Main function of phase shifting transformer is to control load sharing of parallel lines independently of the generation.
Say there are two parallel lines; 1 & 2. Line 1 has 50MW load and line 2 has 50MW. Now say line 2 is in over load situation, then load in line 2 is reduced to 27MW and load in line 1 is increased to 73 MW. This customized load sharing is done by the phase shifting transformer.
How phase shifting transformer works
So all we can do is to shift the phase angle in the sending end or receiving end of a line ; load flow of the line will be changed and so as the load flow of the other line also will be changed as they are in parallel.
In phase shifting transformer there are two sets of transformer, one is shunt unit connected in parallel with the line and another is series unit connected in series with the line. The shunt unit shifts the angle of the power by 90°, apply this phase shifted power to the series unit. Finally the series unit adds the phase shifted power to the line. With the combination of connection and tap change position the phase angle can be modified easily.
Phase shifting transformers are also known as phase angle regulating transformer, phase angle regulator (PAR, American usage), phase-shifting transformer, phase shifter (West coast American usage), or quadrature booster (quad booster, British usage)