Is “undergrounding” becoming a transmission trend?
Over the last few years we have seen more and more interest in high power underground cable connections. The obvious question to ask is: will this be a trend? And if so, what are the consequences for the transmission and distribution grid? There are several reasons why underground cables are preferred to overhead lines, although in general cable connections are (much) more expensive.
The first reason is the limited space availability for new and existing substations with no possibility to expand and the necessary right of way for the transmission corridor. A second reason is the increased customer awareness of CAVE-thinking (Citizens Against Virtually Everything) opposing for various reasons, such as visual impact of transmission lines, nature preservation, and decrease of real estate property in the neighborhood of substations and transmission lines. A third reason is the increased reliability for the transmission grid. This is especially true for the distribution grid as cable connections can be made very reliable, easy to monitor and diagnose, and most of all, are not prone to extreme weather events such as super storms and flooding (in the Netherlands a common practice is to use watertight cables). Countries which have an almost exclusive medium voltage cable distribution grid, like Singapore and the Netherlands, have a very reliable public power supply with an average customer outage rate of less than 30 minutes a year.
Therefore in response to the original question about trends, the answer is yes. Although “above-ground” technologies are often still the method of choice, undergrounding is definitely an upcoming trend. Although more expensive than overhead transmission line, due to the digging and tunneling costs, it is driven by other previously mentioned factors.
There are, of course, some technical issues that need to be addressed and taken care of, such as changes within grid as it behaves more capacitive instead of inductive due to increased high voltage cable connections in the transmission grid; limited transmission capacity of high power cables (presently around 1000MVA for a single cable) compared to overhead line connections; and the reliability of long stretches of such high power transmission cables with many cable joints is point of attention.