Transmission system operators (TSOs) are examining a breakthrough made by ABB that could help to more efficiently and cheaply link renewable energy sources, which tend to be in remote locations, to the rest of the grid.
ABB said its high voltage direct current (HVDC) circuit breaker is the first of its kind to work quickly enough to allow safe operation of an HVDC transmission network, which is seen as a possible way to deal with the growing amount of renewable energy generation.
‘There is a need to transport more energy across Europe, both north/south and east/west,’ said ABB technology manager Per Skytt, speaking at a press conference.
‘Over long distances, DC is more cost effective than AC … It’s also a more efficient use of land because you can use lines that transmit more power in the same space.’
Underwater HVDC cables are currently used to link several European national grids with one another and HVDC is expected to become even more important as a way of connecting multiple wind, solar or hydro-electric plants and transporting their energy across the continent.
Internal HVDC lines are already being rolled out in Germany, where the decision to close down nuclear power plants will leave the energy-intensive industrial south of the country with a shortage of nearby power.
But these kind of lines can only connect two individual points and can’t be linked in a mesh network because pre-existing HVDC circuit breaker technology relies on mechanical systems that interrupt the flow of current for so long (tens of milliseconds) that it would destabilise such a grid.
The problem of how to break DC transmission circuits without significantly disrupting their operation has challenged engineers for over 100 years and was a factor in the dominance of AC transmission for most of the 20th century.
AC circuits are easier to break because the oscillation of their energy flow means current is regularly reduced to zero, at which point the circuit can be broken without causing disruption.
And faults tend to propagate more quickly across HVDC networks because they aren’t interrupted by transformers, increasing the importance of breakers.
To solve the problem for HVDC, ABB developed a hybrid system that resets a circuit in fewer than five milliseconds and with less than 0.01 per cent energy loss, small enough not to cause problems for the network.
When a fault such as a power surge from a lightning strike is detected on the line, the circuit breaker’s switch shifts current flow from its primary path to a secondary path comprised of semiconductors.
A disconnector on the main path driven by magnetic actuators then opens and closes to reset the voltage and the current is shifted again into a bank of arresters, which absorb the excess energy. The current can then be returned to its original path.
‘The disconnector operates around 50 times faster than anything available today,’ said Skytt.
ABB has tested the circuit breaker in the lab at the 320kV level typically used by HVDC lines and plans to test it in the field over the next two years.
Skytt said the company was in ‘serious negotiations’ with European TSOs about deploying the breaker since it was announced in November 2012.
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