The second key aspect of hybrid grids: The mix of transmission and distribution (T&D)
This is the third blog I’ve written in a on series hybrid grids. In my first blog, I defined hybrid grids and introduced their four key aspects: mix of generation, mix of transmission and distribution (T&D), mix of delivery and mix of control. The second blog elaborated on the first key aspect: mix of generation. In today’s blog I’ll discuss the second key aspect of hybrid grids: transmission and distribution, as well as some of the implications it has for the electric power system.
This key aspect answers the question of by which preferred technological means is electricity “transported” from the point where it is generated to the point where it is used? This is usually done by a network of overhead lines and underground cables, but this can also be local generation with solar panels or micro turbines, possibly combined with storage (not necessarily in the form of electricity). Even mobile sources like batteries from electric vehicles can serve as a local T&D system, and wireless is becoming popular for low energy or short distance, e.g., inductive charging of phones, cooking applications and fast car charging.
Moving forward into the future, there are three key words to remember when thinking of transmission and distribution technology:
– Effective (think of losses, for example)
– Reliable (governs things like access and availability)
– Accepted (meets societal preferences)
For the sake of simplicity, I will now focus on the comparison of lines and cables only.
It is good to recognize the distinction in power engineering that lines are overhead and can be found on poles and towers and cables are underground and buried in soil or put in pipes/tunnels. For both lines and cables a wide range of voltages and power ratings is possible.
Over the last few years we have seen more and more interest in high-power underground cable connections. This begs the question of whether or not this is a trend, and, if so, what are the consequences for the power system? There are three main reasons that underground cables are preferred to overhead lines, although in general cable solutions are (much) more expensive.
Reason 1 – Limited Space Above Ground
The available space for new or the expansion of existing transmission lines and substations is not boundless. It is very difficult and costly to acquire the land for substations or establish the necessary right of way for the transmission corridor.
Reason 2 – Public Opinion
Public opinion has been increasingly marked by CAVE-thinking (Citizens Against Virtually Everything). People oppose to transmission lines for various reasons, such as visual impact, nature preservation and decrease of real estate property in the neighborhood of substations and transmission lines.
Reason 3 – Availability & Reliability
The increase in availability and reliability is especially true for the distribution grid as cable connections can be made very reliable, easy to monitor and diagnose, e.g., with smart cable guard technology, and most important of all, are not prone to failure during extreme weather events such as storms and flooding. Where I live in the Netherlands a common practice is to use watertight power cables.
Countries that have an (almost) exclusive medium- and low-voltage cable distribution grid have a very reliable public power supply, with outages only a few times a year which average less than an hour in duration.
However, above-ground technologies are often still the method of choice because cables are much more expensive than lines – for transmission a factor 6 to 20 can be used as a rule of thumb.
In spite of the cost, undergrounding the power system is definitely becoming a trend. For high voltage transmission there are, of course, some technical issues that need to be addressed when the share of cable increases, e.g., the need for qualified (tested) new types of power cable with high transmission capacity (presently around 1000MVA for a single cable). The reliability of long stretches of such high power cables over land with many cable joints is also a factor.
In my next blog I’ll elaborate on the third key aspect of hybrid grids and its implications: mix of delivery.