Short Circuit Calculation

• In this model, grid is modelled with short circuit current of 40kA and X/R ratio of 14, two winding transformer rating of 100 MVA, 220/33kV, %Z is 8.35 and X/R is 13.
• Fault is created at bus 2, the calculations for symmetrical RMS current is made below.

Impedance Correction Factor:

Transformer Impedance:

Grid Impedance:

Three Phase Fault:

Manual Calculations for Unsymmetrical Fault Current

• The same model is considered here, for calculation of unsymmetrical fault, the SLG fault is considered.
• The fault is created at bus 2 under no load condition i.e. Current before fault occurrence is zero.
• Due to unbalance nature in power system due to unsymmetrical fault, there will be positive, negative and zero sequence currents.
• Fault Impedance is considered to be zero.

Positive Sequence network (Z₁):

Negative Sequence Network (Z₂):

Zero sequence Network (Z₀):

Single Line to Ground Fault:

The fault current for SLG fault is

Line to Line Fault:

The fault current for SLG fault is

Double Line to Ground Fault:

The fault current for DLG fault is