Quadrature booster
A phase angle regulating transformer, phase angle regulator (PAR, American usage), phase-shifting transformer, phase shifter (American usage), or quadrature booster (quad booster, British usage), is a specialised form of transformer used to control the flow of real power on three-phase electric transmission networks. For an alternating current transmission line, power flow through the line is proportional to the sine of the difference in the phase angle of the voltage between the transmitting end and the receiving end of the line.