The voltage regulation of the transformer will be negative when the power factor angle (φ) is :

Voltage Regulation

Voltage Regulation of single-phase transformers is the percentage (or per unit value) change in its secondary terminal voltage compared to its original no-load voltage under varying secondary load conditions.

The voltage regulation is given by:

% \(VR={V_{nl}-V_{fl}\over V_{nl}}\times 100\)

Voltage regulation curve:

Explanation

The voltage regulation is calculated by:

\(VR=IR \space cos\phi \space + \space IX \space sin\phi\)

For negative voltage regulation:

\(IR \space cos\phi \space + \space IX \space cos\phi \space <0\)

\( sin\phi \space <{-R\over X}\space cos\phi\)

\( tan\phi \space <{-R\over X}\space \)

\( tan\phi \space >{R\over X}\space \)

\(\varphi>\tan ^{-1}\left(\frac{R}{X}\right)\)