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Compared with a high-voltage alternative current power system, high-voltage direct current (HVDC) provides feasible solutions to interconnect two power systems between countries or regions, as it can send a vast amount of electrical power over long distances.

HVDC provides feasible solutions to interconnect two power systems across borders regions as it can send a vast amount of power over long distances.

HVDC improves power system stability and power transfer efficiency. A HVDC circuit can also be used in an AC system to form a hybrid system. As the technology develops, both voltage source converter (VSC) and line communicated converter (LCC) systems will find a growing market.


Our technical expertise extends throughout the project lifecycle. Examples include:

  • HVDC project engineer experience in the management of interconnection.
  • Network planning and analysis engineers capable of carrying out system studies and assessing HVDC contractors’ detailed system studies and control strategies.
  • Specification engineers and detail design checking engineers capable of providing a full optimisation enabled (OE) service to HVDC asset owners.

Our HVDC team is complemented by our AC transmission, civil and marine experts to provide a complete service to utilities considering new or enhanced HVDC links, or those considering whether proposed links should be AC or DC.


Our services include:

  • Project initiation and development

Optioneering DC interconnections can be a substantial and involved task and requires in-depth understanding of the necessary technology and systems. DC links can affect AC networks in many ways and early optioneering has to take all the relevant factors, risks and through-life costs into account.

  • Specification and tendering

Specification of HVDC systems and tendering has many specific aspects that do not occur in standard AC system projects. There is a limited supplier base, some specialist technology choices and potentially wide implications for connected AC systems. Contract strategy, technology strategy, project phasing and risk allocation all have to be addressed based on a full technical and commercial understanding of the issues.

  • Design and contract management

The complex technical nature of HVDC systems means contract management requires a high degree of attention. HVDC links are critical parts of a network and investment. To mitigate risk, asset owners need engineering experts to act on their behalf to:

  • Manage the HVDC contractor
  • Cover the detail design process
  • Mitigate potentially adverse effects on connected AC systems
  • Test the components and systems
  • Oversee site implementation
  • HVDC offshore

This is being implemented as a way to transfer power from large offshore wind farms. We are at the forefront of offshore wind farm transmission and offers full design and OE services for such links from initial assessment of the connection strategy to offshore and onshore link installation supervision. This covers AC links and DC links of all types.

  • Embedded HVDC

This offers opportunities to strengthen grids with reduced environmental impact compared to upgrading AC transmission. Long distance HVDC connections offer lower capital cost, lower loss and reduced wayleave.

  • Asynchronous HVDC connections

These are often the only practical way to connect systems operating at different frequencies or where it is not possible to make a direct synchronous connection.

Customer benefits

A one-stop service for all transmission and distribution skills.

Savings/value added demonstrated

Optimisation of onshore and offshore facilities to lower cost.

Selected project examples

HVDC interconnection, Philippines - Design and preparation of tender documents for the HVDC link consisting of a 350kV HVDC link, overhead lines, submarine cable, converter stations, electrode lines and electrodes.

Hinkley Point to Seabank DC Link, UK - Feasibility study for +/-300kV DC link and comparison with a 400kV AC equivalent link. Technologies considered included both line-commutated converter (LCC) with mass impregnated non-draining cables and voltage source converter-based HVDC with XLPE cables.

England-Scotland HVDC West Coast interconnector – Procurement process for a +/- 600kV, 2.2GW interconnector covering preparation of tender evaluation processes and tender evaluation for the converter stations and subsea cable, overall design, studies, equipment design, testing, commissioning, operation and maintenance.

UK-France 2000MW 285kV DC interconnector - Re-commissioning of a 2GW interconnector between France and the UK. We produced testing procedures, supervised testing, analysed results and provided staff guidance.

South-West area HVDC link feasibility study, UK - Detailed study to assess the feasibility of installing 2x1GW HVDC links – line commutated or voltage source converter – in the transmission network, and routing two bi-polar HVDC circuits either onshore or offshore. Both onshore and offshore cable studies investigated technical and routing feasibility using geological, environmental and routing constraint desktop methods.

UK-Norway HVDC interconnector - feasibility study Our feasibility study covered the design aspects of HVDC converter stations, subsea cable and landfall issues, land cable design and point of connection substation design. Detailed design of the converter stations and cables for permitting.

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