Isolation transformer HVDC Plus®

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Overview High-voltage direct-current (HVDC) converter transformers provide the crucial interface between AC networks and HVDC transmission systems. As power systems incorporate larger renewable capacities and require long-distance, low-loss transmission, HVDC converter transformers enable precise voltage adaptation, reliable insulation and coordinated operation with converter technologies for onshore and offshore applications.

The role of HVDC converter transformers HVDC converter transformers connect the AC grid to converter stations (LCC and VSC), ensuring correct voltage levels, dielectric separation and stable operation under combined AC/DC electrical stress and harmonic conditions.

Design objectives and capabilities

• Precise voltage adaptation to converter requirements
• Reliable AC/DC insulation and dielectric performance
• Operation under combined AC/DC stress and harmonic distortion
• Optimized efficiency to reduce transmission losses
• Modular, scalable construction for systems up to 1100 kV DC

How an HVDC transformer works In a converter station the HVDC transformer works with power electronic converters to enable AC/DC conversion. Typical process flow:

• AC power enters from the grid
• Transformer adjusts voltage to converter specifications
• Conditioned power is supplied to converter valves
• Converters perform rectification (AC→DC) or inversion (DC→AC)

The coordinated system enables controlled power flow, efficient long-distance transmission and stable grid integration.

HVDC Classic vs HVDC Plus® HVDC converter transformers support both line-commutated converter (HVDC Classic) and voltage-source converter (HVDC Plus®) applications. HVDC Classic is suited to large-scale bulk transmission; HVDC Plus® supports flexible multi-terminal topologies, offshore wind integration and dynamic grid services.

Core functions

• Voltage adaptation for rectifier and inverter operation
• Electrical isolation between AC and DC systems
• Reliable handling of UHVDC power levels
• Efficiency optimization to reduce lifecycle costs

Technical challenges addressed

• Dielectric and thermal stress: insulation and cooling systems for combined AC/DC voltages
• Mechanical durability: structures engineered for electromagnetic forces and fault conditions
• Advanced voltage regulation: precise control for grid stability under varying loads
• Noise and vibration control for environmental compliance
• Environmental robustness for harsh offshore and onshore conditions

Case study (highlight) Changji–Guquan HVDC project — delivery of a world-class HVDC converter transformer. Key data:

• Transformer capability: 1,100 kV DC
• Transmission capacity: 12 GW
• Dimensions: length 37.5 m, height 14.4 m
• Weight: >900 tons
• Efficiency: >99% at rated power
• Enables transmission over 3,284 km with minimal losses

Applications

• Point-to-point long-distance HVDC transmission for large renewable export
• HVDC PLUS® / VSC systems for multi-terminal and offshore integration
• Back-to-back installations for coupling asynchronous grids
• Multi-terminal and meshed HVDC grids for interconnected systems
• Offshore grid connections and subsea transmission

Siemens Energy innovations Siemens Energy HVDC transformer solutions incorporate advanced insulation for combined AC/DC stress, precise voltage regulation, high-efficiency designs, modular scalable construction up to 1100 kV and tailored engineering to meet project-specific requirements.

Technical characteristics / specifications

• Voltage capability: several hundred kV up to 1100 kV DC (UHVDC)
• Example power rating: up to 12 GW (Changji–Guquan)
• Example dimensions: 37.5 m length, 14.4 m height (case study)
• Example weight: >900 tons (case study)
• Example efficiency: >99% at rated power (case study)
• Design requirements: withstand combined AC/DC stress, harmonic distortion and dynamic loading
• Key technologies: advanced insulation, optimized core and winding design, integrated monitoring and diagnostics, modular scalable construction