In an isolated neutral system, when a single line to ground fault occurs

Question

Examoogle · Accepted Answer

Option B: Voltage in the healthy phase rise to full line value causing insulator breakdown

Explanation & Concepts:
In an isolated (ungrounded) neutral system, a single line-to-ground fault causes the neutral potential to shift from zero to the phase voltage, leading to excessive voltage stress and charging currents. This configuration is prone to resonance and arcing phenomena which can damage insulation.

Working Principle: When a phase (e.g., R-phase) touches ground, its potential becomes zero. The healthy phases (Y and B) experience a voltage rise relative to the ground, increasing from the phase voltage $V_{ph}$ to the line voltage $V_{L} = \sqrt{3}V_{ph}$. Simultaneously, the capacitive current $I_c$ flowing through the healthy phases increases by a factor of $\sqrt{3}$ in magnitude, and the total residual current flowing through the fault point becomes $3I_c$ because of the vector sum of charging currents in healthy phases. Intermittent arcing occurs at the fault point due to the repetitive charging/discharging of line capacitance, leading to high-frequency transients.

Key Formulas: $V_{healthy} = \sqrt{3}V_{phase}$ — Rise in healthy phase voltage; $I_{fault} = 3I_{charging}$ — Magnification of capacitive current in the fault path

Key Points: The neutral point potential is no longer clamped at zero.; Healthy phases rise to full line-to-line voltage with respect to ground.; Capacitive charging current in healthy phases increases by $\sqrt{3}$.; Persistent arcing ground causes high-frequency voltage oscillations (arcing ground phenomena).

Advantages: Continuity of service during a single line-to-ground fault.; Low magnitude of fault current initially.

Disadvantages: High risk of insulation breakdown due to overvoltage.; Difficulty in locating faults due to low current.; Risk of arcing ground transients destroying equipment.

Applications: Small capacity distribution networks.; Systems where continuity of supply is critical and sensitivity is low.

Additional Info: The arcing ground occurs because the fault current is capacitive and passes through zero twice per cycle; if the arc does not quench effectively, the system undergoes severe transient overvoltages.. Option B is correct because the healthy phase voltage rises to line voltage, which is $\sqrt{3} \approx 1.732$ times the normal phase voltage.

Answer: D is correct — An isolated system under a single line-to-ground fault suffers from overvoltage on healthy phases, amplified capacitive fault currents, and recurring arcing ground transients.

Exam Tip: Always remember that in ungrounded systems, the fault current is purely capacitive and leads the voltage by $90^\circ$, which distinguishes it from grounded systems where the fault current is inductive/resistive.

Examoogle · Answer

Option A: Persistent arcing ground will be developed

Examoogle · Answer

Option C: Capacitive current in a faulty phase rises to three times its normal value

Examoogle · Answer

Option D: All of above

Correct Answer

Discussion (0)

Correct Answer

Related Questions

Discussion (0)