A power system is subjected to a fault which makes the zero sequence component of current equal to zero. The n

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A power system is subjected to a fault which makes the zero sequence component of current equal to zero. The nature of fault is

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Option B: Double line fault

Explanation & Concepts:
In a power system, the zero-sequence component of current ($I_0$) exists only when there is a path for the current to flow into the ground. A double-line fault (line-to-line fault without ground) does not involve the ground, hence the zero-sequence current is zero.

Working Principle: In symmetrical component analysis, the zero-sequence current is defined as $I_0 = \frac{1}{3}(I_a + I_b + I_c)$. In a balanced system or a line-to-line fault where no current path to the neutral/ground exists, the sum of phase currents is zero, leading to $I_0 = 0$. For faults involving the ground (L-G, LL-G, LLL-G), at least one path to ground exists, causing a non-zero $I_0$.

Key Formulas: $I_0 = \frac{1}{3}(I_a + I_b + I_c)$ — Definition of zero-sequence current component; $I_a + I_b + I_c = 0$ — Kirchhoff's Current Law for non-grounded faults

Key Points: Zero-sequence current is an indicator of the presence of a ground return path.; Double line (LL) faults are balanced in terms of ground involvement (none).; Positive and negative sequence networks are connected in parallel for an LL fault.; Zero sequence network is open-circuited for faults without ground connection.

Advantages: Simplifies analysis of unsymmetrical faults using sequence networks.; Helps in relay coordination by identifying fault types.

Disadvantages: Does not provide information about the exact location of the fault.; Assumes linear system behavior which may not hold during heavy saturation.

Applications: Protective relay setting calculation.; Power system stability analysis.; Fault current level determination.

Additional Info: The zero-sequence network is essentially the path for current flowing through the neutral and ground.. Option C (Line to ground fault) involves a ground return path, so $I_0 
eq 0$.. Option D (Three-phase fault) is a balanced fault where $I_0 = 0$, but it is not an unsymmetrical fault.

Answer: B is correct — In a double line fault, there is no path to the ground, resulting in the sum of phase currents being zero and thus the zero-sequence current component is zero.

Exam Tip: Always remember: If the fault does not involve the earth (ground), the sum of the phase currents $I_a + I_b + I_c$ is always zero, rendering the zero-sequence component $I_0$ zero.

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Option A: Double line to ground fault

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Option C: Line to ground fault

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Option D: Three-phase to ground fault

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