Which of the network gets affected by the method of neutral grounding?

Question

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Option A: Zero sequence network

Explanation & Concepts:
Neutral grounding primarily influences the zero sequence network in a power system. Since zero sequence current ($I_0$) flows through the neutral of a transformer or generator to the ground, any impedance introduced in the neutral path ($Z_n$) is included in the zero sequence equivalent circuit with a factor of $3Z_n$.

Working Principle: In symmetrical component analysis, faults like Single Line-to-Ground (SLG) involve all three sequence networks (positive, negative, and zero) connected in series. The positive and negative sequence networks are independent of neutral impedance. However, the zero sequence network contains the neutral return path; therefore, placing an impedance $Z_n$ in the neutral results in $3Z_n$ being added to the zero sequence impedance $Z_0$ of the machine or transformer.

Key Formulas: $Z_{0,total} = Z_0 + 3Z_n$ — Total zero sequence impedance including neutral grounding; $I_0 = \frac{V_{phase}}{Z_1 + Z_2 + Z_0 + 3Z_n}$ — Fault current equation for SLG fault

Key Points: Neutral grounding affects only the zero sequence network because zero sequence currents are in phase and return through the neutral.; Positive and negative sequence currents have a 120-degree phase shift, summing to zero, hence they do not flow through the neutral point.; The factor 3 arises because the total ground current is $I_g = I_a + I_b + I_c = 3I_0$.; Solid grounding results in $Z_n = 0$, whereas ungrounded systems have $Z_n = \infty$.

Advantages: Limiting fault current; Reduction of overvoltages; Ease of fault detection

Disadvantages: Increases potential for sustained ground faults in ungrounded systems; Requires complex relaying settings

Applications: Transmission lines; Generator neutral protection; Distribution transformer grounding

Additional Info: Option B (Positive sequence) and Option C (Negative sequence) are independent of the neutral path as these currents are balanced and do not return through ground.. The zero sequence impedance $Z_0$ represents the path for the zero sequence current.

Answer: A is correct — Neutral grounding impedance directly affects the zero sequence network impedance as $3Z_n$, while it has no impact on positive or negative sequence networks.

Exam Tip: Always remember: $I_1$ and $I_2$ (positive and negative sequence currents) always sum to zero, so they never appear in the neutral return path. Only $I_0$ returns through the ground.

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Option B: Positive sequence network

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Option C: Negative sequence network

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Option D: All of above

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