Question

In a biased differential relay the bias is defined as ratio of
1. Number of turns of restraining and operating coil.
2. Operating coil current and restraining coil current.
3. Fault current and operating coil current.
4. Fault current and restraining coil current.

Answer 1

Correct Answer - Option 1 : Number of turns of restraining and operating coil.

Operating force of relay coil = n_o(|i₁|-|i₂|)

Opposing force produced by restraining coil \(= {n_r}\left( {\frac{{\left| {{i_1}} \right| + \left| {{i_2}} \right|}}{2}} \right)\)

At balanced (or) boundary condition

\(\begin{array}{l} {n_o}\left( {\left| {{i_1}} \right| - \left| {{i_2}} \right|} \right) = {n_r}\left( {\frac{{\left| {{i_1}} \right| + \left| {{i_2}} \right|}}{2}} \right)\\ \frac{{{n_r}}}{{{n_o}}} = \left( {\frac{{\left| {{i_1}} \right| - \left| {{i_2}} \right|}}{{\frac{{\left| {{i_1}} \right| + \left| {{i_2}} \right|}}{2}}}} \right) = bias \end{array}\)

Where n_o and n_r are the number of turns on operating (or) relay coil and restrained coil respectively.