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Overvoltage causes; solid grounding merits

Short answer

Overvoltages are caused by lightning, induced charges, switching surges, arcing grounds, insulation failure, resonance, and load rejection or Ferranti effect. Solid grounding holds the neutral at earth potential, avoids arcing grounds, and gives a definite fault current; its demerits are very high fault current, communication interference, tripping on every fault, and step/touch hazards.

What are the causes of over voltages on power system? List the merits and demerits of solid grounding.

An overvoltage is any voltage above the maximum normal operating voltage that stresses the insulation of power system equipment.

Causes of overvoltages

A. External causes

  1. Direct lightning strokes on lines, towers or substation equipment, giving surges of several MV.
  2. Indirect (induced) lightning: charged clouds near a line induce bound charges; when the cloud discharges elsewhere, the released charge travels as a surge.
  3. Electrostatic charging of lines by wind, dust, fog and changing atmospheric conditions.

B. Internal causes

  1. Switching surges: opening or closing of CBs; energising long lines, switching off unloaded lines and capacitor banks (restriking), current chopping when breaking transformer magnetising current.
  2. Arcing grounds in ungrounded systems, giving 5–6 times normal voltage.
  3. Insulation failure: sudden phase-to-ground fault produces travelling waves.
  4. Resonance: series LC resonance or ferro-resonance at certain frequencies or harmonics.
  5. Load rejection and Ferranti effect on long lightly loaded lines.

Merits of solid grounding

  1. Neutral is held at earth potential; healthy phases stay near phase voltage during an earth fault, so insulation can be graded to lower levels (cheaper equipment, e.g. 80% arresters).
  2. No arcing grounds, because the fault current is large and the arc is cleared by the CB.
  3. Large, definite fault current makes earth-fault relaying simple, fast and selective.
  4. Simple and cheapest method; no extra equipment in the neutral.
  5. Effective for single-phase loads connected phase-to-neutral (LV distribution).

Demerits of solid grounding

  1. Very high earth-fault current (can exceed three-phase fault current), which stresses equipment thermally and mechanically and needs CBs of high breaking capacity.
  2. Large fault current causes greater interference with nearby communication circuits.
  3. Every earth fault, even transient, causes a CB trip and loss of supply (poor continuity).
  4. Higher step and touch voltages near the fault; danger to persons.
  5. Heavy fault current can damage generator windings, so generators are usually not solidly grounded.

Solid grounding is used at LV (400/230 V) and for EHV systems (132 kV and above) where insulation saving outweighs these demerits.

✓ Answer checked by Claude Opus 5.5 (high effort)

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