When a fault occurs in a power system the zero sequence component of current becomes zero. The type of fault i

Question

When a fault occurs in a power system the zero sequence component of current becomes zero. The type of fault is

Examoogle · Accepted Answer

Option B: Line to line fault

Explanation & Concepts:
In symmetrical component analysis, the zero-sequence current $I_0$ is defined as $\frac{1}{3}(I_a + I_b + I_c)$. In a Line-to-Line (L-L) fault, the fault occurs between two phases without involving the ground, meaning the sum of the currents remains zero (i.e., $I_a + I_b + I_c = 0$), resulting in $I_0 = 0$.

Working Principle: In an L-L fault (e.g., between phases B and C), the currents satisfy $I_b = -I_c$ and $I_a = 0$. Since the zero-sequence component is the average of the phase currents, it follows that $I_0 = \frac{1}{3}(0 + I_b - I_b) = 0$. This condition exists because there is no return path for the fault current to the neutral or ground, preventing the flow of zero-sequence currents.

Key Formulas: $I_0 = \frac{1}{3}(I_a + I_b + I_c)$ — Definition of zero sequence current; $I_0 = 0$ — Condition for faults not involving ground

Key Points: Zero sequence current requires a path to ground to exist.; L-L faults are purely symmetrical regarding ground, hence no zero sequence current.; L-G and D-L-G faults involve the ground, therefore they always have non-zero $I_0$.; Symmetrical components are used to simplify the analysis of unsymmetrical faults.

Advantages: Simplifies analysis of power system networks; Easier determination of relay settings

Disadvantages: Complex calculation for multi-phase faults; Requires knowledge of network sequence impedances

Applications: Protective relaying coordination; Fault current calculation in transmission lines

Additional Info: The zero-sequence network is open-circuited for faults that do not involve the ground (e.g., L-L faults).. Option A and D refer to Line-to-Ground faults, where $I_0$ is maximum. Option C refers to Double-Line-to-Ground, where $I_0$ is also present due to the ground connection.

Answer: B is correct — Line-to-line faults do not involve the ground, resulting in a zero-sequence current component of zero.

Exam Tip: Remember: If a fault does not involve the ground, $I_0$ is always zero. This is a quick way to identify L-L faults in objective questions.

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Option A: phase to ground fault

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Option C: Double line to ground fault

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Option D: Line to ground fault

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