Causes and protection against overvoltage

An overvoltage is a voltage, normally a short-duration surge, above the highest normal operating voltage of the system. Insulation must be protected from it by shielding, by diverting the surge to earth and by controlling switching.

Causes of overvoltage

External:

• Direct lightning stroke on conductors, towers or substation.
• Induced lightning (bound charge released when a nearby cloud discharges).
• Electrostatic build-up on lines due to wind, dust and atmospheric changes.

Internal:

• Switching surges: closing/opening long lines, switching capacitor banks (restrike), current chopping of transformer magnetising current.
• Arcing grounds in isolated-neutral systems.
• Insulation failure (sudden earth fault).
• Resonance and ferro-resonance.
• Load rejection and Ferranti effect.

Methods of overvoltage protection

1. Ground (earth/shield) wires: bare conductors run above the phase conductors and earthed at every tower. They intercept direct strokes and shield the line within a protective angle (about 30°). Low tower-footing resistance is needed to avoid back flashover.
2. Lightning conductors / masts / shield wires over substations: tall rods or overhead wires earthed through a good earth mat protect outdoor equipment from direct strokes.
3. Lightning (surge) arresters: connected between line and earth near equipment. They act as an open circuit at normal voltage, conduct during a surge to divert it to earth, then reseal. Types: rod gap, horn gap, expulsion type, valve type (SiC) and metal-oxide (ZnO) gapless arresters, now standard.
4. Rod gaps and horn gaps: simple spark gaps across insulators; cheap back-up, but they cause a short circuit until the CB trips.
5. Surge absorbers: devices that flatten the steep front of travelling waves before they reach transformer or machine windings, e.g. a capacitor between line and earth, an inductor with a parallel resistor in series with the line, or the Ferranti surge absorber (an air-cored inductor inside an earthed metal sheath, whose eddy-current losses dissipate the surge energy).
6. Neutral grounding: solid, resistance or Peterson coil grounding removes arcing ground overvoltages.
7. Control of switching surges: CBs with pre-insertion/opening resistors, synchronised (point-on-wave) switching, restrike-free breakers (vacuum, SF6), shunt reactors to limit Ferranti rise.
8. Insulation coordination: insulation levels (BIL) of equipment are chosen higher than the protective level of arresters by a safety margin (about 20–25%).
9. Good earthing: low earth-mat and tower-footing resistance so that surge current is discharged safely.

 Line ---+-------------+---- Transformer
         |             |
      [Arrester]   [Surge absorber]
         |             |
        ===           ===
 Shield wire above line, earthed at each tower

A combination of shield wires against direct strokes and metal-oxide arresters near every important item of equipment gives the most reliable protection.