AC Power Transformers (with Tap Changer)
Overview
Power transformers are an essential component of the Alternating Current (AC) electrical power system, enabling the exchange of the electrical power between grids with different voltage levels. Power transformers must be built to withstand severe electrical and mechanical stress from transients and fault currents.
Integrating an On-Load Tap Changer (OLTC) with the transformer allows for the regulation of the output voltage by adjusting the number of turns in one of the transformer windings (the transformation ratio). This ability is needed to keep the voltage in the connected networks in designed bandwidth according to the load condition. While all power transformers (HV/MV) have on-load tap changers, the distribution transformers (MV/LV) were built cost efficiently with off-load tap changers. Nowadays, the ability to also adapt the voltage in real-time in Medium Voltage (MV) and LV 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.
Moreover, in pairing with OLTCs, DSOs utilise Voltage Regulating Distribution Transformers (VRDTs), which are advanced distribution transformers designed to maintain stable voltage levels in electrical distribution networks. By being equipped with on-load tap changers, the transformer's voltage ratio without interrupting the power supply may be achieved. This capability enables VRDTs to respond dynamically to fluctuations in load demand or distributed energy generation, such as from solar panels or electric vehicles.
- Unlike conventional transformers, which either have fixed voltage ratios or rely on de-energized tap changers that require manual intervention and system downtime, VRDTs continuously monitor grid conditions and autonomously adjust their tap positions. This ensures that voltage remains within regulatory and operational limits, improving power quality, grid reliability, and energy efficiency.
- VRDTs are available in various configurations to suit different range of voltage and power ratings.
Benefits
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
- Enhances voltage stability and power quality
- Enables higher DER hosting capacity
- Reduces technical losses and customer complaints
- Supports active network management and grid automation
- Cost-Effective Grid Reinforcement: By mitigating voltage issues locally, VRDTs reduce the need for expensive infrastructure upgrades such as cable reinforcements or substation expansions, offering a more economical path to grid modernization
Challenges
The main challenges of AC power transformers with Tap Changer are:
- Tap Changer are significant points of transformer failure due to mechanical complexity and contact wear, requiring frequent maintenance
- mpact on environment due to insulating fluids used
Current Enablers
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
- Bridge static loading and very extensive transportation route planning
Applications
DSO
| Location: Hungary | Year: 2024 |
|---|---|
Description:
Introduction and descripton of the technology application The Danube InGrid project includes the replacement of conventional MV/LV transformers with OLTC transformers at a minimum of 50 locations. The first three sites (Paloznak, Komárom-Szőny, and Jánossomorja) each represent a distinct use case. These new transformers enable voltage regulation on the low-voltage side, allowing the supply voltage to automatically adapt to changes in load or local generation, ensuring more stable service for end users. Objective of the application The OLTC transformers were deployed at three pilot sites, each selected to address a specific challenge that this technology is designed to solve. The challanges that the distribution network is facing are:
At MV connection points with renewable energy input, voltage on the MV busbar can fluctuate significantly, affecting downstream consumers. OLTC transformers help stabilize these variations, ensuring consistent voltage delivery. In rural areas with increasing photovoltaic installations, LV voltage can exceed acceptable limits. OLTC transformers provide voltage regulation, keeping voltages within safe boundaries and potentially unlocking areas currently restricted from further PV integration. In resort and holiday areas, seasonal electricity demand often increases due to factors like air conditioning, heat pumps, and EV charging. OLTC transformers can automatically respond to these changing load patterns, helping to maintain stable voltage levels throughout the year. | |
| Design: Technical configuration:
Country specific factors:
Difficulties/challenges with implementation:
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| Result: Evaluation of results is ongoing. However, initial observations showed, that OLTC transformers at all sites were able to provide the consumers with a voltage in the correct range. Decoupling the low voltage side from the mid voltage side is possible with the technology. Figure 1 shows the voltages of a pilot location, left : transformer side, right: end of low voltage conductor; upper graphs: without voltage control, lower graphs: with voltage control.  | |
| Technology Readiness Level (TRL): TRL 8 | |
| References: | |
| Location: Netherlands | Year: 2015 |
|---|---|
| Description: The first VRDT commissioned in the Alliander grid was in 2015. By using VRDT, Alliander aims to overcome potential voltage problems proactively in short (locally-5 year) and long term (neighbourhood level, until 2050). This proactive stance will permit a better prioritization of operational labour capacity and cost efficiency, especially in low loaded long grid topologies. Simply put, digging ground for installing new cables is labour intensive, street work disrupts our clients and this all brings considerable additional initial costs. To reach/test the initial objective, VRDTs were installed in key locations, and the voltage quality monitored. The benefits were clear, and after the pilot-project phase, more VRDTs were installed throughout the grid. | |
| Design: The installation procedure of a distribution transformer with OLTC is practically the same as of a conventional transformer. | |
| Result: Alliander confirms that Voltage Regulating Distribution Transformers (VRDTs) are technically and economically viable in both rural areas with long, low-load cable networks and in densely populated urban areas where space for distribution transformers is limited and voltage regulation is challenging. Grid design studies and practical experiences confirm viability of VRDT for a lot of cases. There will be a future study where Alliander will also explore if this is feasible for all cases. For different aspects of the operational effects, existing papers should be considered. The VRDT helps a 100% to keep the voltage within the bandwith of the given setpoint. | |
| Technology Readiness Level (TRL): TRL 9 | |
| References: | |
TSO
| Location: Germany | Year: 2014 |
|---|---|
| Description: 420 kV power transformer (rated power of 400 MVA) for a substation for TransnetBW (TSO state of Baden-Württemberg) to link the 380 kV voltage level with the 110 kV grid. | |
| Design: Power transformer with ester oil as insulation. All permissible (over)temperatures have been rated according to IEC 60076-2. | |
| Result: 420 kV extra-high voltage level using natural ester. Due to the lower flammability, the transformer also has a higher fire protection class (K instead of O), so that the equipment can be used in densely populated areas. | |
| Technology Readiness Level (TRL): TRL 9 | |
| References: Franco Pizzutto, Jur Erbrink, and B. Kooij, “Techno-economic analysis of single-phase, versus three-phase load dependant, dynamic set-point control in voltage regulating distribution transformers (VRDT) to increase hosting capacity of low voltage networks,” IET conference proceedings., vol. 2024, no. 5, pp. 781-785, Jan. 2025, doi | |
R&D Needs
- Develop guidelines for proper On Load Tap Changer (OLTC) selection and transformer design integration
- Enhance the dielectric and thermal performance of insulating fluids used in transformers and OLTCs
- Ensure OLTCs can handle bidirectional power flows and voltage fluctuations caused by renewables and DERs
- Integrate sensors and analytics for condition monitoring and lifetime estimation
- Develop transformers and OLTCs that are easier to transport, install, and maintain
- Investigation of use of alternative oils for improved environmental impact
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. Moreover, in line with DSO Entity Technical Vision 2025, the technology highlights the need for intelligent, flexible, and automated distribution systems. VRDTs are key enablers of this vision by supporting local voltage control and grid observability.
Technology Readiness Level (TRL)
TRL 9 for HV AC and extra HV AC in the operational applications
TRL 4 to 7 for the topics covered in the R&D section
TRL 9 for VRDTs