What is a Phase Shifting Transformer & Its Working

Day by day, the energy market and complex power generation is growing; the capacity to control the power flow is very important. For that, a phase-shifting transformer (PST) is used. As they defend transmission lines, they make power grids more reliable as well as decrease losses in transmission. The classification of this transformer can be done based on the symmetrical otherwise asymmetrical & direct otherwise indirect character of this transformer. This article discusses an overview of the phase-shifting transformer, construction, and its types.

What is a Phase Shifting Transformer?

A special kind of transformers like PST or Phase Shifting Transformer is used to control the active power flow on 3-phase transmission networks. This can be done by changing the difference of voltage phase angle among the system nodes. The working principle of a phase-shifting transformer mainly depends on an injection of a phase-shifted voltage source into the line using a series-connected transformer. This can be fed through a shunt transformer. These two transformer’s configuration mainly induces the phase shift.

Phase Shifting Transformer
Phase Shifting Transformer

A phase-shifting transformer or PST is an essential component that is used to enhance the efficiency of the AC network. When the transmitted energy increases then it will push the network to the edge, enhancing the threat of network insecurity.

These transformers mainly allow for controlling the flow of power within the transmission grid separately for the generation. By avoiding overloads of the system as well as instabilities, these transformers defend and HV equipment as well as transmission lines from thermal overload, so that it improves the capacity of the transmission system & controls the flow of power among various networks for parallel cables or overhead lines.

These transformers are extremely specialized devices that need leading-edge construction & manufacturing skills united through stringent quality control. As compared to a traditional transformer, PSTs are extremely complex power transformers including more tap changers, windings as well as a number of connections among the 3-phases.


The theory of phase-shifting transformer is, the flow of power throughout a transmission line can be proportional toward the sine of the disparity within the phase angle of the voltage among the end of transmitter & the receiver of the transmission line.


So the phase angle can be shifted within the ends of the transmitter and receiver of a line. The flow of load flow in the line will be modified & also other transmission lines’ load flow can also be changed because they are arranged in parallel.

The construction of a phase-shifting transformer mainly includes two sets of the transformer. The first one is the shunt unit that is connected in parallel through the transmission line whereas the second one is the series unit that is connected within series through the transmission line.

Construction of Phase Shifting Transformer
Construction of Phase Shifting Transformer

The shunt unit is used to shift the power angle by 90° to apply this power toward the series unit. Lastly, the series unit includes the phase-shifted power toward the transmission line. The modification of phase angle can be done by the blend of connection as well as the position of tap change. These kinds of transformers are also called phase angle regulator (PAR), phase angle regulating transformer, phase shifter, phase-shifting transformer otherwise quadrature booster.

The Function of Phase Shifting Transformer

The function of phase-shifting transformers includes the following.

  • These transformers are used to control the flow of power among two power systems that are big in size.
  • To modify the efficient phase displacement among the i/p voltage as well as the o/p voltage for a transmission line, therefore controlling the quantity of active power that can supply within the line.
  • The loading can be balanced when the power systems are allied jointly in the above one point so that loops will exist & the impedances within parallel lanes result in the undesired flow of power distribution within the lanes.
  • To guard from thermal overload as well as to progress the stability of the transmission system.
  • The phase-shifting transformers applications mainly include high voltage systems like 420 KV & the throughput power rating can be quite high like 1630 MVA.

Types of Phase Shifting Transformer

The classification of the phase-shifting transformer can be done based on the characteristics like the following.

Direct PSTs

The direct PSTs or direct phase-shifting transformers mainly depend on the three-phase core and its phase shift can be obtained by connecting the windings within a suitable way.

Indirect PSTs

The indirect PSTs or indirect phase-shifting transformers mainly depend on a design using two separate transformers like variable tap exciter and another one is the series transformer. So, variable tap exciter is used to change the quadrature voltage’s amplitude whereas the series transformer is used to insert quadrature voltage within the correct phase.

Asymmetrical PSTs

Asymmetrical phase-shifting transformers are used to generate an o/p voltage through changed amplitude as well as phase angle as compared to the i/p voltage.

Symmetrical PSTs

Symmetrical phase-shifting transformers generate an output voltage through a changed phase angle as compared to the i/p voltage, however with a similar amplitude.

Why Should we Perform Tests?

Because of the power system’s inductive nature, an active flow of power among sources as well as loads should be attained through a phase lag among terminals. These transformers are an ideal tool to attain this target.

The WACC cost of this transformer is high. But, the gain within elasticity, operation speed also in the loss in line justifies the investment. So the cost of PSTs can recover very quickly if they are placed under stable supervision as well as maintenance.

Generally, these transformers are used within critical nodes of the network. If any malfunction occurs then the time taken for maintenance is extremely significant. So both the TSOs (transport system operators) as well as the DSOs (distribution system operators) must be tested. To maintain these transformers permanently, then advanced devices are used to control the flow of power.

How do we Test PSTs?

The transformers exciting unit’s terminals are connected within the field are generally not accessible. So, specific EU & SU tests are generally performed simply within the factory. Some of the tests performed for this transformer include the following.

  • Turns Ratio
  • Dynamic Winding Resistance
  • Phase Shift
  • DC Winding Resistance
  • Exciting Current

The above-mentioned tests are appropriate to illustrate the characteristic performance of the transformer. To perform the measurement of a DC Winding Resistance as well as Dynamic Winding Resistance, a test like winding resistance can be used.

To perform other tests like turns ratio, phase shift tests, and exciting current through the necessary accuracy, beneath the international principles will also be very easy even for fresher’s through the High Test Turns Ratio Tester


The advantages of phase-shifting transformers include the following.

  • The phase shift transformer allows the operator of the grid to manage loop flows that occur suddenly, so these transformers are utilized for congestion relief.
  • These transformers manage power flow among two grids and they cannot enhance the capability of the lines, however, if some lines are overloaded when capacity is still obtainable on others which are parallel to them, then optimizing the flow of power these transformers can enhance the overall capacity of the power grid.
  • These transformers are frequently the most economic as well as consistent approach for power supply management & system design allowing the operators of the transmission systems to acquire more out of their accessible assets.
  • The transmission lines which exist can be loaded up toward the thermal edge without being overloaded. So, the investment within new lines can be postponed.


The following key applications of the PST technology can be mentioned:

These transformers are used to control the power flow within transmission lines. An electrical power system can be used to control the flow of active power among two points by changing the phase angle difference for equivalent voltage.

This phase angle shift can be attained by locating this transformer within a shunt mode with respect to the terminals of the line so that, by connecting the voltages, the o/p voltage phase can be shifted through an angle difference with respect to the input in the transformer. Thus, these transformers can be utilized to take benefit of an accessible capacity on the network or to make an interconnection more secure.

These transformers can also be utilized to match contract responsibilities from trading actions at a broad regional level through physics laws. They offer essential tools to address non-anticipated supplies & lowering the requirement of counter trading.

Other applications are made possible through including series reactive elements targeting at the uprating substation, reserve sharing in the substation, decoupling of the network, control of power flow in line through APSTs (Assisted PSTs) & de-icing of HV transmission lines. These applications mainly depend on the association of conventional phase-shifting transformers & reactive elements to meet abnormal objectives for these transformers.

Thus, this is all about an overview of the phase-shifting transformers (quadrature booster). This kind of transformer allows the increased utilization of renewable energy. These transformers optimize the usage otherwise losses which occur in transmission lines. PSTs mainly belong to the collection of FACTS devices and these are the valve within the grid. Here is a question for you, what are the disadvantages of a phase-shifting transformer?