Page 592 - Basic Electrical Engineering

We know regulation can be negative only when the power factor is leading

and when the expression for regulation is

Regulation

Regulation will be zero when the numerator of the above expression will be

zero. That is

The power factor at which regulation will be zero is cos 20.6° = 0.94 leading
Regulation will be negative if the power factor angle is more than 20.6°

leading. To verify, let the angle of lead of load current be higher than 20.6°.
If we assume this angle as 37°, the power factor is 0.8 leading. The regulation
at 0.8 leading is calculated as

Regulation