Causes of high gradient electric fields

A high gradient field is a region where the electric field (voltage gradient, kV/cm) is much higher than the average value $V/d$. Such regions start corona, partial discharge and finally breakdown, so they decide the size and shape of HV equipment. The main causes are listed below.

1. Small radius of curvature (sharp points and edges)

Near a surface of radius $r$, the field is roughly $E \approx V/[r \ln(d/r)]$, so a small radius gives a very high gradient. Examples: sharp edges of electrodes, bolt heads, conductor strands, needle points, and thin conductors on EHV lines. This is why EHV lines use large-diameter or bundled conductors and fittings are given corona rings.

2. Non-uniform electrode geometry

Rod–plane, sphere–plane and point–plane gaps, and coaxial systems produce strongly non-uniform fields. The field is highest at the electrode with the smaller radius. The field utilisation factor $\eta = E_{av}/E_{max}$ is much less than 1 for such geometries.

3. Surface irregularities on conductors

Scratches, burrs, broken strands, dust, insects and water drops on conductors locally raise the gradient. This is why corona inception falls in rain or fog (irregularity factor $m < 1$).

4. Voids and cavities in solid insulation

A gas-filled void in a solid of relative permittivity $\varepsilon_r$ carries a field about $\varepsilon_r$ times that in the solid, and the gas is also weaker. This leads to partial discharges.

5. Interfaces of different permittivities (composite dielectrics)

In series dielectrics, the field divides inversely with permittivity ($E_1\varepsilon_1 = E_2\varepsilon_2$). The lower permittivity material (often air or oil) is over-stressed. Example: an air gap between a bushing and its insulation.

6. Triple junctions

The point where an electrode, a solid insulator and a gas meet has a very high local field. Discharges often start there and creep along the surface.

7. Conductive particles and contamination

Metallic particles in GIS or oil, and moisture and salt on insulator surfaces, distort the field and create local high stress, leading to flashover.

8. Space charge and terminations

Charges left by previous discharges, and the ends of cable screens and shields, concentrate the field at the termination edge. Stress cones are therefore used at cable ends.

9. Overvoltages

Lightning and switching surges raise the voltage, and therefore every local gradient, far above normal values.

Remedies (in brief)

• Use large radii, grading (corona) rings and stress cones.
• Use bundled conductors.
• Make insulation void-free and keep it clean.
• Choose permittivities that grade the field properly.