Phase Shifting Transformer

Phase Shifting Transformers (PST), able to control active power by regulating the voltage phase angle difference between two nodes of the system. The operation principle is voltage source injection into the line by a series connected transformer, which is fed by a tapped shunt transformer. So, overloading of lines can be eliminated. A typical phase shifting transformers may be capable of a variable phase angle shift up to 20°. However, the speed of phase shifting transformers for changing the phase angle of the injected voltage via the taps is very slow. Phase shifting transformers and similar devices using mechanical taps can only be applied for very limited tasks with slow requirements under steady state system conditions.

Quadrature booster 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 favourable paths. Alternately, where an interchange partner is intentionally causing significant “inadvertent energy” to flow through an unwilling interchange partner’s system, the unwilling partner may threaten to install a phase shifter to prevent such “inadvertent energy”, with the unwilling partner’s tactical objective being the improvement of his system’s stability at the expense of the other system’s stability. As power system reliability is really a regional or national strategic objective, the threat to install a phase shifter is usually sufficient to cause the other system to implement the required changes to his system to reduce or eliminate the “inadvertent energy”.

 

A phase shifting transformer typically consists of two separate transformers: a shunt unit and a series unit. The shunt unit has its windings connected across the phases. If it produces output voltages proportional to, and in phase with, the primary-side phase voltage then series transformer will produce a change in voltage magnitude. Alternatively, shunt transformer can be constructed so that its output voltages proportional to the primary-side phase-to-phase voltage. Since the line voltage between two phases in a three-phase system is always in quadrature with the voltage of third phase, the series transformer will introduce a small change in voltage magnitude but a large change in voltage angle.

The overall output voltage is hence the vector sum of the supply voltage and the phase shifted component. Tap connections on the shunt unit allow to control the output voltage and thus the power flow on the circuit containing the phase shifting transformer may be increased (boost tapping) or reduced (buck tapping) as shown in figure below.