The output voltage of transformer is related to its internal impedance and load power factor. It could be increased when load power factor is leading (capacitive).
Regulation is defined as the change (increase) in the output voltage that occurs when the load on the transformer is reduced from rated load to no load while the input voltage is held constant. It is typically expressed as a percentage, or per unit, of the rated output voltage at rated load. A general expression for the regulation can be written as:
where VNL is the voltage at no load and VFL is the voltage at full load. The regulation is dependent upon the impedance characteristics of the transformer, the resistance (r), and more significantly the ac reactance
(x), as well as the power factor of the load. The regulation can be calculated based on the transformer impedance characteristics and the load power factor using the following formulas:
where p is the power factor of the load and r and x are expressed in terms of per unit on the transformer base. The value of q is taken to be positive for a lagging (inductive) power factor and negative for a leading (capacitive) power factor.
It should be noted that lower impedance values, specifically ac reactance, result in lower regulation, which is generally desirable. However, this is at the expense of the fault current, which would in turn increase with a reduction in impedance, since it is primarily limited by the transformer impedance.
The regulation increases as the power factor of the load becomes more lagging (inductive) and vice versa the regulation should be decreases as the load power factor becomes more leading (capacitive).