Neutral grounding transformer sizing

A neutral grounding transformer (NGT) is a single-phase distribution-type transformer connected between the generator neutral and earth, with a resistor on its secondary. The resistor, reflected to the primary as a high resistance, gives high-resistance grounding of the generator.

   Generator (star)
     \ | /
      \|/  neutral
       |
   NGT primary (e.g. 11 kV/sqrt3)
     |||
   NGT secondary (e.g. 240 V)
       |
      [R]  loading resistor
       |    + 59N relay across R
      ===  earth

Purpose: limits stator earth fault current to about 5–15 A, preventing core damage; limits transient overvoltages during arcing faults; allows sensitive stator earth fault protection (59N relay across the resistor).

Steps to find optimal size of NGT

1. Total capacitance to earth per phase $C_0$: stator winding, generator bus duct/cables, GSU LV winding, surge capacitors and VT windings.
2. Capacitive earth fault current:

$$I_C = 3\omega C_0 V_{ph}, \quad V_{ph} = \frac{V_L}{\sqrt{3}}$$

3. Choose resistor current $I_R \geq I_C$ (resistance not more than the capacitive reactance), so that transient overvoltage stays below about 2.6 p.u.:

$$R_N \leq \frac{1}{3\omega C_0}$$

4. NGT voltage ratio: primary rated at generator phase-to-neutral voltage (often line voltage for safety margin), secondary e.g. 240 V; turns ratio $n = V_1/V_2$.
5. Secondary resistor value: $R_2 = R_N / n^2$.
6. NGT kVA rating: $S = V_{ph} \times I_R$ (short-time rating for 1 min or until protection trips), giving a smaller, cheaper transformer than a continuous rating.
7. Check: total fault current $I_f = \sqrt{I_R^2 + I_C^2}$ is within the allowed limit (typically below 10–15 A) and the relay sensitivity covers about 95% of the winding.