Paralleling devices for main-tie-main configuration - Coordination study

[Magic Gorge]

I have a feeder breaker relay feeding a 500kVA transformer, which is daisy chained to another 500kVA transformer. Both transformers are fused on the primary. Both transformers feed a Switchgear with main-tie-main breaker configuration, and all breakers are normally closed. When I plot the device curves, would it be proper to plot the main breakers at twice the current? Even though the main-tie-main are all 2000A, this would show the mains like 4000A devices.

 

[Ingenieur]

2 xfmrs in parallel feeding a common bus each thru a 2000 cb
the bus is segmented with a 2000 cb tie
correct?
a single line would help

depends where the fault occurs
but regardless they are 2000 and the curve(s) will remain the same
only the fault current will change based on location and type, ie, single gnd, phase-phase, or variations

 

[Magic Gorge]

2 xfmrs in parallel feeding a common bus each thru a 2000 cb
the bus is segmented with a 2000 cb tie
correct?
a single line would help

depends where the fault occurs
but regardless they are 2000 and the curve(s) will remain the same
only the fault current will change based on location and type, ie, single gnd, phase-phase, or variations

I'm attaching a single line for reference. I remember being told that paralleling generators can sometime be treated this way, in reference to the feeder breakers, but maybe not with the tie between. The philosophy is that the current divides equally between the two branches, so the mains can be plotted at twice the current compared to the feeders.

 

[Ingenieur]

Thanks
so you are talking about the 208 feeder cb's?
what are they, uv or shunt trip with relaying?
or the mv cb feeding the xfmrs?

if the fault is on the left feeder both xfmrs will supply fault current and vice versa
but the breaker should still be treated as a 2000

I guess I'm not clear on what you are asking

coordination is the issue
feeder
tie
main
???

 

[Magic Gorge]

These are overcurrent devices, so I'm considering a three phase fault on the load side of the feeder breakers. Trying to coordinate the main breakers with the tie and the feeders. I'm not certain about the shunt trip question, but all of these devices are GE low voltage power circuit breakers.

 

[Ingenieur]

These are overcurrent devices, so I'm considering a three phase fault on the load side of the feeder breakers. Trying to coordinate the main breakers with the tie and the feeders. I'm not certain about the shunt trip question, but all of these devices are GE low voltage power circuit breakers.

so on the xfmr secondary they are all non-adjustable inverse time molded case cb's
or do the instantaneous trip level and short time adjustments?

this is a fairly simple study
you need
xfmr parameters including pu Z
cable lengths
curves for the cb's

if not comfortable with this it may be better to pay someone to do it

 

[Magic Gorge]

Thanks for your reply, I think I can handle the coordination aspect, it just seems that after looking over a couple curve sheets, there's no way the switchgear main breakers, as well as the upstream xfmr primary fuses, will coordinate with the larger feeder breakers in the switchgear. I think a study was done 30-40 years ago for this building, but a lot has changed with the design over the years. I thought about the paralleling aspect, but I suspect I'll have to view it radially as you've suggested. The fuses at the xfmr primary are 30A and the low-voltage main is 2000A, and there are a couple 1600A feeders in the switchgear. I feel like they added the second xfmr later and increased the load. Might need to upgrade the fuses, but I'll have to give it a closer look.

 

[Ingenieur]

The primary fuses are tight
flc is 22 A
you are allowed 250-300% based on xfmr pu Z and location, supervised or not
30/22 ~ <140%

secondary 125-250% depending on location
or none with a primary fuse less than than 250%
xfmr sec 2000
bus tie 2000
208 feeder 1600

look at increasing the primary fuse, rating or time delay
you have inherent coordination with the feeder and main 1600 vs 2000
they should see essentially the same fault so the 1600 should trip first
be nice if the tie could split the 2

alot depends on xfmr Z and cables, eg minimum fault current
as well as maximum for cb ratings

 

[steve66]

I have a feeder breaker relay feeding a 500kVA transformer, which is daisy chained to another 500kVA transformer. Both transformers are fused on the primary. Both transformers feed a Switchgear with main-tie-main breaker configuration, and all breakers are normally closed. When I plot the device curves, would it be proper to plot the main breakers at twice the current? Even though the main-tie-main are all 2000A, this would show the mains like 4000A devices.

I wouldn't figure the primary breakers at twice their rating, but if you are looking at this right, each primary breaker would only have half the total primary current flowing through it. (I'm assuming everything is balanced - transformers are the same, very short cable between the two transformers, etc.)

If you are using SKM or a similar program, it seems like it should take care of the two transformers being in parallel.

 

[Bugman1400]

For a m-t-m config, I typically take out xfmr 1 and coordinate the feeder bkr from bus 1 with the tie, mains 2, xfmr fuse 2, and then the primary MV bkr. If the tie is closed during normal operation, the feeder bkrs will need to be rated to handle the fault current from both xfmrs.

 

[Pwrgeek]

Back of the napkin calculations can be had by assuming the transformers will share the fault current evenly. You can therefore plot the main and tie breakers on your TCC at half current vs the feeder breaker. What I would do in this situation is coordinate with the tie open. Then I would know I had coordination with it closed as the main current would be less but the feeder current would be the same. Philosophy in this case depends on the normal operating condition and how often and for how long it is in the abnormal condition. It's a trade off of clearing time vs reliability.


Sent from my iPhone using Tapatalk

 

[Bugman1400]

Back of the napkin calculations can be had by assuming the transformers will share the fault current evenly. You can therefore plot the main and tie breakers on your TCC at half current vs the feeder breaker. What I would do in this situation is coordinate with the tie open. Then I would know I had coordination with it closed as the main current would be less but the feeder current would be the same. Philosophy in this case depends on the normal operating condition and how often and for how long it is in the abnormal condition. It's a trade off of clearing time vs reliability.


Sent from my iPhone using Tapatalk

The fault currents will not be even because of daisy chained cable impedance. This will be more or less pronounced depending on the cable length. I would coordinate with the tie bkr closed and one of the mains open.