The expression for voltage regulation of a short transmission line for lagging power factor is given by: ___, where, 'I' is the line current, 'V_r' is the receiving end voltage, 'R' is the line resistance and 'X' is the line reactance.

• A. (IX cosφ_r - IR sinφ_r)/V_r
• B. (IR cosφ_r - IX sinφ_r)/V_r
• C. (IX cosφ_r + IR sinφ_r)/V_r
• D. (IR cosφ_r + IX sinφ_r)/V_r

Answer: (D)

For lagging power factor = (IR*cosφ + Ixsinφ)/Vr
For leading power factor = (IR*cosφ - Ixsinφ)/Vr
Zero voltage regulation occurs on leading power factor only.