Question

An 11 kV, 100 MVA generator is provided with differential scheme of protection. The percentage of generator winding to be protected against phase to ground fault is 80%. The relay is set to operate when there is 15% out of balance current. The value of the resistance to be placed in the neutral to ground connection is ______ohms.

• A. 6.44
• B. 2.788
• C. 0.4025
• D. 1.61

Answer 1

The correct option is D 1.61

Primary earth-fault current at which the relay operates,

$=\frac{100\times {10}^6}{\sqrt{3}\times 11\times {10}^3}\times \frac{15}{100}=787.29$ A

The percentage of winding which remains unprotected is

P = 100 - 80 = 20%


The fault current $=\frac{20}{100}\times \frac{11\times {10}^3}{\sqrt{3}{R}_n}$

Where $R_n$ is the resistance in the neutral to ground connection

$\frac{20}{100}\times \frac{11\times {10}^3}{\sqrt{3}{R}_n}=787.29$

$\therefore$ $R_n=\frac{20\times 11\times {10}^3}{100\times \sqrt{3}\times 787.29}=1.61$ ohms