HVDC Technology

Direct current (DC) is the preferred technology for moving large amounts of power across long distances. DC results in overall higher efficiency and reliability than an equivalently-sized alternating current (AC) system moving the same amount of power.

Historically, the transfer of electricity between regions has been over high voltage alternating current (AC) transmission lines (typically along overhead wires and pylons), which means that both the voltage and the current on these lines move in a wave-like pattern along the lines and are continually changing direction. In Europe, this change in direction occurs 50 times per second (defined as 50 hertz [Hz]). The electric power transmitted over AC transmission lines is exactly the same as the power we use every day from AC outlets, but at a much higher voltage.

Unlike an AC transmission line, the voltage and current on a direct current (DC) transmission line are not time varying, meaning they do not change direction as energy is transmitted. DC electricity is the constant, zero-frequency movement of electrons from an area of negative (-) charge to an area of positive (+) charge.

The first commercial electric power system built by Thomas Edison in the late nineteenth century carried DC electricity, but given some early advantages, AC power eventually became the primary power system worldwide. Some of these advantages are no longer applicable (e.g. technology has advanced to allow efficient conversion from AC to DC), and DC transmission is the preferred solution for moving large amounts of power over long distances with considerable social and commercial advantages.

Two converter stations connected with a submarine cable

HVDC delivers power from one grid system to the other by converting power to DC, using power electronic switches called thyristors. The resulting DC power is then transmitted over hundreds of miles along a pair of buried cables before being converted back to AC. This system comprises two converter stations at either terminal, with an underground or submarine cable link between them (as shown on the first illustration). Cable-laying process is demonstrated on the illustration below. After the DC power is converted back to AC it is transformed to the common voltage of the grid to which it is being connected.

Innovations in almost every area of HVDC have been constantly adding to the reliability of this technology with economic benefits for users throughout the world.

A major advantage of DC power lines is their efficiency – less energy is lost as it is transmitted and there is no need for reactive compensation along the line. Because DC flows steadily through the wires without changing direction many times each second and through the entire conductor rather than at the surface, DC transmission lines typically lose less power than AC transmission lines.

DC has harmless magnetic fields, cables are hidden underground and installation has extremely small footprint. DC converter station looks like a warehouse building which occupies relatively little space. It is almost entirely noiseless and is completely pollution free.

Cable-laying process
Cable-laying process

Key Features

  • High Controllability & Flexibility in Comparison to Classic HVDC
  • Allows for Independent & Fast Control of Active & Reactive Power
  • Black Start Capability
  • Firewall Against Disturbance
  • Inherent STATCOM Functionality
  • Low Environmental Impact & Footprint
  • Easy Assembly & Maintenance

Key Solutions

  • Back-to-Back Connection
  • Undersea Cable Transmission
  • HVDC Overhead Transmission
  • De-Icing Applications
  • Renewable Energy Integration
  • Adjustment of the AC Network Voltage & Contribution to Improved Power Quality
  • Supply for Offshore Loads
  • In-Feed to City Centres
  • Multi-Terminal Interconnection
  • Feeding into Passive Networks
  • Grid Interconnection & Power Trading
  • Wind Farm Integration (especially for farm with a transmission distance over 50-100 km)

Europagrid’s interconnectors are built with HVDC Flexible transmission technology based on a voltage source converter (VSC). In contrast to classic HVDC, power reversal in HVDC Flexible does not involve a change of DC voltage polarity, but is done via a change of DC current direction. This function makes HVDC Flexible a very attractive solution for multi-terminal systems. HVDC Flexible generates a controlled output voltage, the magnitude and phase angle of which can be changed rapidly and flexibly.

As a result, HVDC Flexible allows for independent and fast control of active and reactive power.

Valve module
Valve module

We use standardised designs with compact transportable modules, which are factory assembled and pre-tested. This speeds the process up, with delivery times sometimes as short as 12 months.

Our converter stations are designed to blend seamlessly into the surrounding architectural environment and are virtually maintenance free.

Valve tower
Valve tower

HVDC Flexible also has a very low contribution towards the short circuit rating of network equipment, further minimising network impacts. This technology has a proven track record and ensures extremely high levels of availability – greater than 98%.

The combination of this innovative technology coupled with our unique marine capabilities is a successful template for the rapid deployment of subsea network assets across Europe.


Key Advantages of DC

  • More efficient: Over long distances, DC transmission can move more power with less electrical losses than an equivalent AC transmission line.
  • Lower Cost: Higher efficiency means a lower transmission cost.
  • Improved Reliability: HVDC transmission can enhance system stability, allow the operator complete control over power flow, and facilitate the integration of wind from different resource areas.
  • Smaller Footprint: DC transmission lines require narrower right-of-way footprints, using less land, than equivalent AC lines.