What is Voltage Regulation ?

The voltage regulation is the percentage of voltage difference between no load and full load voltages of a transformer with respect to its full load voltage.

Explanation of Voltage Regulation of Transformer

Say an electrical power transformer is open circuited, means load is not connected with secondary terminals. In this situation, the secondary terminal voltage of the transformer will be its secondary induced emf E2. Whenever full load is connected to the secondary terminals of the transformer, rated  current  I2 flows through the secondary circuit and voltage drop comes into picture. At this situation, primary winding will also draw equivalent full load current  from source. The voltage drop in the secondary is I2Z2 where Z2 is the secondary impedance of transformer.Now if at this loading condition, any one measures the voltage between secondary terminals, he or she will get voltage V2 across load terminals which is obviously less than no load secondary voltage E2 and this is because of I2Z2 voltage drop in the transformer.

Expression of Voltage Regulation of Transformer

Expression of Voltage Regulation of Transformer, represented in percentage, is

Voltage Regulation of Transformer for Lagging Power Factor
Now we will derive the  expression of voltage regulation in detail. Say lagging power factor of the load is cosθ2, that means angle between secondary  current  and voltage is  θ2

Here, from the above diagram,

Angle between OC and OD may be very small, so it can be neglected and OD is considered nearly equal  to OC i.e.