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Electricity | Transmission vs Distribution

Electricity supply around the world is split up into three categories: generation; transmission; and distribution. Transmission and distribution are commonly overlooked as being the same thing; however there are quite distinct differences between the two. Below is an image illustrating the process of energy transportation in an easy to understand manner:

So, in a nutshell...

  1. Generation: The electricity produced from the source e.g. power plants, wind turbines etc.
  2. In order to reduce resistance in transmission lines, the electricity is passed through a step-up transformer to increase the voltage and therefore reduce the current. This allows the electricity to be transported over large distances without losing as much electricity in the process.
  3. The electricity is transmitted across the country bidirectionally to allow the grid’s needs to be fulfilled. This reduces the risk of power cuts and allows electricity to be sent to where it is needed.
  4. Because the voltage of the transmission network is way too high to be distributed directly into homes and businesses, the electricity is passed through a step-down transformer to reduce the voltage.
  5. The electricity is now transferred to low voltage distribution lines that are managed by area-specific distribution companies. This electricity is now transported over small distances, usually within small localities.
  6. You can put the kettle on and watch your favourite tele programme.

Transmission

Transmission is the stage between the energy creation source and the wooden pylon lines that are dotted around your residential localities. Transmission pylons can commonly be seen next to motorways and in open green spaces. They look a little like this:

These metal structures are used to carry extremely high voltages of electricity up and down the country at high speeds. For safety reasons they need to be large structures that allow the electricity to transported at a safe distance from the ground. The height of transmission pylons commonly ranges between 15m-55m; however, they are constructed up to 190m in the UK. There are two voltage categories for these structures: those that carry between 50,000V - 200,00V, called ‘high voltage lines’; and those that carry over 200,000V called ‘extreme high voltage lines’. Transmission lines in the UK use alternating current (AC) as their form of electricity transportation. It is vital that the network runs in such a way that allows bidirectionality in order to provide energy to where it is needed. In the USA, over the larger distances that cross state lines (etc.), high voltage direct current (HVDC) is used to efficiently transport electricity unidirectionally; however, this would not make sense in the UK because of its relatively small size and comprehensive AC network. As such, transmission lines pertain to the bidirectional AC network.

As shown in the map below, the transmission network in the UK is primarily managed by National Grid; however, in Scotland it is split into two sections: SP Energy Networks; and Scottish and Southern Energy.

Maintenance

The maintenance of transmission lines and pylons is an extremely long and difficult process; however, it has the ability to add up to 50 years of life onto its functional duration. It also improves the performance of electricity transmission, allowing its transportation to be smoother and lose less energy in the process. This task is carried out by extremely skilled engineers and can take hours, days and even weeks.

Distribution

Distribution lines are local networks of small, (usually) wooden pylons that distribute lower voltage electricity over shorter distances. They look a little like this:

Because the electricity that they carry is of a considerably lesser voltage, distribution line pylons do not need to be constructed in the same manner of transmission lines. The line height of distribution pylons are not that much over 10m, which in comparison to transmission lines that in the UK have been constructed up to 190m, is rather tiny. There are two voltage categories for distribution lines: those that carry between 0V - 1000V, called ‘low voltage lines’; and those that carry between 1000V - 50,000V, called ‘medium voltage lines. The voltage that enters our homes in the UK is 230V, which means the entry point comes from the common ‘low voltage line’. Distribution lines, just like transmission lines in the UK, use AC to transport electricity to our homes. This allows networks of electricity to be much more forgiving and adaptive.

As shown in the map below each area of the UK has its own Network Distribution Operator (DNO) which manages the distribution of electricity to each household under their operation: