Download Transformer Impedance

Transformer Impedance
In Simple Terms
• Transformer impedance is the % of rated
voltage required to push 100% of the rated
secondary current when the secondary
winding is short circuited.
Simplified Equivalent Circuit

Transformer impedance
Z = R + jX

Z is represented in percent of rated primary
voltage to cause rated secondary current in a
short-circuited secondary.
Transformer Impedance
•Transformer Impedance Is A Major Parameter
To Enable Parallel Operation Of Transformers
•For Proper Paralleling The Following Are
Required:
– Same Percentage Impedance For All Units (With
Some Tolerance)
– Same Phase Relationship Between Windings
– Same Voltage Ratios
– Similar Power Ratings
Maximum available fault current
• Calculate the secondary (LS) current at the
given MVA
• Calculate what the impedance related voltage
is based on the % impedance.
• Calculate how many times the calculated
voltage will go into the rated primary voltage.
• Multiply this number by the full load current
at the MVA
Now The Easy Method
• Fault Amps = Full Load Amps ÷ Impedance
• FA = 656 ÷ 0.0791
• FA = 8293