A system is said to be effectively grounded if its

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Examoogle · Accepted Answer

Option D: Ratio of $\frac{X_0}{X_1} < 3.0$

Explanation & Concepts:
A power system is considered effectively grounded (or solidly grounded) if the ratio of the zero-sequence reactance ($X_0$) to the positive-sequence reactance ($X_1$) is less than 3.0. Additionally, the ratio of the zero-sequence resistance ($R_0$) to the positive-sequence reactance ($X_1$) must be less than 1.0.

Working Principle: Effective grounding is designed to limit the overvoltage on the unfaulted phases during a single line-to-ground fault. By maintaining a low $X_0/X_1$ ratio, the neutral point of the system remains close to the ground potential, which significantly reduces the displacement of the neutral and protects the insulation levels of the equipment.

Key Formulas: $\frac{X_0}{X_1}; $\frac{R_0}{X_1}

Key Points: Effective grounding minimizes Transient Recovery Voltage (TRV).; It allows the use of graded insulation for transformers.; It restricts the rise in voltage of sound phases to approximately 80% of line-to-line voltage during a ground fault.

Advantages: Reduced insulation costs for equipment.; Easier detection of ground faults via relaying.; Significant reduction in transient overvoltages.

Disadvantages: Higher fault current magnitudes require circuit breakers with higher ratings.; Increased risk of inductive interference with communication circuits.

Applications: Extra High Voltage (EHV) transmission systems.; Systems where standard insulation levels are to be kept economical.

Additional Info: Option B (Ratio > 3.0) represents an ungrounded or Petersen coil grounded system condition.. Standard industrial practice defines effective grounding to ensure protection coordination simplicity.

Answer: D is correct — The condition for effective grounding is that the ratio of zero-sequence reactance to positive-sequence reactance must be less than 3.0, ensuring controlled phase-to-ground voltages.

Exam Tip: Always remember that $X_0$ is highly dependent on the grounding method, whereas $X_1$ and $X_2$ are inherent to the machine/line geometry and remain relatively constant.

Examoogle · Answer

Option A: Neutral is grounded directly

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Option B: Ratio of $\frac{X_0}{X_1} > 3.0$

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Option C: Ratio of $\frac{R_0}{X_1} > 2.0$

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