Transformer impedance in ohms

[mshields]

If given the kVA and the percent impedance; say 112kVA @5% impedance, how would you determine the actual impedance in ohms?

Note: Actual problem I'm looking to solve is: I need to provide step up (from 208 to 480V transformer for x-ray machine for which the minimum line impedance needs to be less than 120milliohms.

So even after I determine what the transformer ohms are, if that plus the negligible ohms in the line less than 120milliohms, am I good. Or do I have to worry about the 208V line as well back to the utility transformer?

 

[Ingenieur]

pu = act / Z base or act = pu x Z base

S = v^2 / Z base or Z base = v^2 / S
v can be prim or sec, the beauty of pu

as far as the second part
not sure, need to ask equip mfg
I would assume sec only

 

[mshields]

OK - Hold the phone a moment

OK - Hold the phone a moment

So in this case, I've got a 75kVA step up transformer 208 to 480/277V
Impedance is, let's say 5%
So S = 75000
pu = 480V squared/75,000 = 3.07

Actual I assume is actual Impedance which is therefore 5.00 time 3.07 = 17.652

Is that right?

 

[Ingenieur]

So in this case, I've got a 75kVA step up transformer 208 to 480/277V
Impedance is, let's say 5%
So S = 75000
pu = 480V squared/75,000 = 3.07

Actual I assume is actual Impedance which is therefore 5.00 time 3.07 = 17.652

Is that right?

No
pu is 0.05 (5 is in percentage) or 0.05 x 3.072 = 0.1536 Ohm line-n
line-line is 0.1536 Ohm x 1.732 = 0.2660 Ohm

to confirm
std sc i calc = 75000/(1.732 x 480 x 0.05) = 1804.3 A

check
480 vac / 0.2660 Ohm = 1804.3 A

if the Z was 17. 7 Ohm losses would be
3 x (75000/(480 x 1.732))^2 A x 17.7 Ohm = 430 kw...on a 75 ka xfmr
not plausible

 

[Sahib]

Can't a line power conditioner, instead of trying to keep line impedance within limit, solve the problem?

 

[steve66]

Getting the 120 milliohms while using a step up xformer could be difficult, although a larger transformer should help. (The manufacturer usually lists a minimum size.)

And yes, the impedance of the wire on the 208V side will also count since it will cause a voltage drop on the primary side of the transformer. Actually, it will probably have more effect than the wiring on the secondary side since its at a lower voltage and a higher current.

I would cheat and model it in SKM. And you might have to use a power conditioner to get the results you need.

 

[Ingenieur]

we do not have enough info
is this a v drop issue
X/R limitation issue
does the primary matter? it is electrically isolated

we do not even know the load, steady state, inrush, etc.
assume 90% loading on the 112.5 kva or 120 A
assume loop Z, vdrop = 0.120 x 120 = 14.4 V or 3%

I don't think the internal xfmr Z should be included
it's accounted for in the xfmr rating, representative of full load operation