A phase shifting transformer (PST) is a specialised type of transformer used to control the flow of active power in three-phase electric transmission networks. It does so by regulating the voltage angle and the voltage amplitude between two nodes of the system. The regulation range of angle and amplitude depend on the PST type.

These functions can be realised either with one transformer (direct PST) or with two transformers (indirect PST), where one is a variable exciter to regulate the amplitude and voltage angle and one is a series transformer to inject the voltage angle into the circuit.

The benefits of PSTs are listed below: 

• PSTs basically allow the loadability of the mashed electrical networks to be increased, where between the load and generation many parallel electrical circuits of different length exist. The application of PST allows the loads of longer circuits to be increased and thereby avoid the overload of shorter circuits. 
• The PSTs are also used to adapt the electrical power flow with the market-based power flows, especially between different countries. 
• PSTs are system passive devices, which have been used for decades. The technology is very compact, robust and reliable, with minimised maintenance efforts. 
• Depending on the requested abilities, different technologies can be used. Typical PSTs have higher ability to reduce the load flow (e.g. +10°) and a smaller ability to increase the load flow (e.g. -2°). 
• Preventive and curative control strategies are implemented for power flow controllability. In the preventive mode, the permanent phase shift allows the power flows to be redistributed 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 lines to be exploited closer to their thermal limits.

The enablers of PSTs are listed below: 

• PSTs are quite heavy devices for which transportation to the destination substation needs to be permitted. For the transportation, specialised trailers and trains are needed.
• For transportation using roads, all kinds of bridges need to be carefully considered. This is one of the reasons why PSTs should be split into two transformers per phase to reduce the transportation weight.
• Transportation using railways is challenging, especially due to the change of railway profile at the railway stations with a high (equalised) train entrance. In such a case, special contracts with railway companies are needed to design and maintain routes for transportation for decades.

Several R&D activities, listed below, can contribute to further improving the technology: 

• A meaningful reduction of the number of different technology types (alignment of specifications) could accelerate the availability on the market; 
• A reduction of the transportation weight, electrical losses and noise emissions by application of the innovative, high-quality core materials; 
• Increasing the control angle while keeping the weight; 
• Management and optimisation of the transportation roots for permits with digital solutions; 
• Generally, the reduction of the environmental footprint of the transformer by new optimised processing of raw materials, use of CO2-neutral materials, improved assembly and improved losses; 
• Development of models and methods for planning the optimal position of PST in the mashed network; and 
• New solutions in some special network locations for the contribution to dynamic stability, compensating for the prolongation of line in the event of a short circuit (no voltage angle control available).

The technology is in line with milestone “Demonstration of innovative technologies for power flow control and increasing grid efficiency” under Mission 1 of the ENTSO-E RDI Roadmap 2024-2034.

Examples

TRL 9 for the conventional technology at high voltage and extra high voltage.

1. Amprion. “The Giant of Amprion” 

2. Red Electrica de Espana. “Red Electrica commissions the Arkale phase shifter to assist in the flow of international exchanges.”