/reactor. I'll need to investigate what's going on.

 

[Besoeker]

it will not saturate until 15-20 x rated current

I note your opinion but it doesn't accord with mine or my experience so let's not get into this. OK?

 

[Ingenieur]

Thank you both of you. That answers my question about the THD reactor.

Regarding your calculation, I agree it works when doing it by hand...for some reason I have ~37KA available at the line side of the reactor in series with a 30ft 400MCM feeder, but SKM is giving me about 30kA at the load side of the feeder/reactor. I'll need to investigate what's going on.

is the 37 ka before or after the 400 mcm?

 

[Ingenieur]

there may be some confusion
asymmetric vs peak vs rms?

 

[ish1284]

is the 37 ka before or after the 400 mcm?

The 37kA is before. So there's a 37kA available upstream of the feeder/reactor combo.

It is 37kA symmetrical.

Also just found some vendor literature on the THD and it states inductance curve values, 100% inductance at up to 150% rated current, and then 50% inductance at 350% rated current and also states that 50% inductance is the minimum which I interpret as it won't go below 50%.

 

[Ingenieur]

The 37kA is before. So there's a 37kA available upstream of the feeder/reactor combo.

It is 37kA symmetrical.

Also just found some vendor literature on the THD and it states inductance curve values, 100% inductance at up to 150% rated current, and then 50% inductance at 350% rated current and also states that 50% inductance is the minimum which I interpret as it won't go below 50%.

good research!
so you can use 50% x 160 uH = 80 mH
which is actually greater than the i limiting value of 54.7
that will make a big difference: 134.7 vs 54.7 uH
using 277 vac infinite bus 5 ka vs 13 ka

is it a peak or rms value?
what source x/r are you using? or infinite bus?

 

[Ingenieur]

assume 37 ka
fault mva = 30.76 mva
system Z = 480^2 / mva = 0.0075 ohm (277/0.0075 = 37 ka)
30' of 400 mcm x 2 = 0.0029 ohm
i limiting Z = 0.0206

assume a gnd fault infinite bus no v drop
at reactor line side 277 / (0.0075 + 0.0029) = 26.6 ka
after reactor 277/(0.0075 + 0.0029 + 0.0206) = 8.9 ka

something does not add up???

 

[ish1284]

assume 37 ka
fault mva = 30.76 mva
system Z = 480^2 / mva = 0.0075 ohm (277/0.0075 = 37 ka)
30' of 400 mcm x 2 = 0.0029 ohm
i limiting Z = 0.0206

assume a gnd fault infinite bus no v drop
at reactor line side 277 / (0.0075 + 0.0029) = 26.6 ka
after reactor 277/(0.0075 + 0.0029 + 0.0206) = 8.9 ka

something does not add up???

Well I found out what wasn't adding up. Like I said, I agree that by hand the math worked out. What was causing the problem is that in the SKM model I entered the reactance in terms of ohms while the program was expecting the reactance in "per unit". I changed the units to "ohms" in the component editor and now everything looks as it should. Despite my silly error, I've learned a lot about line reactors now. Thanks for the help both of you!

 

[Ingenieur]

Well I found out what wasn't adding up. Like I said, I agree that by hand the math worked out. What was causing the problem is that in the SKM model I entered the reactance in terms of ohms while the program was expecting the reactance in "per unit". I changed the units to "ohms" in the component editor and now everything looks as it should. Despite my silly error, I've learned a lot about line reactors now. Thanks for the help both of you!

cool
what was the final ka after the fault? limiting reactor

 

[ish1284]

cool
what was the final ka after the fault? limiting reactor

For a 3 phase fault it turned out to be ~4500A. This included 50% of the impedance of the THD reactor which is why it's lower than what you calculated.

 

[Ingenieur]

For a 3 phase fault it turned out to be ~4500A. This included 50% of the impedance of the THD reactor which is why it's lower than what you calculated.

thanks

I re-did it at work for a gnd fault
saw I did not add the filter 80 uH or 0.0302
was just doing a sanity check

277/(0.0075+0.0029+0.0302+0.026)=4.2 ka

imo always worth doing a check like you did