Question

For the impedance relaying schemes, calculate the impedance using the
1. Fundamental component of the post-fault current
2. Transient component of the post-fault current
3. Fundamental as well as transient component of the fault current
4. None of the above

Answer 1

Correct Answer - Option 3 : Fundamental as well as transient component of the fault current

The Impedance relay is so designed that it closes its contacts whenever impedance of the protected section falls below the set value i.e. Z in this case. Under threshold condition,

\({Z_{set}} = \sqrt {\frac{{{K_1}}}{{{K_2}}}} \) 

Where, K₁ & K₂ are constants.

When relay is operating,

Z_seen < Z_set

The condition Z_seen < Z_set, occurs when the fundamental component of the fault current is flowing. The impedance reduces and the relay is triggered.