Function
An HVDC system converts and transmits the electrical power generated by the wind turbines. For example, turbines generate at 66kV AC, this is transformed to 132kV AC and then converted 320 kV DC which is transmitted to shore via the export cables. Equipment in the onshore substation converts the voltage back to say 275kV or 400kV for connection to the onshore transmission grid.
Who supplies them
A&T Enclosures, Acrastyle, Hornbill Engineering, Hitachi Energy, GE Grid Solutions and Siemens Energy.
Key facts
Key components of an HVDC system include:
- HV switchgear sets to isolate and protect each array and export connection to the substation.
- Converters to convert AC to DC at a higher voltage for onward transmission.
- Earthing systems including lightning protection connecting electrical components and the substation structure.
- Cable trays, tracks, clamps, and supports to protect electrical items.
An HVDC transmission system, including the export cables and offshore and onshore substations, typically offers a lower lifetime cost (when also taking into account the lower electrical losses of an HVDC system over these distances) than the equivalent HVAC system for wind farms where the distance to the onshore substation is greater than about 80 to 100 km. The factors used in choosing between HVAC and HVDC are, however, complex.
HVDC systems use relatively new technology and systems that are custom designed for the transmission of high power, say over 750 MW, over long distances. HVDC systems currently only operate point-to-point and require the use of a matched pair of converters at each substation (one onshore and one offshore).
Cost reductions have been seen in recent years and are expected to continue, driven by new technology and the increase in the use of interconnectors, as well as by offshore wind.
What’s in it
- HV AC and DC switchgear
- Transformers
- Converters
- Passive and active reactive power compensation
- Earthing systems
- Auxiliary electrical, control and monitoring systems
- Industrial waterproof enclosures
- Cable trays, tracks, clamps and supports to protect electrical items