gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
How to calculate if one transformer of a three phase Star/Delta bank is overloaded?
- Thread starter LongCircuit
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LongCircuit
Member
- Location
- Trinidad & Tobago
This is convincing. I'd like to see how I can arrive at that identity matrix though. I know it's a bit extra writing, but do you mind posting the steps. it will help be grasp it better.
I did it thru inspection
ia = -1 iac + 0 icb + 1 iba
so first row = [-1 0 1]
and so on
the inverse is indeterminate
So no go
took another approach (matrix but simpler)
think I got it
using my conventions from above
and Ia = 300, Ib = 275 and Ic = 250 at 0, 120, 240 deg respectively
I get iac = 161 ang -51
a few others had a solution
I'll post mine after they share lol
Ooooh, matrix math! Careful, your geek is showing!
I read that and checked my fly :lol:
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
GentlePeople...
Wrong, wrong, wrong!
One doesn't need to know more than arithmetic! Hogwash to identity matrix, and xfmr %Z! And, more importantly ignore the Load!
Will reveal answer tomorrow... must first check with my lawyer to insure I'm not releasing proprietary info!
Phil
Wrong, wrong, wrong!
One doesn't need to know more than arithmetic! Hogwash to identity matrix, and xfmr %Z! And, more importantly ignore the Load!
Will reveal answer tomorrow... must first check with my lawyer to insure I'm not releasing proprietary info!
Phil
LongCircuit
Member
- Location
- Trinidad & Tobago
LongCircuit
Member
- Location
- Trinidad & Tobago
Easy to say 'wrong' when you post no answer
just give 1 phase current, show no work
no one will know your proprietary method
I did not use xfmr Z or load, both are accounted for in the line currents
assumed no neutral
V does not matter (until kva is calculated)
phasing 0/120/240
256A, 294A, 341A
which is line i a,b, c?
what kva is associated with each phase i ac, i cb, i ba
kwired
Electron manager
- Location
- NE Nebraska
I took it to mean he has a patent, top secret information, or even something that may effect national security on how to calculate this and only he/his company has rights to use those methods.:huh:
Unfortunately I don't have an advanced math degree of any sort and this is all somewhat over my head on how to determine this, but Phil apparently is the only person that has legal rights to the solution anyway
Please do the math here: Page 108 to 110 of this reference LINK
I did my own calcs using the %Z = 1.81% for each of the 2-50kVA transformer and 1.784%Z for the 75 kVA transformer and my phase currents are:
Phase A and B (50's) = 57.5966% of line current
Phase C (75kVA) = 58.01% of the line current.
Above calculations are based on a condition where the line currents are equal.
checked my math, made a small math error
all 3 phases offset by -67 deg to normalize ph at 0
assumed V = 240
iab = 160/0...... 38.5 kva
ibc = 185/120...... 44.4 kva
ica = 178/-140...... 42.7 kva
curious what others come up with
the sequence varies from my previous because I oriented ph seq differently
the avg of the above Ph currents x sqrt3 = 301 A
the avg of the measured line currents = 297 A
all 3 phases offset by -67 deg to normalize ph at 0
assumed V = 240
iab = 160/0...... 38.5 kva
ibc = 185/120...... 44.4 kva
ica = 178/-140...... 42.7 kva
curious what others come up with
the sequence varies from my previous because I oriented ph seq differently
the avg of the above Ph currents x sqrt3 = 301 A
the avg of the measured line currents = 297 A
Last edited:
LongCircuit
Member
- Location
- Trinidad & Tobago
Yes. V = 240 Forgot to mention that.
This same diagram is in a 2013 thread
http://forums.mikeholt.com/showthread.php?t=155496
and here
http://cr4.globalspec.com/thread/85...-Delta-Bank-Supplying-Both-3-ph-and-ph1-Power
http://forums.mikeholt.com/showthread.php?t=155496
and here
http://cr4.globalspec.com/thread/85...-Delta-Bank-Supplying-Both-3-ph-and-ph1-Power
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Ladies and Gentlemen,
Although InJunEar's calculation of the load's phase-currents is an incorrect approach, he's on to something!
Now, calculate total kVA using the average of Line-currents, Ib, Ir, Iw!
Phil
Although InJunEar's calculation of the load's phase-currents is an incorrect approach, he's on to something!
Now, calculate total kVA using the average of Line-currents, Ib, Ir, Iw!
Phil
gadfly56
Senior Member
- Location
- New Jersey
- Occupation
- Professional Engineer, Fire & Life Safety
So far you've been all hat and no cattle. You said you were going to share some super duper analytical method. Time to pony up before others here head back to the chalk board.
when I check my work by adding my phase currents up to calc line currents they are within an acceptable margin of error
he is not helpful, not sure why
he makes claims
says people are wrong
but never offers a solution
must not have talked to his lawyer
avg line current 297
kva = 123.4 kva
not sure what that does for us
if you take the avg line current of 297 and do this:
ph current = 1/1.732 x act line current/avg line current x act line current you get
Iac = 0.577 x 256/297 x 256 = 127 A....30.5 kva
Ibc = 168 A....40.3 kva
Ica = 226 A....52.2 kva
total kva 123
same as the avg line current yields
but this not take into account phasing
Your phase vectors aren't adding up to the line vectors. For example, should work out that Iab - Ica = Ia
160V@0° – 178V@-140° = 318V@21° (approx.)
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
SmartA, InJunEar..
The only voltage any where are the supply voltages, i.e., 240 V@ 0, -120, +120 Deg!
Phil
The only voltage any where are the supply voltages, i.e., 240 V@ 0, -120, +120 Deg!
Phil
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