I first learned about how the electricity grid works in the early 1980s, which meant I was taught the conventional wisdom of the times: high-voltage AC (alternating current) transmission was ubiquitous because it was the most efficient way to send electricity the long distances needed by the electricity grid.
So I was intrigued to hear that projects such as Sun Cable, the proposal to export solar power generated in the Northern Territory to Singapore, will be designed around high-voltage DC (direct current) technologies.
To find out why, I chatted to Sun Cableâ€™s Andrew Coffey about the project.
â€śAC is quite hard to transmit over very long distances,â€ť he explained.
Thatâ€™s because of the reactive way AC interacts with the cables that transmit it:
â€śAn overhead cable acts as an inductor, so you have to put a bunch of capacitors along the route to counteract that,â€ť he said.
The opposite is true of an underground cable, Coffey said: the cable acts as a capacitor, so designers have to respond by installing inductors along the cable route.
â€śThereâ€™s a tipping point where the cost of converting the signal from AC to DC outweighs the cost of all the capacitors or inductors you have to installâ€ť, he said.
Direct current isnâ€™t affected by capacitance or inductance, so while there are some losses from the resistance of the cable, in a high-voltage application losses are less than in AC transmission systems.
â€śWhen youâ€™re doing long distance, very high power, HVDC is more cost efficient than AC transmission,â€ť Coffey said.
For a more detailed comparison of AC and DC transmission systems, Section 2.2 ofÂ this document (PDF) by Americaâ€™s Energy Information Administration is handy. Its assessment is HVDCâ€™s economic advantages come from the reactive losses Coffey mentioned, but also lower loss from skin effect (the non-uniform distribution of current in the conductorâ€™s cross-section) and lower right-of-way requirements because HVDC has a smaller footprint (you only need two conductors, compared to six conductors for a high-voltage AC system).
Importantly, the EIA paper notes, HVDC is asynchronous â€“ it doesnâ€™t depend on multiple networks being synchronised (operating at the same voltage, system frequency, and system timing) â€“ meaning HVDC is also an ideal technology to interconnect different AC systems.
And in submarine applications, the EIA paper notes HVDC can often be the only option: at some point, the capacitance of an AC system limits its maximum transmission distance.
Coffey said the systems under evaluation by Sun Cable are 525 kV, but theyâ€™re also modelling a 600 kV design in case that technology becomes available in time for the planned construction.
The company isnâ€™t daunted by the engineering involved, he said, because:
â€śWhat weâ€™re doing has been done before over shorter distances. Weâ€™re not doing anything new in that sense, weâ€™re just using a lot more cable.â€ť
â€śThere just hasnâ€™t been a situation where you have both a very low cost electricity generation source, which we have in northern Australia; and you have a customer who has a desire for more renewable energy electricity in their system and limited opportunities to do that in-country.â€ť
The particular dynamic of generating power in northern Australia for consumption in Singapore, â€śhasnâ€™t really occurred in historyâ€ť, Coffey said, because the traditional model is to transport the fuel (gas or coal) and burn it in a generator at the destination.
â€śThere are no technical barriers,â€ť he said, â€śthereâ€™s just the commercial question about whether it makes sense to deploy such a long cableâ€ť.
Coffey told SolarQuotes the biggest technology development enabling Sun Cable is the steady increase in the maximum voltages HVDC systems can carry, because higher voltages reduce losses.
â€śTechnology development here is largely materials science,â€ť he said. â€śNew polymers and enhancing those polymers to ensure they provide the necessary insulation to carry the very high voltages.â€ť
â€śHigher voltages reduce the metal content in the cable, and that reduces the weight, meaning you can lay cables in much deeper water.â€ť
Currently the deepest submarine HVDC cable in the world connects Sardenia to Italy, with sections as deep as 1,600 metres â€“ nowhere near the depth that optical fibre telecommunications cables operate at, but deep enough that Sun Cable can navigate a route to Singapore.
However, the huge weight of the HVDC cable gives it a relative advantage over a fibre optic cable: itâ€™s far too heavy to suffer the storm damage that regularly shuts down communications links. That leaves shipping as the main external risk to a cableâ€™s operation.
Europe is the hotspot for HVDC submarine cable manufacture, Coffey said, with vendors includingÂ ABB,Â Prysmian,Â Nexans, andÂ NKT. Japanâ€™sÂ Sumitomo also manufactures HVDC systems, and while China is home to more than one manufacturer, its systems are mostly produced for local projects.
Sun Cable is still working out the commercial details of the project, as well as optimising the size of the solar energy plant and battery to ensure it works with the expected customer loads, and finally, doing the necessary environmental work.
â€śIt all looks set to be very good,â€ť Coffey told us. â€śThe exciting thing is that the site and the solar farm being very large at 10GW, is such a tiny proportion of the available energy resource in Central Australia. So thereâ€™s no barrier to increasing the renewable energy in the whole South-East Asia system in the next decades.â€ť