Phase Shifting Transformers

A phase shifting transformer (PST) is a specialised type of transformer, typically used to control the flow of active power on three-phase electric transmission networks. It does so by regulating the voltage phase angle difference between two nodes of the system. The principle relies on a phase shifted voltage source injection into the line by a series connected transformer, which is fed by a shunt transformer. The configuration of the shunt and series transformer unit induces the phase shift.

It is a simple, robust and reliable technology. Preventive and curative control strategies are implemented for power flow controllability. In the preventive mode, the permanent phase shift allows redistributing the power flows and relieves network stresses in the event of line outage. In the curative mode, the phase shift is small (sometimes down to zero) in normal operation, but it is automatically controlled to reduce the power flow on the overloaded lines and to avoid a tripping out. The active redirection of power flows allows exploiting lines closer to their thermal limits.

Technology Types

PSTs can be classified based on the following characteristics:

  • Direct PSTs are based on one 3-phase core. The phase shift is obtained by connecting the windings in an appropriate manner.
  • Indirect PSTs are based on a construction with two separate transformers: one variable tap exciter to regulate the amplitude of the quadrature voltage and one series transformer to inject the quadrature voltage in the right phase.
  • Asymmetrical PSTs create an output voltage with an altered phase angle and amplitude compared to the input voltage.
  • Symmetrical PSTs create an output voltage with an altered phase angle compared to the input voltage, but with the same amplitude.

Components & enablers

The components are comparable to traditional transformers:

  • Laminated core
  • Windings
  • Insulating materials
  • Transformer oil
  • Tap changer
  • Bushings

Advantages & field of application

The liberalisation of the European electricity market and the ever-increasing penetration of variable renewable generation have increased the need for this mature technology. Hence, the number of PSTs in the transmission grid is expected to rise. PSTs enable the grid operator to control unexpected loop flows, thus allowing the existing system to be used more efficiently. PSTs are used for congestion relief.

The PST system provides a means to control power flow between two grids. PSTs do not increase the capacity of the lines themselves, but if some lines are overloaded while capacity is still available on others parallel to them, optimising the power flows with PSTs can increase the overall grid capacity.\

Provided that there is free capacity on parallel paths that can be used, these slow devices are better suited for power flow control in the event of no continuous congestion and low congestion volatility.

PSTs are often the most economic and reliable approach to power flow management and system design, enabling TSOs to get more out of their existing assets. Existing transmission lines can be loaded up to the thermal limit without being overloaded. The investment in new lines can be postponed or even avoided.

A strong need for coordination emerges among the TSOs operating PSTs which are placed at the extremities of congested cross-border -tie-lines.

The following are key applications of the PST technology:

Preventive/curative power flow control in transmission lines PSTs are used in electrical power systems to control the active power flow between two points by regulating the corresponding voltage phase angle difference. The phase angle shift is obtained by opportunely placing the PST transformer in a shunt mode in respect to line terminals so that, by combining the voltages, the output voltage phase is shifted by an angle difference respect to the one as input in the PST. PSTs can thus be used to take advantage of an existing capacity margin on the network or to make an interconnection more secure.

Handle market flows in a physical meshed grid As a consequence of the power flow controllability, PSTs can also be used to match contract obligations from trading activities at wide regional level with laws of physics. They provide for crucial tools to address non-anticipated flows and lower the need for countertrading or redispatch.

Other innovative applications Innovative PST applications are enabled by adding series reactive elements aiming for substation uprating, substation reserve sharing, network decoupling, line power flow control using assisted PSTs (APSTs) and HV transmission lines de-icing. All these applications rely on the connection of conventional PSTs and reactive elements to meet unusual objectives for PSTs.

Technology Readiness Level

TRL 9 – System ready for full scale deployment

Research & Development

Current fields of research:

For this mature technology, research efforts focus more on enabling issues than on technological ones: the development of standards, com-mon PST models and protocols for the inter-TSO coordinated control of PSTs.

Innovation Priority:

Speed of switching, tap changer design, insulation fluid, reduction of losses, new materials

Best practice performance

Rated through put power: up to 1,630 MVA

Rated voltage levels: up to 420 kV

Maximum phase angle: <+/- 85 degrees

Best practice application

Border Germany / Poland / Czech Republic


Strongly increased wind power capacity in Northern Germany led to stress on the Polish and Czech transmission system and loop flows. PST were installed for a better control of cross-border exchanges. Thus, the required reserve capacity of the interconnectors can be reduced, and more interconnector capacity is available for the market.

CZ-DE: four PST (380 kV) in Hradec (CZ), two PST in Röhrsdorf (DE) PL-DE: four PST in Mikułowa (PL), four PST in Vierraden (DE) (two commissioned, two planned).

Increasing power system efficiency and reliability at the German-Polish border.

Border Spain / France


Commissioned on 30 June 2017, the PST helps to reinforce international electricity exchanges in South-West Europe at a boundary with limited number interconnection links.

220 kV and 550 MVA PST.

Increase the security of supply and reinforcement of international exchanges power flows.


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