The method of neutral grounding affects the

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Option C: Zero sequence network

Explanation & Concepts:
Neutral grounding is the method of connecting the neutral point of a star-connected transformer or generator to the earth. This connection directly affects the impedance path offered to zero-sequence currents ($I_0$) during an unsymmetrical fault, thereby influencing the zero-sequence network.

Working Principle: In symmetrical component theory, fault current is determined by the interconnection of positive, negative, and zero-sequence networks. Since the neutral is the return path for zero-sequence currents, the impedance of the grounding device (resistor, reactor, or solid ground) appears as $3Z_n$ in the zero-sequence equivalent circuit, where $Z_n$ is the neutral grounding impedance. Positive and negative sequence networks remain unaffected by the neutral impedance because these sequence currents are balanced and sum to zero at the neutral point.

Key Formulas: $Z_{0(total)} = Z_0 + 3Z_n$ — Total zero sequence impedance; $I_0 = \frac{V_a}{Z_1 + Z_2 + Z_0 + 3Z_n}$ — Zero sequence current for L-G fault

Key Points: Neutral grounding impedance ($Z_n$) only affects the zero-sequence network.; Positive and negative sequence networks are independent of the neutral connection.; Solid grounding results in $Z_n = 0$, leading to maximum fault current.; Resistance or reactance grounding is used to limit the fault current ($I_{LG}$).

Advantages: Limits earth fault current; Reduces overvoltage stresses on equipment; Facilitates earth fault relaying

Disadvantages: Adds complexity and cost to the system; May require additional insulation ratings if grounding is high-impedance

Applications: Power transmission systems; Industrial distribution networks; Generator neutral grounding

Additional Info: Positive sequence networks relate to the rotating magnetic field and are independent of neutral connection.. Negative sequence networks represent the reverse rotation and are also independent of neutral path.. The factor of 3 arises because the neutral point carries the sum of the three-phase zero-sequence currents, which is $3I_0$.

Answer: C is correct — The method of neutral grounding modifies the zero-sequence network by adding an impedance equivalent to three times the grounding impedance ($3Z_n$) in series.

Exam Tip: Always remember that neutral impedance only affects zero-sequence currents because positive and negative sequence currents sum to zero at the neutral point, making the neutral point a virtual short for those sequences.

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Option A: Positive sequence network

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Option B: Negative sequence network

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Option D: None of above

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