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A 11 kV 100 MVA alternator is grounded through a resistance of 4 Ω. The C.T.s have a ratio of 500/5. The relay is set to operate when there is an out of balance current of 2 A. What percentage of generator winding will be protected by the percentage differential protection scheme?

A 11 kV 100 MVA alternator is grounded through a resistance of 4 Ω. The C.T.s have a ratio of 500/5. The relay is set to operate when there is an out of balance current of 2 A. What percentage of generator winding will be protected by the percentage differential protection scheme? Correct Answer 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%

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