Given transformer data 30 /40 / 50 MVA Single Phase rating
at 115 / 32 kV with a CT 1200 / 5 A at secondary side.
Solve for secondary Ampere and CT output current at 30 MVA , 40 MVA, and
50 MVA. Can you still use the same CT 1200/5 A for 40 and 50 MVA explain your answer.
Valid for 1 Φ only
Secondary Ampere
 = 
1Phase MVA
kV secondary voltage

A ,CT Pri. Rating
A, CT Sec. Rating

= 
A, CT pri. measured
A, CT sec. measured

Use 30 MVA as your base. When there is a short circuit the secondary voltage goes down very low. At that instant the current is approaching its maximum value. For example when the secondary voltage drop to 2 kV due to short circuit, estimate the secondary short circuit current ampere.
When you download the transformer differential relay or breaker relay data you will see close to 62.5 A primary ampere reading during fault which is 62.5 A x 240 the CT ratio equals to 15,000 ampere.
Same Problem as above but now it is 3 phase rating:
Given transformer data 320 base MVA three phase rating
at 525 / 230 kV with a CT 2000 / 5 A at primary side.
Solve for primary ampere and CT output current.
What is the primary short circuit current when the primary voltage goes down to 50 kV as reported by differential relay data? What is the primary short circuit current when the primary voltage goes down to 12 kV? Use the base MVA in your calculation.
Valid for 3 Φ only
Primary ampere
 = 
3 Phase MVA
kV * 1.732 primary voltage

A , CT pri. Rating
A, CT sec. Rating

= 
A, CT pri. measured
A, CT sec, measured
