Power transformers are an essential component of the Alternating Current (AC) electrical power system, enabling an exchange of the electrical power between different voltage levels. Power transformers must be built to withstand severe electrical stress from fault currents and transients.

Integrating an On-Load Tap Changer (OLTC) with the transformer allows for the regulation of the output voltage by adjusting the number of transformer windings (the transformation ratio). This ability is needed to keep the voltage in the connected networks in designed bandwidth independently of the load condition. While all high voltage power transformers have tap changers, the low voltage transformers were built cost efficiently without tap changers. Nowadays, the ability to also adapt the voltage in medium voltage and low voltage networks becomes important due to the enormous growth of the renewable energy generation.

Key functions of power transformers with tap changers are:

• Voltage step-up and -down to reduce the currents needed for transmitting the same amount of electrical power. Step-up transformers are used to minimise transmission line losses. Step-down power transformers are used to bring down transmission voltages to usable voltage level for end-customer connections.
• Slow dynamic regulation to adjust to changing network conditions supporting the voltage stability of the AC-grid.

The benefits of AC power transformers are listed below:

• Key components of AC electrical systems;
• Simple connection of electrical systems on different voltage levels for power exchange;
• Possibility for galvanic isolation of systems for easier fault treatment;
• Contribution to voltage stability by application of tap changers; and
• Very robust and proven technology.

The enablers of AC power transformers are listed below:

• Availability of copper for transformer windings;
• Availability of special magnetic steel (with low magnetic losses) for transformer cores;
• Bigger size of transformer due to energy efficiency versus transportation limits;
• Railway profiles which allow for transformer transportation; and
• Bridge static loading and very extensive transportation route planning.

Since early 2020, an active Working Group has aimed to address the following issues raised by OLTCs [1]:

• Guidelines for the proper selection of OLTCs and transformer design considerations;
• Actions on OLTCs during the lifetime of regulated transformers; and
• Failure modes. A new Working Group at CIGRE is expected to address the weak points of insulating liquids [2], viz.:
• CIGRE paper 11066_2022 (A2-PS2) – Qualification of insulating liquids for power transformers and tap changers.
• CIGRE Brochure TB856 – Dielectric performance of insulating liquids for transformers.
• IEC TC14 TR60076-26 – Functional requirements of insulating liquids.

The technology is in line with milestone “Advanced reconfiguration and control of network and assets” under Mission 1 of the ENTSO-E RDI Roadmap 2024-2034.

This technology is wide spread in the European transmission network. As an example, consider the following description.

Examples

TRL 9 for high voltage AC and extra high voltage AC in the operational applications. 
TRL 4 to 7 for the topics covered in the R&D section. 
To find more information on TRL definition for the Technopedia, read here

1. Axel Kraemer (Maschinenfabrik Reinhausen), “On-load tap-changer best practices” on going CIGRE Study Committee (WG N° A2.61) 

2. Lars LIDEN (HITACHI Energy), proposal for the creation of a new working group “Modern Insulating Liquids Qualification for OLTC, Bushings and other Accessories”, CIGRE Study Committees A2 and D1 (July 2023)