Why and how the neutral is grounded

Neutral grounding (earthing) means connecting the neutral point of a star-connected generator, transformer or system to earth, either directly or through an impedance. An ungrounded (isolated neutral) system has serious problems, so modern systems above a few kV are grounded.

Why the neutral is grounded

1. Stable neutral voltage: the neutral is held at earth potential, so phase voltages to earth stay balanced and near V_ph under normal and fault conditions.
2. No arcing grounds: in an ungrounded system, an earth fault draws capacitive current that produces intermittent arcs; repeated charging/discharging raises voltage to 5–6 times normal. Grounding removes this.
3. Reduced overvoltage: healthy-phase voltages do not rise to √3 V_ph during an earth fault; insulation can be graded for lower levels, which saves cost.
4. Reliable protection: a definite earth-fault current flows, which operates earth-fault relays quickly and selectively.
5. Safety: equipment frames and persons are protected; static and induced charges are discharged.
6. Lightning: surges are discharged to earth through arresters connected to a grounded neutral.

Methods of neutral grounding

1. Solid (effective) grounding: neutral connected directly to earth with no impedance. Used at LV (400 V) and at high voltages (132 kV and above).
2. Resistance grounding: a resistor in the neutral limits earth-fault current (e.g. to rated full-load current). Used for 3.3–33 kV systems and generators.
3. Reactance grounding: a reactor in the neutral; used where solid grounding gives too high a fault current but X₀/X₁ must stay low.
4. Resonant grounding (Peterson coil / arc suppression coil): a tuned reactor whose inductive current cancels capacitive fault current, L = 1/(3ω²C). Used on overhead lines with many transient faults (e.g. 33 kV).
5. Grounding transformer grounding: where no neutral is available (delta system), a zig-zag or star-delta earthing transformer provides an artificial neutral.

Resistance grounding in detail

   Generator / transformer star
        a   b   c
         \  |  /
          \ | /
           (N)
            |
          [ R ]  neutral resistor
            |
           ===  earth

• Working: during a fault on phase a, fault current flows from the phase to earth, through R, back to neutral. R limits it to a safe value, usually 1 to 2 times full-load current, while still enough to operate earth-fault relays.
• The resistor also damps the oscillations that cause arcing grounds, and reduces burning of machine cores and the electromagnetic stress at the fault.
• Advantages: controlled fault current, no arcing ground, lower damage, less interference with communication lines, reliable relaying.
• Disadvantages: healthy-phase voltages rise above normal (between V_ph and √3 V_ph), so insulation must be slightly higher; resistor losses (I²R) during fault; costlier than solid grounding.
• Use: 3.3 kV to 33 kV systems, and generator neutrals (high-resistance grounding through a distribution transformer).