per unit calculation gives wrong results

I have a transformer tr with :%Ztr=21 %, Str=150MVA ,220KV/66KV /21 KV

and a reactor connected to its tertiary (21 KV) ,Z reactor =22 ohm per phase.

source impedance infinite 

On 21 kv side of TR as bases  values I took KVbase1=21 KV    MVAbase1=100 MVA

--> Zpu reactor==Z reac/Zbase2=5pu

On 220 kv side of TR  as bases  values I took  KVbase2=220 KV , MVAbase2=100 MVA  --> Z2pu TR=Ztr/Zbase1=0.14 pu

conversion to base1 gives Z1pu TR=0.14*(100/100)*(220/21)2 

                                                      Z1pu TR =15.36 pu

Z putot=5pu+15.36 pu=20.36pu-->Isc=KVAbase/(1.732*21kv*20.36pu)=135A

which is wrong because the MVA methid give 535 A. Could you help me to find the error?

asked 3/26/2018
  • I found the error ,i should not convert the p.u of the transformer because it will be the same on both side of it
    so Z pu total=z pu reactor + zpu tr=5+0.14=5.14 pu on base 21 kv , 100 MVA
    so Isc=KVAbase/(1.732*21kv*5.14pu)=534.89 A
    The MVA method already gives 535 A.
    Thank you any way - f13tun 3/26/2018

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