Zero sequence fault currents are absent when the fault is

Question

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Option B: Line to line

Explanation & Concepts:
Zero sequence currents ($I_0$) depend on the presence of a path to the ground for the fault current to flow. In a line-to-line (L-L) fault, the fault involves only two phases with no connection to the neutral or ground, hence the sum of the phase currents remains zero, resulting in zero sequence current.

Working Principle: In symmetrical component analysis, the phase currents are decomposed into positive, negative, and zero sequence components. The zero sequence current is defined as $I_0 = \frac{1}{3}(I_a + I_b + I_c)$. In a line-to-line fault, since there is no ground path, $I_a + I_b + I_c = 0$, leading to $I_0 = 0$.

Key Formulas: $I_0 = \frac{1}{3}(I_a + I_b + I_c)$ — Definition of zero sequence current; $I_a + I_b + I_c = 3I_0$ — Relationship between phase currents and zero sequence current

Key Points: Zero sequence currents only exist if the fault involves ground.; L-G and LL-G faults involve ground, so they contain zero sequence components.; L-L faults and three-phase balanced faults do not involve ground, resulting in $I_0 = 0$.; Positive and negative sequence components exist in all unsymmetrical faults.

Advantages: Simplifies fault analysis by identifying zero sequence network conditions.; Helps in determining relay settings for ground fault protection.

Disadvantages: Does not provide information about the magnitude of positive or negative sequence currents.

Applications: Power system relay coordination; Grounding transformer design; Fault current studies

Additional Info: In a perfectly balanced 3-phase system, $I_a + I_b + I_c = 0$, meaning $I_0 = 0$.. Option A (L-G) and Option C (LL-G) involve the ground, therefore they produce $I_0 
eq 0$.

Answer: B is correct — A line-to-line fault does not involve the ground, therefore the sum of the phase currents is zero, which results in zero sequence current being absent.

Exam Tip: Remember: If a fault does not touch the ground, the zero sequence network is effectively open-circuited.

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Option A: Single line to ground

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Option C: Double line to ground

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Option D: Any of above

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