Fault Calculation

To obtain more accurate results calculation has to be carried out in different time ranges

• Sub Transient- large current 50 ms
• Transient - after 0.5 s
• Steady state- after 1 s

When we analyze large systems with different voltage levels we use base quantities and per unit values .Decaying DC component is considered using the asymmetric factor. Understanding the positive, negative and zero sequence components are required in unbalanced faults.

• For transmission lines positive and negative impedance are same and equal to the per unit impedance of the line. Zero impedance depends on the grounding configuration. Therefor obtain it from manufacture data sheet. 
• Balanced generator only generate positive sequence voltage.
• For motors has same positive and negative impedance. Since most of the motors are ungrounded it doesn't has a zero impedance.

L-L-L or L-L-L-G  => positive sequence
L-G => positive, negative, zero in series
L-L => positive, negative in parallel
L-L-G => positive, negative, zero in parallel

Line to ground voltage = line to line voltage / root(3)