Compact substation HVDC PLUS®

Overview
High-voltage direct current (HVDC) transmission systems are increasingly central to modern power systems due to decarbonization, digitalization and the rise of distributed and inverter-based resources. HVDC PLUS® from Siemens Energy is an MMC-based VSC HVDC solution enabling efficient long-distance transmission, renewable integration and advanced grid-stabilizing functions.

Key proposition
HVDC PLUS® transmits large power quantities over long distances with high efficiency, supports integration of renewables and offers advanced control features to enhance grid stability. It complements FACTS and storage and supports grid modernization and resilience.

Use cases and main applications

• Point-to-point long-distance power transmission via DC cables or overhead DC lines.
• Connecting remote renewable generation (offshore/onshore wind farms) or remote loads to AC grids.
• Facilitating renewable integration and enabling cross-border energy trading.
• Embedded HVDC links for city-centre infeeds and large network interconnectors.
• Connecting asynchronous or different-frequency grids and providing a firewall against cascading blackouts.
• Multiterminal HVDC for collecting energy from multiple sites and distributing to several load centers.

HVDC PLUS® configurations
Symmetric Monopole
The symmetric monopole uses fully insulated DC conductors at equal and opposite voltages to ground, reducing earth return currents and allowing use of standard AC transformers. It limits DC pole-to-ground fault infeed from AC grids and offers proven scalability.

• Well suited for point-to-point HVDC transmission with two converter stations.
• Applicable with submarine/underground DC cables or overhead DC lines.

Bipole
The bipole increases capacity and flexibility versus symmetric monopole. Main variants include:

• Bipole with Metallic Return (DMR) — metallic conductor used as return path to reduce insulation requirements.
• Bipole with Electrode — earth/sea electrodes used during asymmetric monopole operation for maintenance or faults.
• Rigid Bipole — two conductors with opposite polarities and no normal return path; cost-efficient for long-distance cable transmission.

Each bipole design incorporates dedicated DC switchgear to improve redundancy and allow continued operation if one pole is out of service.

Fields of special application

• Multiterminal HVDC — flexible collection and distribution from multiple converter stations (e.g., multiple offshore farms to multiple load centers).
• Back-to-back HVDC — proximate converter stations for frequency conversion or asynchronous interconnection without a DC transmission line.

HVDC PLUS® features and benefits

• Independent, flexible control of active and reactive power (four-quadrant operation).
• Precise AC voltage control to support weak AC networks; can operate as a standalone STATCOM without a DC line.
• Reliable fault-ride-through capability including reactive fault current contribution.
• Acts as a firewall against system disturbances and cascading blackouts.
• Supports multiterminal systems with half-bridge and full-bridge converters and parallel converters for scalable capacity and redundancy.
• Performs under low short-circuit ratio conditions; suitable for both DC cable and overhead line transmission.
• Enables AC network restoration and black-start / System Recovery Ancillary Services (SRAS).

Technology background
Siemens Energy introduced MMC technology commercially in 2007 and commissioned the first MMC-based HVDC in 2010. MMC-based VSC HVDC has become the industry standard for VSC systems, offering compact footprint, grid-forming capabilities and reduced harmonics compared to legacy LCC (thyristor) approaches.

References and experience
Siemens Energy is an HVDC technology pioneer with over 50 years of experience and nearly 60 HVDC projects in more than 25 countries, including numerous LCC-based Classic® projects and a substantial portfolio of HVDC PLUS® implementations.

Additional materials
Product brochures and reference flyers (PDF) describing HVDC PLUS® features, configurations and references are available.

Technical specifications

• Platform: HVDC PLUS® (MMC-based VSC technology).
• Converter technologies: Modular Multilevel Converter (MMC) with half-bridge and full-bridge options; IGBT-based VSC enabling grid-forming and grid-following modes.
• Configuration options: Symmetric monopole, Bipole (DMR, electrode, rigid), Multiterminal, Back-to-back.
• Capabilities: Active/reactive power control, STATCOM operation, fault-ride-through, low SCR operation, black-start / SRAS support.
• Applications: Long-distance transmission (submarine/underground/overhead), renewable integration, interconnectors, city centre infeeds, asynchronous grid links, multiterminal networks.
• Operational benefits: Reduced transmission losses over long distances, enhanced grid stability, reduced earth return currents (symmetric monopole), scalable capacity and redundancy via parallel converters and DC switchgear.
• Proven project footprint: Decades of HVDC project delivery and multiple HVDC PLUS® deployments worldwide.