Convert Utility Source Impedance to Substation TX Low Side 1

[tmaly1]

Dear Folks,

I'm trying to figure out how the analysis dist. software we use calculate the impedance values for our system. My calculations match the software except for one area.

System/Source Information:
R1 (Utility-Per Unit)=0.0096 @100 MVA Base, @115 kV Base
X1 (Utility-Per Unit)=0.0761 @100 MVA Base, @115 kV Base
By Calculation:
=> R1 (Utility-Ohms)=1.2696, X1 (Utility-Ohms)=10.0774

Substation TX Info:115/13.2 KV,20 MVA Base,%Z=8.0,X/R=15
=>R1 (TX-Per Unit)=0.00532, X1(TX-Per Unit)=0.0791
By Calculation:
=>R1 (Ohms @13.2 kV,20 MVA Base)=0.0463 Ohms (TX Low Side)
=>X1 (Ohms @13.2 KV,20 MVA Base)=0.6601 Ohms (TX Low Side)

How Do I calculate/convert the source impedance values calculated above (at the 115 KV level) and convert/reflect them to the TX 13.2 KV level (in PU and Ohms)??

I tried the Per Unit Formulas bt my numbers don't match the software figures!!!!

Your input and feedback is very much appreciated. Thanks

 

[rbulsara]

Refer to IEEE Red Book.

Rafiq Bulsara

http://www.srengineersct.com

 

[magoo2]

Right now you have 2 sets of impedances on 2 different MVA bases. You need to put them on the same base to get anything meaningful.

Once the impedances are on the same MVA base, you can add them together and calculate the total impedance amd fault current.

 

[redfurry]

Here are the numbers:

Source Impedance (100MVA Base): .0096 + .0761j
Trans Impedance (20MVA Base): .00532 + .07717j
Trans Impedance (100MVA Base): .0266 + .38585j (*100/20)

Total Impedance (100MVA Base): .0362 + .4619j (Add Source + Trans))

@13.2kV ZBase = V^2/S = 1.7424 Ohms
Total Impedance (Ohms): .063 + .8049j (Zpu * Zbase)

 

[sibeen]

Multiply the upstream (HV) impedance by (LV/HV)^2. This will give you the equivalent downstream (LV) impedance.

Note that the LV has to be the no-load phase- phase voltage, nornally 5% higher than the nominal phase - phase voltage.