High-voltage direct-current (HVDC) converter transformers

High-voltage direct-current (HVDC) converter transformers are at the core of modern power transmission systems. As energy systems evolve and renewable integration increases, HVDC technology enables efficient long-distance transmission with reduced losses and enhanced controllability.

HVDC converter transformers act as the critical interface between the AC grid and HVDC systems, ensuring precise voltage adaptation and stable operation under complex electrical conditions.

Siemens Energy HVDC converter transformers combine decades of engineering expertise with advanced design and manufacturing capabilities to deliver high-performance solutions for ultra HVDC (UHVDC) applications (offshore and onshore). With a proven track record in major transmission projects, Siemens Energy supports reliable and efficient power transmission across diverse grid environments.

In the transition toward a more sustainable and interconnected energy system, HVDC transformers play a critical role in enabling reliable power transmission.

HVDC converter transformers are highly specialized high-voltage components used in converter stations, forming the critical interface between AC grids and HVDC transmission systems.

As a key component in UHVDC and long-distance transmission projects, HVDC transformers enable efficient AC/DC power conversion, supporting bulk power transmission at DC-voltage levels such as 800 kV and 1100 kV. This enables efficient long-distance power transmission with reduced losses, particularly for large-scale infrastructure projects.

HVDC converter transformer design ensures:

• Precise voltage adaptation for converter systems
• Reliable insulation between AC and DC systems
• Reliable operation under combined AC/DC electrical stress

While the transformer itself does not perform the AC/DC conversion, it ensures stable converter operation and enables efficient, controllable and low-loss power transmission.

These capabilities make HVDC transformers a key enabler of modern power transmission systems and future-proof grid infrastructure.

To understand this role in more detail, it is important to look at how an HVDC converter transformer operates within a converter station.

Within a converter station, the HVDC transformer operates in close coordination with power electronic converters (e.g. LCC or VSC systems) to enable efficient AC/DC power conversion.

Simplified process flow:

• AC power enters the HVDC transformer from the grid
• The transformer adjusts the voltage to match converter requirements
• The conditioned power is transferred to the converter valves
• The converter performs rectification (AC to DC) or inversion (DC to AC)

This process enables:

• Controlled power flow
• Stable grid integration
• Efficient long-distance transmission with minimized losses

The animation below illustrates how an HVDC transmission system works and highlights the role of HVDC transformers.

HVDC converter transformers are used in both line-commutated converter (LCC) systems, commonly referred to as HVDC Classic, and voltage source converter (VSC) systems, known as HVDC Plus®.

While HVDC Classic is typically applied in large-scale, long-distance bulk power transmission projects, HVDC Plus® enables more flexible applications such as offshore wind integration, multi-terminal systems and dynamic grid stabilization.

As a result, HVDC converter transformers are specifically designed to meet the distinct electrical and operational requirements of each technology, ensuring reliable performance across a wide range of transmission scenarios.

As part of the HVDC converter station, HVDC converter transformers provide:

• Voltage adaptation for rectifier and inverter operation
• Electrical isolation between AC and DC systems
• Reliable handling of UHVDC and power levels
• Optimized efficiency to minimize transmission losses and reduce overall system operating costs

Their design must also withstand:

• Combined AC/DC voltage stress
• Harmonic distortion from converter operation
• Dynamic load conditions

This makes HVDC converter transformers more complex than conventional high-voltage transformers, requiring advanced insulation, winding design and cooling solutions to ensure reliable operation and long-term performance.

HVDC transformers are essential for flexible, efficient and future-ready power transmission.

They enable:

• Integration of renewable energy (e.g. offshore wind, solar)
• Controllable power flow for improved grid stability
• Reduced transmission losses over long distances
• Interconnection of asynchronous grids

Projects such as SuedLink (DC3) in Germany demonstrate how HVDC technology enables large-scale renewable energy transmission.

HVDC systems are widely used for:

• Bulk power transmission
• Offshore and subsea connections
• Cross-border and intercontinental links

Learn more about broader HVDC solutions.

In these applications, HVDC converter transformers ensure reliable and efficient grid operation.

Siemens Energy is a leading manufacturer of HVDC converter transformers, with more than 50 years of experience in design and manufacturing across global transmission projects.

Key innovations in HVDC transformer design:

• Advanced insulation systems
  Designed to withstand combined AC/DC stress conditions and ensure long-term reliability
• Advanced voltage regulation
  Enabling precise voltage control during operation
• High-efficiency design
  Minimizing losses and improving lifecycle performance, reducing total cost of ownership
• Modular and scalable construction
  Supporting applications even up to 1100 kV HVDC systems
• Custom engineering
  Tailored for HVDC Classic and HVDC PLUS® converter technologies, to meet specific project requirements

Explore Siemens Energy’s power transformer portfolio.

HVDC converter transformers are deployed in various HVDC systems designed to meet specific power system requirements:

• Point-to-Point HVDC Transmission
  Connecting large-scale renewable power plants to onshore grids with minimized losses
• HVDC PLUS® / Voltage Source Converter (VSC) Systems
  Enabling flexible power flow control, multi-terminal integration and renewable energy connection
• Back-to-back HVDC installations
  Coupling asynchronous grids and ensuring stable power exchange
• Multi-terminal HVDC grids
  Supporting interconnected and scalable power systems
• Offshore grid connections
  Enabling reliable operation in harsh marine environments

Designing HVDC converter transformers requires addressing complex electrical, thermal and mechanical challenges:

• Dielectric and thermal stress
  Insulation and cooling systems must withstand combined AC/DC voltages and continuous high-power operation
• Mechanical durability
  Structures must resist electromagnetic forces during switching and fault conditions
• Advanced voltage regulation
  Supporting reliable operation and grid stability under changing load conditions
• Noise and vibration control
  Optimized designs reduce environmental impact and ensure compliance near populated areas
• Environmental reliability
  Solutions are designed for harsh offshore and onshore conditions

This ensures reliable operation, reduced downtime and long-term performance across a wide range of operating conditions.

Siemens Energy combines advanced simulation tools, rigorous testing and decades of manufacturing experience to meet these requirements.

Learn how grid stability solutions complement HVDC.

As power systems evolve, HVDC transformer technology continues to advance to support increasingly complex, renewable-driven grids.

Ultra HVDC for long-distance transmission

UHVDC systems operate at voltage levels of up to 1100 kV, enabling efficient bulk power transmission over long distances and supporting large-scale renewable integration.

MVDC and hybrid AC/DC systems

Medium-voltage direct current (MVDC) solutions are increasingly applied in industrial, offshore and urban environments, supporting more flexible and efficient power distribution and enabling new grid architectures.

Digitalization and data-driven operation

Advanced digital solutions, including real-time monitoring, data analytics and digital twin technologies, support predictive maintenance, improved asset performance and optimized lifecycle management

Meshed HVDC grids and multi-terminal systems
Looking ahead, the development of multi-terminal and meshed HVDC grids is gaining momentum. These next-generation networks will require highly reliable and adaptable transformer solutions to support increasingly interconnected DC systems and ensure stable operation under more complex load conditions.

Explore how Siemens Energy supports next-generation HVDC grids.

Choosing Siemens Energy means partnering with a trusted provider of HVDC converter transformers that deliver reliable performance, long-term value and efficient project execution.

Our expertise and global service network help you to:

• Accelerate project timelines with flexible, turnkey solutions 
• Maximize system availability with advanced monitoring and predictive maintenance 
• Build scalable and future-proof HVDC infrastructure 

Whether for large-scale transmission projects or complex grid integration challenges, Siemens Energy supports customers in designing efficient, reliable and future-ready HVDC solutions.