Model Monday: Phase Shifting Transformers

Phase shift transformers (PST) can be used to regulate power flow through transmission systems to avoid the under- or overloading of lines. The following equations demonstrate how active and reactive power on a transmission line are functions of the voltages at the sending and receiving ends, as well as the angle difference between them.

pst_eq

Placing a PST into the system can impact both the output voltage magnitude and phase angle. In this way, a PST impacts both the reactive and active power flows.

PSTs can be classified as direct (one core) or indirect (two cores) and symmetrical or asymmetrical. Symmetrical PSTs only affect the voltage phase angle, and because lines are mainly inductive, only influence active power. Asymmetrical PSTs affect both the votlage phase angle and magnitude, and thus influence both the reactive and active power flows.

The differences between PSTs and standard transformers can create difficulties when applying protection principles. Therefore, models of PSTs have been developed in RSCAD so that system responses and protection methods may be tested in the simulated environment. The RSCAD component library contains four distinct phase shift transformer models, all of which are single phase. These single-phase models can be interconnected as required in order to implement a three-phase PST.

 

Single-phase, four winding PST (rtds_sharc_pst1p4w):

ps1

This model has a fixed turns ratio of 0.411007, allowing the taps to be controlled dynamically by the user in the RunTime interface. This component may be used to model symmetric quadrature PSTs.

Single-phase, three winding PST (_rtds_phase_shifter_1p3w):

ps2

This model allows dynamic control of its tap via the RunTime interface, and supports a short-circuit impedance entry format for leakage inductance data. This component may be used to hexagonal type PSTs.

Single-phase PST (_rtds_PhaseShiftTrf):

ps3This model has fixed tap positions between 1 and 30 degrees, allowing a user-defined turns ratio. This component may be used to model symmetric quadrature PSTs.

Single-phase, four winding, variable tap-changing PST (_rtds_PST1p4w1tap):

ps4

This model has a secondary winding with a single tap and a separated tertiary winding. The tap position may be controlled dynamically by the user in the RunTime interface. Tap position can span between 5 and 95% of the secondary winding.

The following screenshot of an RSCAD Draft case show an example of how single-phase PSTs could be interconnected to create a three-phase PST configuration. The case uses the rtds_sharc_pst1p4w component.

3phs

 

For an overview of the base component models available in RSCAD, please visit the Modelling Libraries page of our website.