Neutral earthing and insulation level

System (neutral) earthing is the connection of the star point of generators/transformers to earth, either solidly or through a resistance/reactance. It decides how high the voltage of the healthy phases rises during an earth fault, and so it decides the rating of surge arresters and the insulation level (BIL) of the line and equipment.

Earthing coefficient

$$C_e = \frac{\text{highest rms phase-to-earth voltage of healthy phase during earth fault}}{\text{rms line-to-line voltage}}$$

• Isolated (ungrounded) neutral: healthy phases rise to the full line voltage, $C_e \approx 1.0$ (100%). Arcing grounds can produce transients as high as 5–6 pu.
• Effectively (solidly) earthed: $X_0/X_1 \le 3$ and $R_0/X_1 \le 1$; the healthy-phase voltage rises to only about 0.8 of the line voltage, $C_e \le 0.8$ (80%).
• Resistance/reactance earthed: in between.

Effect on surge arrester rating

Arrester rated voltage $= C_e \times V_{max}$ (times a small margin):

• 220 kV, $V_{max} = 245$ kV: effectively earthed → $0.8 \times 245 = 196$ kV arrester (≈"80% arrester"); isolated neutral → 245 kV arrester ("100% arrester").
• A lower-rated arrester has a lower protective (residual) level.

Effect on insulation level (BIL)

• BIL $=$ protection ratio × arrester protective level (protection ratio about 1.2–1.4).
• With effective earthing the protective level is about 20% lower, so a reduced BIL can be chosen. Example for 220 kV: full insulation 1050 kV, reduced 950 or 900 kV (850 kV in some practice).
• Power-frequency withstand voltages and switching over-voltages (which also depend on the phase-to-earth voltage) are lower too.

Effect on overhead line insulation

• Fewer discs per string, shorter cross-arms, smaller clearances, shorter towers and a narrower right-of-way. The saving can be 15–20% of insulation cost at 132 kV and above.
• With an isolated neutral, the insulation must be designed for full line voltage to earth and for arcing-ground surges; this is acceptable only at low voltages (≤ 33 kV), where insulation is cheap.

Practice

• 132 kV and above: neutrals are effectively (solidly) earthed so that reduced insulation can be used; this is standard in Nepal and India.
• 11 kV and 33 kV distribution: often resistance or solid earthing; insulation is cheap, so full insulation is common.
• Effective earthing also gives fast, selective earth-fault relaying and avoids arcing grounds, but it causes high earth-fault current and possible interference with communication lines.

Thus neutral earthing directly lowers or raises the voltage stress on insulation and is a key input to insulation coordination and to the number of discs chosen for an overhead line.