Per Unit Analysis Confusion
Per Unit Analysis Confusion
(OP)
I have this problem I'm trying to figure out, doing the problem normally versus using per unit gives me different answers!
So I have a three-phase motor that is rated for 20MVA at 13.8kV and it is connected to a bus at 1<0 degrees pu. The system base Sb=100MVA and Vb=13.8kV.
To find the PU current:
I = S / (sqrt(3)*V) = (20MVA/100MVA) / (sqrt(3)*(13.8kV/13.8kV)) = 0.1155 pu
Another way of doing it is to find the actual current:
I = S / (sqrt(3)*V) = 20MVA /(sqrt(3)*13.8kV) = 836.739 A
The base current = S / (sqrt(3)*V) = 100MVA / (sqrt(3)*13.8kV) = 4183.7 A
The PU current using this approach is 0.2 pu
Comparing the two answers (0.1155 pu) and (0.2 pu, there is a factor of sqrt(3) somewhere...
Can anyone tell me which is right and why the two different approaches yield different answers? Was my assumptions wrong?
Thank you
So I have a three-phase motor that is rated for 20MVA at 13.8kV and it is connected to a bus at 1<0 degrees pu. The system base Sb=100MVA and Vb=13.8kV.
To find the PU current:
I = S / (sqrt(3)*V) = (20MVA/100MVA) / (sqrt(3)*(13.8kV/13.8kV)) = 0.1155 pu
Another way of doing it is to find the actual current:
I = S / (sqrt(3)*V) = 20MVA /(sqrt(3)*13.8kV) = 836.739 A
The base current = S / (sqrt(3)*V) = 100MVA / (sqrt(3)*13.8kV) = 4183.7 A
The PU current using this approach is 0.2 pu
Comparing the two answers (0.1155 pu) and (0.2 pu, there is a factor of sqrt(3) somewhere...
Can anyone tell me which is right and why the two different approaches yield different answers? Was my assumptions wrong?
Thank you






RE: Per Unit Analysis Confusion
RE: Per Unit Analysis Confusion
RE: Per Unit Analysis Confusion
RE: Per Unit Analysis Confusion
http://en.wikipedia.org/wiki/Per-unit_system
RE: Per Unit Analysis Confusion
RE: Per Unit Analysis Confusion
RE: Per Unit Analysis Confusion
RE: Per Unit Analysis Confusion
I base= Sbase/(sqrt3* Vbase)
So your second solution is correct.
your first solution I = S / (sqrt(3)*V) = (20MVA/100MVA) / (sqrt(3)*(13.8kV/13.8kV)) = 0.1155 pu is wrong as it involves direct conversion of values in per unit. Hence, you left the factor of sqrt3.
you should apply following rule to solve P.U problms: For given Sbase and Vbase find Ibase as Sbase/(sqrt3* Vbase)
and from main circuit find current I.
Therefore, I(p.u) is I/Ibase.
Hope this helps :)
RE: Per Unit Analysis Confusion
Can you explain a bit what you mean here? Maybe in terms of my problem?
RE: Per Unit Analysis Confusion
in symmetrical components Ipu=Iline-to-neutral= Iline-to-line/V3
RE: Per Unit Analysis Confusion