Tomorrow's HVDC Grid
HVDC (High Voltage Direct Current) and HVDC Light
HVDC (High Voltage Direct Current) and HVDC Light® are systems for transmission of electric power. Both systems are applied to meet special requirements in power grids, and consist of a cable or line for direct current and two (or more) converter stations.
This page will try to explain where HVDC and HVDC Light can be used and what the benefits are.
With a few exceptions, all power lines you see in your surroundings carry alternating current (AC) that oscillates with 50 or 60 cycles per second - whether they are for (extra) high voltage, for medium voltage or for the low voltage distribution grid. These lines form a large interconnected network that ties the power generation plants (coal, gas, nuclear, hydro, wind, etc.) to the consumers.
But where does HVDC come in? It is used to interconnect separate power systems, where traditional alternating current (AC) connections can not be used.
The classical HVDC technique was first introduced in Sweden (Gotland link) in 1954 by ASEA (a founding company of ABB). Today there are over 100 projects in all parts of the world. Typically, a classical HVDC transmission has a power of more than 100 Megawatt (MW) and many are in the 1,000 - 3,000 MW range. There are classical HVDC transmissions that use overhead lines and that use undersea (and underground) cables (or combinations of cables and lines).
HVDC Light® can be called "The invisible power transmission" since it is based on underground cables, although over head lines also are a possibility. It is a fundamentally new power transmission technology developed by ABB in the 1990's. HVDC Light uses underground or submarine cables. The technology extends the economical power range of HVDC transmission down to just a few tens of Megawatts (MW). In the upper range, the technology can reach 1,200 MW and ±320 kV.
In both classical HVDC and HVDC Light it is possible to transmit power in both directions.
Applications for classical HVDC
In summary we find the use for classical HVDC in a power system:
· Long undersea cable links (> 50 km)
· Long overhead lines (> 600 km)
· Interconnection of different grids or networks
· Where control of transmitted power is of importance
· Combinations of the above
· Applications for HVDC Light®
The same applications as for classical HVDC are valid also for HVDC Light, but due to the smaller power rating of HVDC Light, its underground cable technology and its superior controllability, there are many more potential applications than for classical HVDC.
The fact that it is possible to build a long electric power transmission underground and avoid public opposition and long uncertain approval processes, makes the HVDC Light system very attractive. Some of the HVDC Light applications that are in commercial operation are listed here:
· Long underground cable link (70 km Gotland HVDC Light) from a wind park (Sweden).
· Long underground cable links (59 km Terranora interconnector & 180 km Murraylink) between different grids (Australia)
· Undersea cable link (40 km Cross Sound Cable) to Long Island (NY, USA)
· Long undersea cable link (70 km Troll A) to feed power to an offshore gas production platform (Norway)
· Interconnection of different grids (Eagle Pass)(USA)
One of the characteristics of HVDC Light is its superior ability to stabilize the AC voltage at the terminals. This is particularly important for wind parks, where the variation in wind speed can cause severe voltage fluctuations.
HVDC Light has also the potential of becoming the preferred system for power in feeds to cities, for strengthening of power networks in areas where the public is opposed to new overhead lines, and for evacuation of trapped marginal generation resources where a new extra high voltage AC line would be too costly. But this requires thinking outside the box.
Power stations generate alternating current, AC, and the power delivered to the consumers is in the form of AC. Why then is it sometimes more suitable to use direct current, HVDC, for transmitting electric power?
Each individual transmission project will display its own set of reasons justifying the choice of HVDC, but the most common arguments favoring HVDC are:
· Lower losses
· Long distance water crossing
· Asynchronous interconnections
· Limit short circuit currents
· Lower investment cost