Fault Analysis

Fault types (approximate frequency of occurrence on overhead lines):

Fault	Share
Single line-to-ground (LG)	~70–80 %
Line-to-line (LL)	~8–15 %
Double line-to-ground (LLG)	~5–10 %
Three-phase (LLL) — symmetrical	~2–5 % (but most severe)

Symmetrical fault current: $I_f = \dfrac{V_{pre}}{Z_{th}}$ — in per-unit, $1/Z_{pu}$. Fault MVA: $S_f = \dfrac{S_b}{Z_{pu}}$.

Unsymmetrical faults are solved with symmetrical components by connecting positive, negative and zero sequence networks. For an LG fault: $I_f = \dfrac{3V}{Z_1 + Z_2 + Z_0 + 3Z_f}$.