A phase angle regulating transformer, phase angle regulator (PAR, American usage), phase-shifting transformer, or quadrature booster (quad booster, British usage), is a specialised form of transformer used to control the flow of real power on three-phase electricity 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. Where parallel circuits with different capacity exist between two points in a transmission grid (for example, an overhead line and an underground cable), direct manipulation of the phase angle allows control of the division of power flow between the paths, preventing overload. Quadrature boosters thus provide a means of relieving overloads on heavily laden circuits and re-routing power via more favorable paths.
The capital cost of a quadrature booster can be high: as much as four to six million GBP (6–9 million USD) for a unit rated over 2 GVA. However, the utility to transmission system operators in flexibility and speed of operation, and particularly savings in permitting more economical dispatch of generation, can soon recover the cost of ownership.
Read more about Quadrature Booster: Method of Operation, Arrangement, Illustration of Effect