A phase shifting transformer (PST), also called a phase angle regulating transformer is a special type of transformer used in power systems to control the direction and magnitude of power flow in transmission lines.
It achieves this by introducing a controlled phase angle shift between the input and output voltages.
Principle of Operation
In an AC power system the real power flow (P) between 2 buses depends mainly on the phase angle difference between the voltages.
If the phase angle changes, the power flow also changes.
Power flow equation:
P = (V1V2/X) sin δ
Where:
P - Real power flow
V1, V2 - Bus voltages
X - Line reactance
δ - Phase angle difference
A phase shifting transformer changes the δ thereby controlling how much power flows through a transmission line.
Components of a Phase Shifting Transformer
A PST generally consists of two transformer sections:
1). Series Transformer
- Injects a voltage into the transmission line.
- This injected voltage creates the phase shift.
2). Exciting (Shunt) Transformer
- Supplies the required voltage to the series transformer.
- Provides voltage with adjustable magnitude and polarity.
Together they generate a quadrature voltage component that shifts the phase angle.
Working Procedure
The operation occurs in the following steps:
- The exciting transformer produces a regulated voltage.
- This voltage is applied to the series transformer.
- The series transformer injects this voltage into the transmission line.
- The injected voltage shifts the phase angle of the output voltage.
- The phase shift changes the power flow in the transmission line.
By adjusting tap changers, operators can control the amount of phase shift.
Why Phase Shifting Transformers are used?
Phase shifting transformers are installed to:
- Control power flow in interconnected power systems
- Prevent transmission line overload
- Improve grid stability
- Optimize power sharing between parallel lines
- Reduce loop flows in power networks
Typical Phase Shift Range
Most phase shifting transformers can provide a phase shift of:
±10° to ±40°
depending on design and system requirements.
Applications
Phase shifting transformers are commonly used in:
- Interconnected national power grids
- Cross-border power exchange
- Large transmission networks
- Renewable energy integration
- HVDC and HVAC power corridors