Correct Answer - Option 3 : 87.4
Merz-price differential protection scheme:
- Merz-price differential protection is nothing but a percentage differential protection.
- It works under the principle of the circulating current scheme.
- Merz-price differential protection is used to protect the transformer from internal short circuits, Internal ground faults, and inter-turn short circuits.
Percentage of Winding unprotected = (I0 × Rn / Vph) × 100
Where,
I0 = Minimum pick up current of relay × CT ratio
Rn = Neutral resistance
Vph = Phase voltage of the generator
Percentage of Winding protected = 100 - Percentage of Winding unprotected
Calculation:
Given,
Minimum pickup current of relay = 2 A
CT ratio = 500/5
Rn = 4 Ω
Vph = 11 / √3 kV
I0 = 2 × (500 / 5)
= 200 A
Percentage of winding unprotected = (200 × 4 × √3 / 11 × 103) × 100
= 12.6 %
Percentage of winding protected = 100 - 12.6
= 87.4 %
Therefore, the percentage of generator winding will be protected by the percentage differential protection scheme is 87.4%