What is meant by voltage regulation in a transformer?

The voltage regulation is defined as the change in the magnitude of receiving and sending voltage of the transformer. The voltage regulation determines the ability of the transformer to provide the constant voltage for variable loads. When the transformer is loaded with continuous supply voltage, the terminal voltage of the transformer varies. The variation of voltage depends on the load and its power factor.

Let us understand the voltage regulation by taking an example explained below:
If the secondary terminals of the transformer are open circuited or no load is connected to the secondary terminals, then no current flows through it. If no current flows through the secondary terminals of the transformer, the voltage drops across their resistive and reactive load become zero. The voltage drop across the primary side of the transformer is negligible.
If the transformer is fully loaded, i.e., the load is connected to their secondary terminal, the voltage drops appear across it. The voltage regulation values vary based on type of loads connected across the transformer like resistive, inductive or capacitive. The value of the voltage regulation should always be less for the better performance of transformer.
No exposition on transformer regulation could be called complete without mention of an unusual device called a ferroresonant transformer. “Ferro resonance” is a phenomenon associated with the behavior of iron cores while operating near a point of magnetic saturation (where the core is so strongly magnetized that further increases in winding current results in little or no increase in magnetic flux).