Per Unit calculation problem
Per Unit calculation problem
(OP)
I have a problem when I did Per-Unit calculation. Under the same condition, I got two different P.U. results for the load current by using two methods, which doesn't make sense to me.
Condition: Terminal voltage is 10 kV, load power is 100 MVA, solidly ground Y system
Method 1: Let base voltage is 10kV and Base complex power is 100 MVA,
then Base line current is 100/(sqrt3*10) = 5.77 kA
in this case, the p.u. load current magnitude is 1 p.u
Method 2: By setting the same base voltage and base MVA as in Method 1,
load current magnitude p.u = complex power_p.u./(sqrt3 * line voltage_p.u.) = 1/(sqrt3 * 1) = 1/ (sqrt3) p.u.
My expertise is telling me Method 1 is right, but I just can't figure out what is wrong with Method 2.
Condition: Terminal voltage is 10 kV, load power is 100 MVA, solidly ground Y system
Method 1: Let base voltage is 10kV and Base complex power is 100 MVA,
then Base line current is 100/(sqrt3*10) = 5.77 kA
in this case, the p.u. load current magnitude is 1 p.u
Method 2: By setting the same base voltage and base MVA as in Method 1,
load current magnitude p.u = complex power_p.u./(sqrt3 * line voltage_p.u.) = 1/(sqrt3 * 1) = 1/ (sqrt3) p.u.
My expertise is telling me Method 1 is right, but I just can't figure out what is wrong with Method 2.






RE: Per Unit calculation problem
Ipu=Spu/sqrt(3)/Upu
By definition Spu=S/Sbase; Upu=U/Ubase. Then:
Ipu=S/Sbase/sqrt(3)/[U/Ubase]=[S/U/sqrt(3)]/[Sbase/Ubase]
Actually Ipu=Iactual/Ibase
Iactual=S/U/sqrt(3) indeed, but Ibase=Sbase/Ubase/sqrt(3) [it is not only Sbase/Ubase].
So, as you can see Method 2 gets not Ipu [load current magnitude p.u]. Actually, in your case is Ipu= 1 as in Method 1.
RE: Per Unit calculation problem
load current magnitude p.u = complex power_p.u./(sqrt3 * line voltage_p.u.) = 1/(sqrt3 * 1) = 1/ (sqrt3) p.u.
As I read your formula:
Complex power_p.u = 1
line voltage_p.u. = 1
You have gone to a lot of work to calculate the reciprocal of √3.
Compare:
100000000(complex power)/(sqrt3* 10 (line voltage)) Method 1
complex power_p.u./(sqrt3 * line voltage_p.u.) Method 2
Complex power/line voltage = 100,000,000/10 = 10,000,000
You have used the same formula but you have used p.u. values when it is inappropriate.
I expect your answer to be off by a ratio of 10,000.
What would be appropriate is if you use the p.u. values to calculate a change in one or more values.
As in:
Let
100 MVA = p.u.1
10,000 V = p.u.1
5.77 kA = p.u.1
When MVA increases 10%. What is the resulting kA?
or
The current drops to 5.5 kA. What is the resulting MVA
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Per Unit calculation problem
Method 1: calculate actual current first and convert it to per unit
Method 2: calculate per unit power and voltage first, then directly calculate the current in per unit domain
They shall agree to each other in terms of per unit current, but they don't.
RE: Per Unit calculation problem
actual current = 5.77 kA
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Per Unit calculation problem
xnuke
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