John Power: Cost is not only factor to consider – reliability also crucial
IRELAND needs a robust electricity transmission system. We also need to decide if we want the lowest-cost infrastructure which will help minimise bills, or a higher-priced alternative that may negatively impact job retention and creation.
Safety, reliability, maintenance and capital cost are among the key factors to be considered when assessing the most suitable design for a significant, large-scale upgrade of the national energy infrastructure. While cost is crucial, it is not always the deciding factor.
There are two technology alternatives: AC (Alternating Current) or DC (Direct Current) – both of which can be placed underground or overhead.
AC technology is the standard for electrical systems worldwide. In common with every other country in Europe, Ireland has an AC transmission system, the vast majority of which is overhead.
DC cables can be put underground. Due to the fact that it is not the standard technology on the Irish power network, a large converter station is needed everywhere it plugs in and out of the existing main AC system – similar to that at the end of the East West Interconnector in Meath.
Each of these stations is about the size of a football pitch and costs about €75m. In the case of Grid25, the DC option impacts on the overall costs of the scheme but, more importantly, has a lower capacity than AC, which presents a barrier to adding new users over time.
DC circuits, unlike the AC network, cannot react instantly to events such as faults or outages on the line. Due to the inflexible reaction, underground DC cables can result in a risk of blackouts and limits how much the circuit can be used. Therefore, DC is not a reliable choice.
While AC lines can be put underground, there is a limit to the length of AC cable that will work at any given voltage or point on the network. Underground AC lines are typically found in urban areas where lower voltages are sufficient.
Underground cables are regularly damaged by third-party activity such as road works and operation of diggers, or internal failures.
Unlike overhead lines, these failures take significant time to repair, which can reduce the ability of the transmission network to efficiently supply power to customers.
JOHN POWER IS A CHARTERED ENGINEER AND DIRECTOR GENERAL AT ENGINEERS IRELAND.