The maximum efficiency in the transmission of bulk AC power will be achieved when the power factor of the load is:

Transmission Efficiency: The efficiency of a transmission line is defined as the ratio of power delivered at the receiving end to the power sent from the sending end.

\(\eta = \frac{{{V_R}{I_R}\cos {\phi _R}}}{{{V_S}{I_S}\cos {\phi _S}}} \times 100\)

\(= \frac{{{V_R}{I_R}\cos {\phi _R}}}{{{V_R}{I_R}\cos {\phi _R}\; + \;losses}} \times 100\)

VR is the receiving end voltage

IR is the receiving end current

ϕR is the receiving end power factor angle

VS is the sending end voltage

IS is the sending end current

ϕS is the sending end power factor angle

For a load of slightly less than unity power factor leading, the reactive power requirement of the transmission line is supplied by the load only. Also, the current flows in the line will reduce which causes the reduction in losses, therefore efficiency in the transmission of bulk ac power increases by providing slightly leading power factor load