Mr. Fintan Slye:

In terms of our analysis of the underground option, it is important to differentiate between the two technologies of AC and DC. They have very different characteristics and uses. Perhaps we have not been good enough at explaining the differences but I will articulate them.

With regard to AC technology, all of the grid is AC. All of the power generated in the generators located around the country is AC power, as is every single part of the power that goes through the transmission and distribution systems, flows into every home and is used for all the appliances. The grid has an alternating current of 50 Hz.

One can underground the voltage that we are talking about. With a backbone transmission of 400 kV, one can underground short sections of AC. One cannot technically underground the lengths we are talking about for any of the projects, be they North-South, Grid West or Grid Link. The lengths are far beyond what is technically possible to underground using AC technology. The longest AC 400 kV cable in the world is in Tokyo but it is in a large purpose-built tunnel that stretches from the outskirts to the heart of the city. No-one else in the world has built anything even close to that length. In London, one cable just short of 20 km has been built in a tunnel.

That is what is possible in terms of the technology. The amount we can put underground is dependent on two things, one of which is the specifics of the circuit. A line from Carrick-on-Shannon to west Mayo, which is what Grid West is, has a different set of characteristics from a line from Meath to Tyrone. The part of the network it is in is very different, as are its characteristics. That dictates how much we can put underground on a specific circuit. There are technical limits. There is a system-wide limit on the overall amount of cable of this type due to issues such as harmonics that arise with large amounts of cable on the system. That is the situation in respect of AC cables. For any specific individual project, we look to see whether it is being developed as an AC overhead line and whether there are specific sections that should be underground for reasons of special scenic beauty. We will look to see if it is appropriate to put it underground. That requires a compound at each end where we come down off the towers. There is a compound with a cable line interface and it goes underground for a number of kilometres before coming back up.

The other technology is called high-voltage direct current, HVDC, technology. It is a different technology, using direct current, and is different from the power that comes out of the all the generators in our transmission system and is used in our appliances. We need large converter stations to convert the power in our grid into this other form of power and to transmit it over cable. The advantage of DC is that it can be transmitted by cable over long distances, and that is primarily why it was developed - to link transmission grids, typically undersea, over long distances. That is its origin and that is largely where it is deployed today. It was deployed in the east-west interconnector, connecting Meath to Wales. It requires large converter stations on each side. This requires putting in a cable and transmitting it over large distances. Inserting it into the middle of a grid means it has a different set of characteristics and uses different technology from an AC connection between two points. There are significant technical limitations to using HVDC in the middle of an AC grid. We can use undergrounding for short sections of AC without materially changing the fundamentals of the solution being deployed, but changing to DC introduces a whole range of technical issues to the solution, notwithstanding the cost issue, which the independent commission pointed out was a factor of three.

Sometimes we are not good enough at explaining the difference between the two underground options. To go for these long projects underground means adopting and inserting this new and different technology into the middle of the grid, as distinct from short lengths, which can be done as part of the AC grid. I hope that explains the difference between the two and the nuances, or what is perceived to be nuance, of our position on it.